Reviewing and Inspecting Non-Structural ESD Stormwater Plans in

Maryland

Tom Schueler

Chesapeake Stormwater Network

andEPA Chesapeake Bay Program

Stormwater Coordinator

Key Topics

• Non-Structural ESD in MD • Maintaining ESD Practices• A Bioretention Case Study • A Note About the Bay TMDL

Linking the Different

Stormwater Communities

Togetherin a

Collaborative NetworkRegulato

rs

Environment

Advocates

Researcher

Designers

Reviewers

MS4Manager

s

CSN

New CSN Initiatives

• Tech Bulletin # 5 Stormwater Design for High Intensity Redevelopment Projects

• Tech Bulletin # 8 The Clipping Point

• Tech Bulletin # 9 Local Nutrient Reduction Accounting

• Tech Bulletin # 10 LID Maintenance

Non-Structural Environmental Site

Design

Tom Schueler Chesapeake Stormwater Networkwww.chesapeakestormwater.net

Step 2: Calculate Site Imperviousness and Water Quality Volume, WQv

Site Area, A (acres) 4Existing Impervious Surface Area (acres) 2.5Proposed Impervious Surface Area (acres) 3

Existing Imperviousness, Ipre 62.5%Proposed Imperviousness, Ipost 75.0%

Development Category Redevelopment

Rainfall Depth, P (in) 1.0Runoff Coefficient, Rv 0.73Water Quality Volume, WQv (ac-in) 2.90Water Quality Volume, WQv (cf) 10,527

Step 4: Calculate Environmental Site Design (ESD) Rainfall Target, PE

% Soil Type A 0%  % Soil Type B 60%  % Soil Type C 40%  % Soil Type D 0%  

Pre-Developed Condition, RCNwoods 61

New DevelopmentSoil Type A ESD Rainfall Target, PE (in) 0.00Soil Type B ESD Rainfall Target, PE (in) 1.32Soil Type C ESD Rainfall Target, PE (in) 0.80Soil Type D ESD Rainfall Target, PE (in) 0.00

Site ESD Rainfall Target, PE (in) 1.80

ESD Runoff Depth, QE (in) 1.31ESD Runoff Volume, ESDv (cf) 18,949

Time to test the spreadsheet!

Environmental and Soil Features

Lots of opportunity to boost the hydrologic function of

urban turf through ESD Credits

Rooftop Disconnection

• Steeper slopes can be broken up with infiltration berms or site grading to meet these limits.

• The depth between the filter path surface and the seasonally high water table cannot be less than two feet in the coastal plain.

• The lateral distance between any two individual

disconnections must exceed 25 feet.•  The filter path shall have a minimum slope of 1%

and a maximum slope of 3%.

Non Roof Disconnect aka Grass Filter Strip

•A gravel diaphragm shall be installed at top of filter strip and an infiltration berm at the toe. •Heavy equipment must be kept out of the filter strip area during construction, unless the soils are restored.

•The depth between the filter path surface and the seasonally high water table cannot be less than two feet in the coastal plain.

Sheet flow to Conservation Area (CA)

Sheet Flow to Conservation Area Rules

• If runoff is directed to the Critical Area Buffer, a grass filter strip must extend at least 75 feet from the nearest contributing impervious cover.

• A grass filter strip with compost amended soils may be suitable to treat small areas of impervious cover, up to a maximum of 5000 square feet.

• The filter strip needs to be equipped with a gravel diaphragm, infiltration berm or engineered level spreader to spread flows.

• The water table must be at least 18 inches below surface the surface of the strip.

• Designers must perform a site reconnaissance to confirm topography, slope, and soil conditions prior to design.

• The boundary zone shall consist of ten feet of level grass, and have a maximum entrance slope of less than 3% in the first ten feet of filter.

• Long term reforestation plan that is capable of creating 75% forest canopy in 10 years

•Soil restoration is a required component of the reforestation credit.

•The planting plan must be approved by the appropriate local forestry or conservation authority, including any special site preparation needs

•After 10 years, the required density of native trees is 300 stems per acre. •Planting plans must include at least 5 different native tree species.

•Under urban conditions, planting plans should emphasize balled and burlapped native tree stock from 1 to 4 inches in diameter.

P Credit for Reforestation/Soil Restoration

•In rural or suburban settings, planting plans should include at least a minimum 10% of larger stock (1” caliper or more).

•The reforestation area must be protected by a perpetual stormwater easement or deed restriction which stipulates that it cannot be cleared unless it is fully mitigated.

•The construction contract should contain a care and replacement warranty extending at least 3 growing seasons.

• Control of invasive tree species should be a major part of the initial maintenance plan

•The reforestation area shall be shown on all construction drawings and erosion and sediment control plans during construction.

P Credit for Reforestation/Soil Restoration

Impervious Cover Conversion

A project is eligible for additional phosphorus removal credit for the pervious area if it is designed to provide further infiltration or bioretention.

The pervious area must be planted with an acceptable vegetative cover,

The conversion shall be permanent, and accompanied by a deed or covenant that specifies that the area cannot be rebuilt in the future (unless it is adequately mitigated).

The maintenance plan shall specify that the vegetative condition of the pervious area shall be regularly inspected and maintained to ensure no soil erosion occurs.

 

Impervious Cover Conversion

The minimum surface area for the impervious cover conversion credit is 250 square feet.

Site plans shall show the specific areas where concrete or asphalt will be removed and recycled.

Underlying compacted soils shall be deep tilled and amended with compost to restore porosity,

The new pervious area can be graded to accept runoff from adjacent hard surfaces.

Green Roof

Submerged Gravel Wetland

Rainwater Harvesting

Rainwater Harvesting

• A spreadsheet available to determine the ESD volume actually captured based on indoor and outdoor demand at www.chesapeakestormwater.net

• Designers should consult Bay-wide Design Specification No. 6 for Rainwater Harvestingg.

Grass Channels

At least its not a credit anymore!

Grass Channels

Not allowed for use on parking lots or rooftops The minimum width of the grass channel is 4 feet. The water table must be at least 12 inches below

the channel bottom. The grass channel must provide at least 10 minutes

of residence time for one inch storm event prior to any discharge to an inlet, pipe or stream.

One foot of restored soil along channel bottom is required for C and D soils and mass graded B soils.

Bio-Swales = Dry Swales

The Old Pond Maintenance Model

One big pond

The New ESD Maintenance Model

24 disconnections18 swale sections14 rain gardens5 bioretention areas4 tree planting areas6 sheet flow credits

The Changing Maintenance ParadigmConventional Practices ESD Practices

Example of Practice Pond Disconnects/rain garden

Number of practices?

A few at each site Dozens

Size of practices? Large drainage area Micro-drainage areaWhen to construct? During site construction After site is stabilizedWho is responsible? Homeowner association HomeownerWho does inspection?

Public sector engineer Trained contractor

Who does maintenance

Specialized contractor Landscape contractor

How long does it take?

Hour or more 10 minutes

What is the goal? Prevent dam failure andand public nuisances

Maintain hydrologic function and landscaping

Sediment cleanouts? On a 30 to 50 year cycle (if ever)

Annual cleanouts at pretreatment devices

Maintenance Triggers

After catastrophic failure

When it looks like crap

Increased Role for Erosion and Sediment Control Inspectors During ESD construction

• Protect areas during site construction• Authorize ESD construction• Inspect and Accept ESD credits• Certify Vegetative Stability and Soil Amendments

At what point is a ESD practice ready for final inspection and acceptance ?

• Perform final inspection at end of establishment phase

• Usually 6 to 12 months after installation for most vegetative ESD practices

• Developer or builder responsible for this first year of maintenance

What exactly does an ESD as-built look like?

• Engineered surveys ?• Digital photo and GPS coordinates ?• Depends on type and scale of ESD

practices

As-builts for micro ESD practices (CDA less than 5000 sf)

• Digital photo and GPS coordinates

• Vegetative cover and stability

• Confirm ponding elevation and flow paths

• No survey work

Larger ESD Practices (CDA more than 5000 sf)

•Limited survey work to confirm inlet and outlet elevations, flow paths and ponding depths

•Confirm underdrain depth and outflow

•Ensure landscaping meets design objectives

•Verify boundaries of stormwater easement

•Check overflow to downstream conveyance system

•Digital photo after establishment phase

Verifying Disconnection and Buffer Credits

• Look for signs of concentrated flow

• Verify distance• Vegetative density• Screwdriver test • Exposed soil• Digital photo

Maintaining On-lot Practices

Disconnections and dry wells are OK

Rain gardens OK if there is a homeowner association that can enforce private maintenance

May want to shift them to expanded street right of way

May want to avoid use in side and backyards

Where to Locate Residential ESD Practices

On – lot• Front yard• Back yard ?• Side yard ?

Off - lot • In expanded street right of way• HOA open space

Localities should clearly define the appropriate conditions under which LID practices can be located within a residential lot.

Is there a superior legal instrument to compel maintenance of smaller ESD practices that an owner can really understand ?

• Depends on whether it is to be maintained by homeowner, HOA or property manager

• Key is an owners maintenance guide w/ ESD map with digital photo of installed practices

Key Issues in Maintenance Agreements

• Identify landscape contractor or other party to perform maintenance

• Require annual self-inspection

• Reference the specific annual maintenance tasks that must be performed

• Provide ESD locator map to find practices

• Provide photos of the established ESD practices

Moving beyond Checklists to Punchlists

Maintenance Task Order Clean out curb cuts and inlets for accumulated grit,

leaves, and debris that are blocking inflow Spot weeding, trash removal, and mulch raking Prune trees at SE corner Add reinforcement planting to cell 3 Remove invasive plants and dead yellow poplar at

point B Stabilize and reseed the exposed soils on

bioretention buffer at point D Supplement mulch in devoid areas to maintain a 3

inch layer Prune trees and shrubs Remove sediment in pre-treatment cell

Clear “triggers” to compel maintenance tasks

How Does ESD Change Performance Bonds?

• ESC and SWM Bonds ensure the design is properly installed

• Price bonds for ESD practices and credits based on landscape contractor

• Front end load the ESC Bonds• On-lot LID (builder) Off-lot LID (developer)

Training Landscaping Contractors on ESD

Many crews may not realize the stormwater objectives of the landscaping areas they are maintaining

How do we do visual inspections to rapidly assess ESD performance?

5 or 10 minutes max

Schedule within 24 hours after a decent storm

Visual indicators

Certification of Hydrologic Performance

Post-construction field evaluation of ESD practices to ensure they are operating properly and are achieving nutrient reduction

Local BMP Reporting to state for Bay-wide TMDL and MS4 Permits

Bay BMP credits are tied to periodic recertification (5 years)

How do we track ESD in our local database ?

Each site would provide data to store in the system:

• Site map with ESD Locator• Responsible party for

maintenance• BMP design info for larger

ESD practices• Maintenance and

inspection reports• Digital photos of accepted

ESD practices• Nutrient and runoff

reduction credit

Linking Maintenance Inspections to state permit and TMDL tracking

• Maintain a project file for each LID project installed.

• The file should be maintained for the lifetime for which the nutrient removal credit will be claimed.

• The typical duration for the credit will be approximately 25 years

• Locality required to conduct a performance inspection at least once every five years to verify that the system is being adequately maintained and operating as designed.

Bioretention Maintenance

Performance Issues Observed in Field

General Performance Problems with Bioretention (n = 40)

8%

8%

15%

18%

18%

23%

25%

33%

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

Clogged Soil Media

Inappropriate Media

Excessive Vegetation

Sediment Deposition

Short-Circuiting of Treatment

Inadequate Vegetation

No Pre-Treatment

Need Maintenance

Maintaining the “Bio” in Bioretention

• Prune woody matter, remove invasive plants, and manage vegetative growth to maintain landscape.• Add reinforcement planting to maintain desired vegetation density.

2000

2003

VegetationManagementIs Key Maintenance Task

Many Future Maintenance Problems are Rooted in Poor Design

• Lose the filter fabric on the bottom

• Max 12 inch ponding depth

• Flat slope• Max Contributing

Drainage Area (CDA)• Maximize Flow Path and

Length to Width Ratio• Turf cover

Provide Pretreatment

• Nature of pretreatment depends on size of bioretention area and type of flow it experiences– Concentrated flow:

two cell design with a small trapping “forebay” and level spreader

– Sheet flow: grass filter strip, stone diaphragm, stone ring berm

No Pre-treatment: Maintenance Problem

Maintenance Considerations During Installation

• Protect bioretention during site construction

• Don’t install until site is stabilized

• Make sure the original design still works

• Excavate from the side• Make sure ponding depth

meets design• Water and maintain

plantings

Construction Inspection

• Under drain and stone sump installation

• Verify the actual contributing drainage area boundaries

• Confirm inlet and outlet elevations

• Side-slope stabilization

* Subtle changes in grading, paving and drainage can really screw up an otherwise fine design

The establishment phase through first growing season

Make sure vegetatively Stabilized

Landscaping contract covers first year after installation Regular watering first few months Spot re-seeding and remove/replace dead plants Remove sediment accumulation at inletsRepair erosion on side-slopes

Wrap-up