Hydrologic Analysis of Hurricane Matthew’s Impact …...2018/08/14 · Hydrologic Analysis of...

Hydrologic Analysis of Hurricane Matthew’s Impact on Dam Safety in North Carolina and South Carolina

August 2018

Federal Emergency Management Agency Department of Homeland Security 500 C Street, SW Washington, DC 20472

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page ES-1

Executive Summary In early October of 2016, Hurricane Matthew brought heavy rains to North Carolina and South Carolina, making landfall on October 8, 2016, near McClellanville, SC. Hurricane Matthew’s extreme precipitation fell on soils already saturated by previous inland storms, resulting in widespread severe flooding that led to 67 breached or overtopped dams in North Carolina and South Carolina. The Federal Emergency Management Agency (FEMA) commissioned this hydrological analysis to help characterize the severity of Hurricane Matthew in North Carolina and South Carolina and provide a basis for estimating the magnitude of storm or flood events that led to dam breaches after Hurricane Matthew.

Hurricane Matthew’s extreme flooding resulted in breaches at 17 dams and overtopping of at least 23 dams in North Carolina. In addition, a week prior to Hurricane Matthew, localized heavy rainfall in northern Cumberland County, NC, resulted in the breach of two additional State-regulated dams, bringing the total to 19 breached or partially breached dams in North Carolina. In South Carolina, Hurricane Matthew resulted in breaches at 25 dams. Table ES-1 summarizes the dam breaches and overtopping from Hurricane Matthew in North Carolina and South Carolina.

Table ES-1: Summary of Dam Breaches and Overtopping from Hurricane Matthew

Dam Hazard

North Carolina South Carolina Total by Hazard Breached Overtopped Breached Overtopped

High-Hazard 12 18 0 0 30

Intermediate-Hazard or Significant-Hazard

2 (1 unregulated)

0 15 0 17

Low-Hazard 5 (5 unregulated)

5 (2 unregulated)

5 0 15

Undefined Hazard Classification 0 0 5 0 5

Total by Damage 19 23 25 0 67

Additional information on each of the dams is included in Appendix A

Precipitation. Total precipitation in September 2016 was above normal for the 12 counties in North Carolina and 10 South Carolina counties where dams were breached. There was especially heavy precipitation in isolated areas of Hoke and Cumberland Counties in North Carolina during the final 3 days of September. The higher-than-normal precipitation in the month leading up to Hurricane Matthew was followed by Hurricane Matthews’s extreme precipitation in early October. The duration and recurrence interval of observed precipitation depths at 114 precipitation gages in the 22 impacted counties in North Carolina and South Carolina were evaluated to characterize the severity of Hurricane Matthew rainfall. Additionally, the precipitation depths at 52 of the 67 impacted dams in North Carolina and South Carolina were estimated in order to characterize the severity of Hurricane Matthew rainfall at the breached and overtopped dams.

Table ES-2 shows the characteristics of the 1-day and 3-day total precipitation for 114 precipitation gages in 22 impacted counties in North Carolina and South Carolina and the estimated 1-day and 3-day precipitation depths computed at the 44 breached dams and 8 of the 23 overtopped dams.


Table ES-2: Summary of Precipitation Depths and Recurrence Intervals

Characteristic 1-day total precipitation 3-day total precipitation

Precipitation recurrence interval – measured at 114 precipitation gages

Estimated computed precipitation depths 3.9 inches to a maximum of 16.6 inches

4.5 inches to a maximum of 19.0 inches.

Average and median precipitation depth Average: 9.9 inches Median: 9.8 inches

Average: 11.1 inches Median: 11.1 inches

Estimated precipitation recurrence intervals 3 years to a maximum of 3,160 years

2 years to a maximum of 2,910 years

Average rainfall recurrence interval 353 years 296 years

Precipitation depth and recurrence – measured at 44 breached dams and 8 of the 23 overtopped dams

Estimated precipitation depths 4.0 inches to a maximum of 15.5 inches

4.5 inches to a maximum of 15.8 inches

Average and median precipitation depth Average: 10.1 inches Median: 10.4 inches

Average: 11.8 inches Median: 12.5 inches

Estimated precipitation recurrence intervals 13 years to a maximum of 2,500 years

3 years to a maximum of 1,600 years

Average recurrence interval 438 years 427 years

Peak discharge. Recurrence interval flooding at 25 U.S. Geological Survey (USGS) stream gages in the impacted areas was evaluated to determine the severity of flooding across the impacted counties.

The recurrence interval flooding at USGS stream gages was used to estimate peak discharge recurrence interval at the 52 analyzed dams in North and South Carolina. The recurrence interval of peak discharge at the 52 analyzed dams ranged from a low of 9 years to a maximum of 420 years. The median recurrence interval of peak discharge was 230 years. The recurrence interval of peak discharge was greater than 100 years at 73 percent of the 52 analyzed dams but did not exceed 500 years at any of the 52 analyzed dams.

Table ES-3 summarizes the distribution of recurrence Intervals for the 114 precipitation gages, 52 dams, and 25 USGS stream gages.

Table ES-3: Summary of the distribution of Recurrence Intervals at Precipitation Gages, Dams, and USGS Stream Gages

Average Recurrence Interval

Distribution of Recurrence Intervals

> 100 Years > 500 years > 1,000 years

Precipitation Gages (percentage of 114 precipitation gages)

1-day total precipitation 61 18 9


Dams (percentage of 52 analyzed dams)




Average Recurrence Interval

Distribution of Recurrence Intervals

> 100 Years > 500 years > 1,000 years

USGS Stream Gages (percentage of 25 stream gages)

Peak Discharge 48 36 0

Dams (percentage of 52 analyzed dams)

Peak Discharge 71 0 0

Probable Maximum Precipitation. Design criteria use computed maximum precipitation as design storms to determine spillway criteria. Therefore, calculating the Probable Maximum Precipitation (PMP) for Hurricane Matthew is useful as part of determining whether a dam performed according to its design criteria and for future design or planning conditions. Computed PMP depths at the impacted dams ranged from a low of 36.2 inches to a high of 45.9 inches, with an average PMP of 43.6 inches and a median PMP of 44.1 inches.

All except one of the 44 breached dams were assumed to have built or known to have been built prior to the adoption of current design standards for North Carolina and South Carolina and as such, the magnitude of the design storms is known only for Mt. Vernon Estates Dam in Cumberland County, NC. The design storm for Mt. Vernon Estates Dam, which is a medium-size, high-hazard dam, is one-half PMP. The reported 3-day total precipitation at Mt. Vernon Estates Dam was 32.7 percent of the computed PMP, indicating that the dam failure occurred at less than the specified design storm.

Although the design storms for the remaining 43 breached dams are not known, those dams are all medium-size or smaller and have hazard classes ranging from low to high. Assuming that the design storms specified by the current design standards would equal or exceed the design storms in use prior to adoption of existing standards, it is assumed that the largest potential design storm used for the 43 dams built prior to current standards would be the one-half PMP. Comparing this to the breached dams:

• North Carolina

o Two of the older breached dams Rhodes Lake Dam (NC-5) and H.F. Lee Power Station Cooling Lake Dam (NC-25) are medium-size high-hazard dams for which the current design standard would be the one-half PMP event. At these dams, the observed 3-day precipitation was 31.4 percent and 29.0 percent of the PMP, respectively. Although the actual design event used for these dams is unknown, if the current design standards had been used the dam failures would have occurred at less than the specified design storm.

o Nine of the remaining 16 older dams are small size dams, high-hazard for which the current design standard would be the one-third PMP event. The observed 3-day precipitation at these 9 dams ranged from 25.9 percent to 34.4 percent of the PMP; the second greatest ratio of observed precipitation to PMP was 30.3 percent. Although the actual design event used for these 9 dams is unknown, if the current design standards had been used, the dam failures would have occurred at or slightly less than the specified design storm Seven of the remaining 16 older dams are small size dams, intermediate- or low-hazard dams for which the current design standard would be the 50-year or 100-year storm, respectively. The observed 3-day precipitation at these 7 dams ranged 124 years to more than 1,300 years. Although the actual design event used for these 7 dams is unknown, if the current design standards had been used, the dam failures would have all at greater than the specified design storm


• South Carolina

o The dates of construction and storage capacity for the 25 breached dams are not known, but the largest is intermediate size.

o The spillway design storm for very small-, small-, or intermediate-size dams of low- or significant-hazard ranges from not specified up to a maximum of one-half the PMF to PMF.

o The spillway design storms for the five unregulated dams that breached are not known, but are assumed to be less than the 100-year storm.

Conclusions and recommendations. The rainfall and flooding were less than one-half the PMP and the PMF; this amount of rainfall and flooding come close to or exceed the existing spillway design floods at many, if not most, of the impacted dams. However, many of the impacted dams are older than the current state-regulated design criteria and may have been designed and constructed to lower standards than the current criteria. As such, the magnitude of the rainfall and flooding approached or exceeded current spillway design criteria and likely exceeded the spillway design criteria at many of the older dams. The dam breaches and overtoppings that occurred highlight the importance of understanding the existing design limits for dams and monitoring dams that are believed to have been designed to less than existing standards. Table ES-4 shows a summary of the conclusions and recommendations based on the study findings and analysis.

Table ES-4: Summary of Conclusions and Recommendations

Topic Number Conclusion Recommendation

Preparedness No. 1 Greater-than-normal antecedent rainfall and the extreme precipitation from Hurricane Matthew combined to exacerbate the flooding in response to Hurricane Matthew.

Extended wet periods should trigger increased awareness of the potential for damaging flooding and provide incentives to prepare, as needed.

Precipitation Recurrence

No. 2. The number of breached and overtopped dams is largely a result of the extreme nature of the 1-day precipitation event and the wide extent of area that received the extreme precipitation.

The estimated recurrence interval of the precipitation observed during Hurricane Matthew should be used to guide warning and alert thresholds for forecast precipitation.

Flood Recurrence

No. 3. The number of breached and overtopped dams is correlated to the locations where the recurrence of the peak discharge was greatest.

The observed magnitude and intensity of Hurricane Matthew precipitation should be used to help guide warning and alert thresholds for future forecast peak discharge.

North Carolina Dam Hazard

No. 4 Mt. Vernon Estates Dam , known to have been constructed after the adoption of the current design standards in 1980 and assumed to have been designed to pass the one-half PMP event, breached at 32.7 percent of the PMP, or an approximately one-third PMP event. The observed precipitation at the other 18 dams was less than 34.4 percent of the computed PMP; a 34.4 percent PMP event would equal or exceed the current North Carolina standard spillway design floods for these dams.

Older small-to-medium dams with unknown design capacity or those designed to pass events significantly less than the PMP or PMF should be evaluated to determine the magnitude of the critical precipitation or discharge event. Smaller, high- and significant-hazard dams identified as potentially vulnerable to smaller precipitation or flood events should consider developing emergency action plans and possibly for mitigation actions to reduce vulnerability.


Topic Number Conclusion Recommendation

South Carolina Dam Hazard

No. 5 The observed precipitation at the 25 breached dams was less than 34.4 percent of the computed PMP. A 34.4 percent PMP event would equal or exceed the current South Carolina standard spillway design floods for very small dams of all hazard classes and small low-hazard dams, accounting for the majority of the 25 breached dams.

Same as Recommendation No. 4 for North Carolina

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page i

Table of Contents Executive Summary .............................................................................................................. ES-1

Acronym List .............................................................................................................................. iv

1.0 Purpose, Scope, and Audience ....................................................................................... 1

2.0 Meteorological Conditions ............................................................................................... 5

2.1 Antecedent Conditions ............................................................................................ 5

2.2 Storm Development .............................................................................................. 13

2.3 Storm Intensity and Size ....................................................................................... 13

2.4 Storm Track, Extent, and Speed ........................................................................... 14

2.5 Post-Event Meteorological Conditions .................................................................. 14

3.0 Precipitation Analysis .....................................................................................................15

3.1 Overview of Hurricane Matthew Precipitation ........................................................ 15

3.2 Precipitation Severity ............................................................................................ 22

3.3 Estimated Probable Maximum Precipitation at Impacted Dams ............................ 30

4.0 Analysis of Peak Flood Discharges ................................................................................34

4.1 Frequency Analysis of Stream Gages in Vicinity of Impacted Dams ..................... 34

4.2 Peak Discharges at Impacted Dams ..................................................................... 41

4.3 Peak Discharge Recurrence Intervals at Impacted Dams ..................................... 45

4.4 Land Cover and Basin Characteristics on Peak Discharges at Impacted Dams .................................................................................................................... 45

4.5 Spillway Design Storms ........................................................................................ 46

5.0 Observation Summaries, Conclusions, and Recommendations ......................................50

6.0 References .....................................................................................................................53

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page ii

List of Figures Figure 1a: Locations of breached and overtopped dams, associated upstream drainage

areas, and nearby stream and precipitation gages in North Carolina ......................... 3

Figure 1b: Locations of breached dams, associated upstream drainage areas, and nearby stream and precipitation gages in South Carolina .......................................... 4

Figure 2a: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina ............................................................................... 8

Figure 2b: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina ............................................................................... 9

Figure 2c: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina ............................................................................. 10

Figure 2d: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in South Carolina ............................................................................ 11

Figure 2e: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in South Carolina ............................................................................ 12

Figure 3: Map of Hurricane Matthew track in North Carolina and South Carolina (Source: NOAA 2017, Figure 1) ............................................................................... 13

Figure 4: Locations of Hurricane Matthew precipitation depths and depth-duration-frequency at breached and overtopped dams in North Carolina............................... 24

Figure 5: Locations of Hurricane Matthew precipitation depths and depth-duration-frequency at breached dams in South Carolina ....................................................... 25

Figure 6: Map of estimated Probable Maximum Precipitation (PMP) at breached and overtopped dams in North Carolina ......................................................................... 32

Figure 7: Map of estimated Probable Maximum Precipitation (PMP) at breached dams in South Carolina ......................................................................................................... 33

Figure 8: Locations of stream gages and average recurrence intervals for Hurricane Matthew in North Carolina ....................................................................................... 38

Figure 9: Locations of stream gages and average recurrence intervals for Hurricane Matthew in South Carolina ....................................................................................... 39

List of Tables Table 1: Summary of Dam Breaches and Overtopping from Hurricane Matthew ......................... 1

Table 2: Comparison of September 2016 with Normal Precipitation for Counties with Dams that Breached During Hurricane Matthew ........................................................ 5

Table 3: Precipitation Gages in Counties Impacted by Hurricane Matthew in North Carolina and South Carolina .................................................................................... 16

Table 4: Maximum Observed Precipitation Depths for 1-, 2-, and 3-Day Durations at Gages ...................................................................................................................... 22

Table 5: Summary of Precipitation Recurrence Intervals at Gages............................................ 22

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page iii

Table 6: Upstream Drainage Areas, Observed Matthew Precipitation Depths and Computed Precipitation Statistics for Breached and Overtopped Dams in North Carolina and South Carolina .......................................................................... 26

Table 7: Summary of Precipitation Recurrence Intervals at Impacted Dams ............................. 30

Table 8: Summary of Ratio of Estimated 3-day precipitation to Computed Probable Maximum Precipitation (PMP).................................................................................. 31

Table 9: Stream Gages in Counties Impacted by Hurricane Matthew in North Carolina and South Carolina .................................................................................................. 36

Table 10: Summary of Maximum and Minimum Peak Discharges and Recurrence Intervals at USGS Stream gages impacted by Hurricane Matthew .......................... 40

Table 11: Estimated Peak Discharges and Peak Discharge Frequency for Breached and Overtopped Dams in North Carolina and Breached Dams in South Carolina ........... 42

Table 12: Summary of maximum and minimum recurrence intervals at USGS Stream dams impacted by Hurricane Matthew ..................................................................... 45

Table 13: Spillway Design Criteria for Dams in North Carolina and South Carolina ................... 46

Table 14: Size Classification for Dams in North Carolina and South Carolina ........................... 47

Table 15: Specified Spillway Design Criteria for Dams in North Carolina .................................. 47

Table 16: Specified Spillway Design Criteria for Dams in South Carolina .................................. 48

Appendix Appendix A: Table A-1: Hurricane Matthew Breached and Overtopped Dams in North Carolina and South Carolina

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page iv

Acronym List ARI Average Recurrence Interval of Precipitation

CoCoRaHS Community Collaborative Rain, Hail, and Snow Network

FEMA Federal Emergency Management Agency

mph miles per hour

NOAA National Oceanic and Atmospheric Administration

NWS National Weather Service

OFR Open-File Report

PFDS Precipitation Frequency Data Server

PMF Probable Maximum Flood

PMP Probable Maximum Precipitation

SIR Scientific Investigation Report (USGS)

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page 1

Hydrologic Analysis of Hurricane Matthew’s Impact on Dams in North Carolina and South Carolina

1.0 Purpose, Scope, and Audience Beginning on October 4, 2016, while Hurricane Matthew still lay off the coast of Cuba, North Carolina and South Carolina began receiving heavy rains from the storm. Over the next 5 days, Hurricane Matthew continued to bring heavy rains to North Carolina and South Carolina as it moved north along the coastlines of Florida and Georgia. Hurricane Matthew made landfall on October 8, 2016, near McClellanville, SC, and then moved north, skirting the coast of South Carolina and North Carolina before moving offshore again on October 9, 2016. Hurricane Matthew’s extreme precipitation fell on soils already saturated by previous inland storms, causing widespread severe flooding in North Carolina and South Carolina. This flooding resulted in breaches and overtopping of dams as shown in Table 1. The locations of the dams are shown in Figures 1a and 1b.

Table 1: Summary of Dam Breaches and Overtopping from Hurricane Matthew

Dam Hazard

North Carolina South Carolina Total by Hazard Breached Overtopped Breached Overtopped

High-Hazard 12 18 0 0 30


2 (1 unregulated)

0 15 0 17

Low-Hazard 5 (5 unregulated)

5 (2 unregulated)

5 0 15

Undefined Hazard Classification 0 0 5 0 5

Total by Damage 19 23 25 0 67

Additional information on each of the dams is included in Appendix A

Purpose. The purpose of this report is to describe the methods used to perform a hydrologic analysis of the impacts of Hurricane Matthew on breached and overtopped dams in North Carolina and South Carolina and present the results of that analysis. The results provide technical information that helps characterize the severity of Hurricane Matthew in North Carolina and South Carolina and provide a basis for estimating the magnitude of storm or flood events that led to dam breaches after Hurricane Matthew. This information can provide insight that can be applied to dams in other areas, which can increase the general understanding of flood-related dam vulnerabilities and risks. Increased understanding of flood-related dam vulnerabilities and risks can be used to help improve dam-related preparedness and planning efforts. The results in this report will also help inform the development of mitigation strategies to counter identified vulnerabilities and risks or to help in developing emergency operations plans to allow effective response and recovery in the event of a dam breach or dam incident.

Scope. The report includes a description of the meteorological conditions leading up to and existing during Hurricane Matthew; analysis of the precipitation that occurred during Hurricane Matthew, including an estimation of precipitation recurrence intervals; and analysis of flooding, including estimation of flooding recurrence intervals. The contents include the following:

• Meteorological conditions

o A description of antecedent meteorological conditions for Hurricane Matthew


o A description of the development of Hurricane Matthew

o A summary of the intensity and size of Hurricane Matthew

o A summary of the track, extent, and speed of Hurricane Matthew

o A description of post-event meteorological conditions for Hurricane Matthew

• Precipitation analysis

o An overview of Hurricane Matthew precipitation

o A discussion of the precipitation severity, including a summary of precipitation depth and duration at impacted locations across North Carolina and South Carolina, the estimated precipitation recurrence intervals in the watersheds of the breached or overtopped locations in both states, and a comparison of rainfall duration at gages with rainfall duration at the impacted dams

o Computation of Probable Maximum Precipitation (PMP) at the dam locations and comparison of observed precipitation amounts at the breached or overtopped dams to the computed PMP values

• Analysis of the peak flood discharges

o Estimation of flood frequency analyses at impacted stream gages in the vicinity of the breached and overtopped dams.

o Estimation of peak flood flows at impacted dams

o Estimation of recurrence intervals for peak flood flows at impacted dams

o Summary of the effect of land cover, topography, and basin characteristics on peak flood flows at impacted dams

o Review of the spillway design storms at impacted dams

Audience. The results of this report are intended for use by the Federal Emergency Management Agency (FEMA) and the respective State Dam Safety agencies, along with other stakeholders and interested parties.


Figure 1a: Locations of breached and overtopped dams, associated upstream drainage areas, and nearby stream and precipitation

gages in North Carolina

Figure 1b: Locations of breached dams, associated upstream drainage areas, and nearby stream and precipitation gages in South

Carolina



2.0 Meteorological Conditions This section describes the meteorological conditions that led to the severe rainfall and flooding during Hurricane Matthew. Antecedent meteorological conditions; storm development; storm intensity and size; the track, extent, and speed of the storm; and the post- storm meteorological conditions are described.

2.1 Antecedent Conditions Precipitation in September 2016 was higher than normal in eastern North Carolina and South Carolina, with departure from normal ranging from 0.3 inches to 12.1 inches due to the passage of two named tropical systems, Hermine and Julia, through or just offshore of the Carolinas. Table 2 shows a comparison of observed precipitation in September 2016 to normal (defined as the 30-year average for period 1981-2010) September precipitation. Tropical Storm Hermine made landfall on the Florida Gulf Coast on September 2 and weakened to tropical storm status as it passed across coastal South Carolina and North Carolina on September 2 and September 3. The resulting total storm precipitation amounts ranged from 6 to 14 inches. Tropical Storm Julia tracked along the east coast of Florida from September 13 to September 14 before moving off the South Carolina coast and degrading to a tropical depression on September 17. It then moved slowly north over the next 3 days before coming ashore near Ocracoke Island, North Carolina, as an extratropical system on September 20. This system dissipated over northeastern North Carolina on September 21. Although Tropical Storm Julia did not bring as much precipitation as Tropical Storm Hermine, the total precipitation depths from Julia ranged from 3 inches to more than 5 inches across coastal areas of North Carolina and South Carolina.

Table 2: Comparison of September 2016 with Normal Precipitation for Counties with Dams that Breached During Hurricane Matthew

County September 2016 Total Precipitation (inches)

September Normal* Precipitation (inches)

Deviation from Normal (inches)

Percent Deviation from Normal

North Carolina Counties

Cumberland 11.1 4.1 7.0 170% Duplin 11.3 5.9 5.4 91%

Harnett 12.7 4.3 8.4 194%

Hoke 15.5 4.3 11.2 261% Lenoir 13.5 5.7 7.8 136%

Moore 8.3 4.2 4.1 97%

New Hanover 15.9 7.8 8.0 102% Robeson 8.4 4.5 3.9 86%

Sampson 10.8 5.1 5.7 111% Wake 4.6 4.4 0.3 6%

Wayne 10.9 6.0 4.9 82%

Wilson 7.4 5.0 2.4 47% Average Deviation - North Carolina 5.7 115%

Maximum Deviation - North Carolina 11.2 261%

Minimum Deviation - North Carolina 0.3 6% South Carolina Counties

Chesterfield 8.0 4.1 3.9 96%

Clarendon 10.2 4.2 6.0 144%


County September 2016 Total Precipitation (inches)

September Normal* Precipitation (inches)

Deviation from Normal (inches)

Percent Deviation from Normal

Darlington 8.7 4.0 4.7 118% Dillon 10.3 4.3 6.0 140%

Florence 9.8 3.7 6.2 168%

Horry (Conway)

9.6 5.5 4.0 73%

Horry (North Myrtle Beach)

18.4 6.3 12.1 193%

Lee 5.8 3.5 2.4 68%

Lexington 6.8 3.5 3.3 92% Marion 9.9 6.1 3.8 62%

Marlboro 8.0 4.1 3.9 95% Average Deviation - South Carolina 5.1 113%

Maximum Deviation - South Carolina 12.1 193%

Minimum Deviation - South Carolina 2.4 62%

*Normal is defined as the 30-year average for period 1981-2010 Sources of data: Compiled from climate summaries provided by National Weather Service Forecast Offices for Raleigh, Wilmington, and Newport/Morehead City in North Carolina; Charleston and Columbia in South Carolina

In addition to the tropical systems, a stalled low-pressure system over parts of both Carolinas resulted in more precipitation over southeastern North Carolina and northeastern South Carolina in the last 3 days of September. A training thunderstorm event on the evening of September 28 and early morning of September 29 over isolated areas of Hoke and Cumberland Counties in the Sandhills region of North Carolina caused precipitation totals of up to 10.5 inches, resulting in local flooding and a dam breach at Long Valley Farm Lake Dam.

The total precipitation in September was above normal for both North Carolina and South Carolina:

• Total precipitation in September was above normal for the 12 counties in North Carolina where dams were breached during Matthew. The total precipitation for the 12 North Carolina counties ranged from 6 percent above normal to 261 percent above normal and was, on average, 115 percent greater than normal September precipitation, as shown in Table 2.

• Total precipitation in September was also above normal for the 10 South Carolina counties where dams were breached. The total precipitation for the 10 South Carolina counties ranged from 62 percent above normal to 193 percent above normal and was, on average, 113 percent greater than normal September precipitation (Table 2).

Analysis results. As a result of the heavy precipitation during September, much of the area affected later by Hurricane Matthew was under much wetter-than-normal antecedent moisture conditions. Plots of precipitation intensity versus time (called hyetographs) were compared to plots of discharge versus time (called hydrographs) for selected rain and stream gages in the impacted areas, shown in Figures 2a – 2e. The comparison examined the effect of these wetter-than-normal antecedent moisture conditions on Hurricane Matthew’s flood discharges.


A review of these plots shows that there was little precipitation during the period from October 1 to October 7 in the areas affected by Hurricane Matthew in both states, and the discharges at nearby stream gages were generally receding in the days leading up to Hurricane Matthew. However, flow levels had not yet returned to base flow stages, indicating that runoff flows from the September precipitation events was continuing prior to Hurricane Matthew.

Figure 2a: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina



Figure 2b: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina


Figure 2c: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in North Carolina


Figure 2d: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in South Carolina


Figure 2e: Hyetographs and hydrographs from selected rain and stream gages in impacted areas in South Carolina


2.2 Storm Development Hurricane Matthew began as a tropical wave off of the west coast of Africa on September 23, 2016, and formally became a tropical storm on September 28 off the coast of Barbados. Matthew reached hurricane strength on September 29 while centered in the eastern Caribbean Sea. Hurricane Matthew developed into a rare 160-miles-per-hour (mph) Category 5 hurricane in the central Caribbean during the first week of October. Before moving north out of the Caribbean, Hurricane Matthew heavily damaged portions of Haiti and the Bahamas with torrential rains and 145 mph winds. In the days just prior to landfall, Hurricane Matthew battered the east coast of Florida and Georgia as the hurricane tracked northwards towards South Carolina (Figure 3).

Figure 3: Map of Hurricane Matthew track in North Carolina and South Carolina

(Source: NOAA 2017, Figure 1)

2.3 Storm Intensity and Size Hurricane Matthew reached its peak strength on October 1, and was the first South Carolina hurricane to make landfall since Hurricanes Charley and Gaston came ashore in 2004. It made landfall on the South Carolina coast near McClellanville on October 8, 2016, at 10:45 a.m. Eastern Daylight Time, as a Category 1 hurricane with 75 mph winds.


2.4 Storm Track, Extent, and Speed Hurricane Matthew made landfall in Haiti, Cuba, and Grand Bahama as a major hurricane before it made landfall along the central coast of South Carolina as a Category 1 hurricane. Beginning on October 7, 2016, Hurricane Matthew moved north, skirting the Florida and Georgia coasts throughout the morning hours of October 8 before making landfall in South Carolina. The eye of the storm had moved back offshore by 2:00 p.m. on October 8 and continued to move northeast along the coast of North Carolina throughout the evening of October 8 and morning of October 9. As the storm tracked northeast, the center remained offshore, with the northwestern edge of Hurricane Matthew’s large eyewall extending inland. It brought hurricane-force wind gusts and heavy rains to coastal regions of the Carolinas throughout the early morning of October 9 before weakening and losing its tropical characteristics by 8:00 a.m. The remnants of the storm were located about 200 miles east of Cape Hatteras, North Carolina, by 8:00 p.m. on October 9.

2.5 Post-Event Meteorological Conditions Precipitation from Hurricane Matthew ended across both North Carolina and South Carolina by the evening of October 9, 2016. There was very little additional precipitation in the affected areas in either State until October 21 and October 22; on those dates there was scattered precipitation across the affected areas in North Carolina, ranging from a trace to more than 1 inch in the central Coastal Plain counties.


3.0 Precipitation Analysis The precipitation resulting from Hurricane Matthew was widespread over much of the Caribbean and the southeast United States, including eastern Florida, coastal areas of Georgia, North Carolina, South Carolina, and the Tidewater of Virginia. The following sections provide an overview of Hurricane Matthew precipitation, a description of precipitation depth and duration at the dams impacted by Hurricane Matthew, a description of the estimated precipitation recurrence intervals at the impacted dams, and a description of the probable maximum precipitation at the impacted dams. The data sources and methods used to characterize precipitation from Hurricane Matthew are presented as well.

3.1 Overview of Hurricane Matthew Precipitation Hurricane Matthew brought torrential rains over large areas of central and eastern North Carolina and eastern South Carolina, with precipitation depths exceeding 10 inches across the impacted areas.

Precipitation gages included in study. Observed precipitation depths and durations were obtained from 114 National Weather Service (NWS) and CoCoRaHS rain gages across the impacted areas of North Carolina and South Carolina. Characteristics of these precipitation gages are presented in Table 3 and the locations are shown in Figures 1a and 1b.

Table 3: Precipitation Gages in Counties Impacted by Hurricane Matthew in North Carolina and South Carolina Gage Information Hurricane Matthew Precipitation at Gages

Map Network Site

Number Site Name County Latitude Longitude

1-Day Maximum

Precipitation (inches)

2-Day Maximum


3-Day Maximum


Peak 1-Day Precipitation Return Period

(years)


(years)


(years)

North Carolina

NC-R1 CoCoRaHS NC-WK-77 Apex 2.2 N Wake 35.7600 -78.8400 6.65 7.54 7.69 50 50 40

NC-R2 CoCoRaHS NC-WK-4 Apex 3.4 ESE Wake 35.7000 -78.7900 7.89 8.57 8.68 130 100 80

NC-R3 CoCoRaHS NC-PT-37 Ayden 3.7 S Pitt 35.4200 -77.4300 6.10 6.85 7.84 10 10 20

NC-R4 CoCoRaHS NC-NS-20 Battleboro 7.2 W Nash 36.0400 -77.8800 12.44 12.79 12.86 550 400 370

NC-R5 CoCoRaHS NC-PR-2 Belvidere 5.4 N Perquimans 36.3500 -76.5500 9.62 9.69 10.13 110 60 60

NC-R6 CoCoRaHS NC-BD-1 Bladenboro 2.0 NNW Bladen 34.5700 -78.8100 6.75 11.31 11.36 20 140 120

NC-R7 CoCoRaHS NC-FK-7 Bunn 1.2 N Franklin 35.9800 -78.2500 8.03 9.06 9.06 100 100 80

NC-R8 CoCoRaHS NC-PD-22 Burgaw 0.3 NE Pender 34.5500 -77.9200 8.23 9.39 9.66 25 25 25

NC-R9 CoCoRaHS NC-DP-3 Mount Olive 2.4 SW Duplin 35.1700 -78.1000 11.92 12.19 12.19 240 160 140

NC-R10 CoCoRaHS NC-CL-3 Chadbourn 1.1 NNW Columbus 34.3400 -78.8400 9.30 12.90 13.05 80 210 190

NC-R11 CoCoRaHS NC-JH-46 Clayton 6.8 ESE Johnston 35.6200 -78.3400 8.93 9.83 9.97 140 130 110

NC-R12 CoCoRaHS NC-SM-17 Clinton 0.8 NE Sampson 35.0100 -78.3200 10.26 10.39 10.47 120 80 70

NC-R13 CoCoRaHS NC-HR-34 Coats 0.2 ESE Harnett 35.4100 -78.6700 9.40 10.72 10.72 200 200 160

NC-R14 CoCoRaHS NC-GT-5 Corapeake 1.3 NE Gates 36.5500 -76.5600 12.00 12.00 12.00 340 160 140

NC-R15 CoCoRaHS NC-DR-24 Duck 1.1 SSE Dare 36.1500 -75.7500 9.50 9.50 10.28 70 40 40

NC-R16 CoCoRaHS NC-CW-22 Edenton 2.0 W Chowan 36.0600 -76.6400 11.75 11.88 12.23 270 140 140

NC-R17 CoCoRaHS NC-BD-2 Elizabethtown 6.2 NW Bladen 34.4900 -78.6900 13.55 18.38 18.85 470 1420 1450

NC-R18 CoCoRaHS NC-BD-15 Elizabethtown 6.6 NW Bladen 34.6900 -78.7000 10.71 12.71 13.00 190 260 250

NC-R19 CoCoRaHS NC-WN-15 Elm City 7.1 ESE Wilson 35.7600 -77.7600 9.80 10.10 10.54 130 90 90

NC-R20 CoCoRaHS NC-CL-21 Evergreen 3.3 NE Columbus 34.4400 -78.8600 14.20 18.95 18.95 700 2100 2000

NC-R21 CoCoRaHS NC-PT-47 Farmville 3.1 NW Pitt 35.6300 -77.6300 10.02 10.74 11.43 130 90 110

NC-R22 CoCoRaHS NC-CM-68 Fayetteville 1.4 SW Cumberland 35.0500 -78.9100 12.05 13.67 13.82 920 1000 910

NC-R23 CoCoRaHS NC-CM-1 Fayetteville 2.4 S Cumberland 35.0400 -78.9100 14.04 15.62 15.69 2800 2830 2150

NC-R24 CoCoRaHS NC-JH-65 Four Oaks 5.7 NW Johnston 35.5100 -78.4900 10.00 10.67 10.72 310 220 200



Gage Information Hurricane Matthew Precipitation at Gages

Map Network Site


1-Day Maximum


2-Day Maximum


3-Day Maximum



(years)


(years)


(years)

NC-R25 CoCoRaHS NC-BD-4 Garland 11.2 W Bladen 34.8100 -78.5900 16.30 18.52 18.52 1730 2100 2060

NC-R26 CoCoRaHS NC-CM-39 Godwin 2.1 SW Cumberland 35.2000 -78.7000 14.32 16.28 16.32 2070 2170 1890

NC-R27 CoCoRaHS NC-WY-1 Goldsboro 4.4 E Wayne 35.3700 -77.9000 12.08 12.94 13.31 240 200 210

NC-R28 CoCoRaHS NC-WY-22 Goldsboro 8.4 WNW Wayne 35.4200 -78.1100 15.48 15.48 15.48 1210 760 700

NC-R29 CoCoRaHS NC-PR-5 Hertford 5.2 SSE Perquimans 36.1200 -76.4300 10.12 10.43 10.81 120 70 70

NC-R30 CoCoRaHS NC-CM-13 Hope Mills 4.8 SE Cumberland 34.9300 -78.8900 14.71 17.00 17.05 2460 3330 2910

NC-R31 CoCoRaHS NC-CM-34 Hope Mills 8.0 SSE Cumberland 34.8700 -78.8800 15.80 15.80 15.89 3160 1660 1570

NC-R32 CoCoRaHS NC-DR-10 Kill Devil Hills 0.9 WNW Dare 36.0200 -75.6800 11.50 11.50 12.31 180 80 100

NC-R33 CoCoRaHS NC-LR-26 Kinston 1.3 NNW Lenoir 35.2900 -77.6000 13.50 15.00 16.50 350 330 500

NC-R34 CoCoRaHS NC-CM-56 Linden 4.2 S Cumberland 35.1900 -78.7400 10.85 12.15 12.18 350 360 290

NC-R35 CoCoRaHS NC-RB-9 Lumberton 2.3 NE Robeson 34.6600 -78.9900 12.73 15.01 15.09 800 1140 1000

NC-R36 CoCoRaHS NC-CN-64 New Bern 7.0 E Craven 35.1200 -76.9600 5.27 6.28 7.66 4 5 9

NC-R38 CoCoRaHS NC-CC-10 Point Harbor 0.2 WSW Currituck 36.0800 -75.8000 10.70 10.73 11.73 120 60 80

NC-R39 CoCoRaHS NC-HK-3 Raeford 9.6 E Hoke 34.9600 -79.0600 10.36 12.88 12.91 400 840 640

NC-R40 CoCoRaHS NC-WK-90 Raleigh 3.8 SSW Wake 35.7700 -78.8800 8.17 9.17 9.40 190 200 170

NC-R41 CoCoRaHS NC-NS-21 Rocky Mount 9.2 SW Nash 35.8500 -77.9100 11.95 12.35 12.91 380 290 320

NC-R42 CoCoRaHS NC-WS-5 Roper 0.6 WSW Washington 35.8800 -76.6282 9.30 9.81 10.22 60 40 40

NC-R43 CoCoRaHS NC-SM-16 Salemburg 2.5 N Sampson 35.0500 -78.5000 15.85 16.56 16.60 1590 1260 1150

NC-R44 CoCoRaHS NC-GN-7 Snow Hill 3.1 NNE Greene 35.5000 -77.6600 10.10 10.90 12.00 110 90 130

NC-R45 CoCoRaHS NC-CM-75 Stedman 0.4 WSW Cumberland 35.0000 -78.7000 13.20 15.30 15.34 880 1190 1060

NC-R46 CoCoRaHS NC-CM-19 Stedman 1.0 NW Cumberland 35.0195 -78.7127 13.33 15.03 15.09 970 1110 1000

NC-R47 CoCoRaHS NC-WK-188 Wake Forest 1.9 NNE Wake 36.0000 -78.5100 6.85 7.72 7.84 60 60 50

NC-R48 CoCoRaHS NC-BF-2 Washington 7.6 SE Beaufort 35.4800 -76.9500 6.85 7.88 8.78 20 20 20

NC-R49 CoCoRaHS NC-WN-17 Wilson 3.3 NNW Wilson 35.7800 -77.9400 9.05 9.68 10.30 100 80 90

NC-R50 CoCoRaHS NC-JH-62 Zebulon 9.1 S Johnston 35.6900 -78.3300 8.21 8.91 9.10 100 80 70

NC-R51 NWS/FAA KFAY Fayetteville Regional Airport

Cumberland 34.9894 -78.8800 14.30 15.04 15.04 2240 1670 1340



Map Network Site


1-Day Maximum


2-Day Maximum


3-Day Maximum



(years)


(years)


(years)

NC-R52 RAWS FBRN7 Fort Bragg Cumberland 35.1451 -79.0708 8.77 8.97 9.04 150 80 60

NC-R53 NWS/FAA KFBG Fort Bragg / Simmons Army Airfield

Cumberland 35.1314 -78.9316 9.75 9.99 9.99 270 140 110

NC-R54 NWS/FAA KPOB Pope Air Force Base Cumberland 35.1753 -79.0077 n.a.1 n.a.1 n.a.1 n.d.2 n.d.2 n.d.2

NC-R55 APRSWXNET

E4573 Holly Springs Wake 35.6402 -78.8336 8.89 9.15 9.26 280 140 120

NC-R56 APRSWXNET

E4349 Raeford Hoke 34.9798 -79.2251 8.17 9.44 9.48 90 110 80

NC-R57 HADS VASN7 Flat Creek near Inverness NC

Hoke 35.1828 -79.1775 7.25 7.51 7.54 50 30 20

NC-R58 RAWS MKLN7 Horseshoe House Moore 35.4693 -79.3809 7.65 8.27 8.30 80 50 40

NC-R59 NWS/FAA KSOP Pinehurst/Southern Pines

Moore 35.2333 -79.4000 6.87 7.24 7.26 30 20 20

NC-R60 HADS RDDN7 Northeast Cape Fear River at US Highway

74/133 at Wilmington NC

New Hanover 34.2517 -77.9503 6.84 7.57 7.58 8 7 6

NC-R61 NWS/FAA KILM Wilmington International Airport

New Hanover 34.2668 -77.8999 5.93 6.83 6.84 5 4 4

NC-R62 HADS PBMN7 Lumber River at State Highway 710 near

Pembroke NC

Robeson 34.6800 -79.2358 n.a. n.a. n.a. n.d. n.d. n.d.2

NC-R63 RAWS WCON7 Lumberton Robeson 34.5951 -79.0849 12.40 12.58 12.59 680 360 300

NC-R64 NWS/FAA KLBT Lumberton Municipal Airport

Robeson 34.6082 -79.0590 12.80 12.98 12.98 830 430 370

NC-R65 NWS/FAA KCTZ Sampson County Airport

Sampson 34.9752 -78.3628 6.43 6.56 6.56 20 10 8

NC-R66 HADS WALN7 Walnut Creek at Sunnybrook Drive near

Raleigh NC

Wake 35.7583 -78.5831 7.41 7.59 7.70 80 40 40

NC-R67 HADS OWFN7 Crabtree Creek at Old Wake Forest at Raleigh

Wake 35.8158 -78.6258 8.55 8.77 8.91 240 120 100

NC-R68 NWS/FAA KRDU Raleigh Durham Wake 35.8922 -78.7819 6.90 7.25 7.46 70 50 40


Map Network Site


1-Day Maximum


2-Day Maximum


3-Day Maximum



(years)


(years)


(years)

NC-R69 HADS RBRN7 Rocky Branch at Pullen Park at Raleigh

Wake 35.7800 -78.6664 7.71 7.98 8.09 110 60 50

NC-R70 HADS APSN7 Swift Creek near Apex 5E

Wake 35.7189 -78.7522 6.31 6.64 6.64 30 20 20

NC-R71 HADS LCPN7 Lake Crabtree Park near Morrisville

Wake 35.8389 -78.7822 7.04 7.40 7.59 80 50 50

NC-R72 HADS RLHN7 Crabtree Creek at Hwy 70 at Raleigh

Wake 35.8381 -78.6742 7.21 7.48 7.61 80 50 40

NC-R73 RAWS GBON7 Finch's Station Wayne 35.4280 -78.0228 10.77 11.15 11.16 160 120 100

NC-R74 NWS/FAA KGSB Seymour-Johnson Air Force Base

Wayne 35.3446 -77.9458 14.89 15.47 15.47 649 495 449

NC-R75 APRSWXN ET

E9077 Lucama Wilson 35.6437 -78.0083 8.96 9.91 9.96 87 83 71

South Carolina

SC-R1 CoCoRaHS SC-BF-35 Beaufort 3.6 NE Beaufort 32.4800 -80.6718 11.00 11.03 11.03 163 79 59

SC-R2 CoCoRaHS SC-OR-24 Branchville 2.5 SE Orangeburg 33.2300 -80.7800 10.15 10.15 10.15 177 93 77

SC-R3 CoCoRaHS SC-CL-6 Canadys 0.4 NW Colleton 33.0600 -80.6300 10.25 10.29 10.31 138 74 59

SC-R4 CoCoRaHS SC-HR-80 Conway 4.4 SE Horry 33.7900 -79.0100 6.65 11.27 12.17 12 76 94

SC-R5 CoCoRaHS SC-CL-16 Cottageville 5.8 WSW Colleton 32.9000 80.5700 11.34 11.89 11.91 226 142 112

SC-R6 CoCoRaHS SC-DR-4 Darlington 7.3 SSW Darlington 34.2000 -79.9100 8.21 12.71 13.06 60 233 238

SC-R7 CoCoRaHS SC-BF-23 Daufuskie Island 1.7 SW Beaufort 32.1000 -80.8800 14.02 14.10 14.10 463 200 162

SC-R8 CoCoRaHS SC-DL-4 Dillon 3.8 NW Dillon 34.4600 -79.4100 7.15 13.85 13.88 33 640 543

SC-R9 CoCoRaHS SC-GT-13 Georgetown 4.9 NNE Dillon 33.4300 -79.2600 6.50 11.50 11.83 7 60 50

SC-R10 CoCoRaHS SC-CL-14 Green Pond 1.3 S Colleton 32.7100 -80.6100 11.66 11.68 11.68 230 110 90

SC-R11 CoCoRaHS SC-BF-2 Hilton Head Island Beaufort 32.2500 -80.7400 16.56 16.58 16.58 1360 560 420

SC-R12 CoCoRaHS SC-WL-2 Kingstree 7.9 NW Williamsburg 33.7400 -79.9300 9.75 13.56 13.70 110 260 250

SC-R13 CoCoRaHS SC-HR-33 Loris 2.9 WSW Horry 34.0400 -78.9400 6.97 11.15 11.88 16 75 87

SC-R14 CoCoRaHS SC-HR-27 Myrtle Beach 8.4 WNW Horry 33.7200 -79.0300 6.27 12.19 12.69 9 104 109




Map Network Site


1-Day Maximum


2-Day Maximum


3-Day Maximum



(years)


(years)


(years)

SC-R15 CoCoRaHS SC-CR-78 North Charleston Clarendon 32.8900 -79.9900 11.00 11.02 11.03 160 80 60

SC-R16 CoCoRaHS SC-CR-49 NOAA NWS Charleston Charleston 32.9000 -80.0300 9.70 12.39 12.40 80 160 120

SC-R17 CoCoRaHS SC-BF-53 Parris Island 1.7 N Beaufort 32.3500 -80.6900 11.00 11.00 11.00 150 70 50

SC-R18 CoCoRaHS SC-DC-18 Reevesville 1 SSE Dorchester 33.1900 -80.6400 12.90 13.05 13.05 600 320 270

SC-R19 CoCoRaHS SC-JS-3 Ridgeland ESE Jasper 32.4400 -80.8900 12.46 12.46 12.46 330 150 110

SC-R20 CoCoRaHS SC-OR-22 Santee 6.9 WSW Orangeburg 33.4300 -80.5900 9.70 10.61 10.84 130 110 100

SC-R21 CoCoRaHS SC-CD-1 Summerton Clarendon 33.5300 -80.2400 9.65 11.69 11.72 120 160 130

SC-R22 CoCoRaHS SC-CL-1 Yemassee Colleton 32.7800 -80.7700 10.65 10.68 10.70 140 70 50

SC-R23 HADS CSFS1 Black Creek below Chesterfield SC

Chesterfield 34.6633 -80.2117 5.31 5.93 5.93 10 8 5

SC-R24 RAWS JEFS1 Carolina Sandhills Chesterfield 34.6619 -80.2742 5.64 6.15 6.15 13 10 8

SC-R25 HADS MBES1 USGS Rain Gage near McBee SC

Chesterfield 34.5067 -80.2183 6.40 7.05 7.05 23 18 15

SC-R26 HADS PAGS1 USGS Rain Gage near Pageland SC

Chesterfield 34.7447 -80.3397 4.39 5.29 5.29 5 5 4

SC-R27 NWS/FAA KMNI Manning / Cooper Regional

Clarendon 33.5833 -80.2167 8.42 10.18 10.18 60 80 60

SC-R28 RAWS SPLS1 Santee NWR Clarendon 33.5575 -80.4414 8.02 9.70 9.70 60 70 60

SC-R29 NWS/FAA KUDG Darlington County Jetport Airport

Darlington 34.4492 -79.8903 6.79 7.40 7.40 30 20 20

SC-R30 SCAN PDEE Pee Dee Site 2037 Darlington 34.3000 -79.7333 11.90 12.67 12.69 330 240 220

SC-R31 NWS/FAA KFLO Florence - Florence Regional Airport

Florence 34.1878 -79.7308 7.44 8.08 8.08 40 30 30

SC-R32 HADS EFFS1 Lynches River at Effingham SC

Florence 34.0514 -79.7542 10.94 11.49 11.50 220 150 130

SC-R33 HADS BCLS1 Buck Creek near Longs SC

Horry 33.9533 -78.7200 10.15 11.07 11.07 80 60 50

SC-R34 RAWS HRAS1 Horry Horry 33.9368 -79.0849 10.04 11.52 11.52 90 90 80

SC-R35 HADS GALS1 Little Pee Dee River at Galivants Ferry SC

Horry 34.0569 -79.2472 11.97 12.96 12.96 320 240 210



Map Network Site


1-Day Maximum


2-Day Maximum


3-Day Maximum



(years)


(years)


(years)

SC-R36 HADS ACWS1 Waccamaw River above Conway SC

Horry 33.8508 -78.8972 9.80 11.31 11.31 70 80 60

SC-R37 HADS BISS1 Lynches River near Bishopville SC

Lee 34.2500 -80.2139 4.94 5.72 5.72 7 7 6

SC-R38 NWS/FAA KMAO Marion County Airport Marion 34.1810 -79.3350 10.05 10.63 10.67 160 110 90

SC-R39 RAWS MRWS1 Marion 01 Marion 34.1858 -79.3362 14.36 14.97 15.02 1160 730 620

SC-R40 NWS/FAA KCAE Columbia Metropolitan Airport

Lexington 33.9419 -81.1181 3.90 4.46 4.46 3 2 2

SC-R41 NWS/FAA KMMT McIntyre Air National Guard Base

Richland 33.9167 -80.8000 5.73 6.22 6.22 20 10 8

n.a. – data not availablen.d. – not determined due to lack of data


Maximum observed precipitation at gages. Although the precipitation from Hurricane Matthew occurred over a 3-day period, most of the 3-day total precipitation occurred during a single day. The 3-day total precipitation was less than 110 percent of the 1-day total precipitation at 52 percent of the 114 precipitation gages and less than 120 percent of the 1-day total at more than 81 percent of the 114 precipitation gages. The highest 1-day, 2-day, and 3-day precipitation recorded at stations in North Carolina and South Carolina are summarized in Table 4.

Table 4: Maximum Observed Precipitation Depths for 1-, 2-, and 3-Day Durations at Gages

Duration Maximum Recorded

Depth County Rainfall Gage Identification

North Carolina

1 day 16.30 inches Bladen County Garland 11.2 W CoCoRaHS gage (NC-R25)

2 day 18.95 inches Columbus County Evergreen 3.3 NE CoCoRaHS gage (NC-R20)

3 day 18.95 inches Columbus County Evergreen 3.3 NE CoCoRaHS gage (NC-R20)

South Carolina

1 day 16.56 inches Beaufort County Hilton Head Island CoCoRaHS gage (SC-R11)



Note: The maximum recorded depths are for the 114 precipitation gages analyzed in North Carolina and South Carolina for precipitation during Hurricane Matthew

Precipitation recurrence intervals at gages. Recurrence intervals for the 1-day observed precipitation across North Carolina and South Carolina during Hurricane Matthew ranged from less than 5 years to more than 3,100 years and recurrence intervals for the 3-day observed precipitation ranged from less than 5 years to more than 2,900 years. A summary of the distribution of the recurrence intervals at the 114 precipitation gages in North Carolina and South Carolina is shown in Table 5.

Table 5: Summary of Precipitation Recurrence Intervals at Gages

Summary Statistic 1-Day Duration 3-Day Duration


Percent gages that exceeded 100-year recurrence interval 61% 46%

Percent gages that exceeded 500-year recurrence interval 18% 15%

Percent of gages that exceeded 1,000-year recurrence interval 9% 9%

Note: The precipitation recurrence intervals are for the 114 precipitation gages analyzed in North Carolina and South Carolina for precipitation during Hurricane Matthew

3.2 Precipitation Severity The following section describes the precipitation depth and duration at the dams impacted by Hurricane Matthew, the estimated precipitation recurrence intervals at those dams, and a comparison of rainfall duration at gages across the affected area versus at the impacted dams.


3.2.1 Precipitation Depth and Duration at Impacted Dams Observed 1-day, 2-day, and 3-day precipitation depths at gage stations (see Figures 4 and 5) were used to develop precipitation depth surfaces for the 1-day, 2-day, and 3-day durations. These precipitation surfaces were then used to estimate precipitation depths for the 1-day and 3-day durations for each dam location as presented in Table 6.

Precipitation depths and duration for each dam site were determined by interpolating observed precipitation depths obtained from the 114 National Weather Service (NWS) and CoCoRaHS rain gages. The rain gages used for this analysis are presented in Table 3, and the locations of these gages are presented in Figures 1a and 1b. Daily precipitation data for the gages was obtained from the MesoWest weather data aggregation and archive site, which provides climate and weather data from a variety of sources.1

1 http://mesowest.utah.edu/html/help/main_index.html

http://mesowest.utah.edu/html/help/main_index.html

Figure 4: Locations of Hurricane Matthew precipitation depths and depth-duration-frequency at breached and overtopped dams in North

Carolina



Figure 5: Locations of Hurricane Matthew precipitation depths and depth-duration-frequency at breached dams in South Carolina


Table 6: Upstream Drainage Areas, Observed Matthew Precipitation Depths and Computed Precipitation Statistics for Breached and Overtopped Dams in North Carolina and South Carolina

Dam Information 1-Day 3-Day Probable Maximum

Precipitation (inches) at

Dam

Dam Map

Number

State Dam

Number

Area Drained (square miles)

Precipitation Depths (inches)

at Dam

Precipitation ARI (years) at

Dam

Return Period Precipitation Depths at Breached and Overtopped Dams (inches) Precipitation

Depths (inches) at Dam


Dam 10-

Year 25-

Year 50-

Year 100-Year

500-Year

1,000-Year

Breached Dams in North Carolina and South Carolina

NC-1 CUMBE-025 6.0 11.5 910 5.5 6.6 7.4 8.3 10.5 11.6 12.7 920 47.3

NC-2 CUMBE-034 25.0 10.6 560 5.4 6.5 7.3 8.2 10.4 11.5 11.4 590 44.2

NC-3 CUMBE-050 6.6 13.8 1000 5.7 7.0 8.0 9.2 12.3 13.9 15.5 1380 47.4

NC-4 CUMBE-052 2.5 12.3 1250 5.5 6.6 7.5 8.4 10.7 11.9 13.8 1220 40.0

NC-5 CUMBE-053 53.2 13.2 1670 5.5 6.6 7.5 8.5 10.9 12.1 14.9 1350 47.6

NC-7 CUMBE-077 1.5 12.6 1430 5.5 6.6 7.5 8.5 10.9 12.1 14.3 1280 47.3

NC-8 CUMBE-086 2.2 15.5 2500 5.6 6.8 7.8 8.9 11.7 13.2 15.8 1600 48.4

NC-9 CUMBE-088 0.4 11.5 910 5.5 6.6 7.5 8.4 10.6 11.7 12.6 900 47.3

NC-10 CUMBE-099 0.6 11.5 770 5.5 6.6 7.5 8.4 10.7 11.9 13.0 950 47.3

NC-11 DUPLI-016 1.6 12.0 220 6.0 7.5 8.8 10.2 14.4 16.7 13.2 280 48.4

NC-12 DUPLI-017 5.3 11.7 180 6.0 7.5 8.9 10.3 14.6 17.0 12.9 260 48.3

NC-13 HARNE-047 3.9 9.0 220 5.3 6.4 7.2 8.1 10.2 11.3 10.0 210 47.6

NC-15 HOKE-028 3.4 10.5 460 5.5 6.6 7.4 8.4 10.7 11.9 12.9 690 48.4

NC-16 LENOI-003 7.1 12.1 210 6.0 7.5 8.8 10.3 14.5 16.8 13.6 300 47.9

NC-20 SAMPS-016 6.2 14.6 910 5.7 7.0 8.2 9.4 12.9 14.8 15.4 1040 48.0

NC-21 SAMPS-047 15.3 11.7 400 5.6 6.9 7.9 9.1 12.2 13.8 12.5 550 46.4

NC-24 WAYNE-008 3.8 12.7 320 5.8 7.2 8.5 9.9 13.9 16.1 13.2 300 48.5

NC-25 WAYNE-009 0.9 13.8 590 5.7 7.1 8.3 9.6 13.4 15.5 14.1 490 48.5

NC-27 WILSO-009 35.1 9.5 150 5.2 6.5 7.6 8.7 12.1 13.9 10.7 120 42.6

SC-1 D 3550 4.6 9.1 370 4.4 5.4 6.2 7.1 9.6 10.9 11.3 170 50.0




Dam

Dam Map

Number

State Dam

Number



at Dam


Dam




Dam 10-

Year 25-

Year 50-

Year 100-Year

500-Year

1,000-Year

SC-2 D 3554 1.0 11.8 310 5.4 6.7 7.9 9.2 13.0 15.1 12.6 220 50.0

SC-3 D 3553 23.0 11.8 300 5.4 6.8 7.9 9.3 13.1 15.2 12.6 220 46.5

SC-4 D 3602 12.0 9.4 310 4.7 5.7 6.6 7.6 10.3 11.7 12.7 290 48.1

SC-5 D 3583 0.5 10.5 200 5.5 6.8 7.9 9.2 12.7 14.6 12.0 220 50.1

SC-6 D 3580 1.3 10.4 200 5.5 6.8 7.9 9.1 12.6 14.5 11.9 210 50.1

SC-7 D 3569 0.7 10.2 160 5.4 6.7 7.9 9.2 13.1 15.3 11.3 140 50.1

SC-8 D 3612 3.4 10.4 140 5.9 7.2 8.4 9.7 13.2 15.1 12.8 290 50.1

SC-9 D 3571 2.4 9.7 130 5.4 6.7 7.8 9.1 12.7 14.7 10.7 150 50.0

SC-10 D 3595 1.5 8.0 60 5.5 6.7 7.7 8.8 11.8 13.4 13.3 450 49.9

SC-11 D 0511 18.4 11.2 280 5.7 7.0 8.1 9.3 12.5 14.2 12.6 250 47.0

SC-12 D 2426 4.1 6.4 20 5.4 6.7 7.9 9.2 13.1 15.3 7.6 49 49.8

SC-13 D 2417 13.4 5.6 13 5.3 6.5 7.7 8.9 12.5 14.5 6.5 21 47.6

SC-14 D 3453 13.9 9.0 130 5.4 6.5 7.5 8.6 11.5 13.0 11.5 240 46.6

SC-15 D 3431 2.4 7.1 35 5.4 6.6 7.6 8.7 11.6 13.1 8.8 100 48.4

SC-16 D 3616 1.8 10.6 140 6.0 7.4 8.6 10.0 13.6 15.5 12.4 180 50.1

SC-17 D 0852 1.8 9.0 96 5.6 6.9 7.9 9.1 12.2 13.8 12.7 280 49.9

SC-18 D 3598 17.2 7.3 36 5.6 6.8 7.9 9.0 12.0 13.6 12.1 130 47.1

SC-19 D 3490 6.2 9.5 98 5.6 6.9 8.1 9.5 13.3 15.4 11.4 130 50.1

SC-20 D 3562 7.0 9.6 140 5.4 6.7 7.8 9.0 12.4 14.2 10.9 160 49.2

SC-21 UnReg4 0.3 6.5 22 5.4 6.7 7.8 9.0 12.3 14.1 7.7 57 48.4

SC-22 UnReg3 1.0 8.0 58 5.5 6.8 7.8 8.9 10.1 10.7 13.0 360 49.9

SC-23 UnReg5 0.2 8.6 72 5.4 6.7 7.9 9.2 13.0 15.1 12.0 190 50.0




Dam

Dam Map

Number

State Dam

Number



at Dam


Dam




Dam 10-

Year 25-

Year 50-

Year 100-Year

500-Year

1,000-Year

SC-24 UnReg1 17.0 7.3 37 5.4 6.7 7.8 9.2 13.0 15.1 9.1 96 46.5

SC-25 UnReg2 7.3 4.0 < 10 5.3 6.4 7.3 8.3 11.1 12.6 4.5 < 10 49.7

Overtopped Dams in North Carolina

NC-14 HOKE-023 2.8 10.6 500 5.5 6.6 7.4 8.4 10.6 11.7 12.8 720 47.3

NC-17 MOORE-040 95.0 7.7 68 5.5 6.5 7.3 8.2 10.3 11.4 8.1 97 39.9

NC-18 NEWHA-003 n.d. 6.9 < 10 7.1 8.9 10.5 12.4 17.7 20.6 7.8 44 n.d.

NC-19 ROBES-004 n.d. 12.2 500 5.6 6.9 8.0 9.1 12.2 13.8 14.1 730 n.d.

NC-22 WAKE-034 66.3 7.8 120 5.1 6.1 6.8 7.6 9.5 10.5 8.3 68 40.9

NC-23 WAKE-035 6.9 7.4 91 5.0 6.0 6.7 7.5 9.3 10.2 7.8 52 46.9

NC-26 WILSO-007 40.0 9.5 140 5.2 6.5 7.6 8.8 12.2 14.0 10.7 120 42.7

ARI = average recurrence interval of precipitation; ARI at dam locations is interpolated from ARI of observed precipitation at rainfall gages in. = inches n.d. = not determinedmi. = miles SC = South Carolina NC = North Carolina UnReg = Unregulated


3.2.2 Estimated Precipitation Recurrence Intervals at Impacted Dams The National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 2 Version 3.0: Delaware, District of Columbia, Illinois, Indiana, Kentucky, Maryland, New Jersey, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, provides precipitation frequency estimates for durations ranging from 5 minutes up to 60 days. The computed 10-, 25-, 50-, 100-, 200-, 500-, and 1,000-year annual recurrence interval precipitation depths for the 1-day, 2-day, and 3-day durations from NOAA’s Atlas 14 were obtained for each gage location from the Precipitation Frequency Data Server (PFDS) on the National Weather Service’s Hydrometeorological Design Studies Center Web site.2 The computed recurrence intervals were compared to observed precipitation depths and used to estimate the recurrence intervals for the observed Hurricane Matthew 1-day, 2-day, and 3-day maximum precipitation depths at each precipitation gage.

Precipitation recurrence intervals at dams. The computed recurrence intervals for the 1-day, 2-day, and 3-day durations at the precipitation gage locations were used to create precipitation recurrence intervalsurfaces for the 1-day, 2-day, and 3-day durations. These precipitation recurrence interval surfaces werethen used to estimate recurrence intervals for the 1-day, 2-day, and 3-day durations at each damlocation.

The estimated precipitation depths and recurrence intervals described above were based on observed precipitation and computed recurrence intervals at the network of precipitation gages across North and South Carolina. In addition to these estimates, precipitation recurrence intervals for each dam location were developed directly from the Atlas 14 recurrence interval estimates. Rasters of 24-hour precipitation depths for each recurrence interval were obtained from the NWS PFDS3 and were intersected with the dam locations to determine the 24-hour depth for 10-, 25-, 50-, 100-, 200-, 500-, and 1,000-year annual recurrence intervals at each dam (Table 6).

Precipitation recurrence intervals for the 1-day precipitation depths and 3-day precipitation depths were calculated, described below:

• Estimated 1-day precipitation depths at the 52 analyzed dams in North Carolina and SouthCarolina ranged from a low of 4.0 inches to a maximum of 15.5 inches. The average 1-dayprecipitation depth was 10.1 inches and the median 1-day precipitation depth was 10.4 inchesat the 52 analyzed impacted dams. The estimated 1-day precipitation recurrence intervals at the52 analyzed dams in North Carolina and South Carolina ranged from a low of 13 years to amaximum of 2,500 years.

• Estimated 3-day precipitation depths at the 52 analyzed dams in North Carolina and SouthCarolina ranged from a low of 4.5 inches to a maximum of 15.8 inches. The average 3-dayprecipitation depth was 11.8 inches and the median 3-day precipitation depth was 12.5 inchesat the 52 analyzed impacted dams. Estimated 3-day precipitation recurrence intervals at the 52analyzed dams in North Carolina and South Carolina ranged from a low of 3 years to a maximumof 1,600 years.

Summary statistics for the 1-day and 3-day precipitation depths at the 52 analyzed dams in North Carolina and South Carolina are shown in Table 7.

2 https://hdsc.nws.noaa.gov/hdsc/pfds/index.html 3 https://hdsc.nws.noaa.gov/hdsc/pfds/index.html

https://hdsc.nws.noaa.gov/hdsc/pfds/index.html

https://hdsc.nws.noaa.gov/hdsc/pfds/index.html


Table 7: Summary of Precipitation Recurrence Intervals at Impacted Dams

Summary Statistic 1-Day Duration 3-Day Duration


Percent of dams in that exceeded the 100-year recurrence interval 71% 81%

Percent of dams that exceeded the 500-year recurrence interval 23% 29%

Percent of dams that exceeded the 1,000-year recurrence interval 10% 13%

Note: The precipitation recurrence intervals are for the 52 analyzed dams in North Carolina and South Carolina impacted by Hurricane Matthew

Precipitation duration at dams. The average of the estimated precipitation recurrence intervals at the impacted dams in North Carolina and South Carolina were greater than the average of the precipitation recurrence intervals observed at the precipitation gages in the impacted counties in North Carolina and South Carolina. In addition, the percentages of impacted dams that received precipitation in excess of the estimated 100-, 500-, and 1,000-year recurrence intervals are larger than the percentages of precipitation gages at which observed precipitation was in excess of the same recurrence interval thresholds. These results can be explained as follows:

• The greater occurrence of 100-, 500-, and 1,000-year recurrence intervals at the dam locations islikely a result of the selection of impacted dams as being breached or overtopped; this greateroccurrence suggests that these dams were located in the most severely impacted areas.

• The 114 precipitation gages included in the study, however, include all available precipitationgages in the counties that were impacted by Hurricane Matthew. As such, gages in the studyinclude gages that are not located in the areas of greatest depth of precipitation.

3.2.3 Comparison of Rainfall Duration at Gages to Dams The rainfall duration was generally longer at the impacted dam locations than across the entire impacted area represented by the precipitation gages. This is evidenced by the larger differences between the 1-day and 3-day precipitation depths at the 52 analyzed impacted dams than at the 114 precipitation gages, as follows:

• Precipitation amount measured at gages: The 1-day total precipitation observed at the 114gages was greater than 90 percent of the 3-day total precipitation at 52 percent of the gagesand greater than 80 percent of the 3-day total at 87 percent of the gages

• Precipitation depth measured at dams: At the 52 analyzed dams the estimated 1-dayprecipitation depth was greater than 90 percent of the 3-day depth at 25 percent of theimpacted dams and greater than 80 percent of the 3-day depth at 87 percent of the dams.

3.3 Estimated Probable Maximum Precipitation at Impacted Dams Design criteria use computed maximum precipitation for design storms to determine spillway criteria. Therefore, calculating the Probable Maximum Precipitation (PMP) for Hurricane Matthew is useful as part of determining whether a dam performed according to its design criteria and for future design or planning conditions.

Probable Maximum Precipitation (PMP) estimates at the breached and overtopped dams were calculated using the methods described in NWS Hydrometeorological Report No. 52 (HMR-52)


“Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian” (NWS, 1982) as applied by the U.S. Army Corps of Engineers – Hydrologic Engineering Center computer program HMR52 (USACE, 1987).

The PMP depth estimates, along with the estimated 1-day, 2-day and 3-day precipitation depths at the impacted dam locations in North Carolina and South Carolina are presented in Table 8; locations of the dams are shown in Figures 6 and Figure 7. Note that the PMP was computed at only 50 of the 52 analyzed dams because the Sutton Cooling Pond Dam (NC-18) in New Hanover County and the Weatherspoon Cooling Pond Dam (NC-19) in Robeson County are constructed impoundments with no defined drainage area and therefore cannot be included in the PMP computations.

Computed PMP depths at the 50 impacted dams are summarized as follows:

• PMP depths ranged from a low of 39.9 inches to a high of 50.1 inches.

o The low PMP of 39.9 inches was computed at Woodlake Dam (NC-17) in Moore County, NC.

o The high PMP of 50.1 inches was computed at several dam locations, all in South Carolina:Lakewood Pond Dam (SC-19) in Clarendon County; Floyd Pond Dam (SC-5), Bowling PondDam (SC-6), Blackwell Pond Dam (SC-7), all in Florence County; Graham Mill Pond Dam (SC-16), in Horry County; and Stroud Pond Dam (SC-8), in Marion County.

• The average PMP depth was 47.6 inches and the median PMP depth was 48.0 inches.

The ratio of the estimated 3-day precipitation depths to computed PMP depth estimates indicates the percent of the PMP represented by the observed 3-day precipitation depth. The maximum, minimum, average, and median ratio of the estimated 3-day precipitation depths to computed PMP depth estimates are provided in Table 8. As shown in Table 8, the average ratio of 3-day precipitation depth to PMP is 24.8 percent and the median ratio of 3-day precipitation depth to PMP is 25.6 percent. The maximum ratio of 3-day precipitation depth to PMP is 34.6 percent at Arran Lakes Dam (NC-4) in Cumberland County and the minimum ratio is 9.1 percent of the PMP at Baywater Drive Dam (SC-25) in Lexington County. This comparison indicates that dam breaches during Hurricane Matthew occurred as result of 3-day precipitation that ranged from less than one-tenth of the PMP to, at most, slightly more than one-third of the PMP. Dam breaches during Hurricane Matthew resulted from 3-day precipitation that was, on average, about one-quarter the PMP.

Table 8: Summary of Ratio of Estimated 3-day precipitation to Computed Probable Maximum Precipitation (PMP)

Summary Statistic Ratio of 3-Day

Precipitation to PMP Dam Location County, State

Maximum ratio of 3-day precipitation to computed PMP

34.4% Arran Lakes Dam (NC-4)

Cumberland County, NC

Minimum ratio of 3-day precipitation to computed PMP

9.1% Baywater Drive Dam (SC-25)

Lexington County, SC

Average ratio of 3-day precipitation to computed PMP

24.8% Not applicable

Median ratio of 3-day precipitation to computed PMP

25.6% Not applicable


Figure 6: Map of estimated Probable Maximum Precipitation (PMP) at breached and overtopped dams in North Carolina


Figure 7: Map of estimated Probable Maximum Precipitation (PMP) at breached dams in South Carolina


4.0 Analysis of Peak Flood Discharges Analysis of the peak flood discharges that resulted from Hurricane Matthew included a review of data and computed flood frequency analyses at impacted stream gages in the vicinity of the breached and overtopped dams. A search of active U.S. Geological Survey (USGS) stream gages with small to medium drainage areas and primarily natural (unregulated) streamflow in the vicinity of the impacted dams in North Carolina and South Carolina showed 32 USGS stream gages; these stream gages are presented in Table 9. Seven of these 32 stream gages are on tidally affected streams. The effects of tides at these gage locations are such that determination of discharge is not performed and only water surface elevation, or stage, is reported at these seven locations, so estimates of peak discharge recurrence intervals are not available at these seven locations.

As part of this analysis, Compass consulted published data regarding the peak stage, streamflow, and associated recurrence intervals at steam gages impacted by Hurricane Matthew in North Carolina and South Carolina. The USGS Open-File Report (OFR) 2016-1205, Preliminary Peak Stage and Streamflow Data at Selected Streamgaging Stations in North Carolina and South Carolina for Flooding following Hurricane Matthew, October 2016 (Weaver et al. 2016) provides a summary of the peak stages and flows at affected stream gages across North Carolina and South Carolina, including 22 of the 32 stream gages identified for analysis for this report. The peak stages and streamflow for the remaining 10 stream gages included in this report were obtained from the USGS National Water Information System Web Interface.4 In addition, the USGS published estimated recurrence intervals for Hurricane Matthew peak discharges at selected gages in North Carolina in a second report, USGS OFR 2017-1047, Characterization of Peak Streamflows and Flood Inundation at Selected Areas in North Carolina Following Hurricane Matthew, October 2016 (Musser et al. 2017).

In summary, the following gages were included in this study:

• 32 stream gages are located in the area studied for this analysis

• 25 of the 32 USGS stream gages are included in the peak discharge recurrence analysis; 7 of the 32 gages are tidally affected and are therefore not included

• 32 USGS stream gages are included in the estimated recurrence interval analysis (22 from Weaver et al. [2016] and 10 from the USGS Web Interface)

4.1 Frequency Analysis of Stream Gages in Vicinity of Impacted Dams Reviews of stream gage records provide a general picture of the recurrence interval of a large-scale flood event. However, it can be very difficult to determine recurrence interval estimates specific to the breached or overtopped dams because none of these dams are on gaged streams. Variability in precipitation depths and the runoff response to precipitation across the affected area can result in a large variation in observed flood flows and the associated flood recurrence intervals across the affected area. In addition, differences in the size, land cover, topography, and other basin characteristics between gaged locations and the breached or overtopped dam drainage areas can add to the variation between observed flows at gage locations and estimates of peak flood flows and recurrence intervals at the ungaged breached or overtopped dam locations. Maps of the stream gage return period for North Carolina and South Carolina are presented in Figures 8 and 9, respectively.

4 https://waterdata.usgs.gov/usa/nwis/nwis

https://waterdata.usgs.gov/usa/nwis/nwis


Recurrence intervals. Musser et al. (2017) provided estimated recurrence intervals for Hurricane Matthew peak discharges at 11 of the 32 stream gages identified for analysis as part of this report and listed in Table 9. Recurrence interval estimates at 14 of the 32 stream gages were determined by performing updated frequency analyses using a combination of 1) existing frequency estimates reported in USGS Scientific Investigation Report (SIR) 2009-5156, Magnitude and Frequency of Rural Floods in the Southeastern United States, through 2006: Volume 2, South Carolina (Feaster et al. 2009) and USGS SIR 2009-5158, Magnitude and Frequency of Rural Floods in the Southeastern United States, through 2006: Volume 2, North Carolina (Weaver et al. 2009), as well as 2) updated frequency analyses performed using data that included the Hurricane Matthew peak flood discharge. The updated frequency analyses were performed using the annual maximum peak discharges for each gage location, which were updated through the 2017 water year. The frequency analyses were performed according to the guidelines in Bulletin 17B (USGS 1982) using the USGS PeakFQ v7.1 software.5 Where indicated by a review of the annual peak records, the appropriate historical adjustment was applied. The weighted skew option was used, along with the generalized skew values and the associated estimated standard error of the generalized skew reported in USGS SIR 2009-5156 (Feaster et al. 2009) and USGS SIR 2009-5158 (Weaver et al. 2009).

5 https://water.usgs.gov/software/PeakFQ/

https://water.usgs.gov/software/PeakFQ/


Table 9: Stream Gages in Counties Impacted by Hurricane Matthew in North Carolina and South Carolina Stream Gage Information Hurricane Matthew Information

Gage Map Agency Site Number Site Name County Latitude Longitude

Drainage Area

(square miles)

Peak Date

Gage

at Peak

Discharge at Gage (cfs)

Peak Discharge ARI at Gage

(years)

North Carolina

NC-S1 USGS 0208726005 Crabtree Creek at Ebenezer Church Road near Raleigh

Wake 35.8453 -78.7244 76 10/8/2016 5,740 12

NC-S2 USGS 0208735012 Rocky Branch below Pullen Drive at Raleigh

Wake 35.7800 -78.6664 1.17 10/8/2016 1,370 2

NC-S3 USGS 02087359 Walnut Creek at Sunnybrook Drive near Raleigh

Wake 35.7583 -78.5831 29.8 10/8/2016 5,960 33

NC-S4 USGS 02088000 Middle Creek near Clayton Johnston 35.5708 -78.5906 83.5 10/09/16 20,600 >500

NC-S5 USGS 02088500 Little River near Princeton Johnston 35.5114 -78.1603 232 10/10/16 9,960 99

NC-S6 USGS 02090380 Contentnea Creek near Lucama Wilson 35.6911 -78.1097 161 10/09/16 12,000 244

NC-S7 USGS 02091000 Nahunta Swamp near Shine Greene 35.4889 -77.8061 80.4 10/09/16 13,600 >500

NC-S8 USGS 02102192 Buckhorn Creek near Corinth Chatham 35.5597 -78.9736 76.3 10/08/16 1,710 2

NC-S9 USGS 02102908 Flat Creek near Inverness Hoke 35.1828 -79.1775 7.63 10/08/16 733 167

NC-S10 USGS 02103000 Little River at Manchester Cumberland 35.1933 -78.9856 348 10/10/16 10,600 108

NC-S11 USGS 02104220 Rockfish Creek at Raeford Hoke 34.9997 -79.2147 93.1 10/09/16 5,490 >500

NC-S12 USGS 02106500 Black River near Tomahawk Sampson 34.7550 -78.2886 676 10/10/16 39,100 >500

NC-S13 USGS 02108000 Northeast Cape Fear River near Chinquapin

Duplin 34.8289 -77.8322 599 10/11/16 18,200 53

NC-S14 USGS 02132320 Big Shoe Heel Creek near Laurinburg Scotland 34.7506 -79.3867 83.3 10/10/16 1,480 500

NC-S15 USGS 02133500 Drowning Creek near Hoffman Richmond 35.0611 -79.4939 183 10/09/16 5,620 49

NC-S16 USGS 02133624 Lumber River near Maxton Robeson 34.7728 -79.3319 365 10/11/16 6,790 175

NC-S17 USGS 02134170 Lumber River at Lumberton Robeson 34.6203 -79.0111 708 10/10/16 14,600 175

NC-S18 USGS 02134480 Big Swamp near Tar Heel Robeson 34.7103 -78.8364 229 10/09/16 19,400 >500


Stream Gage Information Hurricane Matthew Information

Gage Map Agency Site Number Site Name County Latitude Longitude

Drainage Area

(square miles)

Peak Date at Gage

Peak Discharge at

Gage (cfs)

Peak Discharge ARI at Gage

(years)

South Carolina

SC-S1 USGS 02110400 Buck Creek near Longs Horry 33.9535 -78.7197 49.4 10/09/16 4,490 211

SC-S2 USGS 02110701 Crabtree Swamp at Conway Horry 33.8610 -79.0411 17.8 Tidal affected, stage-only gage

SC-S3 USGS 02130840 Black Creek below Chesterfield Chesterfield 34.6633 -80.2117 51.7 10/09/16 834 <2

SC-S4 USGS 02130900 Black Creek near McBee Chesterfield 34.5140 -80.1831 108 10/10/16 1,170 <2

SC-S5 USGS 02130910 Black Creek near Hartsville Darlington 34.3974 -80.1498 173 10/09/16 1,060 4

SC-S6 USGS 02130980 Black Creek near Quinby Florence 34.2438 -79.7448 438 10/08/16 5,730 18

SC-S7 USGS 021313485 Buffalo Creek at Mount Pisgah Kershaw 34.5822 -80.4569 10.7 Tidal affected, stage only gage

SC-S8 USGS 02131500 Lynches River near Bishopville Lee 34.2502 -80.2137 675 10/12/16 3,900 <2

SC-S9 USGS 02131510 Lynches River at I-95 above Olanta Sumter 34.0382 -79.9861 n.a. Tidal affected, stage-only gage

SC-S10 USGS 02134900 Lumber River at Nichols Horry 34.2260 -79.1342 n.a. Tidal affected, stage-only gage

SC-S11 USGS 02135501 Black River at I-95 near Manning Clarendon 33.8339 -80.1316 n.a. Tidal affected, stage-only gage

SC-S12 USGS 21355015 Tearcoat Branch at I-95 near Manning Clarendon 33.8130 -80.1516 n.a. Tidal affected, stage-only gage

SC-S13 USGS 02135615 Pocotaligo River at I-95 above Manning Clarendon 33.7296 -80.2202 n.a. Tidal affected, stage-only gage

SC-S14 USGS 02173212 Cedar Creek near Thor Lexington 33.6900 -81.2542 44.1 10/08/16 91 <2

ARI = average recurrence interval of discharge; ARI at gage locations are based on flow frequency analysis of peak discharge records at gage locations. ARIs greater than 500 years are shown as “>500” due to uncertainties involved in computation of large ARIs. cfs = cubic feet per second n.a. – drainage area not availablemi. = miles USGS = U.S. Geological Survey


Figure 8: Locations of stream gages and average recurrence intervals for Hurricane Matthew in North Carolina


Figure 9: Locations of stream gages and average recurrence intervals for Hurricane Matthew in South Carolina


Table 10 shows the range of peak discharges observed at the stream gages impacted by Hurricane Matthew. In addition, the maximum, minimum, and distribution of the recurrence intervals for the stream gages impacted by Hurricane Matthew are also presented in Table 10. Stream gages with long-term flow records are not as common as those with long-term precipitation gage records. The lack of long-term flow records and the relatively short periods of available flow records introduce a high degree of uncertainty to the computation of recurrence intervals 500 years and greater. As a result of the uncertainty associated with the determination of recurrence intervals of 500 years or more, recurrence intervals greater than 500 years are shown as “> 500” in Table 10.

Table 10: Summary of Maximum and Minimum Peak Discharges and Recurrence Intervals at USGS Stream gages impacted by Hurricane Matthew

Discharge Hurricane Matthew Minimum Hurricane Matthew Maximum Hurricane Matthew Median

Peak flood discharge

91 cfs (USGS gage 02173212 Cedar Creek near Thor [SC-S14])

39,100 cfs (USGS gage 02106500 Black River near Tomahawk [NC-S12])

Not applicable

Peak discharge recurrence interval

Less than 2 years Greater than 500 years 99 years (>100 years at 48% of the [x number] gages and >500 years 24% of the gages)

Note: Data is based on xxx[number] of USGS stream gages located in the area of study in North Carolina and South Carolina cfs = cubic feet per second


4.2 Peak Discharges at Impacted Dams Initial estimates of Hurricane Matthew site-specific peak flood flows at the impacted dams were derived using comparisons with the observed peak discharges at USGS stream gages in the areas impacted by Hurricane Matthew. The observed peak discharges, drainage areas, and estimated impervious cover at the stream gage locations were used to develop a simple regression model that was then applied to the impacted dams. The estimated rainfall return period was compared to the peak discharge return period at each of the selected stream gages. The results of these comparisons were then used to adjust the estimated peak discharge for the impacted dams. The estimated peak discharges at breached or overtopped dams are presented in Table 11.


Table 11: Estimated Peak Discharges and Peak Discharge Frequency for Breached and Overtopped Dams in North Carolina and Breached Dams in South Carolina

Dam Information Peak Discharges at Breached and Overtopped Dams (cfs)

Dam Map Number

State Dam Number


Estimated Recurrence Interval (RI, in years) 10-Year RI 25-Year RI 50-Year RI 100-Year RI 500-Year RI

Breached Dams in North Carolina and South Carolina

NC-1 CUMBE-025 6.0 250 576 758 900 1,060 1,440

NC-2 CUMBE-034 25.0 110 637 829 971 1,110 1,460

NC-3 CUMBE-050 6.6 420 558 771 960 1,170 1,650

NC-4 CUMBE-052 2.5 290 1,070 1,330 1,530 1,740 2,190

NC-5 CUMBE-053 53.2 250 2,020 2,730 3,360 4,030 5,580

NC-7 CUMBE-077 1.5 250 394 472 524 575 685

NC-8 CUMBE-086 2.2 420 287 401 503 615 882

NC-9 CUMBE-088 0.4 240 284 342 384 427 528

NC-10 CUMBE-099 0.6 320 206 253 286 319 392

NC-11 DUPLI-016 1.6 230 228 320 402 493 710

NC-12 DUPLI-017 5.3 210 486 674 840 1,020 1,450

NC-13 HARNE-047 3.9 310 401 558 697 851 1,210

NC-15 HOKE-028 3.4 390 369 514 643 785 1,120

NC-16 LENOI-003 7.1 210 581 804 1,000 1,220 1,720

NC-20 SAMPS-016 6.2 370 535 740 922 1,120 1,590

NC-21 SAMPS-047 15.3 280 934 1,280 1,590 1,920 2,690

NC-24 WAYNE-008 3.8 260 396 551 688 839 1,200

NC-25 WAYNE-009 0.9 230 168 237 299 368 532

NC-27 WILSO-009 35.1 240 1,560 2,120 2,610 3,150 4,380

SC-1 D 3550 4.6 31 443 616 768 937 1330

SC-2 D 3554 1.0 28 175 247 311 382 553


Dam Information

Dam Map Number

State Dam Number



SC-3 D 3553 23.0 28 1,200 1,640 2,030 2450 3420

SC-4 D 3602 12.0 230 806 1,110 1,370 1660 2340

SC-5 D 3583 0.5 130 107 152 193 238 347

SC-6 D 3580 1.3 110 402 525 615 703 924

SC-7 D 3569 0.7 55 200 282 354 435 628

SC-8 D 3612 3.4 240 135 191 241 298 432

SC-9 D 3571 2.4 31 371 517 646 789 1130

SC-10 D 3595 1.5 260 296 415 519 636 910

SC-11 D 0511 18.4 170 225 316 397 487 700

SC-12 D 2426 4.1 14 1,050 1,430 1,770 2140 3000

SC-13 D 2417 13.4 9 412 574 716 874 1240

SC-14 D 3453 13.9 220 882 1,210 1,500 1820 2550

SC-15 D 3431 2.4 170 112 147 173 198 261

SC-16 D 3616 1.8 220 254 356 446 547 786

SC-17 D 0852 1.8 250 247 346 434 532 765

SC-18 D 3598 17.2 250 1,000 1,370 1,700 2060 2880

SC-19 D 3490 6.2 56 535 740 922 1120 1590

SC-20 D 3562 7.0 72 579 801 997 1210 1710

SC-21 UnReg4 0.3 78 25 33 39 45 59

SC-22 UnReg3 1.0 240 270 335 381 427 530

SC-23 UnReg5 0.2 38 109 134 152 169 208

SC-24 UnReg1 17.0 21 998 1,370 1,690 2050 2870

SC-25 UnReg2 7.3 31 423 552 655 774 1070

Peak Discharges at Breached and Overtopped Dams (cfs)


Dam Information Peak Discharges at Breached and Overtopped Dams (cfs)

Dam Map Number

State Dam Number



Overtopped Dams in North Carolina

NC-14 HOKE-023 2.8 400 325 455 569 696 994

NC-17 MOORE-040 95.0 180 1,750 2,270 2,650 3,020 3,960

NC-18 NEWHA-003 n.d. 290

NC-19 ROBES-004 n.d. 210

NC-22 WAKE-034 66.3 240 5,720 7,280 8,460 9,640 12,500

NC-23 WAKE-035 6.9 27 1,910 2,380 2,730 3,070 3,890

NC-26 WILSO-007 40.0 250 1,690 2,290 2,830 3,400 4,720

ARI = average recurrence interval of discharge; ARI at gage locations are based on flow frequency cfs = cubic feet per second analysis of peak discharge records at gage locations. ARIs greater than 500 years are shown n.a.= drainage area not available as “>500” due to uncertainties involved in computation of large ARIs. mi. = miles

RI = recurrence interval USGS = U.S. Geological Survey


4.3 Peak Discharge Recurrence Intervals at Impacted Dams The computed recurrence intervals for the Hurricane Matthew peak discharges at the USGS stream gage locations were used to create a recurrence interval surface for the peak discharges in the areas impacted by Hurricane Matthew. This peak discharge recurrence interval surface was then used to estimate peak discharge recurrence intervals for each dam location. The resulting 10-, 25-, 50-, 100-, and 500-year annual recurrence intervals for each dam location are presented in Table 11. Table 12 showsthe maximum, minimum, and distribution of the recurrence intervals for the dams impacted byHurricane Matthew.

Table 12: Summary of maximum and minimum recurrence intervals at USGS Stream dams impacted by Hurricane Matthew

Hurricane Matthew Minimum

Hurricane Matthew Maximum Hurricane Matthew Median

Peak discharge recurrence interval

9 years (Floyd Pond Dam (SC-13), Lee County

420 years (Mount Vernon Estates Lake Dam [NC-8] and Smith Lake Dam [NC-3], Cumberland County)

230 years (>100 years at 73% of the 52 analyzed dams and <500 years at all dams)

4.4 Land Cover and Basin Characteristics on Peak Discharges at Impacted Dams

Land cover and basin characteristics affect peak discharges during flooding. The impacted dams had different drainage areas and different amounts of impervious area, both of which directly affect peak discharges. Areas with a greater than 10 percent impervious area are considered to be urban; urban areas are generally subject to higher peak discharges due to the high percentage of impervious area. In general, however, during intense rainfall events, which exceed the infiltration capacity of the soil and saturate the soils, effect of increased impervious area to increase peak discharges is reduced.

Analysis. The drainage area sizes and percent imperviousness of the dams in the impacted area are as follows:

• Drainage areas

o Range = 0.2 to 95.0 square miles

o Average = 11.2 square miles

• Percent impervious

o Range = <1 percent to 27 percent

o Ten dams >10 percent

Seven of the impacted dams in North Carolina and three of the impacted dams in South Carolina are considered urban. These 10 dams are located in suburban, mostly residential settings. All 10 dams were constructed in 1970 or earlier. Records of land cover and other basin characteristics for these basins are not available, but it is likely that these drainage basins continued to develop and urbanize after the design and construction of these dams. It is assumed that the potential for peak discharge has increased since their construction.


4.5 Spillway Design Storms Dams in North Carolina are required by the Subchapter 2K, Section .0205 of the North Carolina Administrative Code to have a spillway system with the capacity to pass a flow resulting from a specified design rainfall event, or spillway design storm. The spillway design storm is defined in terms of either the Probable Maximum Precipitation (PMP) rainfall event, a fraction of the PMP, or by the 50- or 100-year recurrence interval rainfall event. The specified spillway design storm for a given dam depends on a combination of the dam’s hazard classification and size. Table 13 shows the spillway design criteria for dams in North Carolina and South Carolina and Table 14 shows the size criteria used to classify dams in order to determine the spillway design criteria.

Table 13: Spillway Design Criteria for Dams in North Carolina and South Carolina

PMF = probable maximum flood PMP = probable maximum precipitation

Spillway Design Storms for Dams in North Carolina

Dam Size Class C (High Hazard)

Class B (Intermediate Hazard)

Class A (Low Hazard)

Small One-third PMP 100-year event 50-year event

Medium One-half PMP One-third PMP 100-year event

Large Three-quarters PMP One-half PMP One-third PMP

Very Large PMP Three-quarters PMP One-half PMP

Spillway Design Flows for Dams in South Carolina

Dam Size Class I (High Hazard)

Class II (Intermediate Hazard)

Class III (Low Hazard)

Very Small 100-year event to one-half PMF Not Specified Not Specified

Small One-half PMF to PMF 100-year event to one-half PMF 50- to 100-year event

Intermediate PMF One-half PMF to PMF 100-year event to one-half PMF

Large PMF PMF One-half PMF to PMF


Table 14: Size Classification for Dams in North Carolina and South Carolina

North Carolina Dam Size Classification

Dam Size Total Storage Dam Height

Small <750 acre-feet <35 feet

Medium >750 and <7,500 acre-feet >35 and <50 feet

Large >7,500 and <50,000 acre-feet >50 and <100 feet

Very Large >50,000 acre-feet >Greater than or equal to 100

South Carolina Dam Size Classification

Dam Size Total Storage Dam Height

Very Small 50 acre-feet <25 feet

Small >50 and <1,000 acre-feet >25 and <40 feet

Intermediate >1,000 and <50,000 acre-feet >40 and <100 feet

Large >50,000 acre-feet >100 feet

>= greater than or equal to > = greater than< = less than

Analysis. As noted in Section 4.4, comparison of the estimated 3-day precipitation depths to computed PMP depth estimates show that the average ratio of 3-day precipitation depth to PMP is 24.8 percent and the median ratio of 3-day precipitation depth to PMP is 25.6 percent. The ratio of 3-day precipitation depth to PMP ranges from a maximum of 34.4 percent at Arran Lakes Dam (NC-4) in Cumberland County to a minimum ratio of 9.1 percent of the PMP at Baywater Drive Dam (SC-25) in Lexington County. Tables 15 and 16 show the specified spillway design criteria for North Carolina and South Carolina, respectively.

The size classification in South Carolina is based on storage in acre-feet. The storage data for the 25 breached dams is not available, but an estimate of storage was made based on the drainage area impounded by the dam. Significant- or low-hazard dams with drainage areas less than 5 square miles are assumed to be very small; while significant- or low-hazard dams with drainage areas greater than 5 square miles are assumed to be small. Unregulated dams are assumed to be very small, with the exception of Andrews Mill Road Dam, which is assumed to be small because it drains 17 square miles.

Table 15: Specified Spillway Design Criteria for Dams in North Carolina

Dam Hazard Classification / Number Dams Breached

Number of Breached Dams by Size / Applicable Current Spillway Design Storms

Small Medium

High-Hazard Number of dams = 9 Current spillway design storm = one-third PMP

Number of dams =3 Current spillway design storm = one-half PMP

Intermediate-Hazard or Significant-Hazard / Low-Hazard

Number of dams = 7* Current spillway design storm = 100-year or 50-year storm


Table 16: Specified Spillway Design Criteria for Dams in South Carolina

Dam Hazard Classification / Number Dams Breached

Number of Breached Dams by Size / Applicable Current Spillway Design Storms

Very Small Small


Number of dams = 2 Current spillway design storm = 100-year event to one-half PMF

Number of dams = 3 Current spillway design storm = One-half PMF to PMF

Low-Hazard Number of dams =10 Current spillway design storm = 50- to 100-year event

Number of dams = 5 Current spillway design storm = 100-year event to one-half PMF

Unregulated Number of dams = 4 Current spillway design storm = none

Number of dams = 1 Current spillway design storm = none

Comparison of Observed Precipitation to PMP – North Carolina. In North Carolina, the only breached dam that was constructed after the adoption of existing design standards in 1980 is Mt. Vernon Estates Dam (NC-8), a medium size, high-hazard dam constructed in 1989. As such, the Mt. Vernon Estates Dam is the only dam which can be assumed with certainty to have been designed in accordance with existing design standards. As such, the Mt. Vernon Estates Dam is assumed to have been designed to pass the one-half PMP event, per criteria for medium size high-hazard dams. However, comparison of the precipitation depth at Mt. Vernon Estates Dam to the computed PMP indicates that the dam breached at 32.7 percent of the PMP, or an approximately one-third PMP event.

The design criteria for the remaining 18 breached dams are unknown, because the construction date is known to pre-date 1980 or the construction date is not known. Although the design criteria for these dams is not known, it can be assumed that the design storms used for these 18 older dams were less than or equal to the current standards. Table 15 shows the current specified spillway design event for the breached dams in North Carolina, classified by size and hazard classification of dam. Discussion and comparison of the remaining 18 breached dams in North Carolina is presented below:

• Two of the older breached dams Rhodes Lake Dam (NC-5) and H.F. Lee Power Station CoolingLake Dam (NC-25) are medium-size high-hazard dams for which the current design standardwould be the one-half PMP event. At these dams, the observed 3-day precipitation was 31.4percent and 29.0 percent of the PMP, respectively. Although the actual design event used forthese dams is unknown, if the current design standards had been used the dam failures wouldhave occurred at less than the specified design storm.

• Nine of the remaining 16 older dams are small size dams, high-hazard for which the currentdesign standard would be the one-third PMP event. The observed 3-day precipitation at these 9dams ranged from 25.9 percent to 34.4 percent of the PMP; the second greatest ratio ofobserved precipitation to PMP was 30.3 percent. Although the actual design event used forthese 9 dams is unknown, if the current design standards had been used, the dam failures wouldhave occurred at or slightly less than the specified design storm.

• Seven of the remaining 16 older dams are small size dams, intermediate- or low-hazard dams forwhich the current design standard would be the 50-year or 100-year storm, respectively. Theobserved 3-day precipitation at these 7 dams ranged 124 years to more than 1,300 years.Although the actual design event used for these 7 dams is unknown, if the current designstandards had been used, the dam failures would have all at greater than the specified designstorm


Comparison of Observed Precipitation to PMP – South Carolina. The design standards for all 25 breached dams in South Carolina are not known because the construction dates are unknown. Although the design criteria for these dams is not known, it can be assumed that the design storms used for the South Carolina breached dams were less than or equal to the current standards. Discussion and comparison of the 25 breached dams in South Carolina is presented below:

• The dates of construction and storage capacity for the 25 breached dams are not known, but thelargest is intermediate size.

• The spillway design storm for very small-, small-, or intermediate-size dams of low- orsignificant-hazard ranges from not specified up to a maximum of one-half the PMF to PMF.

• The spillway design storms for the five unregulated dams that breached are not known, but areassumed to be less than the 100-year storm.


5.0 Observation Summaries, Conclusions, and Recommendations The following observation summaries, conclusions, and recommendations are based on the findings of the hydrological analysis described in this report.

Observation No. 1 - Preparedness. Beginning on October 7, 2016, and continuing through October 9, 2016, Hurricane Matthew brought extreme rainfall to central and eastern North Carolina and South Carolina. The extreme precipitation associated with Hurricane Matthew was preceded by greater than normal rainfall throughout September 2016, which led to above-normal soil moisture and streamflow conditions that set the stage for the extreme flooding that resulted in 67 breached or overtopped dams in North Carolina and South Carolina.

Conclusion No. 1. Greater-than-normal antecedent rainfall and the extreme precipitation from Hurricane Matthew combined to exacerbate the flooding in response to Hurricane Matthew. Monitoring of climatic conditions and awareness of ongoing wetter than normal conditions, especially during hurricane and tropical storm season, is an important component of awareness of the potential for damaging flooding.

Recommendation No. 1. Dam owners and operators should increase their awareness of extended wet periods even though rainfall amounts may not rise to extreme levels. Such extended wet periods should trigger their increased awareness of the potential for damaging flooding and provide incentives to prepare, as needed.

Observation No. 2 – Precipitation Recurrence. Although the precipitation from Hurricane Matthew occurred over a 3-day period, most of the 3-day total precipitation occurred over a single day. Based on data from 114 precipitation gages, the observed 3-day total was, on average, 115 percent of the 1-day total precipitation. The 1-day precipitation depths had an average recurrence interval of 353 years. The 1-day rainfall recurrence interval exceeded the 100-year event at 61 percent of the gages, exceeded the500-year event at 18 percent of the gages, and exceeded the 1,000-year event at 9 percent of gages.

For the dam locations where the dams were breached or overtopped, the estimated 1-day precipitation recurrence intervals were greater than the recurrence intervals for the total area (described in previous paragraph), with an average estimated 1-day precipitation recurrence interval of 438 years at the impacted dam locations.

Conclusion No. 2. The large number of breached and overtopped dams is largely a result of the extreme nature of the 1-day precipitation event and the wide extent of area that received the extreme precipitation.

Recommendation No. 2. The estimated recurrence interval of the precipitation observed during Hurricane Matthew provides insight to the magnitude and intensity of extreme precipitation events that will result in damage-inducing flooding and should be used to guide warning and alert thresholds for forecast precipitation.

Observation No. 3 – Flood Recurrence. Flooding of recurrence interval in excess of 100 years was reported at 48 percent of the 25 USGS stream gages in the impacted areas and was greater than 500 years at 36 percent of the 25 stream gages. Estimated peak discharge recurrence intervals calculated for 52 of the 67 impacted dams in North and South Carolina ranged from a low of 9 years to a maximum of 420 years. The median peak discharge recurrence interval was 230 years and the peak discharge recurrence interval was greater than 100 years at 73 percent of the 52 analyzed impacted dams, but did not exceed 500 years at any of the 52 analyzed impacted dams.


Conclusion No. 3. The number of breached and overtopped dams is correlated to the locations where the recurrence of the peak discharge was greatest.

Recommendation No. 3. The estimated recurrence interval peak discharges observed during Hurricane Matthew can be used to provide insight to the magnitude of flooding that will result in damages to dams. The observed magnitude and intensity of Hurricane Matthew precipitation should be used to help guide warning and alert thresholds for future forecast peak discharge.

Observation No. 4 –North Carolina Dam Hazard. The 19 dams that breached in North Carolina due to Hurricane Matthew are all classified as either small or medium size. The breached dams include 12 high-hazard dams, 2 intermediate-hazard dams, and 5 low-hazard dams. One of the breached intermediate-hazard dams is an unregulated dam and four of the breached low-hazard dams are unregulated dams.

Based on size and hazard classification of these 19 dams that breached in North Carolina and the current North Carolina spillway design criteria, the maximum spillway design storm under current code for a medium size, high-hazard dam is one-half the PMP. The spillway design storm for small high-hazard dams or small to medium intermediate- or low-hazard dams ranges from 50-year storm to one-third PMP. Current code does not apply to unregulated dams.

Conclusion No. 4a – Newer Breached Dam. Mt. Vernon Estates Dam (NC-8 is a medium size, high-hazard dam constructed in 1989 and the only breached dam in North Carolina known to have been constructed after the adoption of the current design standards in 1980. As such, it is assumed the Mt. Vernon Estates Dam was designed to pass the one-half PMP event, per criteria for medium size high-hazard dams. However, comparison of the precipitation depth at Mt. Vernon Estates Dam to the computed PMP indicates that the dam breached at 32.7 percent of the PMP, or an approximately one-third PMP event.

Conclusion No. 4b- Older Breached Dams. Eighteen of the remaining breached dams are either known to have been constructed prior to 1980 or have unknown construction dates and are assumed to be pre-1980 due to the condition and setting of the dams observed during preparation of this report. Because the design and construction standards of these 18 dams are not known, it is not known whether the observed precipitation exceeded the design standard storm for these dams.

However, the study results show that observed precipitation at the remaining 18 breached dams was less than 34.4 percent of the computed PMP. The 34.4 percent PMP event would equal or exceed the current North Carolina standard spillway design floods for small dams of all hazard classes as well as medium-size intermediate- and low-hazard dams.

Recommendation 4 – NC Dam Hazard. Older small-to-medium dams with unknown design capacity or those designed to pass events significantly less than the PMP or PMF should be evaluated to determine the magnitude of the critical precipitation or discharge event. Some of these smaller dams may not currently have emergency actions plan. Those identified as potentially vulnerable to smaller precipitation or flood events should consider developing emergency action plans. In addition, the smaller, high- and significant- hazard dams identified as potentially vulnerable to smaller precipitation or flood events should be considered for mitigation actions to reduce vulnerability.

Observation No. 5 – South Carolina Dam Hazard. The 25 dams that breached in South Carolina and due to Hurricane Matthew are classified as either very small, small, or intermediate size. The breached dams included 20 State-regulated dams, of which five are classified as significant-hazard dams and 15 as low-hazard dams. Five inventoried, but unregulated, dams with undefined hazard classifications were also breached.


Based on size and hazard classification of the dams that breached in South Carolina and the current South Carolina spillway design criteria, the maximum spillway design storm for an intermediate size, significant-hazard dam is one-half the PMF to PMF. The spillway design storms for the five unregulated dams that breached are not known but are assumed to be less than one-half the PMF. None of the construction dates for the 25 dams that breached in South Carolina are unknown, but they are assumed to be pre-1980 due to the condition and setting of the dams.

Conclusion No. 5 – Breached Dams. Because the design and construction standards of the 25 dams that breached in South Carolina are not known, it is not known whether the observed precipitation exceeded the design standard storm for these dams.

However, the study results show that observed precipitation at the 25 breached dams was less than 34.4 percent of the computed PMP. The 34.4 percent PMP event would equal or exceed the current South Carolina standard spillway design floods for very small dams of all hazard classes and small low-hazard dams, accounting for the majority of the 25 breached dams.

Recommendation 5 – SC Dam Hazard. Older small-to-medium dams with unknown design capacity or those designed to pass events significantly less than the PMP or PMF should be evaluated to determine the magnitude of the critical precipitation or discharge event. Some of these smaller dams may not currently have emergency actions plan. Those identified as potentially vulnerable to smaller precipitation or flood events should consider developing emergency action plans. In addition, the smaller, high- and significant-hazard dams identified as potentially vulnerable to smaller precipitation or flood events should be considered for mitigation actions to reduce vulnerability.


6.0 References Feaster, T.D., A. J. Gotvald, A.J., and J.C. Weaver. 2009. Magnitude and frequency of rural floods in the

Southeastern United States, 2006—Volume 3, South Carolina. U.S. Geological Survey Scientific Investigations Report 2009-5156. https://pubs.usgs.gov/sir/2009/5156/.

Musser, J.W., K.M. Watson, K.M., and A. J. Gotvald. 2017. Characterization of peak streamflows and flood inundation at selected areas in North Carolina following Hurricane Matthew, October 2016. U.S. Geological Survey Open-File Report 2017–1047, Version 2.0, August 2017. https://doi.org/10.3133/ofr20171047.

National Oceanic and Atmospheric Administration (NOAA). 2017. National Hurricane Center Tropical Cyclone Report Hurricane Matthew AL142016. April 3, 2017; revised April 7, 2017. https://www.nhc.noaa.gov/data/tcr/AL142016_Matthew.pdf.

National Weather Service (NWS). 1982. Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian. NWS Hydrometeorological Report No. 52 (HMR-52). http://www.nws.noaa.gov/oh/hdsc/PMP_documents/HMR52.pdf.

U.S. Army Corps of Engineers (USACE). 1987. HMR52 Probable Maximum Storm (Eastern United States) User’s Manual. Published March 1984; revised April 1987. http://www.hec.usace.army.mil/publications/ComputerProgramDocumentation/HMR52_UsersManual_(CPD-46).pdf.

U.S. Geological Survey (USGS). 1982. Guidelines for Determining Flood Flow Frequency. USGS Bulletin 17B. https://water.usgs.gov/osw/bulletin17b/dl_flow.pdf.

USGS. 2018. Flood Frequency Analysis Based on Bulletin 17C and recommendations of the Advisory Committee on Water Information (ACWI) Subcommittee on Hydrology (SOH) Hydrologic Frequency Analysis Work Group (HFAWG). https://water.usgs.gov/software/PeakFQ/.

Weaver, J.C., T.D. Feaster, and A. J. Gotvald. 2009. Magnitude and frequency of rural floods in the Southeastern United States, through 2006—Volume 2, North Carolina. U.S. Geological Survey Scientific Investigations Report 2009–5158. https://pubs.usgs.gov/sir/2009/5158/.

Weaver, J.C., T.D. Feaster, and J.C. Robbins. 2016. Preliminary peak stage and streamflow data at selected streamgaging stations in North Carolina and South Carolina for flooding following Hurricane Matthew, October 2016. U.S. Geological Survey Open-File Report 2016–1205. https://doi.org/10.3133/ofr20161205.

https://pubs.usgs.gov/sir/2009/5156/

https://doi.org/10.3133/ofr20171047

https://www.nhc.noaa.gov/data/tcr/AL142016_Matthew.pdf

http://www.nws.noaa.gov/oh/hdsc/PMP_documents/HMR52.pdf

http://www.hec.usace.army.mil/publications/ComputerProgramDocumentation/HMR52_UsersManual_(CPD-46).pdf

http://www.hec.usace.army.mil/publications/ComputerProgramDocumentation/HMR52_UsersManual_(CPD-46).pdf

http://water.usgs.gov/osw/bulletin17b/dl_flow.pdf

https://acwi.gov/

https://acwi.gov/hydrology/

https://acwi.gov/hydrology/Frequency/

https://water.usgs.gov/software/PeakFQ/

https://pubs.usgs.gov/sir/2009/5158/

https://doi.org/10.3133/ofr20161205

Appendix A Table A-1: Hurricane Matthew Breached and Overtopped

Dams in North Carolina and South Carolina

Hydrologic Analysis of Hurricane Matthew’s Impact on August 2018 Dam Safety in North Carolina and South Carolina Page A-1

Table A-1: Hurricane Matthew Breached and Overtopped Dams in North Carolina and South Carolina

Map Identifier Name

State Identifier State County Dam Status Latitude Longitude

Drainage Area (square miles)

Hazard Classification

Emergency Action Plan

NC-1 Loch Lommond Dam CUMBE-025 NC Cumberland Breached 35.07 -79.00 5.99 High No

NC-2 Long Valley Farm Lake Dam CUMBE-034 NC Cumberland Breached 35.21 -78.98 25 High N.R.1

NC-3 Smith Lake Dam CUMBE-050 NC Cumberland Breached 34.86 -78.73 6.61 Low - Exempt (Not regulated) No

NC-4 Arran Lakes Dam CUMBE-052 NC Cumberland Breached 35.03 -78.98 2.48 High No

NC-5 Rhodes Lake Dam CUMBE-053 NC Cumberland Breached 35.23 -78.65 53.2 High Yes

NC-7 Mirror Lakes Dam CUMBE-077 NC Cumberland Breached 35.05 -78.92 1.53 High Yes

NC-8 Mount Vernon Estates Lake Dam CUMBE-086 NC Cumberland Breached 34.85 -78.88 2.25 High No

NC-9 Devonwood Lower Dam CUMBE-088 NC Cumberland Partially breached 35.08 -79.00 0.433 High Yes

NC-10 Rayconda Upper Dam CUMBE-099 NC Cumberland Breached 35.03 -79.02 0.65 High No

NC-11 Rouse Pond Dam DUPLI-016 NC Duplin Breached 35.15 -77.80 1.55 Low - Exempt (Not regulated) No

NC-12 Maxwell Mill Pond Dam DUPLI-017 NC Duplin Partially breached 35.07 -77.79 5.29 Low - Exempt (Not regulated) No

NC-13 Guy Road Pond Dam HARNE-047 NC Harnett Breached 35.49 -78.71 3.88 Low - Exempt (Not regulated) N.R.1

NC-15 Sunset Lake Dam HOKE-028 NC Hoke Breached 34.94 -79.07 3.39 High No

NC-16 Tull Mill Pond Dam LENOI-003 NC Lenoir Breached 35.16 -77.73 7.08 High No

NC-20 Laurel Lake Dam SAMPS-016 NC Sampson Breached 35.01 -78.49 6.18 Intermediate (Exempt) (Not

Regulated) No

NC-21 House Autry Dam SAMPS-047 NC Sampson Breached 35.19 -78.38 15.3 High Yes

NC-24 Durham Lake Dam WAYNE-008 NC Wayne Breached 35.28 -78.06 3.79 Low - Exempt (Not regulated) N.R.


Map Identifier Name





NC-25 H.F. Lee Power Station Cooling Lake Dam WAYNE-009 NC Wayne Breached 35.37 -78.07 0.945 High No

NC-27 Silver Lake Dam WILSO-009 NC Wilson Breached 35.80 -77.95 35.1 Intermediate (Exempt) (Not

Regulated) No

Not mapped Happy Valley Dam BLADE-017 NC Bladen Dam overtopped but held 36.60 -79.65 Not determined High Yes

Not mapped Rayconda Lower Dam CUMBE-019 NC Cumberland Dam overtopped but held 35.02 -79.02 Not determined Low - Exempt (Not regulated)

NC-6 Smith Lake Dam CUMBE-055 NC Cumberland Dam overtopped but held 35.22 -78.82 1.26 Low - Exempt (Not regulated) N.R.1

Not mapped Arran Lakes West Dam CUMBE-067 NC Cumberland Dam overtopped but held 35.01 -78.99 Not determined High

Not mapped Strickland Bridge Road Dam CUMBE-108 NC Cumberland Dam overtopped but held 35.01 -79.02 Not determined High

Not mapped Wooded Lake Dam CUMBE-114 NC Cumberland Dam overtopped but held 35.14 -78.88 Not determined High

NC-14 Kaco-English Dam (also

known as McLaughlin Lakes Dam)

HOKE-023 NC Hoke Dam overtopped but held 34.99 -79.08 2.76 Low N.R.1

Not mapped Lupo Lake Dam HOKE-024 NC Hoke Dam overtopped but held 34.99 -79.14 Not determined High

Not mapped Scull Lake Dam HOKE-025 NC Hoke Dam overtopped but held 35.04 -79.14 Not determined High

Not mapped Thomas Lake Dam #2 HOKE-027 NC Hoke Dam overtopped but held 34.98 -79.28 Not determined High

NC-17 Woodlake Dam MOORE-040 NC Moore Dam overtopped; Concrete spillway damaged 35.22 -79.19 95 High No

NC-18 Sutton 1972 Cooling Pond Dam NEWHA-003 NC New Hanover Dam overtopped but held 34.30 -78.01 Undefined Low No

NC-19 Weatherspoon Cooling Pond Dam ROBES-004 NC Robeson Dam overtopped but held 34.58 -78.97 Undefined Low No

Not mapped Johnson Pond Dam SAMPS-010 NC Sampson Dam overtopped but held 34.96 -78.36 Not determined High

Not mapped Stafford Pond Dam SAMPS-011 NC Sampson Dam overtopped but held 34.97 -79.58 Not determined High


Map Identifier Name





Not mapped Melva Brook Pond Lower Dam SAMPS-037 NC Sampson Dam overtopped but held 34.96 -78.26 Not determined High

NC-22 Lake Benson Dam WAKE-034 NC Wake Dam overtopped but held 35.66 -78.61 66.3 High Yes

NC-23 Lake Johnson Dam WAKE-035 NC Wake Dam overtopped but held 35.76 -78.71 6.92 High Yes

Not mapped Panther Lake Dam WAKE-047 NC Wake Dam overtopped but held 35.72 -78.69 Not determined High

Not mapped Byrd Dam WAKE-220 NC Wake Dam overtopped but held 35.71 -78.71 Not determined High

Not mapped Underwood Dam WAKE-236 NC Wake Dam overtopped but held 35.74 -78.62 Not determined High

Not mapped Lake Wackena Dam WAYNE-002 NC Wayne Dam overtopped but held 35.30 -77.86 Not determined High

NC-26 Lake Wilson Dam WILSO-007 NC Wilson Dam overtopped but held 35.79 -77.92 40 High No

SC-1 Bethea Baptist Home Dam D 3550 SC Darlington Breached 34.25 -79.83 4.56 Low N.R.

SC-2 Pee Dee Experimental Station Pond Dam D 3554 SC Darlington Breached 34.30 -79.73 1.01 Low N.R.

SC-3 Dargan's Pond Dam D 3553 SC Darlington Breached 34.30 -79.72 23 Low N.R.

SC-4 Pee Dee St Park Dam D 3602 SC Dillon Breached 34.33 -79.26 12 Low N.R.

SC-5 Floyd Pond Dam D 3583 SC Florence Breached 33.98 -79.51 0.455 Low N.R.

SC-6 Bowling Pond Dam D 3580 SC Florence Breached 34.04 -79.54 1.26 Low N.R.

SC-7 Blackwell Pond Dam D 3569 SC Florence Breached 34.05 -79.82 0.663 Low N.R.

SC-8 Stroud Pond Dam D 3612 SC Horry Breached 34.18 -79.12 3.42 Low N.R.

SC-9 Sales International Dam D 3571 SC Florence Breached 34.25 -79.67 2.38 Low N.R.

SC-10 Hamer Pond Dam D 3595 SC Dillon Breached 34.53 -79.43 1.52 Low N.R.

SC-11 Legette Millpond Dam D 0511 SC Marion Breached 34.04 -79.34 18.4 Low N.R.

SC-12 Copeland Pond Dam D 2426 SC Lee Breached 34.17 -80.16 4.06 Low N.R.

SC-13 Floyd Pond Dam D 2417 SC Lee Breached 34.31 -80.27 13.4 Low N.R.

SC-14 Covington Millpond Dam D 3453 SC Marlboro Breached 34.61 -79.63 13.9 Low N.R.


Map Identifier Name





SC-15 Haire Pond Dam D 3431 SC Marlboro Breached 34.76 -79.79 2.39 Low N.R.

SC-16 Graham Mill Pond Dam D 3616 SC Horry Breached 34.02 -79.17 1.85 Significant N.R.

SC-17 Flowers Pond Dam D 0852 SC Dillon Breached 34.36 -79.20 1.76 Significant N.R.

SC-18 Gaddys Millpond Dam D 3598 SC Dillon Breached 34.39 -79.24 17.2 Significant N.R.

SC-19 Lakewood Pond Dam D 3490 SC Clarendon Breached 33.73 -80.09 6.18 Significant N.R.

SC-20 Pepsi Cola Lake Pond Dam D 3562 SC Florence Breached 34.15 -79.61 7.04 Significant N.R.

SC-21 Meadow Street Dam Undefined SC Chesterfield Breached 34.70 -79.96 0.315 Unregulated N.R.

SC-22 Hardy's Lake Dam Undefined SC Dillon Breached 34.42 -79.35 0.969 Unregulated N.R.

SC-23 Hampton Pointe Dam Undefined SC Florence Breached 34.19 -79.86 0.213 Unregulated N.R.

SC-24 Andrews Mill Road Dam Undefined SC Darlington Breached 34.22 -80.07 17 Unregulated N.R.

SC-25 Baywater Drive Dam Undefined SC Lexington Breached 33.96 -81.10 7.32 Unregulated N.R.

C1 =High-Hazard classification for dams in South Carolina mi. = miles C2 = Significant-Hazard classification for dams in South Carolina NC = North CarolinaC3 = Low-Hazard classification for dams in South Carolina N.R. = Not RequiredUnregulated = Unregulated dams in South Carolina SC = South Carolina

Hydrologic Analysis of Hurricane Matthew’s Impact …...2018/08/14 · Hydrologic Analysis of...

Documents

Transcript of Hydrologic Analysis of Hurricane Matthew’s Impact …...2018/08/14 · Hydrologic Analysis of...