Advanced Stormwater Design Webcast Series

Stormwater Wetlands Reconsidered

Photo Credit: South River Federation

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Speaker Info

Albert McCullough, Sustainable Science, LLC, albert@sustainablescience.com

Dave Hirschman, Center for Watershed Protection, djh@cwp.org

Tom Schueler, Chesapeake Stormwater Network, watershedguy@hotmail.com

Cecilia Lane, Chesapeake Stormwater Network, watershedgal@hotmail.com

To learn how you can have access to: Discounted Webcasts

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Visit: www.chesapeakestormwater.net

Chesapeake Bay Stormwater Training Partnership

CSN’s 2014 Webcast Series

No. Date Series Topic

10 June 12 Advanced Stormwater Design Rainwater Harvesting

11 June 26 Advanced Stormwater Design Filter Strips & Disconnections

http://chesapeakestormwater.net/events/categories/2014-webcast-series/

Webcast Agenda

• Evolution of the Practice

• Why a Seldom Used Practice is Poised for a Resurgence

• Constructed Wetland Myths

• New Constructed Wetland Applications

• Feasibility and Design Issues

• Construction and Maintenance

• Establishing the Wetland Plant Community

Poll Question #1

How many people are watching with you today?

• Just me

• 2-5 people

• 6-10 people

• > 10 people

Poll Question #2

Tell us a little about yourselves…who are you representing today?

• Design professional

• MS4 Phase 1

• MS4 Phase 2

• State government

• Federal government

• Other

Poll Question #3

Have you designed a constructed wetland in the last year ?

• Yes

• No

• No, although I did fill in a natural wetland (just kidding, we hope! )

Evolution of the Practice

Copyright 2000, Center for Watershed Protection

The First Generation of Stormwater Wetlands

The Second Generation: Better diversity but still the freshwater emergent model

The Third Generation: The Forested Wetland

0 100 200 300 400 500 600 700 800 900 1000

Green roof

Constructed wetland

Sand filters

Submerged gravel wetlands

Wet ponds

Rainwater harvesting

Compost soil amendements

Extended detention ponds

Permeable pavement

Filter strips

Rain gardens

Dry swale

Infiltration practices

Grass swale/channel

Impervious disconnection

Bioretention

Estimated # of Practices Implemented in the Last Year

Chesapeake Bay Urban BMP Implementation Survey Aggregate Reported BMPs Lasy Year by 240+ Survey Respondents

CSN, 2014

Not a Lot of Constructed Wetlands Being Built These Days

Why are so few constructed wetlands being built ?

• Use discouraged since they are not classified as a ESD, LID or Runoff Reduction Practice in new state stormwater manuals

• Available land (ponds easier)

• Concerns about nuisances (mosquitoes etc) and wetland regulatory status

• Lack of understanding about “vegetation”

• Other Factors ?

Why they may be poised for a resurgence

• Potential for greater nutrient reduction

• Cost effective approach for meeting water quality and peak discharge together

• New design applications

• Strong retrofit potential

• Great solution in the coastal plain

• Promote habitat and ecosystem services

• Lower long-term maintenance efforts

Recent Research on Natural and Constructed Wetlands Informs Design

• Effect of stormwater runoff in degrading of natural wetlands (bounce)

• Most natural wetlands are forested

• While nutrient removal in constructed wetlands is variable, designers have not tried enhancing denitrification (e.g., add carbon and anoxic conditions.

• Have not really measured runoff reduction by constructed wetlands

Debunking Myths About Constructed Wetlands

Myth No. 1 Stormwater Wetlands Breed Mosquitoes

• Not generally a problem unless cattails are present

• Scatter deep pools around wetland and connect them with channels

• Dragonflies

• Other LID practices pose greater risk

Myth No. 2 Stormwater Wetlands Need to Be Deep

to Prevent Them From Drying Out

• Once plant community is initially established, stormwater wetlands will recover in the next growing season even after extended droughts

Myth No. 3 What we plant will persist over time

• Experience has shown that many of the species that are initially planted will not persist over time, due to competition, growing conditions, herbivory, and the arrival of volunteer and/or invasive species

Myth No. 4 Natural wetlands have a lot of standing

water

• Most wetlands found in nature have saturated soils and are only inundated seasonally or after extended wet periods

• Plant diversity declines with increasing ponding depth and water bounce

• Many of the ecological services in the wetlands are due to groundwater interactions

Myth No. 5 Constructed wetlands eventually

become a jurisdictional wetlands which are regulated by the Corps

• The policy for decades has been that they are considered a treatment wetland and NOT a jurisdictional wetland

• The only exception would be for “abandoned” facilities that are no longer maintained for their stormwater function

New Constructed Wetland Applications

• Wooded Wetlands

• Pond Wetland Combo

• Submerged Gravel Wetlands

• Dry Pond Retrofits

• Tax Ditch Restoration

• Linear Wetland

• Regenerative Conveyance Systems

Wooded Wetland

Pond Wetland Design

Pond Wetland Combination

• Space-saver for denser development sites.

• Side by side pond and wetland.

• On-line pond and off-line wetland.

• Wetland has 4 to 6 cells that step down a foot of elevation each.

• Pond bleeds water into wetland during dry weather.

• Pond has 70% of total treatment volume.

Submerged Gravel Wetlands

Photos Courtesy of Univ. of New Hampshire Stormwater Center

Submerged Gravel Wetland

Photo & image: Univ. of New Hampshire Stormwater Center

DRY POND CONVERSION RETROFIT

DRY POND

CONSTRUCTED WETLAND

Coastal Plain Tax Ditch Restoration

Linear Wetlands

Linear Wet Swale

Feasibility and Design Issues

1. Level 1 and 2 Design

2. Water Balance

3. Geometry/Flow Path

4. Maintenance Reduction

1. Level 1 and 2 Wetland Design

Level 1 Level 2

Good design to provide treatment, safety, functionality

Includes design enhancements for runoff reduction and/or pollutant removal: • Size, treatment volume • Flow path/multiple cells/tree

peninsulas • Limited “bounce” above pool

VA Constructed Wetland Performance Credit

35

Level 1 Level 2

Runoff Reduction = 0% TP Reduction = 50%

Runoff Reduction = 0% TP Reduction = 75%

NOTE: Many states do NOT have Level 1 and 2 designs because constructed wetlands are not considered a runoff reduction practice for volume reduction criteria.

Constructed Wetland Performance Credit

Maximize Performance & Optimize Sizing: Provide Runoff Reduction Upstream From

Constructed Wetland

Level 1 Wetland Design (VA)

• Single basin with forebay & micropool

• Some storage allowed above pool

• Surface area < 3% of contributing drainage area

• Length/Width Ration ≥ 2:1

• Emergent plant community

Different Levels of BMP Design

Level 1: Sizing • Treatment Volume (Tv) = Rv x A

• ≥ 50% in pool + forebay + micropool

• ≤ 50% above pool

• Detention storage can also be above pool, as long as “bounce” ≤ 12” for 1-inch rainfall & 1-year storm

State-Specific Sizing Methods May Apply

Different Levels of BMP Design

Level 2: Multi-Cell or Pond Wetland

Different Levels of BMP Design

Level 2: Multi-Cell or Pond Wetland

• Multiple cells or pond + wetland cells;

• Surface area > 3% of contributing DA;

• Length/Width Ratio ≥ 3:1

Different Levels of BMP Design

Level 2: Multi-Cell or Pond Wetland

• No ED or detention storage above pool;

• Add microtopography features;

• Emergent + wooded wetland features.

Minimal Water Level Fluctuation (‘bounce’) = Better Plant Community

2. Water Balance Considerations

• Drought rainfall

• Rainfall/runoff that enters wetland from CDA

• Wetland surface area

• Summer ET

• Infiltration loss

• Maintain small pool of water as safety factor (e.g. 6”)

Water Balance Equation

Bill Hunt, NCSU:

DP = RFm * EF * WS/WL – ET – INF – RES

Where: DP = Depth of pool (inches)

RFm = Monthly rainfall during drought (inches)

EF = Fraction of rainfall entering wetland (CDA Rv)

WS/WL = Ratio of contributing DA to wetland Surface Area

ET = Summer evapotranspiration rate (inches; assume 8 inches)

INF = Monthly infiltration loss (assume 7.2 inches @ 0.01 inch/hour)

RES = Reservoir of water for a factor of safety (assume 6 inches)

Example Calculation

With Basic “Worst-Case” Assumptions:

DP = RFm * EF * WS/WL – ET – INF – RES

• RFm = Rainfall during drought = 0”

• EF = Fraction of rainfall entering wetland = (CDA Rv) = 10*0.45 = 4.5

• WS/WL = Ratio of DA to SA = 10 / 0.3 = 33.3 • ET = summer evapotranspiration = 8” • INF = Summer Monthly infiltration loss = 7.2” • Reservoir of water safety factor = 6”

DP = (0” * 8” * 33.3) – 8” – 7.2” – 6” = 21.2 = 22”

Minimum Pool Depth = 22 inches

• Minimum pool may be less if summer/drought groundwater/ baseflow can be documented

Beware of Pocket Wetlands !

Key Design Considerations

Wetland Without the “Wet”

Adjustable Weir/Flashboard

You Don’t Always Know How Water Balance Will Work Out. . .

Plan for Adaptability

3. Geometry and Flow Path

Length to Width Ratio

Outlet

Structure

L

Inlet

Structure W

Length/Width Ratio = L/W, where:

L = Length from inlet to outlet

W = Average width

Outlet

Structure

L

Inlet

Structure W

Outlet

Structure

L

Inlet

Structure W

Length/Width Ratio = L/W, where:

L = Length from inlet to outlet

W = Average width

Inlet

Outlet

L/W = 3.4:1

Seek Longer Flow Path

Shortest Flow Path Criteria

Outlet

Structure

Curb Inlets

L

SFP

Shortest Flow Path Ratio = SFP/L

SFP = Length of shortest flow path from an inlet to outlet

L = Overall length of facility

Outlet

Structure

Curb Inlets

L

SFP

Outlet

Structure

Curb Inlets

L

SFP

Shortest Flow Path Ratio = SFP/L

SFP = Length of shortest flow path from an inlet to outlet

L = Overall length of facility

Virginia Wetland Geometry Criteria

L/W Ratio Shortest Flow Path Ratio

Level 1 = 2.0

Level 2 = 3.0 or multi-cell

Level 1 = 0.5

Level 2 = 0.8

Internal Micro-topography

Specify at least two mechanisms to create better microtopography

• Snags

• Inverted rootwads

• Gravel layers

• Cobble sand weirs

• Coir fiber logs

• Scattered pools

• Peninsulas

High Marsh/Tree Peninsulas

Multiple Cells: forebay, wetland cells

Photo: Triangle Park Stormwater Treatment Wet Swale, Town of Rising Sun, MD

Photo: Triangle Park Stormwater Treatment Wet Swale, Town of Rising Sun, MD

Imbricated Rock Weir

4. Maintenance Reduction Features:

• Access for maintenance

• Sediment forebay

• Safety benches

• Preventing clogging at riser/weir outlet

Make it Easy to Get to it !

Fencing and Slope Considerations

Don’t worry homeowners, here we come!

Place riser in the embankment instead of open water.

Clogging Reduction

• Low flow orifice ≥ 3” (although that alone is not sufficient)

• Internal orifice plates within pipe

• Trash racks, half-round CMP, reverse slope pipe into micro-pool

Pool Outflow Pipe/Drain:

Make sure the “plumbing can be accessed and is in operable condition.

Construction and Maintenance

Tips

Construction

Critical Points • Ready to Install?

– Conversion from ESC

• Secondary ESC Measures – divert flow around

wetland area

•Embankments & Outlets

•Internal berms/weirs

•Micro-topographic features

•Stabilize exposed areas

Stage 1 Inspection:

Construction

Critical Points • Connect drainage area • Measure & stake planting depth

zones • Planting (April – June) – order

plants months in advance – Container grown or plugs – Wetland seed mixes – Volunteers (remaining areas)

• Goose Protection • Plant wetland fringe & buffer • Check inundation zones/status

Stage 2 Inspection:

Construction

• Dewater

• Dredge

• Regrade to rough design elevations

Conversions From ESC Facilities

Construction

• Orange fence along perimeter

• Criss-cross web of white string over plants

• Keep in place until plants are big enough to not be enticing to geese

Source: City of Waynesboro, VA

After Planting: Protect Plants from Geese Predation

Wetland Maintenance

CWP (2009)

General Performance Problems with Constructed Stormwater

Wetland (n=22)

68%

23%

18%

14%

5%

5%

5%

0%

0%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Vegetation/Habitat

Hydrology

Maintenance

Stability

Aesthetics/Placement

Consistency with Design

Grading/Elevations

Treatment Effectiveness

Structural

Performance Issues in the Field

Initial Maintenance

First 1 – 2 Years:

• Spot reseeding

• Watering (trees, other plants as needed)

• Reinforcement plantings

• Develop early strategy for invasives (e.g., semi-annual removal)

Long Term Maintenance

Routine • Monitor plant community

• Inlets, embankments, berms, spillways, slopes, etc.

Non-Routine • Invasives control (when invasive cover 15% of

wetland cell)

• Thinning/harvesting of woody growth (except wooded wetland features)

• Sediment removal from forebays, cells

Establishing the Wetland Plant Community

Establishing the Wetland Plant Community

• Wetlands are similar to us – Defined by where they

live

– Evolve to perform specific work

– Associate with those they like

– Tolerate life stresses

Stormwater Wetland Vegetation Success Metrics

• Should be similar to nearby natural wetland systems

• Maximize water contact with vegetation for best water quality enhancement

• Formed of species that naturally associate together

• Hydrology design behavior matched to the wetland type

Long, Meandering Flow Path A Worthy Goal Indeed!

Reference Wetland Systems

• Identifies what types of plant species & arrangements

• Provides acceptable hydrology ranges

• Information can be obtained from local wetland delineators and regional wetland plant references

Adjusting Water Flow and Availability

• Design should allow for easy water adjustment

• Maintenance activities include: – Clearing and repairing water control and

conveyance devices

– Removing animal structures and repair damage from tunneling

– Conveying excess water

– Installing stone edging in erosive settings

– Adjusting existing structures

Planting Plan for Depth Zones:

Depth Zones (inches):

1. -6 to -12 below normal pool

2. -6 to normal pool

3. Normal pool to +12

4. +12 to +36 above normal pool

See State Manuals for Plant Lists; Commercially-Available Natives Preferred

Detailed Planting Plan

Plant Schedule & Details

Sustaining Vegetation

• Maintenance activities include: – Watering or irrigating

– Controlling diseases and infestations

– Eliminating invasive species

– Removing wracks, algal mats and other litter

– Replanting and reseeding

– Using animal management techniques

Top 10 Causes of Vegetation Failure

• Unsuitable planting substrate

• Too wet or too dry • Salt buildup in soils • Incorrect elevations and

incorrect species • Poorly graded site • Non-viable seed or poor

seeding techniques • Burning of plants by

fertilizer • Plant float out • Herbivory • Insects & disease

Invasive Plants: A good offense is the best defense

• Make sure planned hydrology does not exceed the planned wetland vegetation community tolerance.

• Select plant material based on nearby natural wetlands and the proposed site conditions.

• Design the wetland planting scheme to coalesce vegetation in one year will provide resistance to invasive encroachment.

• A semiannual monitoring and maintenance plan that includes invasive removal is essential for success.

Wildlife Management Techniques

• Population reductions

• Use of tree guards and fences

• Animal repellents

• Tree peninsulas

• Tree islands

• Wetland buffers above ED zone

Trees (Wetter is Not Better!)

Wetland Vegetation Worthy Life Goals

• Project sponsors should anticipate both expected and unexpected

• Design should strive for self-maintenance

• Maintenance typically required for 1 to 2 years after completion of construction until fully grown in

• Monitoring is needed to identify problems early before becoming expensive to solve

Questions and Comments

EXISTING RETROFITS

BMP CONVERSION

DRY POND

CONSTRUCTED WETLAND

Info From Stormwater Plan/Report

• D.A. = 30.8 acres • Managed Turf = 15.1 acres • Impervious = 15.7 acres • CN = 80 • WQV = 55,560 cubic feet

Incremental Credit

BMP Conversion

• Existing ED Pond (Level 1?) – use VA RR rates

• Convert to Constructed Wetland

BMP Enhancement

• Existing ED Pond (Level 1) – use VA RR rates

• Enhance to Level 2 ED Pond w/ wetland features

Results

TP TN

Load From D.A. (lbs) 43.19 319.95

Existing ED Pond Efficiency (Expert Panel, Level 1)

15% 10%

Existing Load Reduction (lbs) 6.48 32.00

BMP Conversion To Constructed Wetland

Efficiency for Constructed Wetland (ST) 55% 35%

Load Reduction From Curve 18.61 87.73

Delta 12.13 55.73

BMP Enhancement: Wetland Cells, Flowpath

Load Reduction for Level 2 ED Pond 31% 24%

Load Reduction 13.39 76.79

Delta 6.91 44.79

• Phosphorus Removal = 14 pounds • $ = $200K • Cost Effectiveness = $14,000/pound

Design Example for

New/Redevelopment Situations

Design Example

• 15 acres, 25 ¼ acre lots

• Impervious = 3.15 ac. (lots + roads)

• Managed Turf = 3.44 ac.

• Open Space = 6.59 ac.

Design Example

Runoff Reduction Spreadsheet (Virginia)

• Site Treatment Volume (Tv) = 14,831 cubic feet

• TP Load = 9.32 lbs/acre/year

• TN Load = 66.66 lbs/acre/year

• VA TP Reduction to Meet 0.41 lbs/acre/year = 3.17 lbs

102

Design Example

TvBMP Constructed Wetland = 13,610 cf Runoff Reduction = 0

Design Scenario 1: Just Constructed Wetland

Design Example

Dry Swale #1 with additional 1.0 ac. Impervious

Design Scenario 2: Constructed Wetland with Upstream Runoff Reduction

Constructed Wetland (with additional 1.58 ac. Impervious + 3.44 ac. turf)

0.57 ac. Simple Disconnection to amended filter path

Design Example

Wetland TvBMP from immediate DA = 8,195 ft3

Tv remaining from upstream RR BMPs = 2,659 ft3

Wetland Design TvBMP = 10,854 ft3

Volume Reduced by upstream RR = 2,756 ft3

Scenario 2: Constructed Wetland with Upstream Runoff Reduction

Where To Find Design Resources: Virginia

• Laws, Regulations, Permits: Virginia Dept. of Environmental Quality, Stormwater: http://www.deq.virginia.gov/Programs/Water/StormwaterManagement.aspx

• BMP Specifications: Virginia Stormwater BMP Clearinghouse, Non-Proprietary (Constructed Wetland = Specification #13): http://vwrrc.vt.edu/swc/NonProprietaryBMPs.html

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: West Virginia

• Laws, Regulations, Permits: West Virginia Dept. of Environmental Protection: http://www.dep.wv.gov/WWE/Programs/stormwater/Pages/sw_home.aspx

• BMP Specifications: West Virginia Stormwater Management & Design Guidance Manual (Constructed Wetland = Specification 4.2.11): http://www.dep.wv.gov/WWE/Programs/stormwater/MS4/Pages/StormwaterManagementDesignandGuidanceManual.aspx

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: D.C.

• Stormwater Rule: District Department of the Environment: http://ddoe.dc.gov/node/610592

• BMP Specifications: D.C. Stormwater Management Guidebook (Wetlands = Specification 3.11): http://ddoe.dc.gov/node/610622

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: Delaware

• Laws, Regulations, Permits: Department of Natural Resources & Environmental Control: http://www.dnrec.delaware.gov/swc/pages/sedimentstormwater.aspx

• BMP Specifications: Sediment & Stormwater Technical Document (Constructed Wetland = Appendix 3.06.2.12): http://www.dnrec.delaware.gov/swc/Drainage/Pages/Technical_document.aspx

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: Maryland

• Laws, Regulations, Permits: Department of the Environment: http://www.mde.maryland.gov/programs/Water/StormwaterManagementProgram/SedimentandStormwaterHome/Pages/Programs/WaterPrograms/SedimentandStormwater/home/index.aspx

• BMP Specifications: Maryland Stormwater Design Manual (Constructed Wetland = Section 3.2): http://www.mde.maryland.gov/programs/Water/StormwaterManagementProgram/MarylandStormwaterDesignManual/Pages/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.aspx

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: Pennsylvania

• Laws, Regulations, Permits: PA Department of Environmental Protection: http://www.portal.state.pa.us/portal/server.pt/community/stormwater_management/21377

• BMP Specifications: PA Stormwater Best Management Practices Manual (Chapter 6.6.1, Constructed Wetland): http://www.portal.state.pa.us/portal/server.pt/community/best_management_practices_manual/21383

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Where To Find Design Resources: New York

• Laws, Regulations, Permits: NY Department of Environmental Conservation: http://www.dec.ny.gov/chemical/8468.html

• BMP Specifications: NY State Stormwater Management Design Manual: http://www.dec.ny.gov/chemical/29072.html

• Design Guidance for Karst, Coastal, Urban/Redevelopment: Chesapeake Stormwater Network, Technical Bulletins: http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/

Webcast Resources • Virginia’s Stormwater Design Specification No 13: Constructed

Wetlands

• BMP Landscaping & Plant Lists (Appendix F from the WV Stormwater Manual) – Most of the different Bay State Stormwater manuals provide plant lists

• North Creek Nursery’s Landscape Plug Manual – can be purchased here: – http://www.northcreeknurseries.com/index.cfm/fuseaction/resources.li

nks/cat_id/15/index.htm

• Subsurface Gravel Wetland Design Specifications. UNH, 2009

• Stormwater BMPs in Virginia's James River Basin: An assessment of field conditions and programs. CWP, 2009

www.chesapeakestormwater.net

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