choosing the best management practices three years later
TRANSCRIPT
Choosing the Best Management Practices Three Years Later
Presented by: Erin Pande, ERA and Jennifer Boyer, DuPage County
Beyond the Basics
September 29, 2011
Each municipality in DuPage County is required to incorporate Best Management Practices
(BMPs) into their regulatory programs under NPDES Phase II.
Lets be specific
Leaves less to interpretation
Avoids confusion over what might be
approved
Sizing should account for site drainage area
Percent impervious Bypass provision is required
Time of concentration
Design event
Developments requiring a Stormwater Management Permit …shall treat the developed area for pollutants from the site in accordance with accepted practices and procedures…(see BMP Manual).
BMP Regulation
Development Proposal
Regulations
Site Variables
• BMPs lend themselves to this approach.
• Development lends itself to this approach
The selection guide is a guide, not regulation.
If it isn’t fitting with the pollutants of concern, don’t be afraid to set it aside in favor of a more
practical solution.
*
Pollutants of concern listed by importance for a commercial use
Total Suspended Solids
Metals/Hydrocarbons
Nutrients
BMP Selection Guide
Sample BMPs Value Pollutant Limitation
Vegetated Swale, native 2 Metals/Oil
Permeable Pavers 3 None
Dry Detention 1 Oil
Wet Bottom Detention 2 Oil
Wetland Detention (no open water) 2 None
Constructed Wetland (CWD) 3 None
Manufactured Varies Varies
Importance Value (BMP Effectiveness) 1 = Low 2 = Moderate 3 = High
BMPs should …average or exceed a 2.5 average importance value.
*Excerpt example from large scale Single Family Residential
Example – Existing Commercial Development
– Existing native, wet bottom detention pond is in good condition and already treats TSS and Nutrients (Score: 2)
– Additional parking needed
Example solution
– Designer may use an oil separator, such as a snout product that is rated for oil removal (Score: 2)
The BMP plan does not meet the required score of 2.5 in the selection guide, but it does address the pollutants of concern by using a treatment train.
New data
BMP Manual Guidance
2-year 24 hour event 3.04 inches
Proposed Guidance
2 inch rain event
80% of runoff is from storms which are 2 inches or less 80% of the sediment load is from storms 1.78 inches or smaller*
*Based on Wisconsin Data
Rain Gardens
Should rain gardens be included in the BMP Manual?
• Not practical
• Too small
• Too difficult to place easements
• Too difficult to protect
• BMPs must treat 100% of developed area
• Pollutants of Concern – TSS &
Hydrocarbons from Parking lot
– Nutrients from building & landscape
Example -Commercial > 1 acre
• Roof and landscaped area tributary to off-site wet bottom basin
• Pollutants from this area are nutrients
• Pollutant removal limitations for wet bottom basins are hydrocarbons
• Hydrocarbons are not anticipated therefore an importance value of 2 may be assigned
Example -Commercial > 1 acre
• Pollutants of concern for parking lot are TSS & Hydrocarbons
• Rain gardens are equivalent in design to “vegetated swale native with under drain” and have no pollutant treatment limitations.
Example -Commercial > 1 acre
BMPs Just Won’t Fit
• Development scenario
– Too many constraints on a small site
• Example solution
– Demonstrate how a BMP will not work on the site and pay in fee-in-lieu
Example – Commercial > 1 Acre (Fee-in-lieu)
• Existing development with building, driveway, loading dock, and gravel pad
• Special Management Areas – Wetland – Floodplain
Example – Commercial > 1 Acre (Fee-in-lieu)
• Design for expected flow using the entire area tributary to your BMP for the design event.
• BMPs which are sized incorrectly will become overwhelmed and less effective in the design event.
Example – Commercial > 1 Acre (Fee-in-lieu)
• Area east of the building that is proposed to be paved is 8,700 sf. • The developed area west existing building is 5,088 sf. • The total tributary area to the rain garden west of the building would
be 19,914 sf. • Rain garden would treat 6,126 sf more than the developed area.
• As the rain garden is combined with the compensatory storage the ponding depth would only be 1" deep and overflow.
• To prevent overflow an – underdrain is
needed, or – Ponding depth needs
to be increased
Example – Commercial > 1 Acre (Fee-in-lieu)
• If an underdrain were added it would have to connect to the adjacent ditch with associated floodplain, fill with water first and displace the soil layer.
• If the ponding depth was increased it would allow the bottom of the rain garden to infiltrate into the silty sand/gravel layer
Example – Commercial > 1 Acre (Fee-in-lieu)
• Seasonally high groundwater observed at 5 ft depth.
• If ponding depth was increased the bottom of the rain garden would infiltrate into the silty sand/gravel layer
• Groundwater may intercept may lead to ground water contamination.
Example – Commercial > 1 Acre (Fee-in-lieu)
Topo doesn’t work
• Development scenario
– Driveway aprons run to the street, not to the BMP
• Example solution
– Swap apron area for flow from an existing paved surface outside of the developed area
Example -Single Family Residence > 3 acre
• BMPs must treat 100% of developed area
• Pollutants of Concern – Nutrients from
building & landscape
– TSS & Hydrocarbons from driveway
Example -Single Family Residence > 3 acre
• Vegetated swale native with check dam BMP proposed
• No pollutant treatment limitations
• "Traded Areas" concept was used with this BMP design – 3,564 sf are not
tributary to BMP
– 6,235 sf of additional off-site upstream area treated
Example -Single Family Residence > 3 acre
Size to treat:
2”, 24 hour event at velocity < 1 ft/sec
10 yr, 24 hour event at velocity < 7 ft/sec
Ways to reduce velocity:
Reduce channel width
Increase roughness
Decrease slope
Add check dams
2", 24 HR STORM BMP SWALE CALCULATION
CALCULATION OF RUNOFF BASED ON PROPOSED CONDITIONS:
TR-55 METHOD
SOIL TYPE: CN
CN (PERVIOUS): 74
CN (IMPERVIOUS): 98
PROPOSED FLOW ANALYSIS THROUGH SWALE (BMP SWALE):
TRIBUTARY AREA: 0.358 ACRES
IMPERVIOUS AREA: 0.164 ACRES
COMPOSITE CN 85
PRECIPITATION (24 HR, 2 YEAR EVENT) 2 INCHES
RUNOFF (FROM TR-55 FIG. 2-1) 1.60 INCHES
RUNOFF VOLUME 0.05 ACRE-FT
LONG FLOW PATH 180 FT
Tc (TR-55 Calculation (See Spreadsheet)) 0.23 HOURS
P2 = 2.0 INCHES n = 0.24
Ia = 0.353 Ia/P = 0.18
Qu (FROM TR-55 EXHIBIT 4-II) 745.00 CSM/IN
Qp (PEAK DISCHARGE, Qu*AREA*RUNOFF) 0.67 CFS
CAPACITY ANALYSIS OF SWALE (BMP SWALE):
AVERAGE SLOPE OF SWALE 0.0400
MANNING'S n 0.24
FLOW DEPTH, d 1.00 FEET
BOTTOM WIDTH, b 2.00 FEET
Z 8.0
FLOW AREA 10.00
WETTED PERIMETER 18.12
HYDRAULIC RADIUS 0.55
SWALE CAPACITY (USING MANNING'S FORMULA) 8.35 CFS
Velocity 0.8 ft/s
SUMMARY:
PROPOSED SWALE CAPACITY 8.35 CFS
PROPOSED PEAK DISCHARGE THROUGH SWALE 0.67 CFS
DOES ENOUGH CAPACITY EXIST? YES
Single Family Homes
• Exempt from new BMP regulations <1 acre
• Must provide BMPs
• May elect to pay into an in lieu fee program 1-3 acres
• Must provide BMPs > 3 acres
Pretreatment
Can I place my manufactured structure downstream of my detention pond?
Not recommended. The idea of a pretreatment structure is to prevent the oils and metals from getting into the basin where they may infiltrate and contaminate groundwater or cause basin sediments to be classified as special hazardous waste.
Pretreatment
Examples of pretreatment practices: •Manufactured structure placed upstream of the basin or BMP •Detention basin forebay
Regular cleanout of a pre-treatment device is easier and may extend the life of your BMPs and detention basins.
• BMPs must treat 100% of developed area
• Pollutants of Concern
– TSS & nutrients from landscape & exhibits
Example -Commercial > 1 acre
738.45
738.5
738.55
738.6
738.65
738.7
738.75
738.8
738.85
0 10 20 30 40
Duration (hrs)
Sta
ge (
ft)
Series1
Example -Commercial > 1 acre
738.45
738.5
738.55
738.6
738.65
738.7
738.75
738.8
738.85
738.9
738.95
739
0 10 20 30 40 50
Duration (hrs)
Sta
ge (
ft)
Series1
738
738.5
739
739.5
740
740.5
741
0 10 20 30 40 50 60
Duration (hrs)
Sta
ge (
ft)
Series1
.5 Year
1 Year 100 Year
• Inundation/Duration – determine the depth and length of time water will be present in the basin to design plantings appropriately. Provide the .5, 1, 2, 5, 10 & 100 year, 24 hour storm events.
New development? An addition? No Problem!
IF…
• The basin treats the pollutants of concern for the new development
and
• The basin meets the design standards in the BMP manual
and
• The basin is in good condition
Good News
The Morton Arboretum
Those who do are getting signoff by the end of their monitoring period. Some are saving money and time by achieving early signoff.
More developers today are taking basin and BMP maintenance seriously from the start of construction.
Exemptions
Development is limited to:
• Parking lot maintenance
• Trail/bike path
• Regional stormwater or flood control project
• Projects that are their
own BMP (stream bank stabilization,
mitigation, etc.)
• BMPs must treat 100% of developed area
• Pollutants of Concern
– TSS & Hydrocarbons from roadway
Example - Roadway
*Roadway reconstruction may be considered maintenance if in same footprint & does not alter volume, release rate, direction of runoff, etc.
• Size to treat:
– 2”, 24 hour event at velocity < 1 ft/sec
– 10 yr, 24 hour event at velocity < 7 ft/sec
Example - Roadway
2 “ BMP SWALE CALCULATION
CALCULATION OF RUNOFF BASED ON PROPOSED CONDITIONS:
TR-55 METHOD
SOIL TYPE: C
CN (PERVIOUS): 74
CN (IMPERVIOUS): 98
PROPOSED FLOW ANALYSIS THROUGH SWALE (BMP SWALE):
TRIBUTARY AREA: 1.300 ACRES
IMPERVIOUS AREA: 0.920 ACRES
COMPOSITE CN 91
PRECIPITATION (2" 24 HR EVENT) 2 INCHES
RUNOFF (FROM TR-55 FIG. 2-1) 2.11 INCHES
RUNOFF VOLUME 0.23 ACRE-FT
LONG FLOW PATH
use Tc from
structure 112
Tc (SHEET FLOW, TR-55 EQ. 3-3) 0.33 HOURS
P2 = 2.0 INCHES n = 0.24
Ia = 0.198 Ia/P = 0.10
Qu (FROM TR-55 EXHIBIT 4-II) 750.00 CSM/IN
Qp (PEAK DISCHARGE, Qu*AREA*RUNOFF) 3.22 CFS
CAPACITY ANALYSIS OF SWALE (BMP SWALE):
AVERAGE SLOPE OF SWALE 0.0100
MANNING'S n 0.13
FLOW DEPTH, d 1.00 FEET
BOTTOM WIDTH, b 2.00 FEET
Z 4.0
FLOW AREA 6.00
WETTED PERIMETER 10.25
HYDRAULIC RADIUS 0.59
SWALE CAPACITY (USING MANNING'S FORMULA) 4.81 CFS
Velocity 0.8 ft/s
SUMMARY:
PROPOSED SWALE CAPACITY 4.81 CFS
PROPOSED PEAK DISCHARGE THROUGH SWALE 3.22 CFS
DOES ENOUGH CAPACITY EXIST? YES
Artificial turf fields
Can double as a BMP IF…
• 80% of OK110-sized sediment is being trapped in the 2 inch event
• Sediment will not be re-suspended
• May accept sediment from
off the field
Grading filter strips can be tricky
• Grading a filter strip may cause soils to compact, settle, develop rills and concentrate flows.
• Runoff must sheet flow evenly and slowly through the filter strip
• Ideally, place in ungraded soil where topo conditions are already just right.
Healthy, diverse buffers are super
Protects stream and
wetland ecosystems
Filter some pollutants
Provide habitat
Reduce runoff volume,
velocity, and temperature
Why is this important?
Diverse ecosystems are better at everything
• Greater plant diversity supports diverse soil fauna
• Different kinds of plants and organisms assimilate different kinds of pollutants
• Tend to reduce runoff volume more efficiently through evapotranspiration and infiltration
Reduced number
of species
Frequent fluctuations
Warm, polluted runoff
High velocity of
flow
Reduced effectiveness at removing pollutants
• 46,539 sf development west of Spring Brook not tributary to the BMP.
• “Traded Areas" concept used, as the proposed BMP treats additional undeveloped areas.
Example - Open Lands/Parks
Questions?
www.eraconsultants.com
www.dupageco.org/stormwater