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Post-Construction SWP3 Review
Dan Bogoevski, Ohio EPA Northeast District Office
March 12, 2014
Who Reviews Post-Construction BMPs?
Within urbanized areas, the local MS4 operator must review and approve the SWPPP
This includes post-construction BMPs
Use of alternative BMPs on large construction sites, off-site mitigation and non-structural in lieu of structural BMPs requires approval from Ohio EPA
Approval must occur prior to the start of construction
Standard Post-Construction BMPs
Dry Extended Detention Pond
Wet Extended Detention Pond
Constructed Wetland
• Includes practice formerly known as
Wet Enhanced Swale
Bioretention Area
• Includes Turfed Linear Bioretention
aka Dry Enhanced Swale
Infiltration Basin or Trench
Permeable Pavement
Sand & Other Media Filters
Pocket Wetlands
Standard Post-Construction BMPs
Turfed Linear Bioretention Sand Filter
Infiltration Trench Bioretention Area Pocket Wetland
Permeable Pavement
But, the Most Popular BMP Remains…
Dry Extended Detention Basin
w/Forebay and Micropool Wet Extended Detention Basin
Water Quality Pond
Dry Extended Detention Bains
Water Quality Volume (WQv) is detained above the
invert of the outlet structure
Target drain time of brimful WQv is 48 hours
No more than ½ WQv in first 16 hours
Must include a forebay at inlet(s) and micropool at outlet
Minimum size = 10% WQv each
These areas provide the “additional 20% WQv” to store
settled pollutants
Example Site: Industrial Development
Dry Extended Detention
Basin
Drainage Area = 26.97 ac
Imperviousness = 75%
Dry Extended Detention Basin Outlet
Extended Detention Volume
Micropool Extended Detention Volume = WQv
Micropool ≥ 10% WQv
WQv Orifice ≥ 48 hours
WQv Orifice
Checking the Extended Detention Volume
Step 1: Make sure you are looking at the permanent
outlet structure!
Temporary Outlet
Permanent Outlet
Checking the Extended Detention Volume
Step 1: Calculate WQv based on drainage area of pond
Need runoff coefficient “C”. What is the land use?
NPDES permit says, use Table 1 in Part III.G.2.e
Or, use formula C = 0.858i3 – 0.78i2 + 0.774i + 0.04
C = 0.858*0.753 – 0.78*0.752 + 0.774*0.75 + 0.04 = 0.544
WQv = 0.75 * C * DA/12 [=] ac-ft
WQv = 0.75*0.544*26.97/12 = 0.917 ac-ft = 39,944 cubic feet
Multiply ac-ft by 43,560 to get cubic feet
Industrial = 0.8
Checking the Extended Detention Volume
Step 2: What is the extended detention volume (EDv)?
For dry basins, the EDv = WQv
For our example EDv = 39,944 cubic feet
Step 3: Determine elevation at which EDv is stored
Make sure elevation on stage-storage table matches elevation
on detail drawing!
Checking the Extended Detention Volume
Step 4: Determine the elevation at which the EDv is
stored
1130.50 is “zero elevation”. EDv is above the zero elevation.
Thus, add 39,944 to 12,935 = 46,879 cubic ft.
This is the Volume Sum at which EDv is stored.
Interpolate to determine the elevation where EDv is stored
(46,879 – 29,575)/(75,555 – 29,575) = 0.38
Thus, EDv is stored at 1131.00 + 0.38 = 1131.38 elevation
1130.50 is “zero elevation”
46,879 cubic feet between
elevation 1131.00 and
1132.00
Checking the Extended Detention Volume
Does basin have adequate capacity to detain EDv?
Invert of WQv
orifice at
Elevation 1130.50
Invert of next stage
orifice at
Elevation 1131.90
EDv at
Elevation 1131.38
YES
Basin design
meets EDv
requirement
Checking the Drain Time
Step 1: Determine target discharge rate for EDv, i.e., Qavg
Qavg = EDv/tdrain time [=] ft3/sec
Target drain time for dry basin = 48 hrs = 172,800 sec
Qavg = 39,944 ft3/172,800 sec = 0.23 ft3/sec
NOTE: Do NOT add 20% to EDv! In dry basins, this is accounted for in forebay and micropool! In wet basins, this is accounted for in the retained volume.
Step 2: Determine equation to size WQv orifice
Equation for circular orifice
Qavg = CA(2ghavg)0.5
If you have a weir structure rather than circular pipe, you must use a different equation. Equation depends on weir type!
Refer to guidance in Section 2.3 of Georgia Stormwater Manual at www.georgiastormwater.com
Checking the Drain Time
Step 3: Determine havg
This is ½ the height needed to store the EDv
Height to store EDv = 1131.38 – 1130.50 = 0.88 ft
Thus, havg = 1/2*0.88 = 0.44 ft
“Zero Elevation”
= 1130.50
Elevation of EDv
= 1131.38
Checking the Drain Time
Step 4: Size the WQv orifice
Use equation from Step 2, but solve for “A” (area)
A = Qavg/[C*(2ghavg)0.5]
The value of “C” and “g” are constants.
Here, C = roughness = 0.60 for most pipe materials. Do not confuse with
runoff coefficient.
g = speed of gravity = 32.2 ft/sec
Thus, for our example, A = 0.23/[0.6*(2*32.2*0.44)0.5] = 0.072 ft2
Area of circle = π(d/2)2 Solve for d, diameter.
d = 2*(A/π)0.5 = 0.30 ft = 3.63 in
Step 5: Check drain time for ½ EDv
Tdraintime = ½ EDv/Qavg = (0.5*39,944)/0.23 = 86,835 sec = 24 h
Checking the Drain Time
Is the WQv orifice sized to meet drain time requirement?
This review ensures compliance with Ohio EPA water quality requirements. Further review is needed to ensure compliance with local requirements on peak rate of discharge and flood control.
4 inch diameter pipe
provided > 3.63
inches calculated, but
½ WQv drain time is
OK.
Calculated drain
time with 4”
orifice is only
38.3 hrs for
brimful EDv.
Target is 48 hrs.
Need to reduce
size of WQv
orifice.
Check Forebay and Micropool Volume
Each must be at least 10% of WQv
Each must be 0.10 * 39,944 = 3,994 ft3
In this case, both
forebay and
micropool are
below the “zero
elevation”
Micropool
4464 ft3 > 3994 ft3
Forebay
8471 ft3 > 3994 ft3
Did We Get What Was Designed?
Design As Constructed
YES,
We did.
WQv outlet
EDv elevation
Wet Extended Detention Basins
Minimum 0.75 WQv is retained below the normal
waterline elevation (NWL)
Pollutants settle within the retained volume. To meet
“additional 20%” requirement of NPDES permit, must retain
0.95 WQv.
Must detain 0.75 WQv above the NWL. This is EDv for a
wet basin.
The target drain time for EDv is 24 hours
No more than ½ EDv can drain in first 8 hours
Example: Industrial Development
Wet Extended
Detention Basin
Drainage Area =
43.1 ac
Wet Basin Example: Calculation Check
Use City of Canton Extended Detention Orifice Design Spreadsheet at
http://cantonohio.gov/engineering/?pg=510
Wet Basin: Design Check
NWL Elevation = 985.50
NWL Elevation
Retained Volume
Retained Volume = 92,945 ft3
Detained Volume
Wet Extended Detention Basin Outlet
Retained Volume ≥ 0.95 WQv
Detained Volume ≥ 0.75 WQv
WQv Orifice
NWL
Note discrepancy in elevations on
detail vs. stage-storage table
Steps to Reviewing a Wet Basin
Step 1: Make sure you are looking at the permanent outlet
Step 2: Calculate the WQv
Step 3: Use stage-storage data to ensure that retained volume is ≥ 0.95 WQv.
This is volume below the NWL
Step 4: Use stage-storage data to determine elevation at which 0.75 * WQv is detained above the NWL
This is the EDv elevation. One-half this height above the NWL is havg.
This volume is above the NWL. Do not count any volume below NWL.
Ensure that no other stage (outlet or orifice) activates below the EDv elevation.
Step 5: Check orifice sizing.
Use 24 hr for tdraintime
No more than ½ * EDv to drain in 8 hr
Example Basin Summary
Note discrepancy
with value used in
spreadsheet!
Consistent with value
calculated using
spreadsheet
Detail drawing should
specify WQv orifice size on
end cap. Diameter of pipe
should be specified.
Constructed Wetlands
Retained volume is generally shallow and of varying depth
6 to 12 inch depth typical
May have forebays and micropool that are deeper
Retained volume ≥ 20% WQv to meet “additional 20%” requirement
Must detain the full WQv above the retained volume
Outlet to drain WQv in 24 hours
No more than ½ WQv can drain in first 8 hours.
Regardless of Basin Type
Configuration between
inlet and outlet should
maximize the length of
flow path
Minimum length-to-width
ratio = 3:1
Baffle can be used to
lengthen flow path
Use an anti-seep collar!
If discharge is to a wetland
Must provide a level
spreader to dissipate flow
Level Spreader
Regardless of BMP Type
Must have a long-term maintenance plan (LTM)
This is part and parcel of SWPPP, but LTM must be stand-alone
Required content:
Party responsible to conduct inspection and maintenance
Routine and non-routine maintenance tasks and schedule
Legally-binding maintenance easements or agreements, if needed
Map showing location of access and maintenance easements
Suggested content:
Site map with BMPs labeled and type identified
Checklist by which to conduct inspections
Detail drawings of BMPs and outlet structures
Planting plans/photos for bioretention facilities
For More Information
Websites Ohio EPA www.epa.ohio.gov/dsw/storm/index.aspx
US EPA www.epa.gov/npdes/stormwater/construction
NEDO Ohio EPA Contacts
Cuyahoga, Geauga, Lake & Lorain
Dan Bogoevski
(330) 963-1145
Medina, Summit, Portage & Wayne
Molly Drinkuth
(330) 963-1215
Ashtabula, Columbiana, Mahoning, Trumbull &
Holmes
Chris Moody
(330) 963-1118
Stark & Carroll
Phil Rhodes
(330) 963-1136