“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 & rapid Population 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--based based

recreation recreation

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

Str

eam

Qu

ali

ty

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 ProgramGC-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


If existing pipes or channels are not adequate, the applicant shall (continued):

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


If existing pipes or channels are not adequate, the 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 × 𝑅𝑣 × 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 bankfullflows 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)


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



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 TvPollutant Load (TP & TN)

Total Load Reduction Reqd.

Weighted (by HSG) Rv for Forest, Turf, & Imp


15 Acres25 ½ acre lots

Drainage Area Land Cover (Acres)

Land Cover Total½ acre lots

Total ¼ acre lots

Forest 0.87 4.31

Turf 8.32 5.26

Impervious 2.26 1.88

15 Acres25 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 subdivisionconstruction:

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)







• 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 × 𝑅𝑣 × 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 × 𝑅𝑣𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 × 𝐶 × 𝐴 × 2.72 12

𝑅𝑣𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 = 𝑅𝑣𝐼 ×%𝐼 + 𝑅𝑣𝑇 ×%𝑇 + 𝑅𝑣𝐹 ×%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

𝑇𝑣𝐵𝑀𝑃 =𝑃 × 𝑅𝑣𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 × 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

Pre

cip

itati

on

Dep

th (

inch

es)

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

1L

oa

d (

lb/a

c/y

r)

Required Level of Treatment

Allowable Load (lb/ac/yr)

Part II C

RequirementsPart II B

Requirements

Required

Treatment

0.45

0.41


Pollutant Removal

Practices

Minimization/ESDRunoff Reduction Practices







Total BMP Performance:

Runoff Reduction Reported Performance:

𝑹𝒖𝒏𝒐𝒇𝒇 𝑽𝒐𝒍𝑰𝑵 𝒗𝒔 𝑹𝒖𝒏𝒐𝒇𝒇 𝑽𝒐𝒍𝑶𝑼𝑻

+

Pollutant Removal Reported Performance:

𝑬𝑴𝑪𝑰𝑵 𝒗𝒔 𝑬𝑴𝑪𝑶𝑼𝑻

=

Total BMP Performance (reported as Load Reduction):

𝑽𝒐𝒍𝑰𝑵 × 𝑬𝑴𝑪𝑰𝑵 𝒗𝒔 𝑽𝒐𝒍𝑶𝑼𝑻 𝑬𝑴𝑪𝑶𝑼𝑻



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

Center for Watershed 91


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 volumeand TvBMP,

• each successive BMP is not sized on the entire upstreamdrainage area;

• Rather, the BMP is sized by the TvBMP from the directlycontributing drainage area + any remaining runoff fromupstream 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 = 12.09Imp = 7.71Area 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 acTurf = 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 = 831, 2, and 10-year volume (RV) measured in watershed inches =

RV1 = 1.28 inchesRV2 = 1.76 inchesRV10 = 3.30 inches

No RRNo CN Adjustment!

No volumereduction




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 ≤ 𝑞1𝑝𝑟𝑒𝑃𝑟𝑒 𝑉𝑜𝑙1𝑃𝑜𝑠𝑡 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


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


118



MANAGED TURF

BIORETENTION CELLS

RESIDENTIAL LOTS



MANAGED TURF

BIORETENTION CELLS

RESIDENTIAL LOTS



2

4

34

3332

3

1

6

75

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 lbLoad Remaining = 8.12 lb


Credit Area (Direct Runoff) to Bioretention Level 2:

1.89 ac Impervious5.0 ac Turf

Volume from upstream RR practice:12,932 ft3

Runoff Reduction = 18,754 ft3

Runoff Remaining = 4,689 ft3

Load Reduction = 2.71 lbLoad 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 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


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


𝐼𝐹

Must consider the pre- and post- condition drainage areas!




MANAGED TURF

BIORETENTION CELLS

RESIDENTIAL LOTS



2

4

34

3332

3

1

6

75

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

Pe

rce

nt

of

Ru

no

ff P

ea

k R

em

ain

ing

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 − 𝐼𝑎

2

𝑃 − 𝐼𝑎 + 𝑆

𝐼𝑎 = 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



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


One-Year Storm Hydrology Summary: 19.8 acresPre-

DevelopedPost-

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 PondsLarge Scale Infiltration

Shallow Bioretention & Dry Swales

Small-scale Infiltration

Dry ED ponds

Wet Swale Green Roofs Grass Channels

Rain Tanks/CisternsSoil 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

WQvPool 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

TvPool 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


http://www.ecoisp.com/images/resources16a.jpg

http://www.ecoisp.com/images/resources16a.jpg




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. DickensonVirginia 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

IrrigationSeasonal 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
