“Stormwater Management in the Coastal Plain & Beyond" Workshop held on December, 15, 2014 - Part I section

Stormwater in the Coastal Plain – Virginia Beach, December 3, 2014

Non-profit 501(c)3, non-advocacy organization founded
in 1992
Work with watershed groups, local, state and federal
governments
Provide tools to communities to protect lakes, rivers,
streams, and estuaries
20 staff in Ellicott City, MD; Field Offices in
Charlottesville, Richmond, & Leesburg, VA; & Ithaca, NY;
and Philadelphia, PA
www.cwp.org

Welcome and Introductions
Historical Perspective
Today’s Perspective
break
Connecting the VRRM Dots: Quality
lunch
Connecting the VRRM Dots: Quantity
break
Meet the BMPs (Coastal Plain Adaptations)
Design Examples & Discussion

Goal of Today’s Workshop:
Understand the differences (and similarities) between
the old (Part IIC) & the new (Part IIB) criteria;
Become familiar with the new menu of BMPs and their
respective performance credits;
Identify issues and/or specific topics for future training
opportunities

Detailed design instructions for BMPs;
Detailed analysis of life-cycle cost of BMPs;
Computational Hydrology & Hydraulics;
Easy solution for stormwater design on
challenging sites:
• Tidal influenced drainage system;
• High water table;
• Flat terrain.

Be engaged
Ask critical questions
Share your expertise
Respect your colleagues:

Why is it different?
and,
Why do we care?

• Flat Terrain
• High water table
• Poorly drained soils
• Very well drained soils
• Highly altered drainage

Shoreline buffer and critical areas cannot be
used for stormwater practices
Dugout ponds intersect with shallow water
table and connect to receiving estuaries
Unique development patterns (waterfront,
marinas, golf courses)

Highway ditches serve as the primary stormwater
receiving and conveyance system;
Heavy seasonal rainfall (hurricanes & Nor’easters)
Historical drainage intended to serve agriculture
Sea level rise

Where is all that water going?
To the nearest waterway — and it is traveling fast.

The Economic Benefits of Cleaning Up the
Chesapeake
Chesapeake Bay Foundation, 2014
Annual economic benefits derived from the land
and waters of the Chesapeake Bay region
~ $107 billion
Continued/Improved implementation of
multiple targeted programs (Chesapeake Clean
Water Blueprint) are expected to increase the
value 21% (~ $130 billion);

Ecosystem Service Valuation:
Water Flow Regulation;
Air Pollution Treatment;
Food Production;
Water Supply;
Waste Treatment;
Climate Stability;
Aesthetic Value; and
Recreation.

•• Extreme storms eventsExtreme storms events
•• Population growth & rapidPopulation growth & rapid
developmentdevelopment
•• Wetland, contiguous forest,Wetland, contiguous forest,
& upland habitat preservation& upland habitat preservation
•• Wastewater disposalWastewater disposal
•• Shellfish prohibitionsShellfish prohibitions
•• Harmful algal bloomsHarmful algal blooms
•• Pollution preventionPollution prevention
–– NitrogenNitrogen
–– BacteriaBacteria
•• Maintain waterMaintain water--basedbased
recreationrecreation
Coastal ConcernsCoastal ConcernsCoastal Concerns
Impact to Recreation and Economic Resources:
Shellfish bed closures;
Beach closures;
Decreased fisheries;
Algal blooms

“the effects of watershed alteration on coastal
resources are poorly understood and inaccessible
to watershed and coastal resource managers.”
(DeVoe and Kleppel, 2006)
Studies suggest a strong negative
relationship between land
development (multiple metrics) and
the biological, physical and
chemical conditions of coastal water
resources

10% 40%25%
Good
Fair
Poor
Watershed Impervious Cover
StreamQuality
60% 100%
Urban Drainage
Over 200 studies:
• Hydrologic Indicators
• Stream Habitat Indicators
• Water Quality Indicators
• Aquatic Diversity Indicators

Threshold values:
Coastal plain streams: 3.5 – 14%
Tidal creeks: 10 – 30%
Estuariane: 24 – 35%
Coastal Plain Watershed Management; Watershed Protection Techniques Vol. 4; No. 1; 2010

Sustainable Development in the Coastal Plain:
Land use plans should allocate lands to their
most appropriate uses;
Requires knowledge and consideration of
physical constraints;
important natural features and the functions they
provide;
potential hazard areas; and
the capacity of the land to support a given use.
Coastal Plain Watershed Management; Watershed Protection Techniques Vol. 4; No. 1; 2010

VA Erosion & Sediment Control Program
GC-7
MS-19:
Properties and waterways
downstream of
development shall be
protected from sediment
deposition, erosion and
damage due to . . .

. . . . Increases in runoff volume, . . .

. . . . velocity, and peak flow rate of
stormwater runoff . . .
. . . in accordance with the following:

b.The adequacy of all channels and pipes
shall be verified in the following manner:
(1) 1% rule
the applicant shall demonstrate that the total
drainage area to the point of outfall analysis is
100 times greater than the contributing DA of
the project

(2) (a) Natural Channels:
2-yr erosion;
2-yr overtopping the banks.
(b) Manmade Channels:
2-yr erosion;
10-yr overtopping the banks.
(c) Pipes and storm sewers:
10-yr capacity

If existing pipes or channels are not adequate, the
applicant shall:
(1) Improve the channel, or
(2) Improve the pipe system, or

applicant shall (continued):
(3) Develop a site design that does not cause an
increase in the 2-yr peak rate of runoff;

applicant shall (continued):
Or:
(4) “Provide a combination of channel
improvement, stormwater detention, or other
measures satisfactory to the VESCP Authority . . . ”

Assumed C soils; Pre-developed land cover = Woods, good condition

1988 Chesapeake Bay Preservation
Act
(§ 62.1‐44.15:67)
&
The Chesapeake Bay
Preservation Area Designation
and
Management Regulations
(9VAC25-830)

Stormwater BMP Sizing
and
Performance Credits
Performance:
Low to Moderate to High
measured from 0 to 100%
(Nationwide Urban Runoff Program
1979 - ’83)
Sizing:
1st Flush to 2” runoff depth

1998 Amendments to the VA SWM Regulations
Water Quality Compliance:
 Performance-Based Criteria; or
• Development Situations 1 thru 4;
• Compliance with Average Land Cover Condition: 16% IC
• Equivalent Annual TP Load = 0.45 lb/ac/yr
 Technology Based Criteria

Simple Method: Average Annual Load
𝐿𝑜𝑎𝑑 = 𝑃 × 𝑃𝑗 × 𝑅 𝑣 × 𝐶 × 𝐴 × 2.72/12
P = average annual rainfall depth (inches) = 43 inches (VA)
Pj = fraction of rainfall events that produce runoff = 0.9
Rv = volumetric runoff coefficient = 0.05 + (0.009 × I); I = impervious cover
C = flow-weighted event mean concentration (EMC) of TP = (mg/L) = 0.26 mg/L
A = contributing drainage area (acres)
2.72 = unit conversion factor: L to ft3, mg to lb, and acres to ft2
12 = unit conversion factor: rainfall inches to feet

Treatment Volume & BMP Sizing
𝑊𝑄𝑣 = Water Quality Volume
𝐼𝐶 = Impervious Cover in the contributing
drainage area to the stormwater practice
𝑊𝑄𝑣 = 0.5" × 𝐼𝐶

* Innovative or alternate BMPs not included in this table may be allowed at
the discretion of the local program administrator or the Department.

Water Quantity Compliance:
 Properties and receiving waterways downstream of
any land development project shall be protected
from erosion and damage due to increases in
volume, velocity and peak flow rate of stormwater
runoff in accordance with . . .

 Minimum Standard 19 of the ESC Regulations; or
 The plan approving authority may determine that
some receiving stream systems require enhanced
criteria to address increased frequency of bankfull
flows and therefore require:
24-hour Extended Detention of the runoff from
the 1-year 24-hour storm

2-yr post back to pre vs 1-yr ED
Moderate development (32% IC):
• > 2 x increase in runoff volume
• 4 to 8 x increased Frequency of bankfull flows;
. . . . significant reduction in stream channel
erosion below facilities designed for 24-hour ED of
the 1-yr frequency runoff (Galli, MWCOG 1992)

What types of BMPs or BMP strategies have
proven to be effective in coastal plain?

2005 NOIRA: Update Virginia’s Stormwater
Management Regulations:
1) allow for the delegation of the VSMP to localities;
2) develop a framework for the Commonwealth to run
local programs as needed;
3) allow for changes as needed to improve the
administration and implementation; and
4) allow for removal of the out-of-date BMPs (Table 1)

EPA Performance Standards
• Protect and restore the physical, chemical and
biological integrity of receiving waters;
• Treat flow (volume) as a surrogate for other
pollutants (reference the National Research Council:
Urban Stormwater Management in the United
States);
• Replicate pre-development hydrology:
Infiltrate, evapotranspirate, or reuse the
runoff from the 90th percentile rainfall event

2007 NOIRA: Replace & clarify the 2005 NOIRA:
1) Amendments, deletions, or additions to Part I
(Definitions, Purpose, and Applicability);
2) Amendments, deletions, or additions to Part II
(Technical Criteria);
3) Amendments, deletions, or additions to Part III (Local
Programs); and
4) Other technical amendments, deletions, or additions
(as may be needed).

Inspector General Report on efforts to restore the
Chesapeake Bay (2007)
• % increase in Impervious cover 5 times that of
population;
• Pollutant load from urban runoff represents the only
increasing source;
• Cost effective solutions dependent on initial site-
specific assessment of topography, soil conditions,
etc.
• Ineffective use of regulatory programs;

2009 NOIRA:
0.28 lb/ac/yr

Encourage Low Impact Development
Inclusion of land cover type in pollutant and
hydrologic loading factors (good science)
New treatment options with performance credit
breakouts (better science)
RR + PR (EMC)  Mass Load Reduction
Step-wise (iterative) simplified compliance
process.
Environmental Site Design

evapotranspiration:
40-50%
interflow: 20-30%
surface runoff: <1%

evapotranspiration:
~25%
interflow: 0-30%
surface runoff: ~30%

Research shows:
 Extended Filtration mimics the hydrologic characteristics
(meaning stormflow reaching a stream) of an
undeveloped watershed.
 Data also indicates that extended filtration releases
water over a much longer period of time than an
undeveloped (agricultural) watershed; and likely a
comparable period of time as a forested watershed.
Hunt et al., NCSU 2010

Runoff Reduction is not just infiltration!
 Infiltration
 Canopy Interception
 Evaporation
 Transpiration
 Rainwater Harvesting
 Extended Filtration
 Soil Amendments

Compliance Spreadsheet

Codifies & incentivizes minimization and
avoidance
Goes beyond impervious cover as a water
quality indicator
Utilizes latest BMP research for Total
Performance (Total Mass Load Removal)
Credits total BMP performance

Environmental Site Inventory & Assessment
• Forest conservation
• Suitable soils
• Steep slopes
• Drainage
• Wetlands
• Zero-order streams
• Buffers
• Sensitive areas
• Limits of disturbance
• Computed nutrient loads &
treatment volume

A 1990 study for the city of Virginia Beach
compared the costs and benefits of conventional
and smart growth development patterns. The
study found:
smart growth pattern resulted in 45% more land
preserved,
45% less in infrastructure costs to the city, and
50% reduction in impervious surface due to
roads
(Siemon, Larsen and Purdy, et al., 1990)

An Assessment of the Better Site Design Principles
for Communities Implementing Virginia’s
Chesapeake Bay Preservation Act (CWP, 2000)
• 16 Model Development Principles
• 4 development projects
• Average 28% reduction in Total Infrastructure Costs
(47%; 15%; 49%; NC)

The Economic Benefits of Watershed Protection
(CWP, 2001)
1. Watershed Planning
2. Land Conservation
3. Aquatic Buffers
4. Better Site Design
5. Erosion & Sediment Control
6. Stormwater Treatment
7. Non-Stormwater Discharges
8. Watershed Stewardship

1. Site Data Input:
• Site Land cover
• Site level Treatment Volume
(Tv)
• Site level pollutant loads and
Removal Requirement
2. Drainage Area Inputs:
• DA Land Cover
• TvBMP (used for BMP sizing)
• Area treated check
• DA pollutant removal
1. Site Data
Input
2. Drainage
Area Inputs
4. Channel/Flood Protection
Check
3. Water Quality
Check
5. Summary
Print-out

Volumetric Runoff Coefficients
• Land Cover (acres) by HSG
• Definitions Provided in
Guidance
Composite Site Rv
Post-Dev Tv
Pollutant Load (TP & TN)
Total Load Reduction Reqd.
Weighted (by HSG) Rv for Forest, Turf, & Imp

15 Acres
25 ½ acre lots
Drainage Area Land Cover (Acres)
Land Cover Total
½ acre lots
Total
¼ acre lots
Forest 0.87 4.31
Turf 8.32 5.26
Impervious 2.26 1.88
15 Acres
25 ¼ acre lots
Drainage Area Water Quality Requirements
Total
½ acre lots
Total
¼ acre lots
Post-Dev Treatment Vol 14,452 ft3 11,198 ft3
Post-Dev TP Load 9.08 lb/yr 7.04 lb/yr
Pollutant Removal Reqd. 4.39 lb/yr 2.34 lb/yr

Finger printing on large
lot construction:
Save trees, soil, etc.
Finger-printing subdivision
construction:
narrow streets; shorter setbacks, etc.

• Reduced runoff
coefficients for
undisturbed pervious
areas;
• Increased runoff
coefficients for
impacted soils &
managed turf;

Cover HSG A HSG B HSG C HSG D
Forest/Open 0.02 0.03 0.04 0.05
Managed Turf /
Disturbed Soil
0.15 0.20 0.22 0.25
Impervious Cover 0.95 0.95 0.95 0.95
1 Center for Watershed Protection – Technical Memorandum: The Runoff
Reduction Method; 4/18/08
Pitt et al (2005), Lichter and Lindsey (1994), Schueler (2001a, 2001b, 1987),
Legg et al (1996), Pitt et al (1999), and Cappiella et al (2005)

Codifies & incentivizes minimization and
avoidance
Goes beyond impervious cover as a
water quality indicator
Utilizes latest BMP research for Total
Performance (Total Mass Load Removal)
Credits total BMP performance

• Impacts from grading and compaction of
soils

• Impacts from turf management activities

Forest & Open Space:
• Undisturbed portions of residential development;
• Open space left in natural vegetated state that will
not be managed as turf (regular mowing, fertilized,
etc.); includes utility ROW’s with periodic bush-
hogging;
• Surface area of BMPs that have vegetative cover
(not wet ponds, green roof, permeable pavement);
Managed Turf
Impervious Cover

Simple Method – Average Annual Load:
𝐿𝑜𝑎𝑑 = 𝑃 × 𝑃𝑗 × 𝑅 𝑣 × 𝐶 × 𝐴 × 2.72/12
P = average annual rainfall depth (inches) = 43 inches (VA)
Pj = fraction of rainfall events that produce runoff = 0.9
Rv = volumetric runoff coefficient = 0.05 + (0.009 × I); I = impervious cover
C = flow-weighted event mean concentration (EMC) of TP = (mg/L) = 0.26 mg/L
A = contributing drainage area (acres)
2.72 = unit conversion factor: L to ft3, mg to lb, and acres to ft2
12 = unit conversion factor: rainfall inches to feet
𝐿 = 𝑃 × 𝑃𝑖 × 𝑅𝑣 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 × 𝐶 × 𝐴 × 2.72 12
𝑅𝑣 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 = 𝑅𝑣𝐼 × %𝐼 + 𝑅𝑣 𝑇 × %𝑇 + 𝑅𝑣 𝐹 × %𝐹

New Rules (Part IIB)
• C = 0.26 mg/l
From:
Runoff Reduction Method
Technical Memorandum,
April 2008
Center for Watershed Protection

1. Weighted average soil cover derived from STATSGO state-wide soils database soil breakdown for VA outside of the
Chesapeake Bay Watershed.
2. Schueler, T., Fraley-McNeal, L., and Capiella, K. “Is Impervious Cover Still
Important? Review of Recent Research” Journal of Hydrologic Engineering, April 2009
1
2

TvBMP = Design Treatment Volume from contributing
drainage area to stormwater practice (does not include
remaining runoff from upstream practices)
P = 90th Percentile rainfall depth = 1”
Rvcomposite = Composite runoff coefficient
A = Direct contributing drainage area to the
stormwater practice
𝑇𝑣 𝐵𝑀𝑃 =
𝑃 × 𝑅𝑣 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 × 𝐴
12

Washington Reagan Airport
0
1
2
3
4
5
6
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Precipitation Event Percentile
PrecipitationDepth(inches)
90th Percentile rainfall depth
1” annual average: Washington Reagan Airport, Richmond Airport, Harrisonburg, Lynchburg, Bristol

Using the 90th percentile rainfall depth translates to
an annual average reduction
• Represents average over all storms and not individual
single-event modeled storms
• Oversizing practice does not necessarily provide for
increase in “annual” RR or PR performance
• Oversizing can help meet quantity control storage
requirements when modeled on single event basis

0
0.25
0.5
0.75
1
Load(lb/ac/yr)
Required Level
of Treatment
Allowable Load
(lb/ac/yr)
Part II C
Requirements
Part II B
Requirements
Required
Treatment
0.45
0.41

Pollutant Removal
Practices
Minimization/ESDRunoff Reduction Practices

Total BMP Performance:
Runoff Reduction Reported Performance:
𝑹𝒖𝒏𝒐𝒇𝒇 𝑽𝒐𝒍 𝑰𝑵 𝒗𝒔 𝑹𝒖𝒏𝒐𝒇𝒇 𝑽𝒐𝒍 𝑶𝑼𝑻
+
Pollutant Removal Reported Performance:
𝑬𝑴𝑪 𝑰𝑵 𝒗𝒔 𝑬𝑴𝑪 𝑶𝑼𝑻
=
Total BMP Performance (reported as Load Reduction):
𝑽𝒐𝒍 𝑰𝑵 × 𝑬𝑴𝑪 𝑰𝑵 𝒗𝒔 𝑽𝒐𝒍 𝑶𝑼𝑻 𝑬𝑴𝑪 𝑶𝑼𝑻
Center for Watershed

Runoff Reduction (RR) and Pollutant Removal (PR):
• Allows for reductions beyond irreducible
concentrations by reducing the volume;
• Provides for maximum performance through a
“Treatment Train” approach including non-structural
practices

Bioretention, Infiltration, Dry
Swales, Soil Amendments,
disconnection, and Related
Practices Reduce Runoff
Volumes by 50 to 90%
Wet Ponds, ED Ponds and
Constructed Wetlands and
Filters Reduce Runoff
Volumes by zero to 10%

Site
Design
Runoff
Reduction
Pollutant
Removal
1. Rooftop Disconnection  
2. Filter Strip  
3. Grass Channel  
4. Soil Amendments * 
5. Green Roof 
6. Rain Tanks & Cisterns 
7. Permeable Pavement  
8. Infiltration  
9. Bioretention  
10. Dry Swales  
12. Filtering Practices 
13. Constructed Wetlands 
14. Wet Ponds 
15. ED Ponds  

BMP
1,000,000 liters of
stormwater
(multiple storm
events)
100 mg/L
pollutant
(average)
1,000,000 liters of
stormwater
(multiple storm
events)
50 mg/L
of pollutant
(average)
No volume reduction, only EMC reduction
100 kg
Total
pollutant
load
50 kg
Total
pollutant
load
SOURCE: VA DEQ, 2013
50% PR

100 mg/L of
pollutant
(average)
500,000 liters of
stormwater
(multiple storm
events)
50 mg/L
pollutants
(average)
Total Performance = 75% load reduction!
25 kg
Total
pollutant
load
BMP
100 kg
Total
pollutant
load
SOURCE: VA DEQ, 2013 (CORRECTED)
1,000,000 liters of
stormwater
(multiple storm
events)
50% RR
50% PR
+

The VRRM Technical Memo documented the
performance of BMPs:
 RR performance more consistent than PR performance;
 Nutrient PR in stormwater BMPs is notoriously
inconsistent;
 RR rates are an annual average based on the individual
study site water balance.
 The recommended rates are conservative estimates
 The RR rates in the regulations are dependent on
meeting Level 1 or Level 2 criteria.

Level 1 standard features included in all designs:
• Function;
• Safety;
• Appearance;
• Safe conveyance;
• Performance longevity
• Maintenance

Level 2 design enhancements for increased RR, PR or
both:
• Increased Tv sizing (by a factor of 1.1, 1.25 or 1.5
times the Tv);
• Enhanced design geometry;
• Vegetative condition;
• Multiple cells;
• Multiple treatment pathways; and Other bells and
whistles, e.g., increased pretreatment, increased
media depth, etc.

Comparative BMP
Level 1
& Level 2
Performance

Consider guidance to standardize Process Diagrams to
track volume and load through complex treatment trains

Allow for compliance on high density sites;
Provide flexibility on small/tight sites by allowing
multiple smaller BMPs to treat stormwater near the
source;
• as the drainage area incrementally increases (with each RR
practice);
• the RR practices incrementally reduce the runoff volume
and TvBMP,
• each successive BMP is not sized on the entire upstream
drainage area;
• Rather, the BMP is sized by the TvBMP from the directly
contributing drainage area + any remaining runoff from
upstream RR practices.

Level 1 Design (RR 40 TP: 25 ) Level 2 Design (RR: 80 TP: 50)
Sizing (Section 6.1):
TvBMP = [(1)(Rv)(A) / 12] + any remaining
volume from upstream BMP
Sizing (Section 6.1):
TvBMP = [(1.25)(Rv)(A) / 12] + any
remaining volume from upstream BMP
Design Summary Table BMP Design Specification
No. 9: Bioretention

Project Graphic Courtesy of Geosyntec
19.8 acre single Family Subdivision
2.2 acres of R.O.W.
34 lots (avg lot size = ½ acre)

Turf = 12.09
Imp = 7.71
Area Total = 19.8 acres
Site Rv = 0.50
Post Dev Tv = 0.83 ac-ft
Post Dev TP Load = 22.77 lb/yr
Load Reduction Required = 14.65 lb/yr
Site Data Tab
Site Avg BMP Eff. = 14.65/22.77 x 100 = 65%

Credit Area (acres) to
Wet Pond Level 2:
Imp = 7.71 ac
Turf = 12.09 ac
TP Removed = 17.06 lb/yr
0 Runoff Reduction
Area Check: OK
Drainage Area Tab
Remaining Runoff &
TP load

Runoff Reduction = 0
TP Reduction = 17.06 lb/yr
Area Check: OK

1, 2, and 10-year
storm rainfall depths
CN = 83
1, 2, and 10-year volume
(RV) measured in
watershed inches =
RV1 = 1.28 inches
RV2 = 1.76 inches
RV10 = 3.30 inches
No RR
No CN Adjustment!
No volume
reduction

Channel Protection:
Concentrated stormwater flow shall be
released in to a stormwater conveyance
system:
Photo: Williamsburg Environmental Group
Manmade Stormwater
Conveyance System
Restored Stormwater
Conveyance System
Natural Stormwater
Conveyance System

Goals:
• Establish “balance” exerted by pre- and post-
developed 1-yr peak stormwater discharge;
• Incentivize Better Site Design (and volume
reduction);
• Keep it Simple
(Qpeak*Vol)pre (Qpeak*Vol)post

Simple “balance” offsets increase in volume and
peak flow of developed condition hydrology
Post-development
runoff volume increases
Allowable discharge
decreases

Energy Balance
Post (Vol1-yr * Peak Q1-yr) ≤ Pre (Vol1-yr * Peak Q1-yr)
re-written:
IF = Improvement Factor:
(0.8 for sites > 1 acre or 0.9 for sites ≤ 1 acre)
𝑞1𝑝𝑜𝑠𝑡 ≤ 𝑞1𝑝𝑟𝑒
𝑃𝑟𝑒 𝑉𝑜𝑙1
𝑃𝑜𝑠𝑡 𝑉𝑜𝑙1
𝐼𝐹

Use VRRM Spreadsheet to calculate the volume
reduction with a double credit:
• Reduced Volpost1 for Energy Balance Equation; and
• Reduced Curve Number (CN) for computing the
q1post
𝑞1𝑝𝑜𝑠𝑡 ≤ 𝑞1𝑝𝑟𝑒
𝑃𝑟𝑒 𝑉𝑜𝑙1
𝑃𝑜𝑠𝑡 𝑉𝑜𝑙1
𝐼𝐹
Must consider the pre- and post- condition drainage areas!

Roadway
Project Drainage
Area
Residential
Lots
Typical subdivision development:

NEW PERVIOUS PAVEMENT
MANAGED TURF
BIORETENTION CELLS
RESIDENTIAL LOTS
MANAGED TURF
BIORETENTION CELLS
RESIDENTIAL LOTS
ROOFTOP DISCONNECTION

Same as traditional scenario, but with:
• Wet Pond area partially converted
from ‘Impervious Cover’ to
‘Managed Turf’
• BMP areas converted from
‘Managed Turf’ to ‘Forest/Open
Space’
Slight change in Tv, TP Load,
and Reduction Requirement
Center for Watershed 118

2
4
34
33
32
3
1
6
7
5
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
No. 1 Bio L2
No. 2 Bio L2
No. 18 Bio L2
?

Aggregated Credit Area to
Simple Disconnection = 5 ac
Runoff Reduction = 4,311 ft3
Runoff Remaining = 12,932 ft3
Downstream Treatment: Bioretention L2
Load Reduction = 2.71 lb
Load Remaining = 8.12 lb

Credit Area (Direct Runoff) to
Bioretention Level 2:
1.89 ac Impervious
5.0 ac Turf
Volume from upstream RR practice:
12,932 ft3
Runoff Reduction = 18,754 ft3
Runoff Remaining = 4,689 ft3
Load Reduction = 2.71 lb
Load Remaining = 8.12 lb
Load from upstream RR practice:
8.12 lb

Area Check - OK
Runoff Reduction Achieved = 23,065 ft3
TP Load Reduction Achieved = 15.96 lb

1, 2, and 10-year
storm rainfall depths
1, 2, and 10-year volume (RV)
reduction =
RV1 = 1.28” 0.96”
CN1 83 77
RV2 = 1.76” 1.44”
CN2 83 78
RV10 = 3.30” 2.98”
CN10 83 80
Volume Reduction = 23,065 ft3

Original design:
• No Volume Reduction;
• Treat 100% of the site (19.8 ac) with Wet Pond Level 2
• Compliance: exceed reqmt. by 2.4 lb/yr
RR Design:
• Treat 11.9 acres
• Compliance: exceed reqmt. by 2.2 lb/yr
• No wet pond Reqd (for water quality)
• Reduce 23,065 ft3 volume (from site Tv = 34,816 ft3)
• Reduce 1-yr CN from 83 to 77

MANAGED TURF
BIORETENTION CELLS
RESIDENTIAL LOTS
ROOFTOP DISCONNECTION

2
4
34
33
32
3
1
6
7
5
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
Disconnection
No. 1 Bio L2
No. 2 Bio L2
No. 18 Bio L2
??

Challenge:
Provide quantity “credit” for distributed retention
practices
Avoid Complex routing/modeling of multiple practices,
yet simulate single event modeling
Allow designers to target volume as primary metric
(quantity and quality)
Various methods explored by VA TAC

Simplifying Assumptions:
• Assume retention is uniformly distributed if
considering multiple features or sub-areas;
• Assume negligible discharge from under-drains
(if any)

5 Methods Considered:
1. Hydrograph Truncation
2. Hydrograph Scalar Multiplication
3. Precipitation Adjustment
4. Runoff Adjustment
5. Curve Number Adjustment
Excerpted from work by Paul R. Koch, Ph.D., P.E.

0%
20%
40%
60%
80%
100%
0% 20% 40% 60% 80% 100%
Volume Stored, as Percent of Total Runoff
PercentofRunoffPeakRemaining
R_trunc
R_as_P
R_as_Q
CN_adj
Scalar
Method of
Analysis
CN Adjustment
Excerpted from work by Paul R. Koch, Ph.D., P.E.

Runoff Depth Equations (TR-55):
Where:
Q = runoff depth (in)
P = precipitation depth (in)
S = potential maximum retention after runoff begins
Ia = initial abstraction, volume filled before runoff begins.
𝑄 =
𝑃 − 𝐼 𝑎
2
𝑃 − 𝐼 𝑎 + 𝑆
𝐼 𝑎 = 0.2𝑆 𝑆 =
1000
𝐶𝑁
− 10
Eq. 2-1:
Eq. 2-2: Eq. 2-4:

Step 3: 1.28” runoff
Step 1: 2.8” rainfallStep 4: Adjust For
Retention (-0.32”)
Step 5: Adjusted CN ~ 77
Step 2: Original CN = 83

One-Year Storm Hydrology Summary: 19.8 acres
Pre-
Developed
Post-
Developed no
RR
Post-
Developed
with RR
Runoff Curve Number 71 83 77
Runoff Volume (RV) 0.62 in 1.28 in 0.96 in
Runoff Volume 1.02 ac-ft. 2.11 ac-ft. 1.58 ac-ft.
Peak Discharge (q1) 9 cfs 39 cfs 27 cfs
Post Developed EB Allowed Peak
Discharge (cfs)
3.5 cfs 4.7 cfs
Storage Volume Reqd. (ac-ft) 1.16 ac-ft.* 0.76 ac-ft.*
34% Reduction in required 1-yr EB Storage Volume

No RR shown; 34% reduction in volume!

Slight increase in allowable release w/RR

St. Paul’s Boulevard, City of Norfolk, VA

< 1 ac of land disturbance: 10% load reduction
≥ 1 ac of disturbance: 20% load reduction
Increase in impervious cover from existing:
• New impervious acreage is managed as new
development (0.41 lb/ac/yr);
• Remainder of site is managed to a 10% or 20%
reduction (as required).
• Definition of site = area of land disturbance

• Virginia Stormwater Management Handbook Appendix
6C: Stormwater Design in the Coastal Plain
• Chesapeake Stormwater Network Technical Bulletin
No. 2 (v1.0): Stormwater Design in the Coastal Plain
• HRPDC: Land & Water Quality Protection Phase II
Discouraged BMPs
• Center for Watershed Protection: Watershed
Protection Techniques Vol.4, No. 1, 2010

Maximize on-site micro-practices such as:
• Filter Strips;
• Buffers (reforestation),
• Simple Disconnection;
• Alternative Disconnection
— cisterns,
— dry wells,
— rain gardens, &
— compost amended filter path;

• Promote de-nitrification (create adjacent
anaerobic and aerobic zones);
• Avoid infiltration in areas with high water table;
• Utilize native plants;
• Create a rooftop to buffer in-line treatment
train; and
• Relax some design criteria to keep practice
depths shallow!

Maximum bacteria removal:
— long residence time and light exposure for coliform
die off;
— Reduce turf around open water to discourage
geese and waterfowl;
— Use shallow wetlands and benches to create
natural micro-predators;
— Minimize resuspension of bottom sediments

1. Preferred BMPs
2. Accepted BMPs
3. Discouraged BMPs

1. Widespread feasibility in the coastal plain
2. High runoff reduction capacity
3. Moderate to high removal of nitrogen and
bacteria
4. Low mosquito breeding capability when
installed and maintained properly
Preferred means the practice does well on at least three
factors;
Discouraged does not mean prohibited; rather it suggests
ruling out an alternative preferred practice first

Preferred Accepted Discouraged
Constructed Wetland Wet Ponds
Large Scale
Infiltration
Shallow Bioretention
& Dry Swales
Small-scale
Infiltration
Dry ED ponds
Wet Swale Green Roofs Grass Channels
Rain Tanks/Cisterns
Soil Compost
Amendments
Roof Disconnection &
Filter Strips
Sand Filter
Permeable Pavers

Coastal plain research places wet ponds in 2
general groups:
1. Standard ponds: do not meet criteria and have
low to negative nutrient removal performance;
2. Enhanced ponds: performed much better
resulting from design features:
• geometry: L:W ratio; multiple cells;
• macrophytes and other wetland characteristics
(wetland cells, benches, etc.);
• Extended detention time of incoming runoff;

James River Basin BMP type (n=3112)
43%
17%
14%
12%
7%
2%
2%
2%
1%
0%
0%
0%
0%
0%
0%
0% 10% 20% 30% 40% 50%
Wet Pond
Pond, Unidentified Detention
Grass Channel
Infiltration
Other
Pond, Water Quality Detention
Proprietary Device
Bioretention
Constructed Wetland
Underground Storage
Filter Practice
Permeable Pavement
Dry Swale
Level Spreader
Wet Swale
• Hirschman and Woodworth, 2009; and
• Literature Synthesis of SC and NC studies; Drescher at al, 2007)

Review of coastal plain wet ponds (as installed &
maintained):
• A large number fail to meet basic (minimum)
design criteria (and enhanced guidelines);
• Many exhibited functional problems relating to
a lack of maintenance (sediment deposition,
excessive plant growth, trees on embankment);
• worst performing were small (pocket style) with
a small contributing drainage area (squeezed
onto site)

Key design drivers:
• Eutrophication: function of nutrient input and
residence time (defined as pool vol/annual runoff
input); and
• Depth of the anoxic zone (which increases the
nutrient release from the bottom sediments).

• Expected nutrient removal rates are slightly reduced in
the coastal plain due to the influence of groundwater,
• Certain design features are essential to achieving
reduction (multiple cells, benches, flow path, etc.)
• Certain design features can enhance performance
(landscaping, bubblers & fountains, floating wetlands)
can improve their function.
• Wet ponds can produce and or export harmful algal
blooms if they interact with brackish ground or surface
waters

Runoff Reduction = 0
TP Reduction = 17.06 lb/yr
Area Check: OK
New Level 2 TP reduction Credit (65%)
= 14.8 (coastal plain):
Congratulations!!

* Includes: multi-cell design, sediment forebays, pool geometry (L:W = 2:1)
** Aquatic Bench
Part IIC Retention Basin
WQv
Pool Volume
(ac-ft.)
TP Reduction
Credit
Retention Basin I*
0.475 ac-ft.
(3x) = 1.425 40%
Retention Basin II* (4x) = 1.90 50%
Retention Basin III** (4x) = 1.90 65%
Part IIB Retention Basin
Tv
Pool Volume
(ac-ft.)
TP Reduction
Credit
Retention Basin L1*
1.180 ac-ft.
1.180 50% (45%)
Retention Basin L2** 1.77 75% (65%)
* Includes: safety bench, aquatic bench, sediment forebays, L:W = 2:1
** Multi-cell design, 10% surface area wetland; 50% of Tv can be in ED above
normal pool, L:W = 3:1;

Application of a maximum impervious area per
disconnection:
• Alternate Disconnection; or
• Vegetated Filter Strips

Key Design Consideration: Nomenclature!
Conserved Open Space
Width
Width
Length

Key Design Considerations:
• Establish or maintain good vegetative cover;
• Establish and maintain sheet flow
• Thick no-mow (low
maintenance)
vegetation.
http://www.clemson.edu/extension

Key Design Consideration:
Engineered Level Spreader (ELS) combined with forebay,
energy dissipator, rigid lip, & gravel diaphragm
Photo: R. Winston; BAE Stormwater Engineering Group, NCSU

‒Vegetated Filter Strips
‒Rainwater Harvesting
‒Green Roofs
‒TREES!!!!
SWM Benefits of Trees (D. Wible, CH2MHILL)

“Water Spreading”
Trees of the poplar,
cottonwood, and willow
family have been shown
to draw as much as 200
gallons of water per day
(EPA, Introduction to
Phytoremediation)

Stormwater BMP Enhancements

Photo Credit: Jeremy Balousek, P.E., Dane County, WI Land and
Water Resources Department
Photo Credit: Richard McLaughlin, Ph.D., North Carolina State
University
Soil Restoration

S. Day, S. Dickenson; Virginia Tech Departments of Forestry and Horticulture
Managing
Stormwater for
Urban
Sustainability
Using Trees and
Structural Soils

Susan Downing Day
&
Sarah B. Dickenson
Virginia Tech
Departments of Forestry and Horticulture

Type of pavement materials
Pervious Concrete Porous Asphalt
Permeable
Interlocking
Concrete Pavers
Concrete Grid Pavers
Pervious Composites Permeable Rubber
Overlays

Level 1
Level 2 (infiltration)
Level 2 (infiltration sump)

Design Option: ‘Upturned Elbow’
Anaerobic Zone for denitrification

Pavement Structural Design
• Thickness of permeable
pavement and reservoir
layer must be sized to
support structural loads
• Primary design elements:
– Anticipated traffic loads;
– Underlying soil properties;
– Surface and bedding strength
coefficients

Key Design Consideration: External Drainage Areas
In all cases, external
drainage areas should
be limited to
impervious surfaces to
reduce potential
sediment loading

Key Design Consideration: Maintenance

Annual runoff reduction volume credit only
awarded for dedicated year-round water
drawdown/demand
Laundry washing Vehicle washingToilet flushing

Street Sweepers
Vactor Trucks
Public Works

Seasonal uses must be supplemented with runoff
reduction drawdown practice
Irrigation
Cooling tower make-up water

Location
Roof Area
Indoor
Demand

Secondary
Drawdown
Cooling
Towers
Seasonal
Irrigation
Seasonal with or
w/o smart control

Additional Daily
Use:
• Vehicle wash
• Street Sweepers
• Vactor Truck
• Etc.with or
Additional Sources
of water
Losses

Without a Smart Control With a Smart Control
Cumulative Daily Water Use
and Equivalent Year Round Use

Overflow (and dry)
Volume & Days
(all storms)
Typical (rain)
Year

7,000 Gallon Cistern Water Levels and Precipitation during a
Normal Rainfall Year

Annual Runoff
Reduction Credit
(based on cistern
size)

Tank Design 2: Storage Associated with Treatment, Channel Protection and Flood Volume
Tank Design 1: Storage Associated with Treatment Volume (Tv) only

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“Stormwater Management in the Coastal Plain & Beyond" Workshop held on December, 15, 2014 - Part I section