Transcript Environmental Site Design In the Critical Area

Reviewing and Inspecting Non-Structural ESD
Stormwater Plans in Maryland
Tom Schueler
Chesapeake Stormwater Network
and
EPA 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
Reviewers
Regulators
Designers
CSN
MS4
Managers
Researcher
Environment
Advocates
Linking the
Different
Stormwater
Communities
Together
in a
Collaborative
Network
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 Network
www.chesapeakestormwater.net
Step 2: Calculate Site Imperviousness and Water Quality Volume, WQv
Time to test the
spreadsheet!
Site Area, A (acres)
Existing Impervious Surface Area (acres)
Proposed Impervious Surface Area (acres)
Existing Imperviousness, Ipre
Proposed Imperviousness, Ipost
4
2.5
3
62.5%
75.0%
Development Category
Redevelopment
Rainfall Depth, P (in)
Runoff Coefficient, Rv
Water Quality Volume, WQv (ac-in)
Water Quality Volume, WQv (cf)
1.0
0.73
2.90
10,527
Step 4: Calculate Environmental Site Design (ESD) Rainfall Target, PE
% Soil Type A
% Soil Type B
% Soil Type C
% Soil Type D
0%
60%
40%
0%
Pre-Developed Condition, RCNwoods
New Development
Soil Type A ESD Rainfall Target, PE
Soil Type B ESD Rainfall Target, PE
Soil Type C ESD Rainfall Target, PE
Soil Type D ESD Rainfall Target, PE
61
(in)
(in)
(in)
(in)
0.00
1.32
0.80
0.00
Site ESD Rainfall Target, PE (in)
1.80
ESD Runoff Depth, QE (in)
ESD Runoff Volume, ESDv (cf)
1.31
18,949
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.
P Credit for Reforestation/Soil Restoration
• 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.
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 Baywide 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 channe
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 disconnections
18 swale sections
14 rain gardens
5 bioretention areas
4 tree planting areas
6 sheet flow credits
The Changing Maintenance Paradigm
Conventional 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 area
When to construct?
During site construction
After site is stabilized
Who is responsible?
Homeowner association
Homeowner
Who 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 and
and public nuisances
On a 30 to 50 year cycle (if
ever)
Maintain hydrologic
function and landscaping
Annual cleanouts at
pretreatment devices
After catastrophic failure
When it looks like crap
Sediment cleanouts?
Maintenance Triggers
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)
Need Maintenance
33%
No Pre-Treatment
25%
Inadequate Vegetation
23%
Short-Circuiting of Treatment
18%
Sediment Deposition
18%
Excessive Vegetation
15%
Inappropriate Media
8%
Clogged Soil Media
8%
0%
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
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
Vegetation
Management
Is Key
Maintenance
Task
2003
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 inlets
Repair erosion on side-slopes
Wrap-up