Post on 30-Dec-2015
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Stormwater Stormwater Management Planning Management Planning & Design& Design
Mike Novotney, P.E. (MD)Center for Watershed Protection
Dave Briglio, P.E.MACTEC
Georgia StormwaterManagement Manual
Hydrologic Methods & Analysis
Dave Briglio, P.E.MACTEC
Bankfull
Q critical
most most erosiveerosiveflowsflows
Channel Channel Protection CriteriaProtection Criteria
most most pollutedpolluted
flowsflows
Water QualityWater QualityCriteriaCriteria
most most destructivedestructive
flowsflows
OverbankOverbankFlooding CriteriaFlooding Criteria
infiltratedinfiltratedflowsflows
Stormwater Better Stormwater Better Site DesignSite Design
biggest flows biggest flows to considerto consider
Extreme Flood Extreme Flood (Floodplain) Criteria(Floodplain) Criteria
Unified Stormwater Sizing Criteria Water Quality: Capture and treat runoff from
first 1.2 inches of rainfall
Channel Protection: Provide extended
detention of 1-yr, 24-hr storm over 24 hours
Overbank Flood Protection: Provide peak
flow attenuation of 25-yr, 24-hr storm
Extreme Flood Protection: Manage 100-yr
storm through detention or floodplain mgmt
Flood
Control
Channel
WQV
Runoff Reduction
Flood Control
CSS vs. GSMM…
Aquatic Resource Protection
Water Quality
Hydrologic design tasks Runoff volumes
and flow rates– Water Balance
Calculations– Filtration/
infiltration rates
Design Support– Determine Outlet
Sizes– Downstream
analysis– Design diversion
structures
Chapter 2.1
IDF Curves Rational Method SCS Method
– Curve numbers– Peak flows– Hydrographs
Georgia Regression– Peak flows– Hydrographs– Urban and Rural
Water Balance Water Quality
Calcs.– Volumes– Flow Rates
Chapter 2.2
Storage volume calculations Channel protection volume
Chapter 2.3: outlet designChapter 2.3: outlet design
Statewide consistency
Fitted to curves automated methods
Chapter 2.1 - IDF Curves
SCS STORM
0
0.2
0.4
0.6
0.8
1
P/P
tot
0 4 8 12 16 20 24
TIME (HOURS)
6 - HOUR STORM
SCS STORM
6 Hour Storm
The SCS storm is The SCS storm is just an “average” just an “average” balanced storm.balanced storm.
From USGS Urban and Rural Different Regions Peak Flow Urban
– 25 ac. <A< 19 mi2
– 1% <TIA< 62% Hydrographs lag
times
Chapter 2.1- Regression Equations
REGION 1
REGION 2
REGION 3
REGION 4
UNDEFINED AREA
Beware of odd Beware of odd situations that do situations that do not fit the “average” not fit the “average” criteria:criteria:
• odd shaped basins odd shaped basins and lag time and lag time impactsimpacts
• two basins versus two basins versus one big one – peak one big one – peak timingtiming
• storage within the storage within the basinbasin
• ““patchy” urban patchy” urban areasareas
Basic mass balance equation
Localized for Georgia
Very approximate
Chapter 2.1 – Water Balance
P = precipitation * pond surface area Ro = runoff based on watershed efficiency Bf = baseflow, normally zero I = infiltration, either measured or estimated E = evaporation based on free surface map
derived for Georgia Et = evapotranspiration, use free surface
unless lots of emergent vegetation Of = pond overflow when ever pond exceeds
capacity
V = P + Ro + Bf – I – E – Et – Of
Water quality volume calculation – volume based BMPs
Peak discharge – flow based BMPs
Chapter 2.1 - Water Quality
Water Quality Volume Calculation - 85% RuleWQv = (1.2 in) (Rv) (A)/12
where: WQv = water quality volume
1.2 = approx. 85th percentile storm
Rv = 0.05 + 0.009(I)I = percent
imperviousnessA = site area
Athens Airport15 Minute, 6-Hour Storm
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Percent of Storms
Inch
es
of
Rai
nfa
ll 85% rule example85% rule example
Water Quality Volume Calculation Impervious cover can be taken directly
off plans or estimated using TR-55 land use factors
WQv should be calculated and addressed separately for each drainage area on a development site
Off-site drainage areas can be excluded
Graphical method Based on extended
detention – 24-hours
Approximate but proven accurate
Avoids iterative approaches
Chapter 2.2 - Channel Protection
Channel Protection Volume EstimationChannel Protection Volume Estimation
Step 1- Compute Unit Peak Discharge Ia = 0.2S and S = (1000/CN) –10
P = XX inches (1-year storm) from TablesIa/P = 0.2S/P
Tc = developed conditions time-of-concentration
qu = from figure 2.1.5-6 XX
cubic feet per square mile per inch (csm/in)
Step1:Step1:Knowing: IKnowing: Iaa, P, T, P, Tcc
Read: qRead: quu
p. 2.1-30p. 2.1-30
Extended Detention Estimated VolumeExtended Detention Estimated Volume
Step2:Step2:Knowing: qu & T (drawdown time) Knowing: qu & T (drawdown time) Read: qo/qi (ratio of outflow to inflow)Read: qo/qi (ratio of outflow to inflow)
p. 2.2-10p. 2.2-10
qo/qi = 12.03 qu –0.9406qo/qi = 12.03 qu –0.9406
Extended Detention Estimated VolumeExtended Detention Estimated Volume
p. 2.2-10p. 2.2-10
Step3:Step3:Knowing:qo/qi (ratio of outflow to inflow) & Knowing:qo/qi (ratio of outflow to inflow) & Storm Type I or IIStorm Type I or IIRead: Vs/Vr (ratio of storage volume to runoff Read: Vs/Vr (ratio of storage volume to runoff volume – Q in the SCS equation)volume – Q in the SCS equation)
Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) Vs/Vr = 0.683 - 1.43(qo/qi) +1.64(qo/qi) 2 2 - 0.804(qo/qi)- 0.804(qo/qi)33
Extended Detention Estimated VolumeExtended Detention Estimated Volume
WQ Peak Flow1. Back out curve
number
2. Calculate unit peak discharge using SCS simplified peak figures
3. Calculate peak discharge as:
CN = 1000/[10 + 5P +10Qwv - 10(Qwv² + 1.25 QwvP)½]
Qwq = qu * A * Qwv
Ia=0.2S=1000/CN-10
Works for 25-year, 100-year, etc. Works for 25-year, 100-year, etc. Storm Volume:Storm Volume:
For:For:Know QKnow Qinin and Q and Qoutout = q = qoo/q/qi i p. 2.2-10p. 2.2-10
Read VRead Vss/V/Vrr
VVrr= runoff volume= runoff volume
Then VThen Vss= storage volume (af)= storage volume (af)
For multiple outlets multiply VFor multiple outlets multiply Vss by by
safety factor of 1.15.safety factor of 1.15.
A few new things derived for this manual
Downstream AssessmentRequirement
The “poor man’s master plan”.
Look downstream until the flow is small compared to the total flow
Based on modeling numerous locations
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80 90 100
Minutes
Disc
harg
e
Post
Pre Pond
Volume is the issueVolume is the issueSame peaksSame peaks
Different volumesDifferent volumes
Downstream Assessments use “10% Rule”
Ten Percent RuleAldridge Creek, Huntsville, AL
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
0 5 10 15 20 25 30 35
Total Area/Pond Area
Dev
. Q/P
ond
Q
Point where pond controlled area is 10% of the total
drainage area
With pond
No pond
Total Area/Pond Controlled Area
Po
std
evel
op
men
t Q
/Pre
dev
elo
pm
ent
Q
5 acres5 acres
Example 1
20 acres20 acres40 acres40 acres
60 acres60 acres
80 acres80 acres
Example 2
25 acres25 acresBigBig
10% Rule Steps
Determine the 10% point Determine pre-development flows to 10% point Determine post-development flows to 10% point Note any increases Design detention for no increase or negotiate
another solution– Flow easement– Downstream improvements– Regional solution
5 acres5 acres
Example 3
20 acres20 acres40 acres40 acres
60 acres60 acres
80 acres80 acres
CC20 acres20 acresTc=15 minTc=15 minCN = 70CN = 70
AA15 acres15 acresTc=20 minTc=20 minCN = 75CN = 75
BB20 acres20 acresTc=20 minTc=20 minCN = 75CN = 75
43434343
Advantages of Downstream Assessments Fairly easy to accomplish Protects from the liability of downstream
impacts Allows for potential waiver of detention Stops unnecessary or harmful detention Allows for “horse trading” Cheaper than master planning Do not use with extended detention
design
<Insert MEN replacement slides>
Other CSS/GSMM Tools
Site Suitability Design Criteria Design Calculation Forms RRv Criteria Satisfaction CSS Design Credits Coastal Challenges Appendix Information
Site Suitability
Site Suitability
Site Suitability
Design Criteria
Design Criteria
Design Schematics
Step-By-Step…
Design Calculation Forms
CSS Criteria SatisfactionTable 6.4: How Low Impact Development Practices Can Be Used to Help Satisfy the Stormwater Management Criteria
Low Impact Development Practice
Stormwater RunoffReduction
Water Quality Protection
Aquatic Resource Protection
Overbank Flood Protection
Extreme Flood Protection
Alternatives to Disturbed Pervious Surfaces
Soil Restoration
“Credit”:Subtract 50% of any restored areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Subtract 50% of any restored areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Assume that the post-development hydrologic conditions of any restored areas are equivalent to those of open space in good condition.
“Credit”:Assume that the post-development hydrologic conditions of any restored areas are equivalent to those of open space in good condition.
“Credit”:Assume that the post-development hydrologic conditions of any restored areas are equivalent to those of open space in good condition.
Site Reforestation/ Revegetation
“Credit”:Subtract 50% of any reforested revegetated areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Subtract 50% of any reforested/revegetated areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Assume that the post-development hydrologic conditions of any reforested/revegetated areas are equivalent to those of a similar cover type in fair condition.
“Credit”:Assume that the post-development hydrologic conditions of any reforested/revegetated areas are equivalent to those of a similar cover type in fair condition.
“Credit”:Assume that the post-development hydrologic conditions of any reforested/revegetated areas are equivalent to those of a similar cover type in fair condition.
Soil Restoration with Site Reforestation/ Revegetation
“Credit”:Subtract 100% of any restored and reforested/ revegetated areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Subtract 100% of any restored and reforested/ revegetated areas from the total site area and re-calculate the runoff reduction volume (RRv) that applies to a development site.
“Credit”:Assume that the post-development hydrologic conditions of any restored and reforested/ revegetated areas are equivalent to those of a similar cover type in good condition.
“Credit”:Assume that the post-development hydrologic conditions of any restored and reforested/ revegetated areas are equivalent to those of a similar cover type in good condition.
“Credit”:Assume that the post-development hydrologic conditions of any restored and reforested/ revegetated areas are equivalent to those of a similar cover type in good condition.
CSS Criteria SatisfactionTable 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria
Stormwater Management Practice
Stormwater RunoffReduction
Water Quality Protection
Aquatic Resource Protection
Overbank Flood Protection
Extreme Flood Protection
General Application Practices
Stormwater Ponds
“Credit”:None
“Credit”:Assume that a stormwater pond provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads.
“Credit”:A stormwater pond can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:A stormwater pond can be designed to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A stormwater pond can be designed to attenuate the extreme peak discharge (Qp100) on a development site.
Stormwater Wetlands
“Credit”:None
“Credit”:Assume that a stormwater wetland provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads.
“Credit”:A stormwater wetland can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:A stormwater wetland can be designed to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A stormwater wetland can be designed to attenuate the extreme peak discharge (Qp100) on a development site.
Bioretention Areas, No Underdrain
“Credit”:Subtract 100% of the storage volume provided by a non-underdrained bioretention area from the runoff reduction volume (RRv) conveyed through the bioretention area.
“Credit”:Assume that a bioretention area provides an 80% reduction in TSS loads, an 80% reduction in TN loads and a 90% reduction in bacteria loads.
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to attenuate the overbank peak discharge (Qp25).
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to attenuate the extreme peak discharge (Qp100).
CSS Criteria Satisfaction
Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria
Stormwater Management Practice
Stormwater RunoffReduction
Water Quality Protection
Aquatic Resource Protection
Overbank Flood Protection
Extreme Flood Protection
Limited Application Practices
Water Quantity Management Practices
Dry Detention Basins
“Credit”:None
“Credit”:None
“Credit”:None
“Credit”:A dry detention basin can be used to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A dry detention basin can be used to attenuate the extreme peak discharge (Qp100) on a development site.
Dry Extended Detention Basins
“Credit”:None
“Credit”:Assume that underground filters provide an 80% reduction in TSS loads, a 25% reduction in TN loads and a 40% reduction in bacteria loads.
“Credit”:A dry extended detention basin can be used to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:A dry extended detention basin can be used to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A dry extended detention basin can be used to attenuate the extreme peak discharge (Qp100) on a development site.
Multi-Purpose Detention Areas
“Credit”:None
“Credit”:None
“Credit”:None
“Credit”:A multi-purpose detention area can be used to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A multi-purpose detention area can be used to attenuate the overbank peak discharge (Qp25) on a development site.
Underground Detention Systems
“Credit”:None
“Credit”:None
“Credit”:An underground detention system can be used to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:An underground detention system can be used to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:An underground detention system can be used to attenuate the extreme peak discharge (Qp100) on a development site.
CSS Design Criteria
7.4 Better Site Planning Technique Profile Sheets7.4.2 Protection Secondary Conservation
Areas7.4.1 Protect Primary Conservation Areas
CSS Design Criteria
7.4.1 Preserve Primary Conservation AreasKEY CONSIDERATIONSProtects important priority habitat areas from the direct impacts of the land development processHelps maintain pre-development site hydrology by reducing post-construction stormwater runoff rates, volumes and pollutant loadsPreserves a site’s natural character and aesthetic features, which may increase the resale value of the development project Conservation areas can be used to “receive” stormwater runoff generated elsewhere on the development site (Section 6.8.3)
USING THIS TECHINQUE Complete Natural Resources Inventory prior to initiating site planning and design process Ensure that primary conservation areas are maintained in an undisturbed, natural state before, during and after construction
CSS Design Credits
Stormwater Management “Credits”Runoff Reduction/Water Quality ProtectionSubtract any primary conservation areas from the total site area when calculating the runoff reduction volume (RRv) that applies to a development site.
Large Storm EventsAssume that the post-development hydrologic conditions of any primary conservation areas are equivalent to the pre-development hydrologic conditions for those same areas.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia
Site Characterist
ic
How it Influences the Use Potential Solutions
Poorly drained soils, such as hydrologic soil group C and D soils
Reduces the ability of vegetated filter strips to reduce stormwater runoff volumes and pollutant loads.
Use soil restoration (Sect. 7.6.1) to improve soil porosity.Place buildings & impervious surfaces on poorly drained soils or preserve as secondary conservation areas (Sect. 7.4.2).Use small stormwater wetlands (Sect. 8.4.2) to capture and treat stormwater.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia
Site Characterist
ic
How it Influences the Use Potential Solutions
Well drained soils, such as hydrologic soil group A and B soils
Enhances the ability of vegetated filter strips to reduce stormwater runoff volumes and pollutant loads, but may allow stormwater pollutants to reach groundwater aquifers with greater ease.
Avoid the use of infiltration-based stormwater management practices, including vegetated filter strips, at stormwater hotspot facilities and in areas known to provide groundwater recharge to aquifers used as a water supply.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia
Site Characterist
ic
How it Influences the Use Potential Solutions
Flat terrain May be difficult to provide positive drainage and may cause stormwater runoff to pond on the surface of the vegetated filter strip.
Design vegetated filter strips with a slope to promote positive drainage.Where soils are sufficiently permeable, use infiltration practices (Sect. 8.4.5) and non-underdrained bioretention areas (Sect. 8.4.3).Where soils have low permeabilities, use small stormwater wetlands (Sect. 8.4.2)
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia
Site Characterist
ic
How it Influences the Use Potential Solutions
Shallow water table
May cause stormwater runoff to pond on the surface of the vegetated filter strip.
Use small stormwater wetlands (e.g. pocket wetlands) (Sect. 8.4.2) or wet swales (Sect. 8.4.6).
Tidally-influenced drainage system
May prevent stormwater runoff from moving through the vegetated filter strip, particularly during high tide.
Investigate the use of other stormwater management practices to manage stormwater runoff in these areas.
GSMM Appendix Information
CSS Appendix Information Appendix A High Priority Plant & Animal
Species
Appendix B Coastal Georgia Rainfall Analysis
Appendix C Stormwater Management Practice Monitoring Protocol
Appendix D Model Post-Construction Stormwater Ordinance