Introduction to the New Virginia Stormwater Design Specifications

The following is an introduction to the new design specifications for 15 non-proprietary stormwater control measures (BMPs, or Best Management Practices) listed
below for use in the Commonwealth:

Rooftop (and Impervious Area) Disconnection
Sheetflow to Open Space and Grass Filter Areas
Grass Channels
Soils Compost Amendments
Vegetated Roofs
Rainwater Harvesting
Permeable Pavement
Infiltration
Bioretention (including Urban Bioretention)
Dry Swales
Wet Swales
Filtering Practices
Constructed Wetlands
Wet Ponds
Dry Extended Detention Ponds

What’s New?

This section outlines the new methods, concepts and performance standards inherent in the new design specifications. It also includes cross-cutting guidance,
information and specifications that apply more than one of the individual specifications.

1. The Spreadsheet versus the Specifications The new regulations herald a shift to the runoff reduction paradigm, where designers focus on reducing the post-development stormwater runoff volume from a site, as well as meeting more stringent nutrient load reduction requirements. The DCR compliance spreadsheet is used to verify whether runoff and nutrient reduction targets are actually being met at the site (Figure 1).

In most cases, designers will need to analyze a lot of design options with the spreadsheet, and will end up with a system or sequence of multiple practices
across the site. While the compliance spreadsheet helps determine whether a site is in compliance, designers must still meet design criteria for individual
practices at the site.

Figure 1. Output from the DCR Compliance Spreadsheet

The Runoff Reduction Method (RRM) spreadsheet computes the required treatment volume for a site, and analyzes the type and design levels of stormwater practices that are needed to comply with runoff and nutrient reduction targets. Designers then must use the design criteria contained in the new design specifications to ensure the practices will be hydrologically effective.

2. Maximizing Runoff Reduction (RR)and Nutrient Removal The new stormwater regulations put a premium on maximizing the degree of runoff volume reduction and nutrient removal achieved at a development site. Each practice has a different capability to reduce annual runoff volumes, as well as a different treatment efficiency to reduce the event mean concentration (EMC) of nutrients as they pass through the practice. Consequently, designers should carefully review Table 1 to determine which practices (and design levels) maximize annual runoff and nutrient reduction rates.

The computed annual load (lbs/ac/yr) is a product of the reduced volume multiplied by the reduced pollutant concentration. Some practices may achieve reductions solely through pollutant removal and provide no runoff reduction, while others may provide only runoff reduction and no measureable pollutant removal. Therefore, as the practices serve to reduce one or both values, a total annual mass load reduction is achieved. The technical support for these numbers can be found in CWP and CSN (2008) and extensive reviews of BMP performance monitoring studies incorporated into the National Pollution Removal Performance Database (CWP, 2007). Estimates for a few practices should be considered provisional (e.g., filter strips) due to limited data. The table will be updated over time to reflect new stormwater research.

At most sites, designers may need to employ several practices in a “roof to stream” sequence to meet the stringent runoff and pollutant reduction targets
(e.g., rooftop disconnection to frontyard bioretention to dry swale to constructed wetland). Another relatively new feature is the inclusino of managed turf as a land cover that generates a pollutant load. In the spreadsheet, designers must account for the load contributed by managed turf in addition to impervious cover. Designers must also select the most appropriate practices to treat turf areas and turf-intensive land uses, such as sports fields, golf courses, and parkland. In many cases, some of the “lower-tech” approaches, such as Sheet Flow to Vegetated Filters and Conserved Open Space (Specification #2) and Grass Channels (Specification #3) may be appropriate. If the drainage area contains both managed turf and impervious cover, then the full range of practices should be considered.

Table 1. Comparative Runoff Reduction and Nutrient Removal for Practices

Practice	Design Level	Runoff Reduction	TN EMC Removal³	TN Load Removal			TP EMC Removal	TP Load Removal ⁶
Rooftop Disconnect	*1 ²*	*25 to 50 ¹*	0	*25 to 50 ¹*			0	*25 to 50 ¹*
	*No Level 2 Design*
Sheet Flow to Veg. Filter or Conserv. Open Space	1	*25 to 50 ¹*	0	*25 to 50 ¹*			0	*25 to 50 ¹*
	*2 ⁵*	*50 to 75 ¹*	0	*50 to 75 ¹*			0	*50 to 75 ¹*
Grass Channels	1	*10 to 20 ¹*	20		15			23
	*No Level 2 Design*
Soil Compost Amendment	Can be used to Decrease Runoff Coefficient for Turf Cover at Site. See the design specs for Rooftop Disconnection, Sheet Flow to Vegetated Filter or Conserved Open Space, and Grass Channel
Vegetated Roof	1	45	0	45			0	45
	2	60	0	60			0	60
Rainwater Harvesting	1	*Up to 90 ^{3, 5}*	0	*Up to 90 ^{3, 5}*			0	*Up to 90 ^{3, 5}*
	*No Level 2 Design*
Permeable Pavement	1	45	25	59			25	59
	2	75	25	81			25	81
Infiltration Practices	1	50	15	57			25	63
	2	90	15	92			25	93
Bioretention Practices	1	40	40	64			25	55
	2	80	60	90			50	90
Urban Bioretention	1	40	40	64			25	55
	*No Level 2 Design*
Dry Swales	1	40	25	55		20		52
	2	60	35	74		40		76
Wet Swales	1	0	25	25		20		20
	2	0	35	35		40		40
Filtering Practices	1	0	30	30		60		60
	2	0	45	45		65		65
Constructed Wetlands	1	0	25	25		50		50
	2	0	55	55		75		75
Wet Ponds	1	0	*30 (20) ⁴*	*30 (20) ⁴*		*50 (45) ⁴*		*50 (45) ⁴*
	2	0	*40 (30) ⁴*	*40 (30) ⁴*		*75 (65) ⁴*		*75 (65) ⁴*
Ext. Det. Ponds	1	0	10	10		15		15
	2	15	10	24		15		31
*Notes* ¹ Lower rate is for HSG soils C and D, Higher rate is for HSG soils A and B. ² The removal can be increased to 50% for C and D soils by adding soil compost amendments, and may be higher yet if combined with secondary runoff reduction practices. ³ Credit up to 90% is possible if all water from storms of 1-inch or less is used through demand, and the tank is sized such that no overflow occurs. The total credit may not exceed 90%. ⁴ Lower nutrient removal in parentheses apply to wet ponds in coastal plain terrain. ⁵ See BMP design specification for an explanation of how additional pollutant removal can be achieved. ⁶ Total mass load removed is the product of annual runoff reduction rate and change in nutrient EMC.

3. Level 1 and Level 2 Design Standards. Perhaps the most dramatic change in the new specifications is the design level approach. Virtually every practice has two design levels that correspond to different runoff and/or nutrient reduction rates. Each design level contains specific performance standards to improve the internal geometry of practices and enhance their hydrologic and nutrient removal performance. For example, the Level 1 and 2 design standards for bioretention basins are provided in Table 2.

Table 2. Bioretention Basin Design Guidelines

Level 1 Design (RR 40 TP: 25 )	Level 2 Design (RR: 80 TP: 50)
Sizing (Sec. 5.1): Surface Area (ft²) = Tv = [(1.0”)(Rv)(A)/12] – volume reduced by upstream BMP	Sizing (Sec. 5.1): Surface Area (ft²) = Tv = [(1.25”)(Rv)(A)/12] – volume reduced by upstream BMP
Maximum Drainage Area = 2 acres
Maximum Ponding Depth = 6 to 12 inches	Maximum Ponding Depth = 6 to 12 inches1
Filter media depth minimum = 24 inches; recommended maximum = 6 feet		Filter media depth minimum = 36 inches; recommended maximum = 6 feet
Media & Surface Cover (Sec. 5.6) = supplied by vendor; tested for acceptable phosphorus index
Sub-soil testing (Sec. 5.2): not needed if underdrain used; Min infiltration rate > 1.0 inch/hour to remove underdrain requirement;	Sub-soil testing (Sec. 5.2): one per 1,000 sf of filter surface; Min infiltration rate > 1.0 inch/hour to remove underdrain requirement
Underdrain (Sec. 5.7) = Schedule 40 PVC with clean-outs	Underdrain & Underground Storage Layer (Sec. 5.7) = Schedule 40 PVC with clean outs, and a minimum 12” stone sump below invert OR none if soil infiltration requirements are met (Sec. 5.2)
Inflow = sheetflow, curb cuts, trench drains, concentrated flow, or equivalent
Geometry (Sec. 5.3): Length of shortest flow path/Overall length = 0.3 OR other design methods to prevent short-circuiting One cell design (not including pretreatment cell)	Geometry (Sec. 5.3): Length of shortest flow path/Overall length = 0.8 OR other design methods to prevent short-circuiting Two cell design (not including pretreatment cell)
Pretreatment (Sec. 5.4): = pretreatment cell, grass filter strip, gravel/stone diaphragm, gravel/stone flow spreader, or other approved (manufactured) pretreatment structure	Pretreatment (Sec. 5.4) = pretreatment cell + one of the following: grass filter strip, gravel/stone diaphragm, gravel/stone flow spreader, or other approved (manufactured) pretreatment structure
Planting Plan (Sec. 5.8) = planting template to include turf, herbaceous, shrubs, and/or trees to achieve surface area coverage of at least 75% within 2 years	Planting Plan (Sec. 5.8) = planting template to include turf, herbaceous, shrubs, and/or trees to achieve surface area coverage of at least 90% within 2 years. If using turf, must combine with other types of vegetation1.
Building setbacks (Sec. 4): 0 to 0.5 Ac CDA = 10’ down-gradient; 50’ up-gradient 0.5 to 2.5 Ac CDA = 25’ down-gradient; 100’ up-gradient
Deeded maintenance O&M plan (Sec. 7)

4. Defined Flow Path

Many of the design specifications contain standards to assure that a minimum flow path is attained through the stormwater practice.
Figure 2 illustrates how these critically important hydrologic parameters are measured and defined.

Figure 2. Typical BMP Flow Path Parameters

5. Integrating Water Quality Treatment with Control of Larger Storms

Designers must also design stormwater practices to provide channel protection and flood control. The new specs allow for a runoff reduction credit that can be applied to reduce the detention storage volume needed to control larger design storm events. This is generally accomplished using the Runoff Reduction Method design spreadsheet.

The practices listed in Table 3 that provide an RR value do so either through a storage component and/or an elongation of the time of concentration, both of which attenuate the runoff and encourage infiltration and abstraction, resulting in a decrease in the computed release volume and peak discharge. The effectiveness of a practice to provide a reduction in volume or discharge during larger storms is a function of the relative volume of storage provided versus the volume of runoff. As the runoff depth increases, say from a 1-year frequency rainfall to a 10-year frequency event, the effectiveness of the storage at reducing the volume or peak discharge decreases.

The RRM spreadsheet provides the designer with this relative value for controlling larger storms by utilizing the annual RR value as retention storage and computing an adjusted (reduced) curve number using the TR-55 Runoff Equations (Equations 2-1 through 2-4; and/or in conjunction with TR-55 Figure 2-1). This new curve number is then used for computing the peak discharge for the larger storm as required by the channel protection of flooding requirements.

If the practice has a storage component that can be expanded in order to provide a great volume of storage for larger storm events, the designer may increase those components in accordance with guidance provided in the specifications or in the updated SWM Handbook. The designer may than choose to utilize the actual storage volume provided (rather than the RR value) and compute an adjusted curve number directly from TR-55 for the desired storm events.

It should be noted that a curve number must be computed for each storm event due to the diminishing effect of the storage as the rainfall depth increases. It should also be noted that the RR credit assigned in the spreadsheet, and not the actual storage, must be used for the water quality calculations. Additional guidance and computational procedures will be provided in the updated SWM Handbook.

The flow chart shown in Figure 3 outlines the general design process for accounting for channel protection and flood control storm events when runoff reduction practices are employed. In most cases, use of upland runoff reduction practices will greatly diminish or even eliminate the storage volumes needed to manage the larger storm events associated with channel protection and/or flood control.

Figure 3.Design Process for Modeling RR Adjustments for Larger Storm Events

6. Stormwater Hotspots

Stormwater hotspots are operations or activities that are known to produce higher concentrations of stormwater pollutants and/or have a greater risk for spills, leaks or illicit discharges. The actual hotspot generating area may only occupy a portion of the entire proposed site. If a site is designated as a potential stormwater hotspot, designers must prepare a Stormwater Pollution Prevention Plan (SWPPP) that outlines pollution prevention and treatment practices that will be implemented to minimize polluted discharges from the site. Depending on the potential severity of the hotspot, there may also be restrictions on practices that infiltrate stormwater into groundwater (see Table 4). ·

Restricted Infiltration. A minimum of 50% of the total treatment volume must be treated by a filtering or bioretention practice prior to any infiltration. Portions of the site that are not associated with the hotspot generating area should be diverted away and treated by another acceptable stormwater practice.
Infiltration Prohibition.

Table 3. Differences in Practice Sizing for Water Quality and Larger Storm Events

		Treatment Volume			Control of Larger Storm Events
Practice	No	ON/ OFF ¹	Level 1	Level 2	Channel Protection and Peak Discharge Control Capability?
Rooftop Disconnection	1	OFF	1 in *	NA	Partial, Adjust CDA CN using RRM Spreadsheet
Sheetflow to Veg. Filter of Conserved Open Space	2	OFF	1 in	NA	Partial, Adjust CDA CN using RRM Spreadsheet
Grass Channels	3	ON	1 in *	NA	Partial, Adjust CDA CN using RRM Spreadsheet and Increase Tc
Soil Compost Amendments	4	ON OFF	1 in *	NA	None
Vegetated Roofs	5	ON	1 in *	1	Partial, Adjust CDA CN using RRM Spreadsheet
Rainwater Harvesting	6	ON	1 in *	1.1	Partial, Adjust CDA CN using RRM Spreadsheet
Permeable Pavement	7	ON	1 in #	1.1	Partial to Full, Adjust CDA CN using RRM Spreadsheet and Add Storage in Reservoir
Infiltration	8	OFF	1 in #	1.1	Partial to Full, Adjust CDA CN using RRM Spreadsheet and Add Storage below underdrain
Bioretention	9	ON OFF	1 in #	1.25	Partial to Full, Adjust CDA CN using RRM Spreadsheet and add extra storage on surface, in soil, and below underdrain
Urban Bioretention	9A	OFF	1 in *	NA	None.
Dry Swales	10	ON	1 in *	1.1	Partial, Adjust CDA CN using RRM Spreadsheet and Increase Tc
Wet Swales	11	ON	1 in *	1.25	Limited. Adjust Tc
Filtering Practices	12	OFF	1 in	1.25	Partial, Adjust CDA CN using RRM Spreadsheet
Constructed Wetlands	13	ON	1 in	1.5	Full. Detention storage can be provided above pool or max ED level in the basin for channel protection and flood control
Wet Ponds	14	ON	1 in	1.5
Ext. Detention Ponds	15	ON	1 in	1.25
Notes: ¹ Whether the practice is normally designed as an on-line (ON) or off-line (OFF) relative to the primary flow path () indicates the practice may be designed to provide only a fraction of the treatment volume (Tv) when multiple practices are combined together. (#) indicates that small or micro-scale design applications may be designed with only partial treatment volume. Other terms*: CDA= contributing drainage area Cpv = channel protection volume, ED = extended detention Tc= time of concentration CN= curve number NA= not applicable RRM = runoff reduction method

7. Adapting Practices for Unique Terrain

Table 4: Comparison of Practices in Different Water Resource Settings

Practice	Spec No.	Karst Terrain ¹	Coastal Plain ²		Trout Waters ³	Ultra- Urban ⁴	Hotspots ⁵
Rooftop Disconnection	1	Preferred	Preferred		Preferred	Restricted	Accepted
Sheetflow to Veg. Filter or Conserved Open Space	2	Preferred	Preferred		Preferred	Restricted	Restricted
Grass Channels	3	Accepted	Restricted		Accepted	Restricted	Restricted
Soil Compost Amendments	4	Accepted	Accepted		Preferred	Preferred	Restricted
Vegetated Roofs	5	Preferred	Accepted		Accepted	Preferred	Accepted
Rainwater Harvesting	6	Preferred	Preferred		Preferred	Preferred	Accepted
Permeable Pavement	7	Preferred	Preferred		Preferred	Preferred	Prohibited
Infiltration	8	SS: Acc.		SS: Acc.	Preferred	Restricted	Prohibited
Infiltration	8	LS: Pro.		LS: Rest.	Preferred	Restricted	Prohibited
Bioretention	9	SS: Acc	Preferred		Preferred	Preferred	Accepted
Bioretention	9	LS: Rest.	Preferred		Preferred	Preferred	Accepted
Urban Bioretention	9A	Preferred	Accepted		Restricted	Preferred	Accepted
Dry Swales	10	Preferred	Preferred		Preferred	Restricted	Restricted
Wet Swales	11	Prohibited	Preferred		Accepted	Restricted	Restricted
Filtering Practices	12	Preferred	Accepted		Accepted	Preferred	Preferred
Constructed Wetlands	13	Accepted	Preferred		Accepted	Restricted	Restricted
Wet Ponds	14	Restricted	Accepted		Prohibited	Restricted	Accepted
Ext. Detention Ponds	15	Restricted	Restricted		Restricted	Restricted	Restricted
KEY		Preferred Practice: widely feasible and recommended
		Accepted Practice: can work depending on site conditions
		Restricted Practice: extremely limited feasibility
		Prohibited Practice: do not use due to environmental risk
NOTES: SS = small scale applications LS = large scale applications ¹ CSN Tech Bulletin No. 1 ²CSN Tech Bulletin No. 2 ³ CSN Tech Bulletin No. 6 ⁴ CSN Tech Bulletin No. 5 ⁵ CWP (2004)

The selection of the most effective stormwater practice depends on the nature of terrain, the intensity of development, and the sensitivity of the receiving water.
To assist designers, Table 4 presents a comparative matrix on which practices are recommended, acceptable, restricted or prohibited in the Commonwealth.
These areas include karst and coastal plain terrain, trout watersheds, ultra-urban watersheds and stormwater hotspots. 8. Spatial Scale at Which Practices are Applied The matrix provided in Table 5 compares the different spatial scales by which the various stormwater practices can be applied to reduce runoff and remove nutrients.

Table 5. Comparison of Practices Based on Contributing Drainage Area Served

Practice	Spec No.	Micro Scale	Small Scale		Normal Scale	Moderate Scale	Large Scale
Rooftop Disconnection	1	250 to 1000 sf
Sheet Flow to Veg. Filter or Conserved Open Space	2		1000 to 5000 sf		5000 to 25,000 sf
Grass Channels	3				20,000 sf to 250,000 sf
Soil Compost Amendments	4	250 sf to 2 acres
Vegetated Roofs	5	Residential 250 to 2000 sf	Commercial 2,000 to 200,000 sf
Rainwater Harvesting	6
Permeable Pavement	7	250 to 1000 sf	1000 to 10,000 sf		10,000 to 200,000
Infiltration	8	250 to 2500 sf		2500 to 20,000 sf	20,000 to 100,000 sf
Bioretention	9	250 to 2500 sf	2500 to 20,000 sf		20,000 to 100,000 sf
Urban Bioretention	9A	250 to 2500 sf	2500 to 20,000 sf
Dry Swales	10				20,000 to 250,000 sf
Wet Swales	11				20,000 to 250,000 sf
Filtering Practices	12				20,000 to 250,000 sf
Constructed Wetlands	13						10 + more acres, unless favorable water balance
Wet Ponds	14
Ext. Detention Ponds	15

The major change in the new specifications is that most practices are applied at a smaller spatial scale than had been done in the past,
which means that more practices will need to be installed at each site. Note that the area ranges for the contributing drainage area (CDA)
are approximate, and may be greater or smaller depending on design and site conditions. Multiple practices of the same or different kind
may be used in combination to treat a larger CDA.

9. Recommended Construction Sequence Recent studies indicate the importance of proper construction methods to ensure that stormwater practices actually meet their intended
design function (Hirschman et al, 2009). Consequently, each design specification contains extensive information on the proper construction
method for the practice, along with checklists and other construction inspection criteria. 10. Maintenance Inspections Maintenance is essential to ensure that practices achieve their hydrologic and pollutant removal functions over time. The new specifications
include more detailed information on how to conduct maintenance inspections that, in turn, trigger specific tasks that must be done to maintain
performance. An example of these maintenance inspection points (for Permeable Pavement) can be found in Table 6. The specifications also
provide more detail on the minimum elements in required maintenance agreements, which are essential when more and smaller stortmwater
practices are employed at a site.

Table 6. Suggested Annual Maintenance Inspection Points for Permeable Pavements

Activity

· The drawdown rate should be measured at the observation well for three days following a storm event in excess of 0.5 inch in depth. If standing water is still observed in the well after three days, this is a clear sign that that clogging is a problem.

· Inspect the surface of the permeable pavement for evidence of sediment deposition, organic debris, staining or ponding that may indicate surface clogging. If any signs of clogging are noted, schedule a vacuum sweeper (no brooms or water spray) to remove deposited material. Then, test sections by pouring water from a five gallon bucket to ensure they work.

· Inspect the structural integrity of the pavement surface, looking for signs of surface deterioration, such as slumping, cracking, spalling or broken pavers. Replace or repair affected areas, as necessary.

· Check inlets, pretreatment cells and any flow diversion structures for sediment buildup and structural damage. Note if any sediment needs to be removed

· Inspect the condition of the observation well and make sure it is still capped

· Generally inspect any contributing drainage area for any controllable sources of sediment or erosion

11. More Defined Feasibility Criteria

Table 7 compares some common feasibility constraints for the range of stormwater practices, including soil restrictions, maximum slopes, available head, space foot print and minimum depth to water table and bedrock. Designers should consult each individual practice specification for additional restrictions, setbacks and environmental constraints.

Table 7: Comparison of Site Feasibility of Practices

Practice	No.	Soils ¹		Other Site Constraints ²
Practice	No.	HSG A/B	HSG C/D	DEPTH WT/BR³	MIN HEAD⁴	MAX SLOPE⁵	SPACE (%) ⁶
Rooftop Disconnection	1	B only	Yes, w/ 2^nd RR ⁷	2 ft	1 foot	1 - 2%	Nominal
Sheet Flow to Veg. Filter or Conserved Open Space	2	Yes	Yes, w/ CA ⁸	2 ft	1-2 ft	6 - 8% ⁹	15 - 25%
Grass Channels	3	Yes	Yes, w/ Ad. RT ¹⁰	2 ft	2-3 ft	2 - 4%	5 - 15%
Soil Compost Amendments	4	Not on A soils	Yes	1.5 ft	1 ft	10%	Nominal
Vegetated Roofs	5	NA ¹¹	NA	NA	1-2 ft	Varies	Nominal
Rainwater Harvesting	6	NA	NA	NA	Varies	NA	Nominal
Permeable Pavement	7	Yes, w/ Ad IR¹²	Yes, w/ UD ¹³	2 ft	2-4 ft	1 - 3% ¹⁴	Nominal
Infiltration	8	Yes w/ Ad IR ¹²	NO	2 ft	2-4 ft	0 to 5%	1 - 4%
Bioretention	9	Yes	Yes, w/ UD ¹³	2 ft	4-5 ft	1 to 5%	3 - 5%
Urban Bioretention	9A	NA	NA	NA	3-5 ft	NA	Nominal
Dry Swales	10	Yes	Yes, w/ UD ¹³	2 ft	3-5 ft	4%	5 - 15%
Wet Swales	11	No	Yes	0 ft ¹⁶	2 ft	2%	5 - 15%
Filtering Practices	12	NA	NA	2 ft	2-10 ft	NA	0 - 3%
Constructed Wetlands	13	Yes, w/ Liner ¹⁵	Yes	Below ¹⁶	2-4 ft	NA	3%
Wet Ponds	14	Yes, w/ Liner ¹⁵	Yes	Below¹⁶	6-8 ft	NA	1 - 3%
Ext. Detention Ponds	15	Yes, w/ Liner ¹⁵	Yes	2 ft	6-10 ft	NA	1 - 3%
Notes ¹ NRCS Hydrologic Soil Groups (HSG)² These are general ranges only. ³vertical distance from bottom invert of practice and water table and bedrock, may be different in karst and/or coastal plain terrain ⁴vertical distance from inflow to practice and its bottom invert⁵maximum internal slope of the practice⁶ typical footprint of practice as percent of site area ⁷with an acceptable secondary runoff reduction practice such as rain garden, dry well or CA-amended filter path ⁸ filter strip w/ compost amendment (CA) ⁹ 6% for conservation filter and 8% for grass filter strip ¹⁰ grass swale must achieve minimum residence time (RT) of ten minutes ¹¹ Not Applicable (NA) ¹² with minimum measured infiltration rate (IR) of 0.5 inches/hr ¹³ with underdrain ¹⁴ slopes can be broken up by terracing. ¹⁵ depending on borings, a liner may be needed to hold water ¹⁶ for water table only, 2 foot distance to bedrock still required

12. Unified Terminology Table 8 contains a list of the key terminology and abbreviations that are found throughout the specifications.

Table 8. List of Key Units and Abbreviations Used in Specs

A	Site Area (acres)	IP	Interlocking Concrete Pavers
ASTM	A. Society of Testing Materials	P	Annual Precipitation
CA	Compost Amendments	PA	Porous Asphalt
CDA	Contributing Drainage Area	PC	Porous Concrete
CEC	Cation Exchange Capacity	Rv	Runoff Coefficient
Cpv	Channel Protection Volume	RCS	Regenerative Conveyance System
CN	NRCS Curve Number	RRv	Runoff Reduction Volume
CSN	Chesapeake Stormwater Network	SA	Surface Area of Practice
CWP	Center for Watershed Protection	SRP	Secondary RRv Practice
ED	Extended Detention	Tc	Time of Concentration
EMC	Event Mean Concentration	Tv	Water Quality Treatment Volume
ESC	Erosion and Sediment Control	TP	Total Phosphorus
H:V	Horizontal to Vertical (slopes)	TN	Total Nitrogen
HSG	Hydrologic Soil Group	WQv	Really means Tv
IC	Impervious Cover

13. Appendices to this Introduction Document

There are five Appendices attached to this introduction document. The Introduction involves information common to most or all of the BMPs.
Similarly, these five Appendices were actually individual BMP specifications in the 1999 Virginia Stormwater Handbook for omponents of various
pond practices. They include:

Earthen Embankments

Principal Spillways

Vegetated Emergency Spillways
Sediment Forebays
Landscaping

Since these five specifications still apply to more than one other individual BMP design, they are presented here as Appendices A through E.

14. Acknowledgements

These new BMP standards and specifications for complying with the Virginia Stormwater Management Law and Regulations were developed in
partnership with Scott Crafton of VA DCR with assistance from Tom Schueler of the Chesapeake Stormwater Network (CSN), Dave Hirschman,
Kelly Collins, Laurel Woodworth and Greg Hoffman of the Center for Watershed Protection (CWP), Joe Battiata of the Williamsburg Environmental
Group, David Smith of the Interlocking Concrete Pavement Institute (ICPI), and prior efforts by the Northern Virginia Regional Commission (NVRC),
and the Engineers and Surveyors Institute (ESI) of Northern Virginia .

15. References.

Center for Watershed Protection (CWP). 2004. Pollution source control practices. Manual 8 in the Urban Subwatershed Restoration Manual Series.
Center for Watershed Protection. Ellicott City , MD

Center for Watershed Protection (CWP) and Chesapeake Stormwater Network (CSN). 2008. Technical Support for the Baywide Runoff Reduction Method.. Baltimore , MD www.chesapeakestormwater.net

CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection. Ellicott City , MD.

Chesapeake Stormwater Network (CSN). 2009a. Technical Bulletin No. 1. Stormwater Design Guidelines for Karst Terrain in the Chesapeake Bay watershed. www.chesapeakestormwater.net

CSN. 2009b. Technical Bulletin No. 2. Stormwater Design Guidelines for Coastal Plain Terrain in the Chesapeake Bay watershed. www.chesapeakestormwater.net

CSN. 2009c. Technical Bulletin No. 5. Stormwater Design Guidelines for Trout Waters in the Chesapeake Bay watershed. www.chesapeakestormwater.net

CSN. 2009d. Technical Bulletin No. 6. Stormwater Design Guidelines for Ultra-urban areas in the Chesapeake Bay watershed. www.chesapeakestormwater.net

Hirschman, D., L Woodworth and J. Tribo. 2009. Technical Memo: Results of BMP Evaluation Survey in the James River Basin . Center for Watershed Protection. Charlottesville , VA.

Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax , VA.

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond , VA.