Designing Stormwater Ponds for Water Quality - Ohio EPA

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Designing Stormwater
Ponds for Water Quality
Jay Dorsey
ODNR-DSWC
February 13, 2008
Designing Stormwater Ponds
for Water Quality
‣ Water Quality Volume
‣ Outlets
‣ Other Design Considerations
The Purpose of Stormwater
Regulations/Management
‣ Minimize impacts to receiving
waters
‣ Offset or mitigate for the changed
site hydrology and the loss of
natural watershed services
Traditional stormwater management
approaches (e.g., Critical Storm Method)
were aimed at matching postdevelopment
to pre-development peak
discharges for infrequent (extreme)
storm events, resulting in large detention
basins with large outlets
Long-term Rainfall Characteristics
Manmade Stormwater Management Systems
Dayton, OH Rain Events (1950-1999)
4000
3500
3000
2500
2000
1500
1000
500
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.5 3.0 3.5 4.0 5.0
Rain Depth (in)
1

Cumulative Occurrence Probability
%
Stormwater Management
Regulation in Ohio
Problems/concerns with peak discharge control
methods (e.g., Critical Storm Method)
‣ Problem 1: Too little detention time for
effective pollutant removal
‣ Problem 2: Peak discharge control methods
allow smaller rainfall events to become
channel eroding events
Ohio Application of WQv Formula
Water Quality
Volume (WQv)
‣ Urban Runoff Quality
Management, ASCE Manual
of Practice No. 87, Amercan
Society of Civil Engineers,
Reston, VA, (1998).
‣ WQv = C * P * A / 12
Where:
WQv = water quality volume (ac-ft)
‣ C = runoff coefficient
‣ P = 0.75 inch precipitation
‣ A = area draining to the BMP (acres)
Why 0.75” Rainfall Depth?
Clarification of WQv
Dayton, OH Rain Events (1950-1999)
100
90
80
70
60
50
40
30
20
10
0
0 0.5 1 1.5 2 2.5 3
Rain Depth (in)
‣ Runoff Coefficient
‣ Drawdown Requirement
2

Runoff Coefficient
Runoff Coefficient
WQv Runoff Coefficient
WQv Runoff Coefficient
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
WQv Runoff Coefficent
C
C = runoff coefficient
i = watershed
imperviousness ratio
(percent total
imperviousness
divided by 100)
C
0.858i
3
C = runoff coefficient
0.78i
2
0.774i
i = watershed imperviousness ratio (percent total
imperviousness divided 100)
0.04
0.20
0.10
0.00
0.0 20.0 40.0 60.0 80.0 100.0
Impervious Area (%)
C
0.858i
3
0.78i
2
0.774i
0.04
Source: Urban Runoff
Quality Management,
ASCE, 1998, p. 175.
Example
WQv Formula
Determine the Runoff Coefficient, C, for:
‣ 100 acre residential development, 0.5 acre
lots, with 20% impervious area (i = 0.20)
C = 0.17
WQv = C * P * A / 12 = (0.17*0.75*100)/12 = 1.06 ac-ft
From Example 1, using Table 1 value of C = 0.8,
WQv = C * P * A / 12 = (0.3*0.75*100)/12 = 1.88 ac-ft
‣ WQv = C * P * A / 12
Where:
WQv = water quality volume (ac-ft)
‣ P = 0.75 inch precipitation
‣ A = area draining to the BMP (acres)
‣ C = runoff coefficient - ????
Runoff Coefficient Comparison
Runoff Coefficient Comparison
Runoff Coefficent Comparison
Calculated
Published
1.000
0.900
0.800
0.700
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0.0 20.0 40.0 60.0 80.0 100.0
Impervious Area (%)
CGP-C
C
Impervious Runoff Runoff Calc Publ Increase
Area Coefficient Coefficient WQv WQv WQv
SLU - Standard Land Use % C C acre-ft acre-ft %
Urban Open Space 4.9 0.08 0.20 0.5 1.25 163
Urban Parks 9.6 0.11 0.20 0.7 1.25 85
Low Density Residential 20.3 0.17 0.30 1.1 1.875 74
Med Density Res no alleys 37.7 0.27 0.40 1.7 2.5 50
Duplex 39.1 0.27 0.50 1.7 3.125 82
High Density Res no alleys 53.0 0.36 0.50 2.2 3.125 39
Multi-Family Res no alleys 53.1 0.36 0.50 2.2 3.125 39
Medium Industrial 68.5 0.48 0.80 3.0 5 67
Office Park 73.1 0.52 0.80 3.3 5 53
Strip Commercial 90.7 0.74 0.80 4.6 5 8
3

Runoff Coefficient Comparison
C Determination Method
Land Use % Impervious Runoff Coefficient Wqv Increase TSS
TSS
(ac-ft) in Wqv (lb/ac/yr) Reduction
(%)
(%)
No Pond 54.3 -
‣ Recommendation – Use the formula, not the
Table
Urban Open Space 4.9
Calc 0.08 0.5 - 9.0 83.5
Publ 0.20 1.25 163 9.3 83.0
Duplex 39.1
No Pond 280 -
Calc 0.27 1.7 - 59.5 78.8
C
0.858i
3
0.78i
2
0.774i
0.04
Publ 0.50 3.12 82 57.3 79.6
No Pond 703 -
Medium Industrial 68.5
Calc 0.48 3.0 - 139 80.3
Publ 0.80 5.0 67 132 81.2
Adjustments to WQv Formula
Wet Ponds
Adjustments to WQv Formula
Structural BMPs (excl Wet Ponds)
OEPA-CGP p22
OEPA-CGP p22
Discharge Rate
Sediment Settling Process
‣ How quickly do we release the WQv to
meet our stormwater management goals?
Pond
Inflow
v w
v s
Permanent Pool/Sediment Storage
Pond
Discharge
4

Volume (cu ft)
Volume (cu ft)
Pond
Inflow
Sediment Settling Process
Sediment Settling Process
Pond
Inflow
v w
v w
v s
v s
Pond
Pond
Permanent Pool/Sediment Storage
Discharge
Discharge
v sand v silt v clay
OEPA CGP Drawdown Requirements
‣ The OEPA CGP lists drain time
(or drawdown) requirements for
structural BMPs (CGP – Table 2)
Select an appropriate outlet to
meet drawdown requirement for
wet ponds and dry ponds
Volume vs Drawdown Time
Volume vs Drawdown Time
45,000
45,000
40,000
40,000
35,000
35,000
30,000
30,000
25,000
4.5" Outlet
25,000
4.5" Outlet
20,000
8" Outlet
20,000
8" Outlet
15,000
15,000
10,000
10,000
5,000
5,000
0
0
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
Drawdown Time (hr)
Drawdown Time (hr)
5

Volume (cu ft)
Volume (cu ft)
Volume (cu ft)
Volume (cu ft)
Volume vs Drawdown Time
Volume vs Drawdown Time
45,000
WQv
The WQv orifice should be sized to release no more
than one-half the WQv (0.5*WQv) in the first one-third
of the target drawdown period (0.333*Td)
40,000
35,000
30,000
The drawdown curve
should fall above and to
the right of the ½ volume
in 1/3 drawdown time
target
0.5xWQv
25,000
4.5" Outlet
Target
20,000
8" Outlet
15,000
10,000
5,000
0.333xTd
Td
Drawdown Time (hr)
0
0 10 20 30 40 50 60 70 80
Drawdown Time (hr)
WQv Outlet - Primary Considerations
‣Performance
‣Maintenance
Water Quality Volume (WQv) Outlet
‣ For most detention
pond designs, an
orifice needs to be
used to meet the
drawdown
requirements of the
Water Quality Volume
(WQv).
WQv Outlet - Performance
WQv Geometry (Surface Area, Depth) and Orifice Size
‣ Use an appropriately sized orifice
45,000
Volume vs Drawdown Time
WQv = 40,000 cu ft, td = 24 hr
40,000
60 Ac MDR (1/4 ac lots)
Volume vs Drawdown Time
40 Ac Multi-family
35,000
30 Ac Office Park
20 Ac Strip Commercial
WQv
30,000
The WQv orifice should be sized to release no more
than one-half the WQv (0.5*WQv) in the first one-third
of the target drawdown period (0.333*Td)
25,000
20,000
A=1.8 Ac
D=0.5 ft
dia=12 in
A=3.7 Ac
D=0.25 ft
dia=24 in
0.5xWQv
15,000
Target
10,000
A= 0.5Ac
D=2 ft
dia=4.5 in
A=0.9 Ac
D=1.0 ft
dia=6 in
0.333xTd
Td
5,000
A=0.2 Ac
D=4 ft
dia=3.5 in
0
0 10 20 30 40 50 60 70 80
Drawdown Time (hr)
Drawdown Time (hr)
6

TSS - % Reduction
Orifice Diameter (in)
% TSS Reduction by Pond Area
Sediment Settling Process
100
80
30
25
60
20
15
40
10
20
5
0
0
0 1 2 3 4
Pond
Inflow
v s
v w
Permanent Pool/Sediment Storage
Pond
Discharge
Average Pond Area (Ac)
TSS Reduction
Orifice Size
Multi-Stage Outlets
Multi-Stage Outlets
‣ Most detention basins that include a Water
Quality Volume (WQv) require separate outlets
for the WQv and the peak discharge control.
‣ The exception is very shallow extended
detention volumes in large surface area wet
detention basins.
Unprotected WQv Outlets
Unprotected WQv Outlets
7

WQv Outlet - Maintenance
WQv Outlet – Reverse Slope Pipe
‣ Protect the orifice
‣ Protected orifice options:
• Reverse slope pipe
• Perforated tile/pipe with gravel filter
WQv Outlet – Perforated Riser/Gravel Filter
9

WQv Outlet – Perforated Pipe/Gravel Filter
10

Other Design Considerations
‣ Discuss issues related to BMP
selection
‣ Highlight other issues
• Health and safety
• Maintenance
• Performance
WQv BMP Selection
BMP Selection – Drainage Area
‣ Drainage area
‣ Soil type
‣ Performance (Source area/pollutants?
Local TMDL? Target pollutants? Runoff
temperature?)
‣ State/Local Regulations
‣ Outlet
‣ Depth/High Water Table
‣ Soil type
BMP Selection – Soil Type
• HSG-A – 1.2%
• HSG-B – 18%
• HSG-C – 61.2%
• HSG-D – 19.5%
Detention Basin Selection
‣ Wet pond (or wetland ED basin)
• Usually the best choice in Ohio given the
predominance of C & D soils, water quality
treatment performance, maintenance/
aesthetics.
‣ Dry ED basin
• May be a reasonable choice for smaller
development sites (

Dry Basins?
Other Design Considerations
Health and Safety
‣ Discuss issues related to BMP selection
‣ Highlight other issues
• Health and safety
• Maintenance
• Performance
‣ Sideslopes
‣ Safety benches
‣ Inlets/outlets
‣ Mosquitos/West Nile virus
‣ Flood Routing
‣ Freeboard
‣ Emergency spillways
‣ Earthwork (embankments)
Maintenance
Sediment Forebays/Maintenance Access
‣ Sediment pre-treatment (filters and
forebays)
‣ Maintenance access
‣ Pond drains
‣ Inlets/outlets
‣ Dry basins
‣ Permanent Stormwater Maintenance Plan
‣ Responsible Management Entity (RME)
12

Pond Drains
Outlet Maintenance
Poor outlet
designs require
constant attention
to work as
designed
Pond drains allow
rapid draining of wet
ponds (and dry ponds)
to allow maintenance
Dry Basins?
Are Pilot Channels the Answer?
How Big a Concern Is the Wet Spot?
An Attractive Dry Basin?
13

Performance
Pretreatment Opportunities
‣ Tailwater elevations/tailwater analysis
‣ Pretreatment/treatment trains
‣ Sediment forebays
‣ Dry basins – forebays and micropools
‣ Flow path length
‣ Surface area
‣ Outlets that work
‣ Conversion from sed-pond to detention pond
Pretreatment Opportunities?
Pretreatment Opportunities?
Sediment Forebays
Micropools
14

Flow Path Length
Sediment Settling Process
Pond
Inflow
v w
v s
Permanent Pool/Sediment Storage
Pond
Discharge
Flow Path Length
Flow Path Length
Outlets and Performance
Having a
functional Water
Quality pond
depends on
functional outlets
Detention Pond as Sediment Pond
Sed basins must have
appropriate outlet to
drain dewatering
volume in 48-72 hours –
see RLD for guidance
15

Sediment Pond to Detention Pond Conversion
References
- Detention Basin Design -
‣ Rainwater and Land Development, Ohio DNR, Division of Soil &
Water Conservation (2006).
http://www.dnr.state.oh.us/soilandwater/default/water/default/tabid/9185/Default.aspx
‣ Design and Construction of Urban Stormwater Management
Systems, ASCE Manual of Practice No. 77, Amercan Society of Civil
Engineers, Reston, VA, (1992).
Sediment basin to postconstruction
basin outlets
and conversion timing
should be specified in
SWPPP, checked during
inspections
‣ Design of Detention Systems, J.N. Paine and A. Osman Akan,
Chapter 7 in L.W. Mays (ed.), Stormwater Collection Systems
Design Handbook, McGraw-Hill, New York (2001)
‣ Ponds – Planning, Design, Construction, USDA-NRCS Ag
Handbook 590, 1997.
16