Stormwater Management
Planning & Design
Mike Novotney, P.E. (MD)
Center for Watershed Protection
Dave Briglio, P.E.
MACTEC
Hydrologic Methods
& Analysis
Dave Briglio, P.E.
MACTEC
Georgia Stormwater
Management Manual
biggest flows
to consider
Bankfull
Extreme Flood
(Floodplain) Criteria
most
destructive
flows
Overbank
Flooding Criteria
most
erosive
flows
Channel
Protection Criteria
Q critical
most
polluted
flows
infiltrated
flows
Water Quality
Criteria
Stormwater Better
Site Design
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
CSS vs. GSMM…
Runoff
Reduction
WQV
Channel
Water Quality
Flood
Aquatic Resource
Control
Protection
Flood Control
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
Chapter 2.1 - IDF Curves


Statewide
consistency
Fitted to curves
automated methods
SCS STORM
6 - HOUR STORM
1
P /P t o t
0.8
0.6
0.4
0.2
0
0
4
8
12
16
TIME (HO URS )
20
24
SCS STORM
0.2
R a i n fa l l (i n c h e s )
The SCS storm is
just an “average”
balanced storm.
0.15
6 Hour Storm
0.1
0.05
0
0
3
6
9
Time (Hrs.)
12
15
18
Chapter 2.1- Regression
Equations




From USGS
Urban and Rural
Different Regions
Peak Flow Urban
– 25 ac. <A< 19 mi2
– 1% <TIA< 62%

Hydrographs lag times
Beware of odd situations
that do not fit the
“average” criteria:
• odd shaped basins and
lag time impacts
• two basins versus one
big one – peak timing
• storage within the
basin
• “patchy” urban areas
Chapter 2.1 – Water Balance



Basic mass balance
equation
Localized for Georgia
Very approximate
 V = P + Ro + Bf – I – E – Et – Of

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

Et = evapotranspiration, use free surface unless

Of = pond overflow when ever pond exceeds




derived for Georgia
lots of emergent vegetation
capacity
Chapter 2.1 - Water Quality


Water quality volume
calculation – volume
based BMPs
Peak discharge – flow
based BMPs
Water Quality Volume
Calculation - 85% Rule
WQv = (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
imperviousness
A
= site area
Athens Airport
15 Minute, 6-Hour Storm
5
Inches of Rainfall
4.5
4
85% rule example
3.5
3
2.5
2
1.5
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Percent of Storms
0.7
0.8
0.9
1
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
Chapter 2.2 - Channel
Protection




Graphical method
Based on extended
detention – 24-hours
Approximate but
proven accurate
Avoids iterative
approaches
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

Tc = developed conditions time-of-concentration

qu = from figure 2.1.5-6 XX
cubic feet per square mile per inch (csm/in)
Extended Detention Estimated Volume
Step1:
Knowing: Ia, P, Tc
Read: qu
p. 2.1-30
Extended Detention Estimated Volume
Step2:
Knowing: qu & T (drawdown time)
Read: qo/qi (ratio of outflow to inflow)
qo/qi = 12.03 qu –0.9406
p. 2.2-10
Extended Detention Estimated Volume
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 = 0.683 - 1.43(qo/qi) +1.64(qo/qi) 2 - 0.804(qo/qi)3
p. 2.2-10
WQ Peak Flow
1.
Back out curve
number
CN = 1000/[10 + 5P +10Qwv - 10(Qwv² + 1.25 QwvP)½]
2.
3.
Calculate unit peak
discharge using SCS
simplified peak
figures
Calculate peak
discharge as:
Qwq = qu * A * Qwv
Ia=0.2S=1000/CN-10
Works for 25-year, 100-year, etc.
Storm Volume:
For:
Know Qin and Qout = qo/qi p. 2.2-10
Read Vs/Vr
Vr= runoff volume
Then Vs= storage volume (af)
For multiple outlets multiply Vs 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
Volume is the issue
14
12
Post Same peaks
Different volumes
Discharge
10
8
6
Pre
4
Pond
2
0
0
10
20
30
40
50
Minutes
60
70
80
90
100
Downstream Assessments
use “10% Rule”
Ten Percent Rule
A l d r i d g e C r e e k , H u n tsv i l l e , A L
Q
Q/Predevelopment
Postdevelopment
Dev. Q/Pond
Q
1.50
1.45
No pond
1.40
1.35
1.30
1.25
Point where pond controlled
area is 10% of the total
drainage area
1.20
1.15
1.10
1.05
1.00
0
5
10
With pond
15
20
Tota
l Ar e a /P ond
Ar e a Area
Total
Area/Pond
Controlled
25
30
35
Example 1
5 acres
40 acres
20 acres
60 acres
80 acres
Example 2
Big
25 acres
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
Example 3
B
5 acres
20 acres
Tc=20 min
CN = 75
40 acres
C
60 acres
20 acres
80 acres
Tc=15 min
CN = 70
A
2015acres
acres
Tc=20 min
CN = 75
43
43
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
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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 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 recalculate the runoff
reduction volume (RRv)
that applies to a
development site.
“Credit”:
Subtract 50% of any
restored areas from the
total site area and recalculate the runoff
reduction volume (RRv)
that applies to a
development site.
“Credit”:
Assume that the postdevelopment hydrologic
conditions of any restored
areas are equivalent to
those of open space in
good condition.
“Credit”:
Assume that the postdevelopment hydrologic
conditions of any restored
areas are equivalent to
those of open space in
good condition.
“Credit”:
Assume that the postdevelopment 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 postdevelopment hydrologic
conditions of any
reforested/revegetated
areas are equivalent to
those of a similar cover
type in fair condition.
“Credit”:
Assume that the postdevelopment hydrologic
conditions of any
reforested/revegetated
areas are equivalent to
those of a similar cover
type in fair condition.
“Credit”:
Assume that the postdevelopment 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 recalculate 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 recalculate the runoff
reduction volume (RRv)
that applies to a
development site.
“Credit”:
Assume that the postdevelopment 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 postdevelopment 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 postdevelopment 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 Runoff
Reduction
Stormwater Management
Practice
Water Quality Protection
Aquatic Resource
Protection
Overbank Flood
Protection
“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.
“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.
“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).
Extreme Flood Protection
General Application Practices
Stormwater Ponds
Stormwater Wetlands
Bioretention Areas,
No Underdrain
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
“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.
“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.
“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.
“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.
Dry Detention Basins
Dry Extended Detention
Basins
Multi-Purpose Detention
Areas
Underground Detention
Systems
CSS Design Criteria
7.4 Better Site Planning Technique
Profile Sheets
7.4.2 Protection Secondary Conservation Areas
7.4.1 Protect Primary Conservation Areas
CSS Design Criteria
7.4.1 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 predevelopment hydrologic conditions for those same areas.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in
Coastal Georgia
Site
How it Influences the
Potential Solutions
Characteristic
Use
Poorly drained Reduces the ability of
Use soil restoration (Sect.
soils, such as
vegetated filter strips to
7.6.1) to improve soil
hydrologic soil
reduce stormwater runoff porosity.
group C and D volumes and pollutant
Place buildings &
soils
loads.
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
How it Influences the
Potential Solutions
Characteristic
Use
Well drained
Enhances the ability of
Avoid the use of
soils, such as
vegetated filter strips to
infiltration-based
hydrologic soil
reduce stormwater runoff stormwater management
group A and B volumes and pollutant
practices, including
soils
loads, but may allow
vegetated filter strips, at
stormwater pollutants to
stormwater hotspot
reach groundwater
facilities and in areas
aquifers with greater
known to provide
ease.
groundwater recharge to
aquifers used as a water
supply.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in
Coastal Georgia
Site
How it Influences the
Potential Solutions
Characteristic
Use
Flat terrain
May be difficult to provide Design vegetated filter
positive drainage and
strips with a slope to
may cause stormwater
promote positive drainage.
runoff to pond on the
Where soils are
surface of the vegetated
sufficiently permeable, use
filter strip.
infiltration practices (Sect.
8.4.5) and nonunderdrained 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
How it Influences the
Potential Solutions
Characteristic
Use
Shallow water
May cause stormwater
Use small stormwater
table
runoff to pond on the
wetlands (e.g. pocket
surface of the vegetated
wetlands) (Sect. 8.4.2) or
filter strip.
wet swales (Sect. 8.4.6).
TidallyMay prevent stormwater
Investigate the use of
influenced
runoff from moving
other stormwater
drainage
through the vegetated
management practices to
system
filter strip, particularly
manage stormwater runoff
during high tide.
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