Stormwater Management Planning & Design

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


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


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

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 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


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.



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.



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.



CSS Criteria SatisfactionTable 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria

Stormwater Management Practice






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






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


Site Characterist

ic


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


Site Characterist

ic


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


Site Characterist

ic


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

Stormwater Management Planning & Design

Documents

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