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Probable Maximum Flood Determination

Tittabawassee River Hydroelectric Projects

Secord (P-10809)

Smallwood (P-10810) Sanford (P-2785)

Edenville (unlicensed)

Gladwin and Midland Counties, Michigan

Prepared for

Spicer Group, Inc. and Four Lakes Task Force

May 15, 2020

3433 Oakwood Hills Parkway Eau Claire, WI 54701-7698

715.834.3161 • Fax: 715.831.7500 www.AyresAssociates.com

Ayres Associates Project No. 26-1144.00

File: i:\26\spicer group four lakes task force\report may 2020\pmf final report-may2020.docx

Probable Maximum Flood Determination

Tittabawassee River Hydroelectric Projects

Secord (P-10809) Smallwood (P-10810) Sanford (P-2785)

Edenville (unlicensed)

May 15, 2020

i

Contents Page No.

Executive Summary ...................................................................................................................................... 1

Project Background and Descriptions ........................................................................................................... 1

Secord Dam ............................................................................................................................................. 2

Smallwood Dam ....................................................................................................................................... 2 Edenville Dam .......................................................................................................................................... 2

Sanford Dam ............................................................................................................................................ 3

Historic Streamflow Records ........................................................................................................................ 3

Previous Studies ........................................................................................................................................... 4

HEC-HMS Model Development .................................................................................................................... 5

Basin Subdivision .................................................................................................................................... 6 Unit Hydrograph Parameters ................................................................................................................... 8

Hydrologic Loss Functions....................................................................................................................... 8

Quasi-Distributed Loss Accounting: Rationale and Overview ............................................................ 8

Initial Assignment of Loss Rates ........................................................................................................ 9

Channel/Floodplain Routing .................................................................................................................. 10 Reservoir/Spillway Routing .................................................................................................................... 10

HEC-HMS Model Calibration ...................................................................................................................... 11

Flood Flows ........................................................................................................................................... 11

NEXRAD Precipitation Analysis ............................................................................................................ 12

Data Acquisition ................................................................................................................................ 12

NEXRAD Processing and Analysis .................................................................................................. 13 Calibration - Unit Hydrographs and Low Permeability Soil Classes ...................................................... 14

Model Verification: Reproducing the 100 Year Peak Flow .................................................................... 16

Final Model Parameters......................................................................................................................... 17

Baseflow Assumptions ........................................................................................................................... 19

Probable Maximum Storm Development .................................................................................................... 19

Analysis Methodology ............................................................................................................................ 19 Synoptic Storms (Edenville and Sanford) .............................................................................................. 20

Mesoscale Convective System (MCS) Storms (Secord and Smallwood) ............................................. 21

Probable Maximum Flood Hydrographs ..................................................................................................... 22

Reservoir Routing Assumptions ............................................................................................................ 22

Secord Project Probable Maximum Flood ............................................................................................. 23

Smallwood Project Probable Maximum Flood ....................................................................................... 24 Edenville Project Probable Maximum Flood .......................................................................................... 25

Sanford Project Probable Maximum Flood ............................................................................................ 26

ii

Discussion and Comparison to Previous Estimates .............................................................................. 26

List of Exhibits Exhibit 1 ................................................................................................................................ Watershed Map Exhibit 2 ...................................................................................Subbasin Soil Distributions by Minimum Ksat

Exhibit 3 .............................................................................................................. HEC-HMS Calibration Plots

Exhibit 4 ............................................................................................ Probable Maximum Storm Calculations

List of Figures Page No.

Figure 1: Loss Rate Cumulative Distributions, 2020 and 1994 ............................................................ 19

Figure 2: Second Project Probable Maximum Flood Hydrographs ...................................................... 23

Figure 3: Smallwood Project Probable Maximum Flood Hydrographs ................................................. 24

Figure 4: Edenville Project Probable Maximum Flood Hydrographs .................................................... 25

Figure 5: Sanford Project Probable Maximum Flood Hydrographs ...................................................... 26

List of Tables Page No.

Table 1: Tittabawassee Basin Stream Gages and Floods of Record ..................................................... 4

Table 2: Tittabawassee River Projects PMF Flows – 1994 Study (Mead & Hunt) ................................. 5

Table 3: Comparison of 1994 HEC-1 and 2020 HEC-HMS Models ....................................................... 6

Table 4: HEC-HMS Model Subbasins .................................................................................................... 7

Table 5: Constant Loss Rate Distribution by Subbasin ........................................................................ 10

Table 6: Tittabawassee River Calibration Events ................................................................................. 12

Table 7: NEXRAD-Derived Storm Depth by Subbasin, 2014 and 2017 ............................................... 14

Table 8: HEC-HMS Model Calibration Summary of Results ............................................................... 16

Table 9: HEC-HMS Model Results for 72 Hour, 100-Year Storm......................................................... 17

Table 10: Summary of Final Subbasin Model Parameters ................................................................... 18

Table 11: Probable Maximum Storm Depths Creating Edenville/Sanford PMF (850-Square-Mile 72-Hour Storm Centered above Edenville) .............................................................. 21

Table 12: Probable Maximum Storm Depths Creating Smallwood PMF (450-Square-Mile 24-Hour Storm Centered above Smallwood Dam) .................................................. 21

Table 13: Probable Maximum Storm Depths Creating Secord PMF (300-Square-Mile 24-Hour Storm Centered above Secord Dam) ........................................................ 22

iii

Table 14: Comparison of PMF Estimates ............................................................................................. 27

May 15, 2020 1

Executive Summary

The Secord, Smallwood, and Sanford hydroelectric projects and the Edenville Dam (formerly a FERC-licensed hydroelectric project) are located on the Tittabawassee River in east central Michigan. Under Federal Energy Regulatory Commission (FERC) criteria, the Secord, Smallwood, and (if licensed) Edenville facilities are required to safely pass the Probable Maximum Flood (PMF). The Sanford project has been shown to safely pass its Inflow Design Flood, which is much less than the PMF.

PMF and spillway capacity analyses completed for the projects between 1994 and 2017 indicated that the spillway capacities at Secord and Edenville are less than the required PMF discharges, while the capacity at Smallwood exceeds the PMF. (Note that recent analyses by GEI Consultants show otherwise, as discussed below.) The present study was undertaken on behalf of the Four Lakes Task Force, which is presently preparing to acquire the dams. The objective of this study was to re-evaluate the PMF at all four Tittabawassee River facilities using improved precipitation, streamflow, and watershed data, and reflecting updated FERC guidelines. To accomplish this a HEC-HMS model was constructed of the Tittabawassee River watershed from the upstream basin boundary to Sanford Dam. Changes in the 2020 HMS model relative to the previous HEC-1 model include re-delineation of basin and subbasin boundaries using the National Elevation Dataset; aggregation into a single HMS model instead of the 1994 HEC-1/UNET combination; more detailed subbasin division; use of SSURGO soil data to estimate loss potential distributions for each subbasin; and calibration of the model using NEXRAD precipitation records and streamflow data from USGS gages and the hydro projects.

The calculated PMF peak inflows to the Secord, Smallwood, Edenville, and Sanford reservoirs are 29,400 cfs, 41,200 cfs, 80,900 cfs, and 80,600 cfs respectively. These represent increases of 10 percent, 0.5 percent, 30 percent, and 7 percent over the previously accepted values. The increases can be attributed primarily to a decrease in simulated hydrologic loss rates in 2020. The disproportionately large increase at Edenville Dam is also attributed to (a) a more critical storm position than was previously evaluated; and (2) a 2011 study resulting in a decrease in the Edenville estimate, which was not applicable to the other dams.

Project Background and Descriptions

The four Tittabawassee River hydroelectric projects were constructed in 1923 by the Wolverine Power Company. Since 2006 they have been owned and operated by Boyce Hydropower, LLC. There are currently plans to transfer ownership to the Four Lakes Task Force (FLTF), a delegated authority of Gladwin and Midland Counties created for the purpose of managing and maintaining the four impoundments and dams. Prior to beginning required spillway upgrades at the Secord and Edenville projects, the FLTF determined that the PMF should be reviewed and updated if appropriate. This PMF study was conducted by Ayres Associates under contract to the Spicer Group, which has been retained by the FLTF to provide engineering and design services related to FERC dam safety compliance for the four projects.

Pertinent project features are described below. Note that the drainage areas listed are based on the 2020 redelineation of the basin using the National Elevation Dataset (NED) supplemented by county-level LiDAR, and differ from previously reported areas. Project-related elevations use the NGVD29 vertical datum. To convert to NAVD88, subtract 0.5 foot from the NGVD29 elevation for the Secord and Smallwood projects, and 0.6 foot for the Edenville and Sanford projects. Spillway geometry and discharge data were taken from an April 2020 Technical Memorandum prepared by GEI Consultants.

May 15, 2020 2

The dams’ locations in the upper Tittabawassee River basin are shown on Exhibit 1.

Secord Dam

The Secord project (FERC Project 10809) is located in Gladwin County and is the most upstream of the four dams, at a drainage area of 177 square miles. It impounds the 979-acre Secord Lake. At normal pool elevation 750.8, freeboard to the top of the embankments is seven feet.

Flood flows are passed through two tainter gates, one 20.5 feet wide and one 23.6 feet wide. According to investigations by Spicer Group and GEI Consultants in 2020, the smaller gate’s maximum opening height is 7.5 feet and the larger gate opens to 10.5 feet. GEI’s calculations indicate that at zero freeboard (pool elevation 757.8) the total gate capacity is 7,695 cfs. An additional 4,440 cfs flows over the left abutment and the east reservoir rim towards Tea Creek, for a total zero freeboard capacity of 12,135 cfs. Flow over the east reservoir rim crosses private residential properties on which the licensee owns flowage easements.

The Secord watershed is largely undeveloped except for lakeshore properties. Relief is generally low and land cover types include forest, wetland forest, and agriculture.

Smallwood Dam

The Smallwood project (FERC Project no. 10810) is located in Gladwin County at a drainage area of 289 square miles. Impoundment area measurements vary: the project Supporting Technical Information Document (STID) lists the impoundment area as 500 acres, but the Michigan DNR lists the impoundment area as 232 acres. Ayres’ measurement using 2018 aerial imagery and the National Elevation Dataset is 360 acres. Normal pool elevation is 704.8 feet and zero freeboard elevation is 715.7 feet.

Flood flows are passed through two 23.4-foot-wide tainter gates. One of the gates opens to a height of 10 feet and the other to a height of 9.9 feet. At zero freeboard elevation 715.7 the gates discharge 10,185 cfs. “Zero freeboard” refers to the top of a sheetpile wall driven into the upstream faces of the embankments, but a 680-foot length of the left embankment was left unprotected due to its low height and non-critical failure consequences. Overtopping of this length of embankment adds 19,650 cfs in capacity at zero freeboard elevation, for a total capacity of 29,835 cfs.

The watershed at Smallwood Dam is mostly undeveloped, but has more agricultural use in the Sugar River subbasin than the watershed upstream of Secord. Lancer Lake Dam on the Sugar River impounds a 500 acre lake.

Edenville Dam

Edenville Dam is constructed across both the Tittabawassee and Tobacco Rivers just above their confluence, at a combined drainage area of 904 square miles. Its reservoir, Lake Wixom, has a normal surface area of 1,980 acres. Lake Wixom is bifurcated by the Michigan Highway 30 causeway with a 55-foot bridge opening which, for this study, was assumed to allow free exchange of flows and equalized pool levels between the Tobacco River side and the Tittabawassee River side. (HEC-RAS model analyses conducted by Spicer Group, separate from this study, support this assumption.) Normal pool elevation is 675.8 feet and the zero freeboard elevation is 682.1 feet.

May 15, 2020 3

Gated spillways containing three tainter gates each are located on each branch of the reservoir. On the Tittabawassee River (or Edenville) side, two of the gates are 20 feet wide and the third is 23.5 feet wide. On the Tobacco River side, two spillway gates are 23.6 feet wide and the third is 20 feet wide. All of the spillway gates open to a height between 8.9 and 9.6 feet, except the small Tobacco River gate which was found in gate tests to open only 4.5 feet. The total zero-freeboard spillway capacity is 20,670 cfs, of which 10,750 cfs passes through the Edenville gates and 9,920 cfs passes through the Tobacco River gates.

Significant additions to the drainage basin between Smallwood and Edenville include the Tobacco River, draining 458 square miles of mixed forest, wetland forest, and agriculture to the west; and the Molasses River, draining 78 square miles of mostly wetland forest to the east. The Tobacco River drainage includes the cities of Gladwin and Beaverton and two significant impoundments: the 500-acre Wiggins Lake, dammed by Chappel Dam; and the 300-acre Ross Lake, dammed by the Beaverton Dam.

Sanford Dam

Sanford Dam is on the Tittabawassee River at a drainage area of 945 square miles. It impounds the 1,430-acre Sanford Lake. Normal pool elevation is 630.8 feet and the zero freeboard elevation is 636.8 feet. Spillways include four 22-foot-wide tainter gates and two 25.4-foot wide tainter gates, with opening heights ranging from 10.1 to 11 feet. The dam is also equipped with a fuse plug spillway which is first overtopped at elevation 634.8 feet. The fuse plug spillway crest elevation is 631.8 feet. According to GEI’s 2020 calculations, the total spillway capacity is 36,175 cfs.

Dam failure analyses have shown that based on downstream failure impacts, the Inflow Design Flood for Sanford Dam is 37,000 cfs, much less than the PMF. This determination was made in Mead & Hunt’s Addendum 1 to the 1994 Inflow Design Flood Study. Therefore, while Sanford Dam is included in this study, it received a lesser focus than the Edenville, Smallwood, and Secord projects.

Historic Streamflow Records There are two USGS stream gages within the basin upstream of Sanford Dam, and one downstream of Sanford at Midland, Michigan. Table 1 summarizes available stream gage data.

May 15, 2020 4

Table 1: Tittabawassee Basin Stream Gages and Floods of Record

Gage Name USGS Gage Number

Drainage Area (square miles, as reported by USGS)

Period of Record Flood of Record (cfs) and date

South Branch Tobacco River near Beaverton

04152238 160 1987-present 3,280 April 14, 2014

Tobacco River at Beaverton

04152500 487 1948-1982, 2015 - present

7,680 July 1957

Tittabawassee River at Midland (downstream of all projects)

04156000 2,400 1907, 1910-present

39,100 June 24, 2017

In addition, hydrographs and peak inflows at the Edenville Dam in April 2014 and June 2017 were estimated from hourly records of gate openings and pool levels maintained by Boyce Hydro. The estimated peak inflow to Lake Wixom on April 2014 was 18,700 cfs. The estimated peak inflow in June 2017 was 12,100 cfs.

Previous Studies

With the exception of the Edenville project, the accepted PMF values at the dams date from a 1994 study conducted for Wolverine Power by Mead & Hunt. That study used the HEC-1 and UNET models and divided the watershed at Sanford Dam into eight subbasins. Warm- and cool-season Probable Maximum Storms were developed from the 1993 Wisconsin-Michigan Probable Maximum Precipitation Study.

The 1994 unit hydrographs were developed from limited gage records from the South Branch of the Tobacco River and an adjacent gaged watershed, the Rifle River. Clark unit hydrograph parameters for the model subbasins were derived by transferring the channel length/time of concentration relationship from one of the gaged basins to the model subbasins, with the analogous gaged basin being chosen based on apparent hydrologic similarity.

Spatially distributed losses were calculated outside of the HEC1 program by dividing each subbasin into loss classes based on the STATSGO soils database, calculating individual soil class losses and runoff, and summing all of the runoff over a subbasin before returning it to the HEC-1 model. Loss rates were modified for the presence of wetlands. The constant loss rate was assigned based on the STATSGO saturated hydraulic conductivity (Ksat), using the geometric mean of the published range for each soil layer and unit. This is less conservative than current FERC requirements to consider minimum-of-range Ksat values unless higher losses can be justified through field data or model calibration.

Channel routing was also performed outside of the HEC-1 model in the UNET model, which pre-dated the dynamic HEC-RAS routing model.

May 15, 2020 5

The resulting PMF inflows and outflows are listed in Table 1. The Edenville PMF estimate was reduced as a result of a 2011 study, described below. As of April 2020, the PMF flows shown in Table 2 for Secord and Smallwood are the accepted design floods for those dams.

Table 2: Tittabawassee River Projects PMF Flows – 1994 Study (Mead & Hunt)

Project PMF Inflow (cfs) PMF Outflow (cfs)

Secord 27,200 27,100

Smallwood 41,000 40,700

Edenville 74,400* 73,900*

Sanford 75,500 73,200

* Inflow and outflow revised to 62,000 cfs in Mill Road Engineering study, 2011

In 2011, Mill Road Engineering concluded that the 1994 model misrepresented the offset in timing between the Tittabawassee River and Tobacco River contributions to Lake Wixom. The two branches of the reservoir were re-analyzed using a HEC-RAS model, resulting in a peak spillway flow at Edenville of 62,000 cfs.

HEC-HMS Model Development The HEC-HMS model used for this study was informed by the 1994 model structure but incorporated many new elements. A lumped, rather than gridded, model structure was retained due to the large size of the basin and the associated complexity in data management, interpretation, calibration/verification, and model execution. Like the 1994 model, the HEC-HMS model extended downstream to Sanford Dam and used multiple subbasins defined by major dams, divides, and confluences. Table 3 compares the methods used in the two models. The 2020 data and methods are described in more detail in the following sections.

May 15, 2020 6

Table 3: Comparison of 1994 HEC-1 and 2020 HEC-HMS Models

Model Element 1994 HEC-1 Model 2020 HEC-HMS Model

Number of subbasins 8 11 plus two reservoir subbasins Subbasin delineation Manually on USGS quadrangle

maps National Elevation Dataset and county Lidar

Total measured basin area at Sanford Dam (square miles)

968 945

Subbasin hydrograph routing method

UNET dynamic model, outside of HEC-HMS

Muskingum-Cunge routing within HEC-HMS

Number of dams and reservoirs 6: four Tittabawassee River dams , Beaverton, and Sugar Springs/Lake Lancer

7: four Tittabawassee River dams, Beaverton, Lake Lancer, and Wiggins Lake/Chappel Dam

Unit hydrograph derivation Analogy to calibrated Tc and R on South Branch of the Tobacco River and Rifle River (outside of basin)

Initially applied relationship between Tc, R, and main channel length from 1994 study but adjusted by calibration

Hydrologic Loss Calculations Made in spreadsheet outside of HEC-1 using 8 loss classes derived from STATSGO data

Divided subbasins into three loss classes, plus zero losses, within HEC-HMS based on SSURGO data

Model calibration – streamflow data

No whole-model calibration as no flow data were available except at South Fork of the Tobacco

Used recorded gate openings at Edenville from 2014 and 2017 to reconstruct inflow hydrographs; also South Branch Tobacco and Tobacco River USGS gages

Model calibration – Precipitation data

None NEXRAD data for storms of April 2014 and June 2017

Basin Subdivision

The subbasins modeled for this study are shown in Exhibit 1 and summarized in Table 4. Significant differences from the 1994 study were to divide the Tobacco River drainage (formerly modeled as a large subbasin comprising almost half of the entire study basin) into three subbasins, and add the Molasses River watershed as a separate subbasin draining to the Tittabawassee River reach above Wixom Lake.

Table 4 lists the subbasins from upstream to downstream, not in numeric order which is a legacy of the 1994 study. Also, note that in the HEC-HMS model, each of the subbasins listed in Table 4 (except the two separately modeled reservoirs) is actually divided into 3 sub-subbasins to provide separate runoff accounting for each of three runoff potential classes.

May 15, 2020 7

Table 4: HEC-HMS Model Subbasins

Subbasin Number

HEC-HMS Name Area (square miles)

Description

1 Secord 129.1 Upper Tittabawassee River, drains to Secord Lake storage element

8 W Br Tittabawassee

46.3 West Branch Tittabawassee River, drains to Secord Lake storage element

- Secord Res 1.5 Secord Reservoir; 100 percent impervious with nominal (1 hour) time of concentration; drains to Secord Lake storage element

2 Sugar Springs 34.4 Sugar River above Lake Lancer, discharges to routing reach and then Smallwood Lake

4 Smallwood 77.4 Smallwood Reservoir drainage below Lake Lancer and Secord Dam, drains to Smallwood Lake storage element

3a Chappel 117.2 Cedar River above Wiggins Lake (Chappel Dam); dam discharges to routing reach to Ross Lake

3b Beaverton-Cedar 136.9 Lower Cedar River drainage plus North and Middle Branches Tobacco River, drains to Ross Lake storage element

3c Beaverton-Tobacco

153.3 South Branch of the Tobacco River, drains to Ross Lake storage element.

6 Edenville-Tobacco

50.5 Tobacco River drainage below Beaverton Dam, drains to Wixom Lake storage element

5a Molasses 77.9 Molasses River, drains to routing reach between Smallwood and Wixom Lake

5b Edenville-Tittabawassee

76.4 Tittabawassee River drainage below Smallwood Dam, drains to Wixom Lake storage element

- Wixom-subbasin 3.1 Wixom Lake, 100 percent impervious with nominal time of concentration; drains to Wixom Lake storage element

7 Sanford 40.8 Direct drainage to Sanford Lake, includes Sanford Lake as impervious fraction

Note that Wixom Lake and Secord Lake were treated as separate subbasins whereas Smallwood Lake and Sanford Lake were included in the subbasins representing the surrounding drainage areas. This was originally an outcome of calibration efforts at Secord and Edenville Dams, in which rapidly arriving flows from precipitation on the reservoirs were considered potentially significant to the hydrograph shape. For PMF modeling, the small size of Smallwood Lake and early timing of peak flows from the Sanford subbasin (long before the PMF peak originating upstream of Edenville, whether or not the lake is modeled separately) justified retaining this model structure.

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Unit Hydrograph Parameters

The Clark unit hydrograph parameters Tc (time of concentration in hours) and R (storage coefficient, also expressed in hours) were initially estimated using the relationship between Tc, R, and main channel length (L) developed in the 1994 Mead & Hunt study. This relationship is

Tc/L = 0.64 (Tc in hours and L in miles) and R/L = 0.92 (R in hours and L in miles) For subbasins with land cover analogous to the Rifle River (Subbasins 1, 4, 5a, 5b, 7) and

Tc/L = 1.26 and R/L =0.75

For subbasins with land cover analogous to the South Branch of the Tobacco (Subbasins 2, 3a,3b,3c, 4, 8)

The unit hydrograph parameters were later refined as indicated by calibration results, in particular for the second group of subbasins where calibration data were available at the South Branch of the Tobacco and Tobacco River gages. Final subbasin unit hydrograph parameters are detailed in Table 9 in a later section of this report.

Hydrologic Loss Functions

Quasi-Distributed Loss Accounting: Rationale and Overview

Soil permeability types in the watershed range from sands with very high saturated hydraulic conductivity (Ksat) to silty clays and hydric soils with very low Ksat values. While Ksat is not the only determinant of loss potential (land cover, slope, and depth to the water table are some other factors) it is a useful baseline value for the constant loss rate when initial moisture deficits are satisfied.

When very high and very low Ksat values are present in the same subbasin, the effective average constant loss rate is mathematically related to the peak rainfall rate. As a very simple example, if 30 percent of a basin has a Ksat of zero and the remaining 70 percent has a Ksat of 6 inches per hour, for all precipitation events up to 6 inches per hour, no single-valued loss rate would apply. Instead, the calculated basin-averaged loss rate would be 70 percent of the rainfall rate. The calibrated loss rate for a 2-inch-per-hour storm (around a 10 year event in Michigan) will be 1.4 inches per hour, but the appropriate average loss rate for a 6-inch-per-hour storm (typical of a Probable Maximum Storm) would be 4.2 inches per hour. In a lumped hydrologic model, a solution to this is to further divide subbasins into a manageable number of loss (or runoff potential) classes.

For this study, soil hydrologic characteristics were classified in a spatial analysis by Spicer Group, consisting of overlaying the USDA SSURGO database for Gladwin, Midland, Roscommon, Clare, Bay, Ogemaw, Arenac, and Isabella Counties on the subbasin boundaries. Hydrologic losses were modeled in a quasi-distributed manner by modeling each subbasin as three parallel sub-subbasins, one representing high permeability soils, one representing moderately permeable soils, and one representing low permeability soils. All three sub-subbasins within a given subbasin have identical unit hydrograph parameters. Zero-permeability areas were also treated separately by assigning a fixed impervious percentage to each sub-subbasin. In this report we use “permeability” loosely to mean the overall hydrologic loss potential of a given spatial soil unit, which becomes the HEC-HMS “constant loss rate.”

May 15, 2020 9

Ksat refers to the saturated hydraulic conductivity listed in SSURGO, which varies by depth for a given spatial soil unit and is also expressed as a range at each depth, based on soil texture. These ranges are generally logarithmic; when expressed in inches per hour they have values such as 0.06–0.2 inch per hour; 2–6 inches per hour, and 6-20 inches per hour. “Soil unit” refers to the map units in the SSURGO database, which may be a single soil type (e.g. Roscommon fine sand) or an association (e.g. Roscommon-Brevort-Timakwa).

Initial Assignment of Loss Rates

Based on the spatial analysis performed by Spicer Group, soil units in each subbasin were tabulated according to area covered and the minimum of the listed Ksat range for the least permeable layer in the top 60 inches of the soil column. This typically gave 10-15 Ksat classes based on the minimum published Ksat in each soil unit (see Exhibit 2). These were grouped into four more general categories as follows:

Zero Losses: Soils with a minimum-of-range Ksat of 0.0 inch per hour to 0.016 inch per hour in the top 60 inches of the soil column. These were initially assigned a HEC-HMS constant loss rate of zero and input to the model as an impervious percentage of the other loss-class subbasins. The assigned impervious percentage did not change as the result of calibration.

Low Permeability: Soils with minimum-of-range Ksat values from 0.06 inch per hour to 0.2 inch per hour. These were initially assigned a constant loss rate of 0.1 inch per hour, which proved in calibration to be excessively conservative and was adjusted upward to 0.35 inch per hour. (This is still a reasonable representative value for this group of soils, considering that all soils listed as having a minimum-of-range Ksat of 0.2 inch per hour had an overall range of 0.2-0.6 iph).

Moderate Permeability: Soils with minimum-of-range Ksat values ranging from 0.6 inch per hour to 2.0 inches per hour. Almost all of these soils had a minimum-of-range Ksat of either 1.5 or 2.0 inch per hour, so the “moderate permeability” areas were initially assigned a constant loss rate of either 1.5 or 2.0 inches per hour, depending on the dominant type identified for the subbasin in question. This loss rate exceeds the maximum hourly precipitation rates used in model calibration and therefore could not possibly be calibrated upward. The calibration (described in a later section) did not justify a downward change, so these loss rates remained at the initial values in the final PMF model runs. As computed by the HEC-HMS model, these soil classes do not generate runoff during the calibration events but do generate runoff during the PMF, consistent with the concept that watersheds have a “variable contributing area” which expands as precipitation becomes more intense.

High Permeability: Soils with minimum-of-range Ksat values of 6 inches per hour. These were assigned a loss rate of 6 inches per hour, and showed no computed runoff during either the calibration events or the PMF.

Table 5 lists the percentage of each subbasin’s area assigned to the four loss classes listed above. The small subbasins representing Secord Lake and Wixom Lake were considered 100 percent impervious.

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Table 5: Constant Loss Rate Distribution by Subbasin

Subbasin No.

Total Area (square miles)

Percent of Subbasin Assigned to Loss Class Zero

Permeability Low

Permeability Moderate

Permeability High

Permeability 1 129.1 3.9 49.6 26.5 20.1 2 34.4 5.8 61.9 27.0 5.2 3a 117.2 9.0 48.5 30.9 11.6 3b 136.9 16.9 51.6 27.6 3.9 3c 153.3 23.1 45.5 29.1 2.3 4 77.4 8.3 63.6 15.5 12.7 5a 77.9 2.3 29.8 3.3 64.6 5b 76.4 8.1 43.1 7.3 41.5 6 50.5 39.0 17.2 28.1 15.6 7 40.8 22.1 42.6 15.9 19.4 8 46.3 3.2 40.0 42.1 14.7

Entire Basin

945 13.3 (includes 2 lakes)

45.8 23.5 17.4

The methodology described above does not include assigning a zero loss rate to all areas classified as wetlands, as was done in the 1994 study. Preliminary calibration runs found this approach to be excessively conservative, failing to account for the substantial storage and slow infiltration of precipitation in wetland forests.

No initial losses were modeled in either the calibration runs or the PMF modeling.

Channel/Floodplain Routing

River reaches on the Cedar, Tobacco, Tittabawassee, and Sugar Rivers were represented with Muskingum-Cunge routing using a trapezoidal channel/floodplain section with side slopes and longitudinal slopes measured from the National Elevation Dataset. Manning’s “n” values of 0.08 were used to represent predominantly floodplain flow during an extreme overbank event. A check of model results indicated the routing method was producing reasonable results. Calculated flood peak travel velocities during the PMF ranged from four to seven feet per second (three to five miles per hour) and were roughly 1.5 times higher during the PMF than during the calibration floods.

Reservoir/Spillway Routing

Reservoir (storage) routing elements were included in the HEC-HMS model at Secord, Smallwood, Edenville, Sanford, Lake Lancer Dam, Wiggins Lake/Chappel Dam, and Ross Lake/Beaverton Dam. Spillway rating curves for the three dams not owned by Boyce Hydropower were obtained by request from the Michigan Department of Environment, Great Lakes, and Energy (EGLE). Elevation-area curves were developed from the National Elevation Dataset and augmented by county LiDAR, and checked against published values. All reservoirs were assumed to be at their normal maximum operating pool level at the beginning of the PMF. Spillways were assumed to deploy immediately to minimize pool surcharges, providing no intentional flood storage.

May 15, 2020 11

HEC-HMS Model Calibration When the 1994 study was conducted, there were no real opportunities for model calibration. At the time, the only functioning stream gage was on the South Branch of the Tobacco River and represented only a small fraction of the basin; there were few large events in the record; reliable flood flow records at the dams were unavailable; and the only hourly precipitation records within the basin were from Gladwin, in the upper reaches of the Tobacco River watershed. For the present study, additional data sources include the Tobacco River at Beaverton stream gage, reinstated in 2015, and hourly records of pool levels and gate openings at Edenville Dam maintained by Boyce Hydro during the flood events of April 2014 and June 2017. The Edenville flood operation records were used to generate estimated reservoir inflow hydrographs by hourly back-routing. Operating records were also reviewed for Smallwood Dam and Secord Dam, but the back-routed inflow hydrographs created from those records fluctuated dramatically from hour to hour, even when a moving average was applied in an attempt to smooth them. This suggests that the recording precision of both gate opening and pool height probably is not adequate to reliably reconstruct the inflow hydrographs.

In addition, NEXRAD precipitation data, ground-truthed against the hourly gage at Gladwin, provided new and more detailed precipitation time series for the calibration events.

Flood Flows

Table 6 summarizes the 2014 and 2017 flood events and data sources. As noted in Table 1, the flood of April 2014 was the largest event in the 33-year record at the South Branch of the Tobacco River gage. NOAA climate records indicate the last snow in the watershed melted approximately two weeks before this flood, likely leaving high antecedent moisture conditions. (Additional snow fell near the end of the storm on April 14 and 15, but during the days prior to that temperatures ranged from about freezing to the mid-to-upper 40s on each day). The estimated peak inflow to Wixom Lake was 18,700 cfs. In comparison, the Midland County Flood Insurance Study lists a 100-year flood flow of 19,800 cfs “upstream of Sandford [sic] Dam.” The flood of June 2017 was the largest event since 1910 at the downstream Tittabawassee River at Midland USGS gaging station, but does not appear to have been as severe in the Tittabawassee River above Sanford Dam.

The estimated storm precipitation totals listed in Table 6 were calculated from NEXRAD precipitation data. The methodology used to estimate subbasin precipitation from NEXRAD data is addressed in more detail below.

May 15, 2020 12

Table 6: Tittabawassee River Calibration Events

Flood Dates Peak Inflow to Edenville Reservoir (cfs, estimated by back-routing)

Peak Flow at Tobacco River at Beaverton Gage (cfs)

Peak Flow at South Branch Tobacco River Gage (cfs)

Basin Average Precipitation (inches, estimated from NEXRAD)

April 13-16, 2014

18,700 April 14

Gage not yet in service

3,280 April 14

5.0 April 12-14

June 22-25, 2017

12,140 June 23

5,490 June 24

2,410 June 23

3.5 June 22-23

NEXRAD Precipitation Analysis

Data Acquisition

NOAA Next Generation Radar (NEXRAD) Level 3 products from the Gaylord, Michigan National Weather Service (NWS) site were used in this analysis. The NEXRAD products are divided in two data processing levels. The lower Level 2 data are base products at original resolution. Level 2 data are recorded at all NWS and most USAF and FAA WSR-88D radar sites. From the Level 2 quantities, computer processing generates numerous meteorological analysis for Level 3 products. The Level 3 data consists of reduced resolution, low-bandwidth, and base products as well as many derived post-processed products.

For this analysis three NEXRAD Level 3 data products were used. The Level 3 products used in this analysis are briefly described as follows:

1. One-Hour Precipitation (N1P) - A display of estimated one-hour precipitation accumulation on a 1.1-nm x 1-degree grid using the Precipitation Processing System (PPS) algorithm. This product assesses rainfall intensities for flash flood warnings, urban flood statements, and special weather statements.

2. Digital Precipitation Array (DPA) - The Digital Precipitation Array is a format of estimated one-hour precipitation accumulations on the 1/4 LFM (4.7625 km HRAP) grid. This is an 8-bit product with 255 possible precipitation values. This product assesses rainfall intensities for flash flood warnings, urban flood statements, and special weather statements.

3. One-Hour Precipitation (DAA) - One-hour precipitation accumulation is available on a 0.13-nm x 1-degree grid. The dual-polarization QPE algorithm is used and 256 possible data levels are available.

Raw data files from the three NEXRAD Level 3 precipitation products were obtained for the 2014 and 2017 storm periods. Rain gauge data from NOAA’s National Center for Environmental Information (NCEI) Hourly Precipitation Data (HPD) network were also used to compare with the Level 3 NEXRAD products. This was done to determine which of the NEXRAD products most accurately represented both the 2014 and 2017 rainfall events, and if any additional calibration of the NEXRAD dataset(s) was warranted. The rainfall data from the Gladwin rain gauge site was used to determine if calibration of the NEXRAD dataset(s) was warranted.

May 15, 2020 13

NEXRAD Processing and Analysis

The raw NEXRAD data files were processed into TIFF image files using NOAA’s Weather and Climate Toolkit.

The N1P, DAA, and DPA geotif raster images from the top of each hour, for both 2014 and 2017 events, were loaded into ESRI’s ArcGIS Pro Software. A geoprocessing tool using a raster calculation was executed to sum the one-hour precipitation NEXRAD products into 6-hour accumulation periods. The result was a raster image representing the previous 6 hours of NEXRAD rainfall totals at 12:00AM, 6:00AM, 12:00PM, 6:00PM, and so on, through the duration of each rainfall event.

The 6-hour NEXRAD accumulation results were contrasted against the hourly rain gauge data, which was also summarized into 6-hour accumulation totals for the corresponding 6-hour periods, mentioned above, through the duration of both rainfall events.

After analyzing the results, it was determined that the DPA dataset provided the best representation of the storm events when compared to the actual recorded hourly rain gauge totals summarized into 6-hour accumulation periods. It was also determined that no calibration to the DPA NEXRAD datasets was needed to continue the analysis.

Next, the estimated 1-hour precipitation totals for each of the 11 individual sub-watersheds was calculated by averaging the values from the estimated 1-hour precipitation DPA NEXRAD datasets. This data is from the top of each hour, across each of the sub-watersheds, for every hour over the duration of both the 2014 and 2017 rainfall events.

A custom geoprocessing tool was created using ArcGIS Pro software to automate and streamline the analysis. The custom geoprocessing tool used the Zonal Statistics tool and Raster Calculator to perform the calculation. The tool iterated through the DPA NEXRAD datasets from the top of each hour for both the rainfall events and calculating the estimated previous 1-hour average rainfall for each of the 11 sub-watersheds. The result was 126 MS Excel files representing the averaged estimated 1-hour precipitation accumulation that fell across each individual sub-watershed at any given hour over the course of both rainfall events. These files were aggregated into one workbook per storm using a custom MS Excel macro to automate the process and tabulate the results.

Table 7 lists the total and maximum hourly precipitation, by subbasin, extracted from the NEXRAD analysis for the 2014 and 2017 storms.

May 15, 2020 14

Table 7: NEXRAD-Derived Storm Depth by Subbasin, 2014 and 2017

Subbasin April 12 – 15, 2014 June 22-24, 2017

Storm Total (in) Maximum Hour (in) Storm Total (in) Maximum Hour (in)

1 5.45 0.78 2.28 0.36

2 5.48 0.84 2.62 0.63

3a 6.20 1.03 3.26 0.84

3b 6.21 0.83 3.97 0.97

3c 6.22 0.57 4.16 0.92

4 6.43 1.04 3.31 0.75

5a 6.23 0.94 2.82 0.61

5b 5.87 0.74 3.96 0.75

6 6.02 0.71 4.59 0.83

7 4.31 0.47 5.08 0.72

8 5.20 0.78 2.28 0.47

Calibration - Unit Hydrographs and Low Permeability Soil Classes

The hourly NEXRAD precipitation sequences for each subbasin were applied to the HEC-HMS model and the resulting hydrographs at the junction representing the combined Tobacco/Tittabawassee inflow to Lake Wixom, the South Branch of the Tobacco River, and the outflow from Ross Lake/Beaverton Dam were compared to the backrouted Edenville inflow hydrograph, the South Branch of the Tobacco USGS gage, and the Beaverton USGS gage respectively. In the calibration runs, flood hydrographs were allowed to pass through the dams in the model according to their spillway rating curves, without an attempt to reproduce actual spillway operation.

Constant loss rates for the four loss classes (zero, low, moderate, and high permeability) were modeled as segregated sub-subbasins as discussed above. No initial losses were modeled. For both events, NOAA records suggest a high antecedent moisture condition: End-of-season snowmelt two weeks prior to the 2014 flood, and several days of light (less than 0.1 inch per day) rain in the week prior to the June 2017 storm.

May 15, 2020 15

In the initial run, with the subbasin unit hydrograph and loss parameters set as described above, simulated hydrograph timing was slower than observed at the Tobacco River and South Branch Tobacco River gages, and the inflow hydrographs to Wixom Lake appeared too sharply peaked. Simulated runoff volumes in general were significantly higher than the observed.

The reservoir operation data also suggested early, steep increases in observed inflow at both Wixom Lake and Secord Lake which the HEC-HMS model did not capture. These may have been caused by precipitation falling directly on the reservoirs. Boyce Hydro recorded hourly precipitation at the dams during both the 2014 and 2017 events. The operators’ recorded hourly precipitation depths were input as rainfall on the reservoir-only subbasins. These reservoir precipitation increments were significantly more intense than the adjacent subbasin-averaged NEXRAD depths, possibly due to the effect of spatial averaging or to the Boyce rain gage setup or reading method. (The operator-recorded peak precipitation at Edenville Dam was actually listed as occurring several hours earlier than the NEXRAD and Gladwin hourly precipitation data suggested, and was shifted back in time accordingly in the calibration model.) In any event, the reservoir-only precipitation data proved to have only a small effect on the overall calculated hydrograph shape.

Various shifts in the unit hydrograph parameters and loss rates were tested and judged by the visual fit of the calculated hydrograph to the observed. Muskingum-Cunge routing parameters were not varied in the calibration, because they were producing reasonable results and there were no reach-specific data to support adjustments. The unit hydrograph and loss parameters were only adjusted if the adjustment could be made systematically and applied to both events and multiple subbasins. The best fit across all of the calibration points and both events was achieved by making the following modifications to the initial parameters:

• All of the “low permeability” sub-subbasins were assigned a loss rate of 0.35 inch per hour. Although it is a significant increase from the initial value of 0.1 inch per hour, this change keeps the representative loss rate for the “low permeability” soils within the published range (0.2-0.6 inch per hour) for the majority of soils assigned to this category. A loss rate of 0.35 inch per hour still resulted in considerable runoff being generated from the “low permeability” areas in the model, as the peak NEXRAD rainfall rate in each storm was around one inch per hour.

• The “zero permeability” soil groups, represented as a percent impervious in each sub-subbasin, were not changed. The “moderate” and “high” permeability soil groups also were not changed, because their SSURGO minimum-of-range loss rates exceeded the maximum calibration storm rainfall rates and therefore could not be tested by the observed storms. (In other words, lowering the “moderate” or “high” permeability loss rates would mean moving them below the published range; and raising them could not be justified because even at the minimum, they produce no computed runoff in the calibration storms.)

• The unit hydrograph time of concentration for all the Tobacco River subbasins (3a, 3b, and 3c) was reduced by 10 percent, and the storage coefficients reduced proportionately, to achieve a better hydrograph match at the South Branch and Beaverton USGS gages.

• In subbasins 1, 4, 5a, 5b, and 7, whose unit hydrograph parameters were initially assigned based on similarity to the Rifle River, the Clark Tc and R were increased by 15 percent. This provided for a less steeply peaked inflow hydrograph to Lake Wixom.

May 15, 2020 16

Table 8 summarizes the results of calibration. For the 2014 flood, only two calibration points were available. The South Branch of the Tobacco represents only one model subbasin, the 153-square-mile Basin 3c. The Edenville inflow hydrograph represents all of the subbasins except Subbasin 7, and a drainage area of 904 square miles. The Tobacco River at Beaverton gage represents the combined outflows from subbasins 3a, 3b, and 3c, combined and routed through Ross Lake and Beaverton Dam.

Table 8: HEC-HMS Model Calibration Summary of Results

Calibration Point and Date

Peak Flow (cfs) Peak 72-hour Runoff Volume (cfs-days)

Time of Peak (date/time)

Observed/ Backrouted

HEC-HMS Simulated

Observed/ Backrouted

HEC-HMS Simulated

Observed / Backrouted

HEC-HMS Simulated

Lake Wixom Inflow, 4-2014

18,683 18,903 38,503 36,235 4-14 1400 4-14 0500

South Branch Tobacco R. Gage, 4-2014

3,280 3,788 6,422 7,077 4-14 1400 4-14 1200

Lake Wixom Inflow, 6-2017

12,140 8,070 18,999 14,302 6-23 1600 6-23 2100

South Branch Tobacco R. Gage, 6-2017

2,410 2,952 3,378 4,065 6-23 1100 6-24 1100

Tobacco R. at Beaverton Gage, 6-2017

5,490 5,587 10,921 9,973 6-24 0500 6-24 1000

The final calibration plots are attached as Exhibit 3.

While the calibration did not succeed in closely reproducing all of the observed hydrographs, the modified HEC-HMS model was accepted based on reasonable reproduction of runoff volumes, an absence of observable bias (there was no consistent offset in peak, volume, or timing between the observed and simulated hydrographs), and peaks within 20 percent of the observed for four of the five calibration points. Furthermore, for the 2017 flood in particular, operating records for Secord and Smallwood Dams suggested that the upper Tittabawassee River hydrographs may have been affected by dam operation in a way that the HEC-HMS model could not reproduce.

Model Verification: Reproducing the 100 Year Peak Flow

A review of all available stream gage, precipitation gage, and project operation records did not produce any suitable events for model calibration or verification beyond the 2014 and 2017 floods discussed above. As a “reality check” on the HEC-HMS model performance, a run of the calibrated model was made with precipitation inputs consisting of a 72 hour, 100 year storm uniformly distributed across the basin. Although other factors as well as storm rainfall frequency determine the magnitude and frequency of the resulting flood event, the tendency of the HMS model to transform the 100 year storm into the 100 year flood is a good check of reasonableness in the absence of other verification data. The 72-hour, 100 year precipitation total (6.6 inches) at the approximate basin centroid was extracted from NOAA Atlas 14 and temporally distributed according to the 72-hour Probable Maximum Storm temporal pattern from the 1994 study. A 4 percent areal reduction was applied to the Atlas 14 point value

May 15, 2020 17

based on extrapolated depth-area-duration relationships shown in the 1992 Rainfall Frequency Atlas of the Midwest (Atlas 14 does not provide guidance on areal adjustments). The HEC-HMS model results for the Lake Wixom inflow, the Edenville dam outflow, and the combined flow at Sanford dam are shown in Table 9 below in comparison with information from the 2013 Midland County Flood Insurance Study and the 2018 Gladwin County Flood Insurance Study.

Table 9: HEC-HMS Model Results for 72 Hour, 100-Year Storm

Location and Source of Data

Drainage Area (square miles) Published or Calculated Peak Discharge (cfs)

Upstream of Sanford Dam – 2013 Midland County Flood Insurance Study

968 (FIS published) 19,817

At Edenville Dam – 2018 Gladwin County Flood Insurance Study

n/a 19,159

Lake Wixom Inflow – 2020 HEC-HMS

904 (2020 measurement) 19,844

Lake Wixom Outflow – 2020 HEC-HMS

904 (2020 measurement) 18,581

Combined Inflow at Sanford Dam – 2020 HEC-HMS

945 (2020 measurement) 18,967

The Midland County FIS also lists a peak discharge of 14,500 cfs for the Tobacco River but includes notes indicating that this estimate may include some of the Tittabawassee drainage as well. Final Model Parameters

Based on the calibration and 100-year flood comparison discussed above, the calibrated model was adopted for calculating the PMF. Table 10 summarizes the subbasin parameters applied for the PMF analysis.

May 15, 2020 18

Table 10: Summary of Final Subbasin Model Parameters

Subbasin (see Table 4 or Exhibit 1)

Area (sq mi)

Unit Hydrograph Loss Class Percentage Tc (hours) R (hours) Zero Low

(0.35 in/hr) Moderate (1.5–2 in/hr)

High (> 6 in/hr)

1 129.1 13 19 3.9 49.6 26.5 20.1 2 34.4 15 10 5.8 61.9 27.0 5.2 3a 117.2 22 15 9.0 48.5 30.9 11.6 3b 136.9 29 20 16.9 51.6 27.6 3.9 3c 153.3 36 24 23.1 45.5 29.1 2.3 4 77.4 18 25 8.3 63.6 15.5 12.7 5a 77.9 18 25 2.3 29.8 3.3 64.6 5b 76.4 7 10 8.1 43.1 7.3 41.5 6 50.5 14 8 39.0 17.2 28.1 15.6 7 40.8 13 20 22.1 42.6 15.9 19.4 8 46.3 20 14 3.2 40.0 42.1 14.7

Secord 1.5 1 1 100 0 0 0 Wixom 3.1 1 1 100 0 0 0

Figure 1 compares the cumulative distributions of loss rates, as percentages of the entire modeled basin, from the 1994 study and the present (2020) study. In 1994, a greater percentage of the watershed was assigned a zero loss rate than in 2020, probably as a result of the choice to model all wetlands as completely impervious. However, beginning with the “low permeability” loss rate of 0.35 inches per hour, the 2020 model consistently shifts higher percentages of the area to lower loss rates than the 1994 model. This is expected to be particularly influential for PMF calculations (as opposed to calibration runs), because Probable Maximum Precipitation (PMP) intensities significantly exceed the low and moderate-permeability loss rates. Therefore, the computed PMF runoff will be positively affected by the loss rate changes in the 2020 model.

May 15, 2020 19

Baseflow Assumptions

For the PMF calculation, baseflows were set to reproduce a starting baseflow of approximately 2 cfs per square mile, which was the calculated baseflow at Edenville Dam prior to the flood of April 2014.

Probable Maximum Storm Development This analysis focused on the warm-season PMP and PMF. The cool-season PMF was analyzed and found not to control in the 1994 analysis, with computed peak flows being less than the warm-season peaks at all four dams. As will be discussed below, the 2020 estimates of the warm-season PMF exceed the 1994 estimates, primarily as a result of a downward shift in the assumed loss rates. There is no reason to expect a disproportionate increase in cool-season results relative to warm-season results, especially since one significant influence on the computed cool-season runoff is a reduction in loss rates to account for frozen soils – but for the 2020 analysis, some of that reduction has already been made even for the warm-season case, as shown in Figure 1.

Analysis Methodology

The 1994 study utilized PMP data from the 1993 Probable Maximum Precipitation Study for Wisconsin and Michigan conducted by North American Weather Consultants under contract to the Electric Power Research Institute. Isohyetal analysis of candidate storms was conducted in 1994 with the WMPMS program, a FORTAN program adapted in 1993 from the Corps of Engineers’ HMR52 program.

For the 2020 analysis, ArcGIS was used to construct storm isohyets and compute subbasin precipitation sequences following the general guidelines (storm orientation, axis ratio, and temporal distribution) presented in the WMPMS users’ manual. PMP depth-area-duration data were taken from the 1993 Wisconsin and Michigan study and maps and were essentially identical to the 1994 values, with small

May 15, 2020 20

differences related to curve smoothing. Candidate storm areas and axis angles were chosen by reference to the 1994 study and visual inspection of the basin and subbasin layout.

Once a candidate storm area was identified, Ayres used a MS-Excel spreadsheet to develop within-storm and without-storm isohyet depths for twelve six-hour increments, following the tabular data in the WMPMS user manual. These values, the proposed storm centroid location, and the ellipse orientation and axis ratio (a function of storm size) were supplied to GIS specialists at Spicer Group, who calculated precipitation depths for each subbasin and each six-hour increment in a 72-hour (or 24 hour) storm. Finally, Ayres constructed 72-hour synoptic storm sequences and 24 hour mesoscale convective system (MCS) sequences at one-hour intervals, using the 1994 WMPMS output as a template for the temporal pattern.

Exhibit 4 documents the steps in the entire process for the synoptic storm producing the PMF at Edenville and Sanford, and includes summary tables for the critical MCS storms for Secord and Smallwood.

Synoptic Storms (Edenville and Sanford)

Two possible storm sizes and centerings were considered for the Edenville and Sanford projects. The first was the 1994 study’s critical Edenville/Sanford storm, a 1,000 square mile storm centered over the entire 945-square-mile Sanford drainage and oriented 330 degrees from north. Additionally, an 850-square-mile storm positioned to maximize rainfall over only the watershed upstream of Edenville was analyzed. For this event, the long storm axis was oriented 230 degrees from north. Storm isohyet maps are shown in Exhibit 4.

The critical storm for both Edenville and Sanford proved to be the smaller (850-square-mile) storm positioned over the watershed upstream of Edenville. This is a different outcome from the 1994 study, although the 1994 report provides no discussion of storm patterns tested but found to be non-critical. The finding that the smaller storm is critical at Sanford as well as Edenville is unsurprising, given that the 41-square-mile Subbasin 7 (local inflows to Sanford Lake) peaks so early in the event that it has very little impact on the peak flow, which is dominated by the much larger and later flood hydrograph out of Edenville Dam.

Table 11 summarizes the total 72-hour precipitation and maximum 1-hour precipitation for this event by subbasin.

May 15, 2020 21

Table 11: Probable Maximum Storm Depths Creating Edenville/Sanford PMF (850-Square-Mile 72-Hour Storm Centered above Edenville)

`Subbasin 72-hour Depth

(inches) Maximum 1 hour Depth

(inches) 1 - Secord 16.66 2.93

2 - Sugar Springs 15.94 2.77 3a – Chappel/Upper Cedar 16.05 2.82

3b – Lower Cedar, North and Middle Tobacco 17.45 3.16 3c - South Br Tobacco 16.10 2.78

4 - Smallwood 19.35 3.66 5a - Molasses 13.48 2.21

5b - Edenville – Tittabawassee 12.35 2.01 6 – Edenville-Tobacco 15.40 2.63

7 - Sanford 7.88 1.15 8 – W Br Tittabawassee 15.65 2.70

Mesoscale Convective System (MCS) Storms (Secord and Smallwood)

A review of the 1994 study and the modeled basin response time (20 hours from peak rainfall to peak inflow at Smallwood Reservoir) demonstrated that more intense 24-hour MCS type storms would be critical at Secord and Smallwood. For Smallwood, a 450-square-mile MCS storm centered on the four subbasins draining to its reservoir (1, 2, 4, and 8) and oriented 230 degrees from north was modeled. For Secord, the critical storm was a 300-square-mile MCS oriented 305 degrees from north and centered on subbasins 2 and 8, which drain to Secord Lake.

Tables 12 and 13 list 24-hour and peak hourly precipitation depths by subbasin for the critical Smallwood and Secord storms respectively.

Table 12: Probable Maximum Storm Depths Creating Smallwood PMF (450-Square-Mile 24-Hour Storm Centered above Smallwood Dam)

Subbasin 24-hour Depth


(inches) 1 – Secord 15.92 5.13

2 - Sugar Springs 14.52 4.63 4 – Smallwood 14.48 4.61

8 – W Br Tittabawassee 15.10 4.83

May 15, 2020 22

Table 13: Probable Maximum Storm Depths Creating Secord PMF (300-Square-Mile 24-Hour Storm Centered above Secord Dam)

Subbasin 24-hour Depth


(inches) 1 – Secord 16.44 5.72

8 – W Br Tittabawassee 15.75 5.45

Probable Maximum Flood Hydrographs The Probable Maximum Storm hyetographs for each subbasin and dam site were input to the calibrated HEC-HMS model to develop PMF inflow hydrographs. HEC-HMS input and output files accompany this report on electronic media.

Reservoir Routing Assumptions

All of the reservoirs were assumed to be at maximum normal pool at the outset of the Probable Maximum Storm. Antecedent storms 72 hours prior to the PMS were considered but not analyzed, because the calculated hydrographs and the observed 2014 Edenville hydrograph - which was similar in magnitude to the 100-year event – rise and fall within 72 hours, suggesting that pool levels at the beginning of the PMP would not be significantly affected by a storm occurring 72 hours earlier.

All spillway gates at the Tittabawassee dams as well as the tributary dams were assumed to be fully opened to maintain normal pool as long as possible . Spillway rating curves input to the HEC-HMS model were derived from GEI’s April 2020 Technical Memorandum. No dams were assumed to fail, although all were computed to overtop during the PMF.

May 15, 2020 23

Secord Project Probable Maximum Flood

The PMF for the Secord project is the result of a 24-hour mesoscale storm occurring in the 177-square mile watershed upstream of the dam. The calculated PMF inflow peak at Secord Dam is 29,400 cfs. The calculated peak outflow is 28,100 cfs, and the peak stage is 759.0 feet NGVD. The calculated PMF outflow overtops the dam by 1.2 feet, and includes approximately 13,000 cfs flowing over the east reservoir rim and into the Tea Creek drainage. Figure 2 illustrates the computed inflow, outflow, and stage hydrographs at Secord Dam.

May 15, 2020 24

Smallwood Project Probable Maximum Flood

The PMF for the Smallwood project results from a 24-hour mesoscale storm over the 289-square mile watershed upstream of the dam. The calculated PMF peak inflow to the project is 41,200 cfs. The calculated peak outflow is 41,000 cfs, and the peak stage is 716.8 feet NGVD. The calculated PMF outflow would overtop the dam by 1.1 foot. Figure 3 illustrates the computed inflow, outflow, and stage hydrographs at the Smallwood project.

May 15, 2020 25

Edenville Project Probable Maximum Flood

The PMF for the Edenville project results from a 72-hour synoptic storm over the 904-square mile combined Tobacco River and Tittabawassee River watersheds upstream of the dam. The calculated PMF inflow peak to Wixom Lake is 80,900 cfs. The calculated peak outflow is 80,100 cfs, and the peak stage is 686.0 feet NGVD. The calculated PMF outflow would overtop the dam by 3.9 feet. Figure 4 illustrates the computed inflow, outflow, and stage hydrographs at the Edenville project.

May 15, 2020 26

Sanford Project Probable Maximum Flood

The PMF for the Sanford project results from a 72-hour synoptic storm over the 904-square mile combined Tobacco River and Tittabawassee River watersheds upstream of Edenville Dam. This is a more critical storm configuration than a larger storm optimized over the entire 945-square-mile basin at Sanford Dam, because the incremental drainage area below Edenville Dam contributed flows early in the flood hydrograph and has little or no impact on the calculated peak flow. The calculated PMF inflow peak to Lake Sanford is 80,600 cfs. The calculated peak outflow is 79,100 cfs, and the peak stage is 641.2 feet NGVD. The calculated PMF outflow would overtop the dam by 5.6 feet. However, previous dam failure analyses for the Sanford project have shown that the Inflow Design Flood at this project is much less than the PMF. Figure 5 illustrates the computed inflow, outflow, and stage hydrographs at the Sanford project.

Discussion and Comparison to Previous Estimates

Table 14 compares the 2020 HEC-HMS model results to previously reported results. With the exception of the second, lower set of inflow/outflow values at Edenville calculated by Mill Road Engineering, all of the reported previous estimates are from the 1994 Mead & Hunt study.

May 15, 2020 27

Table 14: Comparison of PMF Estimates

Project Previous Estimates of Peak Flows (cfs) 2020 Calculated Peak Flows (cfs)

Inflow Outflow Inflow Outflow

Secord 27,200 27,100 29,400 28,100

Smallwood 41,000 40,700 41,200 41,000

Edenville (1994) 74,400 73,900 80,900 80,100

Edenville (2011) 61,800 61,700

Sanford 75,500 73,200 80,600 79,100

Relative increases in the 2020 calculated peak flows over the previous estimates range from less than one percent at Smallwood to about 30 percent at Edenville. All of the 2020 calculated flows were affected by the change in modeled loss rates, primarily the fact that the soil groups with high Ksat values outside of the feasible calibration range were shifted from mid-SSURGO-range values to minimum-of-range values. Secondly, the application of a smaller and more intense storm more optimally oriented on the basin upstream of Edenville increased the PMF estimates at both Edenville and Sanford. Finally, the 2020 HEC-HMS model structure did not predict the significant offset between the Tobacco and Tittabawassee contributions that was proposed in the 2011 Mill Road study.

At Smallwood, the peak flows were also positively affected by the change in loss rate assumptions, but the 2020 relative increase in peak inflow was very small. This appears to be due to the revised spillway rating curve at Secord. In previous studies, both the gate capacity and the reservoir rim outflow to Tea Creek were reported to be larger for a given reservoir level than the 2020 GEI analysis suggests. The more restrictive outflow rating curve, combined with significant storage in Secord Lake and the fact that the Secord basin is small and produces a hydrograph with a higher peak/volume ratio than the downstream projects, means that computed Secord outflows were affected by reservoir storage to a greater degree than in previous studies. This can be seen in both the Smallwood inflow and the Secord outflow. At Secord, the 1994 analysis gave only a 100 cfs decrease in outflow relative to inflow, whereas the 2020 storage attenuation effect was 1,300 cfs.

Sensitivity analyses were not conducted as part of this study. There is obviously uncertainty in the input HEC-HMS model parameters, but the model calibration against the 2017 and 2014 floods as well as the check on the ability to convert the 100-year storm to the 100-year flood suggest that the model as developed produces realistic results. For parameters that could not be calibrated – specifically loss rates for the moderate and high permeability soil groups – the most conservative values within the published SSURGO ranges were chosen. The PMF values reported above for Edenville and Sanford are a little more than four times the flood of record and the 100-year flood, which is a typical ratio for upper Midwest PMF estimates. Therefore, the PMF values reported above are considered to be realistic and conservative given available information.

DRAFT - MAY 11, 2020

Exhibit 1

Watershed Map

#*

#*

#*

#*

#*

#*

#*

Sugar

E Br Titta

baw assee

Cedar

Tobacco

CedarSugar

Titt abawasse eWBrTittabawassee

M olasses

Tit tabaw assee

MBrTittabawassee

M BrTo b a cco

S Br Tobacco

11

3 a3 a

5 b5 b

3 b3 b

66

3 c3 c

77

2288

5 a5 a

44

Edenville

Sanford

Smallwood

Secord

Lake Lancer

Beaverton

ChappelGladwin

Exhibit 1: Tittabawassee River WatershedTittabawassee River Hydroelectric ProjectsGladwin and Midland Counties, MichiganApril 30, 2020

0 4 8 12 162Miles

±

LegendSubbasinsSubbasins

Rivers

#* Dams


Exhibit 2

Subbasin Soil Distributions by Minimum Ksat

Exhibit 2 sheet 1

SUBBASIN 1 - SSURGO SOIL DATA SUMMARY Permeability Range (60", inches per hour)

Minimum Permeability (60") (inches/hour)

Percentage Designation Model Notes

0-0.06 0.000 3.6% impervious Zero permeability0-0.06 0.001 1.3% 5.0%0.06-0.2 0.060 19.7%0.142-0.567* 0.142 5.6%0.2-0.6 0.198 23.3% low permeability Start at 0.1 inch/hr,

0.2-0.6 0.200 0.3% 49.0%after calibration 0.35 inch/hr

0.567-1.42* 0.567 0.8%1.42-5.67* 1.417 10.1%1.56-5.67* 1.559 2.5%2.0-6.0 1.984 12.7% mid permeability 1.5 or 2 inch per hour,

2.0-6.0 2.000 0.1% 26.2%no change through calibration

≥ 6 5.953 17.6% high permeability 6 inch per hour,

≥ 6 6.001 2.2% 19.8%no change through calibration

Subbasin area based on 2019 boundaries delineated using LiDAR ground elevation data.**Permeability calculated using 2018 SSURGO soil data.

* some SSURGO values are converted from metric scale that uses 1-4, 4-10, 10-40, etc. microns/sec** Note: Subbasin 1, 5a, and 6 percentages differ slightly from report Table 5

because of post-SSURGO-analysis decision to model Lakes Wixom and Secord as separate subbasins

Exhibit 2 sheet 2



PercentageDesignation Model Notes

0-0.06 0.000 5.1% impervious Zero permeability0-0.06 0.001 0.6% 5.7%0.06-0.2 0.060 33.2%0.142-0.567* 0.142 9.8%0.2-0.6 0.198 9.7% low permeability Start at 0.1 inch/hr,






Subbasin area based on 2019 boundaries delineated using LiDAR ground elevation data.Permeability calculated using 2018 SSURGO soil data.

* Some SSURGO values are converted from metric scale that uses 1-4, 4-10, 10-40, etc. microns/sec

Exhibit 2 sheet 3

SUBBASIN 3a - SSURGO SOIL DATA SUMMARY Permeability Range (60", inches per hour)



0-0.06 0.000 2.1% 0-0.06 0.001 6.9% impervious0.014-0.057* 0.014 0.0% 9.0% Zero permeability0.06-0.2 0.060 16.9%0.142-0.567* 0.142 11.5%0.2-0.6 0.198 19.7% low permeability Start at 0.1 inch/hr,








Exhibit 2 sheet 4

SUBBASIN 3b - SSURGO SOIL DATA SUMMARY Permeability Range (60", inches per hour)



0-0.06 0.000 3.2% 0-0.06 0.001 10.1% impervious0.014-0.057* 0.014 3.7% 17.0% Zero permeability0.06-0.2 0.060 18.7%0.142-0.567* 0.142 19.3% low permeability Start at 0.1 inch/hr,


0.567-1.42* 0.567 1.4%1.42-5.67* 1.417 14.7%1.56-5.67* 1.559 1.6% mid permeability 1.5 or 2 inch per hour,






Exhibit 2 sheet 5

SUBBASIN 3c - SSURGO SOIL DATA SUMMARY

Permeability Range (60", inches per hour)

Minimum Permeability (60")

(inches/hour)Percentage Designation Model Notes

0-0.06 0.000 3.2% 0-0.06 0.001 17.7% 0.014-0.057* 0.014 2.1% impervious0.016-0.057* 0.016 0.2% 23.1% Zero permeability0.06-0.2 0.060 14.6%0.142-0.567* 0.142 16.3%0.2-0.6 0.198 14.4% low permeability Start at 0.1 inch/hr,








Exhibit 2 sheet 6

SUBBASIN 4 - SSURGO SOIL DATA SUMMARY




0-0.06 0.000 1.4% 0-0.06 0.001 6.4% impervious0.014-0.057* 0.014 0.4% 8.3% Zero permeability0.06-0.2 0.060 37.1%0.142-0.567* 0.142 1.2% low permeability Start at 0.1 inch/hr,








Exhibit 2 sheet 7

SUBBASIN 5a - SSURGO SOIL DATA SUMMARY




0-0.06 0.000 0.7% impervious0.014-0.057* 0.014 1.6% 2.3% Zero permeability0.06-0.2 0.060 2.4%0.142-0.567* 0.198 26.1% low permeability Start at 0.1 inch/hr,


1.42-5.67* 1.417 0.3%2.0-6.0 1.984 2.0% mid permeability 1.5 or 2 inch per hour,






Exhibit 2 sheet 8

SUBBASIN 5b - SSURGO SOIL DATA SUMMARY



Percentage Designation Model Notes0-0.06 0.000 3.9% 0-0.06 0.001 2.3% impervious0.014-0.057* 0.014 3.7% 9.9% Zero permeability0.06-0.2 0.060 11.6%0.142-0.567* 0.142 0.1% low permeability0.2-0.6 0.198 26.8% Start at 0.1 inch/hr,


0.567-1.42* 0.567 0.6%1.42-5.67* 1.417 4.2% mid permeability 1.5 or 2 inch per hour,






because of post-SSURGO-analysis decision to represent Lakes Wixom and Secord as separate subbasins

Exhibit 2 sheet 9






0.567-1.42* 0.567 0.7%1.42-5.67* 1.417 9.1% mid permeability 1.5 or 2 inch per hour,


≥ 6 5.953 8.5% 6 inch per hour,




because of post-SSURGO-analysis decision to represent Lakes Wixom and Secord as separate subbasins

Exhibit 2 sheet 10







0.567-1.42* 0.567 0.4%0.6 - 2.0 0.600 0.5%1.42-5.67* 1.417 14.8% mid permeability 1.5 or 2 inch per hour,






Exhibit 2 sheet 11





0-0.06 0.000 2.24% impervious0-0.06 0.001 1.09% 3.33% Zero permeability0.06-0.2 0.060 14.80%0.142-0.567* 0.142 5.23%0.2-0.6 0.198 14.26% low permeability Start at 0.1 inch/hr, 0.2-0.6 0.200 5.59% 39.88% after calibration 0.35 inch/hr0.567-1.42* 0.567 8.86%1.42-5.67* 1.417 25.71%1.56 - 5.67 1.559 1.89%2.0-6.0 1.984 5.26% mid permeability 1.5 or 2 inch per hour,







Exhibit 3

HEC-HMS Calibration Plots

Exhibit 3 Sheet 1

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

4/12/14 12:00 4/13/14 12:00 4/14/14 12:00 4/15/14 12:00 4/16/14 12:00

Disc

harg

e, c

fs

Calibration: Edenville Inflows, April 2014

HMS EDENVILLE INFLOW BACKROUTED EDENVILLE INFLOW

Peak

Rai

n 4-

12, 2

300

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4/12/14 0:00 4/13/14 0:00 4/14/14 0:00 4/15/14 0:00 4/16/14 0:00 4/17/14 0:00

Disc

harg

e, c

fs

2014 Calibration: USGS South Branch of the Tobacco River

HEC-HMS FLOW USGS So BR GAGE

Peak

Rai

n4-

12, 2

300

Exhibit 3 Sheet 2

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

6/22/17 0:00 6/23/17 0:00 6/24/17 0:00 6/25/17 0:00

Disc

harg

e, cf

s2017 Calibration: Edenville Inflow

HMS EDENVILLE INFLOW BACKROUTED EDENVILLE INFLOW

Peak

Rai

n 6-

23, 0

600

0

1,000

2,000

3,000

4,000

5,000

6,000

6/22/17 0:00 6/23/17 0:00 6/24/17 0:00 6/25/17 0:00 6/26/17 0:00

Disc

harg

e, c

fs

2017 Calibration: USGS Tobacco River below Beaverton Dam

HMS ROSS OUTFLOW USGS Tobacco R Gage

Peak

Rai

n 6-

23, 0

600

Exhibit 3 Sheet 3

0

500

1,000

1,500

2,000

2,500

3,000

3,500

6/22/17 0:00 6/23/17 0:00 6/24/17 0:00 6/25/17 0:00 6/26/17 0:00

Disc

harg

e, cf

s2017 Calibration: USGS So Br Tobacco

HEC-HMS FLOW USGS So BR GAGE

Peak

Rai

n 6-

23, 0

600


Exhibit 4

Probable Maximum Storm Calculations

STORM NAME Syn-2 PMP FROM DAD CURVES PMP BY 6 HR INCREMENTSIZE 850 1 hr 2 hr 3 hr 6 hr 12 hr 18 hr 24 hr 30 hr 36 hr 42 hr 48 hr 54 hr 60 hr 66 hr 72 hr 1st 6 hr 2nd 6 hr 3rd 6 hrORIENTATION 230 3.6 5.2 6.6 10.3 12.9 13.6 14.2 14.8 15.4 15.9 16.4 16.7 17 17.3 17.6 10.3 2.6 0.7CENTERED 1-6,8 u/s EDVL basin centroidAXIS RATIO 2.7 Figure 1, WMPMS guide

Adjustment Factor - WMPMS Tables 1-4 Isohyet Depths = adjustment factor x 6 hr incremental depth

Isohyet Area (sq mi) 1st 6 hr 2nd 6 hr 3rd 6 hr4th - 12th 6 hr 1st 6 hr 2nd 6 hr 3rd 6 hr 4th 6hr 5th 6hr 6th 6hr 7th 6hr 8th 6hr 9th 6hr 10th 6hr 11th 6 hr 12th 6 hr TOTAL

A 10 1.45 1.15 1.044 1 14.94 2.99 0.73 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 22.66B 25 1.36 1.11 1.032 1 14.01 2.89 0.72 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 21.62C 50 1.28 1.075 1.02 1 13.18 2.80 0.71 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 20.69D 100 1.19 1.045 1.011 1 12.26 2.72 0.71 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 19.68E 175 1.1 1.02 1.004 1 11.33 2.65 0.70 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 18.68F 300 1.01 1 1.001 1 10.40 2.60 0.70 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 17.70G 450 0.94 0.98 0.997 1 9.68 2.55 0.70 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 16.93H 700 0.87 0.96 0.994 1 8.96 2.50 0.70 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 16.15I 1000 0.72 0.85 0.92 0.92 7.42 2.21 0.64 0.55 0.55 0.55 0.46 0.46 0.28 0.28 0.28 0.28 13.95J 1500 0.54 0.71 0.765 0.77 5.56 1.85 0.54 0.46 0.46 0.46 0.39 0.38 0.23 0.23 0.23 0.23 11.02K 2150 0.4 0.585 0.625 0.62 4.12 1.52 0.44 0.37 0.37 0.37 0.31 0.31 0.19 0.19 0.19 0.19 8.56L 3000 0.3 0.48 0.505 0.505 3.09 1.25 0.35 0.30 0.30 0.30 0.25 0.25 0.15 0.15 0.15 0.15 6.71M 4500 0.19 0.35 0.4 0.4 1.96 0.91 0.28 0.24 0.24 0.24 0.20 0.20 0.12 0.12 0.12 0.12 4.75

Check Consistent with PMP DADArea Weighted PMP inches inside isohyet

Isohyet Area (sq mi) Incr. Area 1st 6 hr 2nd 6 hr 12 hrA 10 10 149.35 29.9 179.25B 25 15 217.0725 44.07 261.14C 50 25 339.9 71.0125 410.91D 100 50 636.025 137.8 773.83E 175 75 884.5125 201.3375 1085.9F 300 125 1358.313 328.25 1686.6G 450 150 1506.375 386.1 1892.5 1st 6 hr 2nd 6 hrH 700 250 2330.375 630.5 2960.9 TOTAL INSIDE 700 10.603 2.613I 1000 300 2456.55 705.9 3162.5 TOTAL inside 1000 9.8785 2.535J consistent with 850 sq mi values in row 3K

EXHIBIT 4, SHEET 1SAMPLE PMP CALCULATION FOR EDENVILLE-SANFORD PMS(a) DEVELOP ISOHYET DEPTHS

Axis LengthsFull axis Half axis Isohyet Value (Precip Depth - inches)

IsohyetArea (sq mi)

Short (mi) Long (mi)

Short (mi) Long (mi) 1st 6 hrs

2nd 6 hrs

3rd 6 hrs

4th, 5th, and 6th 6 hrs

7th and 8th 6 hrs

9th - 12th 6 hrs

A 10 2.17 5.86 1.09 2.93 14.94 2.99 0.73 0.60 0.50 0.30B 25 3.43 9.27 1.72 4.64 14.01 2.89 0.72 0.60 0.50 0.30C 50 4.86 13.11 2.43 6.56 13.18 2.80 0.71 0.60 0.50 0.30D 100 6.87 18.54 3.43 9.27 12.26 2.72 0.71 0.60 0.50 0.30E 175 9.08 24.53 4.54 12.26 11.33 2.65 0.70 0.60 0.50 0.30F 300 11.89 32.11 5.95 16.06 10.40 2.60 0.70 0.60 0.50 0.30G 450 14.57 39.33 7.28 19.67 9.68 2.55 0.70 0.60 0.50 0.30H 700 18.17 49.06 9.08 24.53 8.96 2.50 0.70 0.60 0.50 0.30I 1000 21.72 58.63 10.86 29.32 7.42 2.21 0.64 0.55 0.46 0.28J 1500 26.60 71.81 13.30 35.90 5.56 1.85 0.54 0.46 0.39 0.23K 2150 31.84 85.97 15.92 42.99 4.12 1.52 0.44 0.37 0.31 0.19L 3000 37.61 101.55 18.81 50.78 3.09 1.25 0.35 0.30 0.25 0.15M 4500 46.07 124.38 23.03 62.19 1.96 0.91 0.28 0.24 0.20 0.12

EXHIBIT 4, SHEET 2STORM ISOHYETS FOR EDENVILLE-SANFORD PMSBY 6 HOUR INCREMENTS

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th1 9.53 2.51 0.69 0.59 0.59 0.59 0.49 0.49 0.30 0.30 0.30 0.302 9.01 2.42 0.67 0.58 0.58 0.58 0.48 0.48 0.29 0.29 0.29 0.29

3a 9.18 2.41 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.283b 10.29 2.55 0.69 0.59 0.59 0.59 0.49 0.49 0.29 0.29 0.29 0.293c 9.05 2.46 0.68 0.59 0.59 0.59 0.49 0.49 0.29 0.29 0.29 0.294 11.93 2.71 0.71 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30

5a 7.20 2.15 0.61 0.53 0.53 0.53 0.44 0.44 0.26 0.26 0.26 0.265b 6.55 2.00 0.57 0.48 0.48 0.48 0.41 0.41 0.24 0.24 0.24 0.246 8.58 2.37 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.287 3.76 1.41 0.41 0.35 0.35 0.35 0.29 0.29 0.17 0.17 0.17 0.178 8.82 2.38 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.28

Subbasin

850 SQ MI SYNOPTIC STORM CENTERED ABOVE EDVL6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 3EDENVILLE-SANFORD PMS BY SUBBASIN

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th CHECK TOTAL

1 9.53 2.51 0.69 0.59 0.59 0.59 0.49 0.49 0.30 0.30 0.30 0.30 16.662 9.01 2.42 0.67 0.58 0.58 0.58 0.48 0.48 0.29 0.29 0.29 0.29 15.943a 9.18 2.41 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.28 16.053b 10.29 2.55 0.69 0.59 0.59 0.59 0.49 0.49 0.29 0.29 0.29 0.29 17.453c 9.05 2.46 0.68 0.59 0.59 0.59 0.49 0.49 0.29 0.29 0.29 0.29 16.104 11.93 2.71 0.71 0.60 0.60 0.60 0.50 0.50 0.30 0.30 0.30 0.30 19.355a 7.20 2.15 0.61 0.53 0.53 0.53 0.44 0.44 0.26 0.26 0.26 0.26 13.485b 6.55 2.00 0.57 0.48 0.48 0.48 0.41 0.41 0.24 0.24 0.24 0.24 12.356 8.58 2.37 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.28 15.407 3.76 1.41 0.41 0.35 0.35 0.35 0.29 0.29 0.17 0.17 0.17 0.17 7.888 8.82 2.38 0.66 0.57 0.57 0.57 0.47 0.47 0.28 0.28 0.28 0.28 15.65

rank HR

PCT OF 6

HR SB 1 6‐HR SB 1 RAIN SB 2 6‐HR SB 2 RAIN

SB 3a 6‐

HR

SB 3a

RAIN

SB 3b 6‐

HR

SB 3b

RAIN

SB 3c 6

HR

SB 3c

RAIN

SB 4 6

HR SB 4 RAIN

SB 5a 6

HR

SB 5a

RAIN

SB 5b 6‐

HR

SB 5b

RAIN SB 6 6‐HR

SB 6

RAIN

SB 7 6‐

HR

SB 7

RAIN

SB 8 6‐

HR SB 8 RAIN

1 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0472 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.047

12 3 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0474 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0475 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0476 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0477 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.0478 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.047

10 9 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04710 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04711 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04712 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04713 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07814 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.078

8 15 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07816 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07817 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07818 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07819 16.667 0.59 0.084 0.58 0.083 0.57 0.082 0.59 0.084 0.59 0.084 0.60 0.086 0.53 0.076 0.48 0.069 0.57 0.082 0.35 0.050 0.57 0.08220 16.667 0.59 0.091 0.58 0.090 0.57 0.088 0.59 0.091 0.59 0.091 0.60 0.093 0.53 0.082 0.48 0.074 0.57 0.088 0.35 0.054 0.57 0.088

6 21 16.667 0.59 0.092 0.58 0.090 0.57 0.089 0.59 0.092 0.59 0.092 0.60 0.094 0.53 0.083 0.48 0.075 0.57 0.089 0.35 0.055 0.57 0.08922 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09523 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09524 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09525 14.300 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09526 15.500 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.095

4 27 15.600 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09528 16.900 0.59 0.100 0.58 0.098 0.57 0.096 0.59 0.100 0.59 0.100 0.60 0.101 0.53 0.090 0.48 0.081 0.57 0.096 0.35 0.059 0.57 0.09629 18.200 0.59 0.107 0.58 0.106 0.57 0.104 0.59 0.107 0.59 0.107 0.60 0.109 0.53 0.096 0.48 0.087 0.57 0.104 0.35 0.064 0.57 0.10430 19.500 0.59 0.115 0.58 0.113 0.57 0.111 0.59 0.115 0.59 0.115 0.60 0.117 0.53 0.103 0.48 0.094 0.57 0.111 0.35 0.068 0.57 0.11131 12.100 2.51 0.304 2.42 0.293 2.41 0.292 2.55 0.309 2.46 0.298 2.71 0.328 2.15 0.260 2 0.242 2.37 0.287 1.41 0.171 2.38 0.28832 13.000 2.51 0.326 2.42 0.315 2.41 0.313 2.55 0.332 2.46 0.320 2.71 0.352 2.15 0.280 2 0.260 2.37 0.308 1.41 0.183 2.38 0.309

2 33 14.900 2.51 0.374 2.42 0.361 2.41 0.359 2.55 0.380 2.46 0.367 2.71 0.404 2.15 0.320 2 0.298 2.37 0.353 1.41 0.210 2.38 0.35534 17.000 2.51 0.427 2.42 0.411 2.41 0.410 2.55 0.434 2.46 0.418 2.71 0.461 2.15 0.366 2 0.340 2.37 0.403 1.41 0.240 2.38 0.40535 20.000 2.51 0.502 2.42 0.484 2.41 0.482 2.55 0.510 2.46 0.492 2.71 0.542 2.15 0.430 2 0.400 2.37 0.474 1.41 0.282 2.38 0.47636 23.000 2.51 0.577 2.42 0.557 2.41 0.554 2.55 0.587 2.46 0.566 2.71 0.623 2.15 0.495 2 0.460 2.37 0.545 1.41 0.324 2.38 0.54737 8.300 9.53 0.791 9.01 0.748 9.18 0.762 10.29 0.854 9.05 0.751 11.93 0.990 7.2 0.598 6.55 0.544 8.58 0.712 3.76 0.312 8.82 0.73238 13.600 9.53 1.296 9.01 1.225 9.18 1.248 10.29 1.399 9.05 1.231 11.93 1.622 7.2 0.979 6.55 0.891 8.58 1.167 3.76 0.511 8.82 1.200

1 39 18.400 9.53 1.754 9.01 1.658 9.18 1.689 10.29 1.893 9.05 1.665 11.93 2.195 7.2 1.325 6.55 1.205 8.58 1.579 3.76 0.692 8.82 1.62340 30.700 9.53 2.926 9.01 2.766 9.18 2.818 10.29 3.159 9.05 2.778 11.93 3.663 7.2 2.210 6.55 2.011 8.58 2.634 3.76 1.154 8.82 2.70841 16.800 9.53 1.601 9.01 1.514 9.18 1.542 10.29 1.729 9.05 1.520 11.93 2.004 7.2 1.210 6.55 1.100 8.58 1.441 3.76 0.632 8.82 1.48242 12.100 9.53 1.153 9.01 1.090 9.18 1.111 10.29 1.245 9.05 1.095 11.93 1.444 7.2 0.871 6.55 0.793 8.58 1.038 3.76 0.455 8.82 1.06743 20.500 0.69 0.141 0.67 0.137 0.66 0.135 0.69 0.141 0.68 0.139 0.71 0.146 0.61 0.125 0.57 0.117 0.66 0.135 0.41 0.084 0.66 0.13544 19.000 0.69 0.131 0.67 0.127 0.66 0.125 0.69 0.131 0.68 0.129 0.71 0.135 0.61 0.116 0.57 0.108 0.66 0.125 0.41 0.078 0.66 0.125

3 45 17.000 0.69 0.117 0.67 0.114 0.66 0.112 0.69 0.117 0.68 0.116 0.71 0.121 0.61 0.104 0.57 0.097 0.66 0.112 0.41 0.070 0.66 0.11246 15.000 0.69 0.104 0.67 0.101 0.66 0.099 0.69 0.104 0.68 0.102 0.71 0.107 0.61 0.092 0.57 0.086 0.66 0.099 0.41 0.062 0.66 0.09947 14.500 0.69 0.100 0.67 0.097 0.66 0.096 0.69 0.100 0.68 0.099 0.71 0.103 0.61 0.088 0.57 0.083 0.66 0.096 0.41 0.059 0.66 0.09648 14.000 0.69 0.100 0.67 0.094 0.66 0.092 0.69 0.097 0.68 0.095 0.71 0.099 0.61 0.085 0.57 0.080 0.66 0.092 0.41 0.057 0.66 0.09249 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09550 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09551 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.095

5 52 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09553 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09554 16.667 0.59 0.098 0.58 0.097 0.57 0.095 0.59 0.098 0.59 0.098 0.60 0.100 0.53 0.088 0.48 0.080 0.57 0.095 0.35 0.058 0.57 0.09555 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07856 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.078

7 57 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07858 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07859 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.048 0.47 0.07860 16.667 0.49 0.082 0.48 0.080 0.47 0.078 0.49 0.082 0.49 0.082 0.50 0.083 0.44 0.073 0.41 0.068 0.47 0.078 0.29 0.078 0.47 0.07861 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04762 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.047

9 63 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04764 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04765 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04766 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04767 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04768 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.047

11 69 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04770 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04771 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.04772 16.667 0.3 0.050 0.29 0.048 0.28 0.047 0.29 0.048 0.29 0.048 0.30 0.050 0.26 0.043 0.24 0.040 0.28 0.047 0.17 0.028 0.28 0.047

16.674 15.951 16.011

Subbasin

850 SQ MI SYNOPTIC STORM CENTERED AND ORIENTED ABOVE EDENVILLE

6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 4EDENVILLE HOURLY PMS BY SUBBASIN

1st 2nd 3rd 4th1 13.29 1.32 0.66 0.652 11.99 1.26 0.65 0.63

3a 9.58 1.11 0.59 0.583b 7.51 0.93 0.50 0.493c 1.92 0.26 0.14 0.144 11.95 1.26 0.64 0.63

5a 3.76 0.47 0.25 0.255b 2.44 0.31 0.17 0.166 2.82 0.36 0.19 0.197 0.00 0.00 0.00 0.008 12.52 1.29 0.65 0.64

Subbasin

450 SQ MI MCS STORM CENTERED ABOVE SMWD6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 5PMS BY SUBBASIN FOR SMALLWOOD

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th TOTAL1 13.29 1.32 0.66 0.65 15.922 11.99 1.26 0.65 0.63 14.52

3a 9.58 1.11 0.59 0.58 11.863b 7.51 0.93 0.50 0.49 9.443c 1.92 0.26 0.14 0.14 2.454 11.95 1.26 0.64 0.63 14.48

5a 3.76 0.47 0.25 0.25 4.735b 2.44 0.31 0.17 0.16 3.086 2.82 0.36 0.19 0.19 3.567 0.00 0.00 0.00 0.00 0.008 12.52 1.29 0.65 0.64 15.10

rank HRPCT OF 6 HR SB 1 6-HR SB 1 RAIN SB 2 6-HR SB 2 RAIN

SB 3a 6-HR

SB 3a RAIN

SB 3b 6-HR

SB 3b RAIN

SB 3c 6 HR

SB 3c RAIN

SB 4 6 HR SB 4 RAIN

SB 5a 6 HR

SB 5a RAIN

SB 5b 6-HR

SB 5b RAIN

SB 6 6-HR

SB 6 RAIN

SB 7 6-HR

SB 7 RAIN

SB 8 6-HR

SB 8 RAIN

2425 15.100 0.65 0.098 0.63 0.095 0.58 0.088 0.49 0.074 0.14 0.021 0.63 0.095 0.25 0.038 0.16 0.024 0.19 0.029 0 0.000 0.65 0.09826 16.300 0.65 0.106 0.63 0.103 0.58 0.095 0.49 0.080 0.14 0.023 0.63 0.103 0.25 0.041 0.16 0.026 0.19 0.031 0 0.000 0.65 0.106

4 27 16.500 0.65 0.107 0.63 0.104 0.58 0.096 0.49 0.081 0.14 0.023 0.63 0.104 0.25 0.041 0.16 0.026 0.19 0.031 0 0.000 0.65 0.10728 16.600 0.65 0.108 0.63 0.105 0.58 0.096 0.49 0.081 0.14 0.023 0.63 0.105 0.25 0.042 0.16 0.027 0.19 0.032 0 0.000 0.65 0.10829 17.700 0.65 0.115 0.63 0.112 0.58 0.103 0.49 0.087 0.14 0.025 0.63 0.112 0.25 0.044 0.16 0.028 0.19 0.034 0 0.000 0.65 0.11530 17.800 0.65 0.116 0.63 0.112 0.58 0.103 0.49 0.087 0.14 0.025 0.63 0.112 0.25 0.045 0.16 0.028 0.19 0.034 0 0.000 0.65 0.11631 13.100 1.32 0.173 1.26 0.165 1.11 0.145 0.93 0.122 0.26 0.034 1.26 0.165 0.47 0.062 0.31 0.041 0.36 0.047 0 0.000 1.29 0.16932 13.200 1.32 0.174 1.26 0.166 1.11 0.147 0.93 0.123 0.26 0.034 1.26 0.166 0.47 0.062 0.31 0.041 0.36 0.048 0 0.000 1.29 0.170

2 33 13.400 1.32 0.177 1.26 0.169 1.11 0.149 0.93 0.125 0.26 0.035 1.26 0.169 0.47 0.063 0.31 0.042 0.36 0.048 0 0.000 1.29 0.17334 15.700 1.32 0.207 1.26 0.198 1.11 0.174 0.93 0.146 0.26 0.041 1.26 0.198 0.47 0.074 0.31 0.049 0.36 0.057 0 0.000 1.29 0.20335 19.800 1.32 0.261 1.26 0.249 1.11 0.220 0.93 0.184 0.26 0.051 1.26 0.249 0.47 0.093 0.31 0.061 0.36 0.071 0 0.000 1.29 0.25536 24.800 1.32 0.327 1.26 0.312 1.11 0.275 0.93 0.231 0.26 0.064 1.26 0.312 0.47 0.117 0.31 0.077 0.36 0.089 0 0.000 1.29 0.32037 5.800 13.29 0.771 11.99 0.695 9.58 0.556 7.51 0.436 1.92 0.111 11.95 0.693 3.76 0.218 2.44 0.142 2.82 0.164 0 0.000 12.52 0.72638 11.700 13.29 1.555 11.99 1.403 9.58 1.121 7.51 0.879 1.92 0.225 11.95 1.398 3.76 0.440 2.44 0.285 2.82 0.330 0 0.000 12.52 1.465

1 39 17.900 13.29 2.379 11.99 2.146 9.58 1.715 7.51 1.344 1.92 0.344 11.95 2.139 3.76 0.673 2.44 0.437 2.82 0.505 0 0.000 12.52 2.24140 38.600 13.29 5.130 11.99 4.628 9.58 3.698 7.51 2.899 1.92 0.741 11.95 4.613 3.76 1.451 2.44 0.942 2.82 1.089 0 0.000 12.52 4.83341 15.800 13.29 2.100 11.99 1.894 9.58 1.514 7.51 1.187 1.92 0.303 11.95 1.888 3.76 0.594 2.44 0.386 2.82 0.446 0 0.000 12.52 1.97842 10.200 13.29 1.356 11.99 1.223 9.58 0.977 7.51 0.766 1.92 0.196 11.95 1.219 3.76 0.384 2.44 0.249 2.82 0.288 0 0.000 12.52 1.27743 18.700 0.66 0.123 0.65 0.122 0.59 0.110 0.5 0.094 0.14 0.026 0.64 0.120 0.25 0.047 0.17 0.032 0.19 0.036 0 0.000 0.65 0.12244 17.600 0.66 0.116 0.65 0.114 0.59 0.104 0.5 0.088 0.14 0.025 0.64 0.113 0.25 0.044 0.17 0.030 0.19 0.033 0 0.000 0.65 0.114

3 45 16.500 0.66 0.109 0.65 0.107 0.59 0.097 0.5 0.083 0.14 0.023 0.64 0.106 0.25 0.041 0.17 0.028 0.19 0.031 0 0.000 0.65 0.10746 16.400 0.66 0.108 0.65 0.107 0.59 0.097 0.5 0.082 0.14 0.023 0.64 0.105 0.25 0.041 0.17 0.028 0.19 0.031 0 0.000 0.65 0.10747 15.400 0.66 0.102 0.65 0.100 0.59 0.091 0.5 0.077 0.14 0.022 0.64 0.099 0.25 0.039 0.17 0.026 0.19 0.029 0 0.000 0.65 0.10048 15.400 0.66 0.102 0.65 0.100 0.59 0.091 0.5 0.077 0.14 0.022 0.64 0.099 0.25 0.039 0.17 0.026 0.19 0.029 0 0.000 0.65 0.100

Subbasin

450 SQ MI MCS CENTERED AND ORIENTED ABOVE SMALLWOOD6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 6SMALLWOOD HOURLY PMS BY SUBBASIN

1st 2nd 3rd 4th1 13.72 1.36 0.68 0.672 9.30 1.11 0.58 0.58

3a 1.26 0.17 0.09 0.093b 0.00 0.00 0.00 0.003c 0.00 0.00 0.00 0.004 4.18 0.54 0.29 0.29

5a 8.06 0.93 0.49 0.495b 1.50 0.19 0.10 0.106 0.00 0.00 0.00 0.007 0.00 0.00 0.00 0.008 13.08 1.33 0.67 0.67

Subbasin

300 SQ MI MCS STORM CENTERED ABOVE SECD6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 7SECORD PMS BY SUBBASIN

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th TOTAL1 13.72 1.36 0.68 0.67 16.442 9.30 1.11 0.58 0.58 11.58

3a 1.26 0.17 0.09 0.09 1.603b 0.00 0.00 0.00 0.00 0.003c 0.00 0.00 0.00 0.00 0.004 4.18 0.54 0.29 0.29 5.30

5a 8.06 0.93 0.49 0.49 9.975b 1.50 0.19 0.10 0.10 1.896 0.00 0.00 0.00 0.00 0.007 0.00 0.00 0.00 0.00 0.008 13.08 1.33 0.67 0.67 15.75

rank HRPCT OF 6 HR SB 1 6-HR SB 1 RAIN

SB 2 6-HR

SB 2 RAIN

SB 3a 6-HR

SB 3a RAIN

SB 3b 6-HR

SB 3b RAIN

SB 3c 6 HR

SB 3c RAIN

SB 4 6 HR

SB 4 RAIN

SB 5a 6 HR

SB 5a RAIN

SB 5b 6-HR

SB 5b RAIN

SB 6 6-HR

SB 6 RAIN

SB 7 6-HR

SB 7 RAIN

SB 8 6-HR

SB 8 RAIN

2425 15.800 0.67 0.106 0.58 0.092 0.09 0.014 0 0.000 0 0 0.29 0.046 0.49 0.077 0.1 0.016 0 0.000 0 0.000 0.67 0.10626 15.800 0.67 0.106 0.58 0.092 0.09 0.014 0 0.000 0 0 0.29 0.046 0.49 0.077 0.1 0.016 0 0.000 0 0.000 0.67 0.106

4 27 15.800 0.67 0.106 0.58 0.092 0.09 0.014 0 0.000 0 0 0.29 0.046 0.49 0.077 0.1 0.016 0 0.000 0 0.000 0.67 0.10628 17.100 0.67 0.115 0.58 0.099 0.09 0.015 0 0.000 0 0 0.29 0.050 0.49 0.084 0.1 0.017 0 0.000 0 0.000 0.67 0.11529 17.100 0.67 0.115 0.58 0.099 0.09 0.015 0 0.000 0 0 0.29 0.050 0.49 0.084 0.1 0.017 0 0.000 0 0.000 0.67 0.11530 18.400 0.67 0.123 0.58 0.107 0.09 0.017 0 0.000 0 0 0.29 0.053 0.49 0.090 0.1 0.018 0 0.000 0 0.000 0.67 0.12331 12.600 1.36 0.171 1.11 0.140 0.17 0.021 0 0.000 0 0 0.54 0.068 0.93 0.117 0.19 0.024 0 0.000 0 0.000 1.33 0.16832 13.400 1.36 0.182 1.11 0.149 0.17 0.023 0 0.000 0 0 0.54 0.072 0.93 0.125 0.19 0.025 0 0.000 0 0.000 1.33 0.178

2 33 13.400 1.36 0.182 1.11 0.149 0.17 0.023 0 0.000 0 0 0.54 0.072 0.93 0.125 0.19 0.025 0 0.000 0 0.000 1.33 0.17834 15.700 1.36 0.214 1.11 0.174 0.17 0.027 0 0.000 0 0 0.54 0.085 0.93 0.146 0.19 0.030 0 0.000 0 0.000 1.33 0.20935 19.700 1.36 0.268 1.11 0.219 0.17 0.033 0 0.000 0 0 0.54 0.106 0.93 0.183 0.19 0.037 0 0.000 0 0.000 1.33 0.26236 25.200 1.36 0.343 1.11 0.280 0.17 0.043 0 0.000 0 0 0.54 0.136 0.93 0.234 0.19 0.048 0 0.000 0 0.000 1.33 0.33537 5.500 13.72 0.755 9.3 0.512 1.26 0.069 0 0.000 0 0 4.18 0.230 8.06 0.443 1.5 0.083 0 0.000 0 0.000 13.08 0.71938 11.000 13.72 1.509 9.3 1.023 1.26 0.139 0 0.000 0 0 4.18 0.460 8.06 0.887 1.5 0.165 0 0.000 0 0.000 13.08 1.439

1 39 17.300 13.72 2.374 9.3 1.609 1.26 0.218 0 0.000 0 0 4.18 0.723 8.06 1.394 1.5 0.260 0 0.000 0 0.000 13.08 2.26340 41.700 13.72 5.721 9.3 3.878 1.26 0.525 0 0.000 0 0 4.18 1.743 8.06 3.361 1.5 0.626 0 0.000 0 0.000 13.08 5.45441 15.000 13.72 2.058 9.3 1.395 1.26 0.189 0 0.000 0 0 4.18 0.627 8.06 1.209 1.5 0.225 0 0.000 0 0.000 13.08 1.96242 9.500 13.72 1.303 9.3 0.884 1.26 0.120 0 0.000 0 0 4.18 0.397 8.06 0.766 1.5 0.143 0 0.000 0 0.000 13.08 1.24343 18.900 0.68 0.129 0.58 0.110 0.09 0.017 0 0.000 0 0 0.29 0.055 0.49 0.093 0.1 0.019 0 0.000 0 0.000 0.67 0.12744 17.900 0.68 0.122 0.58 0.104 0.09 0.016 0 0.000 0 0 0.29 0.052 0.49 0.088 0.1 0.018 0 0.000 0 0.000 0.67 0.120

3 45 16.800 0.68 0.114 0.58 0.097 0.09 0.015 0 0.000 0 0 0.29 0.049 0.49 0.082 0.1 0.017 0 0.000 0 0.000 0.67 0.11346 15.800 0.68 0.107 0.58 0.092 0.09 0.014 0 0.000 0 0 0.29 0.046 0.49 0.077 0.1 0.016 0 0.000 0 0.000 0.67 0.10647 15.800 0.68 0.107 0.58 0.092 0.09 0.014 0 0.000 0 0 0.29 0.046 0.49 0.077 0.1 0.016 0 0.000 0 0.000 0.67 0.10648 14.800 0.68 0.101 0.58 0.086 0.09 0.013 0 0.000 0 0 0.29 0.043 0.49 0.073 0.1 0.015 0 0.000 0 0.000 0.67 0.099

Subbasin

300 SQ MI MCS CENTERED AND ORIENTED ABOVE SECORD6 hr rain depth, largest to smallest

EXHIBIT 4, SHEET 8SECORD HOURLY PMS BY SUBBASIN

Probable Maximum Flood Determination · 715.834.3161 • Fax: 715.831.7500 Ayres Associates Project...

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