choosing the best management practices three years later

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

Development is evolving.

The field of water quality is evolving.

From Cookbook to Real Life

Regulation is

simple, but broad

Lots of guidance in the BMP Manual X

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

Lessons Learned

Flexibility can be scary

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.

Three years later…so what happened?

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


• 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

Tips and Tricks

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


• 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.


• 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


• 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


• 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.


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


• Pollutants of Concern – Nutrients from

building & landscape

– TSS & Hydrocarbons from driveway


• 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


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.


• Pollutants of Concern

– TSS & nutrients from landscape & exhibits


• Constructed Wetland Detention


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


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.

• Forebay

• Micropool


For the sake of future development, keep that

basin maintained

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

Vegetated basins should be maintained to remove weeds and encourage deep rooted

vegetation.

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.)


• 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.

Example - Roadway

• 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.

Can BMPs and Special Management Areas

Coexist?

Answer: a qualified “yes”.

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

Can BMPs and Special Management Areas Coexist?

• 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

Example - Open Lands/Parks

Questions?

www.eraconsultants.com

www.dupageco.org/stormwater

http://www.eraconsultants.com/

http://www.dupageco.org/stormwater

choosing the best management practices three years later

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