Stormwater Management Planning & Design Mike Novotney, P.E. (MD) Center for Watershed Protection Dave Briglio, P.E. MACTEC

Georgia Stormwater Management Manual Hydrologic Methods & Analysis Dave Briglio, P.E. MACTEC

Bankfull Q critical mosterosiveflows Channel Protection Criteria mostpollutedflows Water Quality Criteria mostdestructiveflows Overbank Flooding Criteria infiltratedflows Stormwater Better Site Design biggest flows to consider Extreme Flood (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 design

Statewide consistency Fitted to curves automated methods Chapter IDF Curves

SCS STORM 6 - HOUR STORM

SCS STORM 6 Hour Storm The SCS storm is just an “average” balanced storm.

From USGS Urban and Rural Different Regions Peak Flow Urban –25 ac. <A< 19 mi 2 –1% <TIA< 62% Hydrographs lag times Chapter 2.1- Regression Equations

Beware of odd situations that do not fit the “average” criteria: odd shaped basins and lag time impactsodd shaped basins and lag time impacts two basins versus one big one – peak timingtwo basins versus one big one – peak timing storage within the basinstorage within the basin “patchy” urban areas“patchy” urban areas

Basic mass balance equation Localized for Georgia Very approximate Chapter 2.1 – Water Balance

P = precipitation * pond surface area R o = runoff based on watershed efficiency B f = baseflow, normally zero I = infiltration, either measured or estimated E = evaporation based on free surface map derived for Georgia E t = evapotranspiration, use free surface unless lots of emergent vegetation O f = pond overflow when ever pond exceeds capacity

Water quality volume calculation – volume based BMPs Peak discharge – flow based BMPs Chapter Water Quality

Water Quality Volume Calculation - 85% Rule WQ v = (1.2 in) (Rv) (A)/12 where:WQ v = water quality volume 1.2= approx. 85th percentile storm Rv= (I) I=percent imperviousness A=site area

85% rule example

Water Quality Volume Calculation Impervious cover can be taken directly off plans or estimated using TR-55 land use factors WQ v 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 Channel Protection

Channel Protection Volume Estimation Step 1- Compute Unit Peak Discharge   Ia = 0.2S and S = (1000/CN) –10   P = XX inches (1-year storm) from Tables Ia/P = 0.2S/P   T c = developed conditions time-of-concentration  q u = from figure XX cubic feet per square mile per inch (csm/in)

Step1: Knowing: I a, P, T c Read: q u p Extended Detention Estimated Volume

Step2: Knowing: qu & T (drawdown time) Read: qo/qi (ratio of outflow to inflow) p qo/qi = qu – Extended Detention Estimated Volume

p Step3: Knowing:qo/qi (ratio of outflow to inflow) & Storm Type I or II Read: Vs/Vr (ratio of storage volume to runoff volume – Q in the SCS equation) Vs/Vr = (qo/qi) +1.64(qo/qi) (qo/qi) 3 Extended Detention Estimated Volume

WQ Peak Flow Back out curve number Calculate unit peak discharge using SCS simplified peak figures Calculate peak discharge as: Ia=0.2S=1000/CN-10

Works for 25-year, 100-year, etc. Storm Volume: For: Know Q in and Q out = q o /q i p Read V s /V r V r = runoff volume V r = runoff volume Then V s = storage volume (af) For multiple outlets multiply V s by safety factor of 1.15.

A few new things derived for this manual

Downstream Assessment Requirement The “poor man’s master plan”. Look downstream until the flow is small compared to the total flow Based on modeling numerous locations

Post Pre Pond Volume is the issue Same peaks Different volumes

Downstream Assessments use “10% Rule” Point where pond controlled area is 10% of the total drainage area With pond No pond Total Area/Pond Controlled Area Postdevelopment Q/Predevelopment Q

5 acres Example 1 20 acres 40 acres 60 acres 80 acres

Example 2 25 acres Big

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 acres Example 3 20 acres 40 acres 60 acres 80 acres C 20 acres Tc=15 min CN = 70 A 15 acres Tc=20 min CN = 75 B 20 acres Tc=20 min CN = 75

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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

Other CSS/GSMM Tools Site Suitability Design Criteria Design Calculation Forms RRv Criteria Satisfaction CSS Design Credits Coastal Challenges Appendix Information

Site Suitability

Design Criteria

Design Schematics

Step-By-Step…

Design Calculation Forms

CSS Criteria Satisfaction Table 6.4: How Low Impact Development Practices Can Be Used to Help Satisfy the Stormwater Management Criteria Low Impact Development Practice Stormwater Runoff Reduction 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 (RR v ) 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 (RR v ) 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 (RR v ) 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 (RR v ) 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 (RR v ) 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 (RR v ) 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 Satisfaction Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria Stormwater Management Practice Stormwater Runoff Reduction 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 (ARP v ). “ Credit ” : A stormwater pond can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater pond can be designed to attenuate the extreme peak discharge (Q p100 ) 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 (ARP v ). “ Credit ” : A stormwater wetland can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater wetland can be designed to attenuate the extreme peak discharge (Q p100 ) 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 (RR v ) 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 (ARP v ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the overbank peak discharge (Q p25 ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the extreme peak discharge (Q p100 ).

CSS Criteria Satisfaction Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria Stormwater Management Practice Stormwater Runoff Reduction 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 (Q p25 ) on a development site. “ Credit ” : A dry detention basin can be used to attenuate the extreme peak discharge (Q p100 ) 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 (ARP v ). “ Credit ” : A dry extended detention basin can be used to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A dry extended detention basin can be used to attenuate the extreme peak discharge (Q p100 ) 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 (Q p25 ) on a development site. “ Credit ” : A multi-purpose detention area can be used to attenuate the overbank peak discharge (Q p25 ) 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 (ARP v ). “ Credit ” : An underground detention system can be used to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : An underground detention system can be used to attenuate the extreme peak discharge (Q p100 ) on a development site.

CSS Design Criteria 7.4Better Site Planning Technique Profile Sheets Protection Secondary Conservation Areas Protect Primary Conservation Areas

CSS Design Criteria Preserve Primary Conservation Areas KEY CONSIDERATIONS  Protects important priority habitat areas from the direct impacts of the land development process  Helps maintain pre-development site hydrology by reducing post-construction stormwater runoff rates, volumes and pollutant loads  Preserves 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 Protection Subtract any primary conservation areas from the total site area when calculating the runoff reduction volume (RRv) that applies to a development site. Large Storm Events Assume 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 Characteristic 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 ) to improve soil porosity.  Place buildings & impervious surfaces on poorly drained soils or preserve as secondary conservation areas (Sect ).  Use small stormwater wetlands (Sect ) to capture and treat stormwater.

Coastal Challenges Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia Site Characteristic 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 Characteristic How it Influences the Use Potential Solutions Flat terrainMay 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 ) and non- underdrained bioretention areas (Sect ).  Where soils have low permeabilities, use small stormwater wetlands (Sect )

Coastal Challenges Challenges Associated with Using Vegetated Filter Strips in Coastal Georgia Site Characteristic 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 ) or wet swales (Sect ). 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