Proposed Federal Stormwater Guidance
Vol. 18 No. 5

May 3, 2010

The Chesapeake Bay Executive Order 13508 (Chesapeake Bay Protection and Restoration) Section 502 Draft Guidance for Federal Land Management in the Chesapeake Bay Watershed, dated March 15, 2010, (here after referred to as "Section 502") outlines implementation measures to control nonpoint source nutrient and sediment pollution from federal lands within the Chesapeake Bay watershed. This is very likely to become a requirement for all development and public works projects in the Chesapeake Bay watershed either through updated MS-4 permits or through upcoming state stormwater regulations (since they require U.S. Environmental Protection Agency [EPA] approval).

Section 502 Chapter 3, Technical Guidance on Controlling Urban Runoff in the Chesapeake Bay, describes two options for "Implementation Policies to Preserve and Restore Predevelopment Hydrology:"

  • Option 1, "Retain the 95th Percentile Rainfall Event;" and
  • Option 2, "Site-Specific Hydrologic Analysis."
Wetland Studies and Solutions, Inc. (WSSI) has reviewed both Section 502 Chapter 3 and EPA's publication, "Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act." The above-mentioned documents indicate that Option 1, "Retain the 95th Percentile Rainfall Event," will require the retention, infiltration, or reuse of the 95th percentile rainfall event, or approximately 1.7" in the Washington. D.C. area. Our review of these documents raised a number of questions as to the definitions of "retain" and "predevelopment hydrology," and the practicality and implications of the proposed guidance. The following sections discuss each of these items in greater detail.

Issue 1: Defining "Retain"

WSSI was unable to locate a definition for "retain" in the above-mentioned documents, and it was unclear in the documentation whether use of the word, "retain," allowed for slow release of the stormwater over time (as is typically done in civil engineering design) or if all stormwater must be used onsite. After contacting the EPA, "retention" as per the Section 438 guidance was clarified to mean "that the water will be evapotranspired, infiltrated or used onsite and not temporarily detained and discharged slowly over some predetermined period" (per e-mail correspondence from Robert Goo; April 6, 2010). Based on this definition, in the Washington, D.C. area, storms less than or equal to 1.7" must be used onsite – i.e., no water can be released on the surface from 24-hour storm events less than 1.7".

Issue 2: Defining "Predevelopment Hydrology"

WSSI was also unable to locate a definition for the term "predevelopment hydrology." This term was also clarified in the e-mail from Mr. Goo: "Predevelopment hydrology should be considered the hydrology of the site using the historic land cover typology of the site as a reference condition, e.g., mature deciduous hardwood forest or meadow. If more specificity is desired, soils, geology, slope, etc. can be factored in. Pre-development hydrology is not the existing conditions on the site directly before the development or redevelopment activity takes place." Therefore, predevelopment hydrology should be considered pre-settlement hydrology.

Issue 3: Period of Time over Which Retained Water Must be Used

The above-mentioned documents do not appear to provide a period of time over which the water must be retained, infiltrated, or reused. (We have contacted the EPA to clarify this issue, but we have not received a response back at the time of this publication.) For instance, in terms of a rainwater harvesting cistern which provides water for toilets, water can only be reused as quickly as people flush:

  • WSSI currently captures 0.25" of rainwater from our office roof for use in our toilet system, and we are unable to use all of the water between storm events. In fact, during the summer of 2009, the cistern did not empty despite a long dry spell in August. WSSI captures an additional 0.5" of rainwater from our office roof to irrigate our landscaping (although we could easily use a larger cistern for summertime irrigation). In the winter when irrigation is not needed, rainwater bypasses the irrigation cistern into a raingarden (which has an underdrain because of clay soils that are present on the site).

  • For a typical townhouse with a 700 s.f. roof, 1.7" of rain equates to approximately 750 gallons of water. With "new" standard toilets that consume 1.6 gallons per flush, a family would have to flush 470 times to use the captured water. With 2.5 people (average) per townhouse flushing 4 times per day, it would take 47 days to empty the cistern if it does not rain for those 47 days (which is very unlikely!). We also note that, currently, in most parts of Virginia, reuse of rainwater in residential toilets is prohibited.
Therefore, a system would need to be greatly oversized to capture back-to-back storms, unless the second storm is allowed to bypass the requirement for retention, infiltration, or reuse. Based on the typical hydrology of undeveloped land, the second of two back-to-back storms would tend to run off the site due to the already saturated ground, even if it is a smaller storm than would typically produce runoff.

We acknowledge that toilet flushing is certainly not the only reuse for captured rainwater. Other uses such as irrigation, cooling towers, car washing, etc, however, also have practical issues that need to be considered. (For example, landscaping only needs irrigation during part of the year in the D.C. area, and cooling towers are only used for certain HVAC systems that are not used in many building types.) Unfortunately, for sites with low-permeability soils, infiltration (the least expensive option) is simply not physically possible. Thus, practicable alternatives are needed when significant volumes of water cannot be reused, infiltrated, or evapotranspired.

Another possible way to intercept rainwater is through the use of green roofs. Conservatively assuming 30% void space in the planting medium on a green roof, a roof with 5.7" of soil will capture approximately 1.7" of rainwater. (Depending on the type of vegetation, there will also be some uptake by the plants, although it will not occur quickly enough to help with brief, intense storms.) Based on our own experience, this is also the approximate depth at which typical green roof plants (such as sedums) flourish; we find that a minimum of 6" of soil medium, versus the more typical extensive green roof soil depth of 2 to 4 inches, produces more vigorous, hardy plants.

(We note that there may be some drainage during storm events depending on the antecedent moisture conditions and rainfall intensity, but hopefully this practice will be deemed to meet the retention requirements.)

The Section 502 guidance should also promote an expansion of pervious pavement and rain garden usage. However, in clay/silty soils, underdrains will need to be employed to ensure success of the technique – how underdrains relate to EPA guidance is unknown at this time.

WSSI provided comments to the EPA on Section 502 regarding this issue. (The final day for comments was April 23, 2010.) We recommend providing language in the guidance document that allows for a slow release of any water that cannot be practicably retained, infiltrated, or reused, such as:

The 95th percentile storm event shall be captured on the site and retained, infiltrated, evapotranspired, or reused to the maximum extent practicable based on soil type and on building and landscaping needs. After the captured water has been retained, infiltrated, evapotranspired, or reused to the maximum extent practicable, excess water shall be allowed to release from the site at a rate less than or equal to the runoff rate of a 1-year, 24-hour storm event from a forest in good condition. Flows from rain events in excess of the 10-year event will be exempt from this requirement and will be allowed to release from the site without additional detention or rate reduction.
This language provides developers with a strong economic incentive to reuse, infiltrate, or evapotranspire captured stormwater because increasing the reuse rate will reduce the size of storage tanks required to detain any overflow for release. It also allows design engineers a way to meet the Section 502 requirements with less ambiguity.

The Section 502 guidance will likely greatly increase the cost of stormwater management in many projects in this area (i.e., those with low permeability soils), but the language above avoids the current version's conundrum of creating an impossible-to-meet criterion for projects on clayey soils.

Implications for Projects in Varying Soil Types

As part of WSSI's review of these documents, we analyzed the implications of this guidance on sites with varying soil types (hydrologic soil groups A, B, C, and D) using Technical Release 55 "Urban Hydrology for Small Watersheds" (TR-55)1. Using Equation 2-4, we can solve for the Potential Maximum Retention After Runoff Begins (S):



- 10


where, S = Potential Maximum Retention After Runoff Begins (in.)
CN = Curve Number (a number assigned to a particular soil type and condition indicating the runoff potential. A higher curve number equates to greater runoff potential; for instance, the CN for asphalt is 98, while the CN for a forest with sandy soils is 30.)

WSSI used standard Curve Number (CN) values for forested landuse in good condition for soil types A-D2 to solve for S, which was then used to back-calculate the point where runoff (Q) begins using TR-55 Equation 2-3:

(P - 0.2S)2
(P + 0.8S)
where, Q = Runoff (in.)
P = Rainfall (in.)
S = Potential Maximum Retention After Runoff Begins (in.)
The figure below shows that, for soil type A, Option 1 ("Retain the 95th Percentile Rainfall Event") should be used because runoff occurs at 4.71" of precipitation (greater than the 1.7" cut-off); however, for soil types C and D, Option 2 ("Site-Specific Hydrologic Analysis") should be used because runoff is initiated at 0.88" and 0.61" respectively (less than the 1.7" cut-off). Runoff is initiated at 1.66" for soil type B (very close to the 1.7" cut-off); therefore, if a site is characterized by primarily type B soils, the engineer will want to assess which method will best satisfy the project's stormwater management goals on a case-by-case basis based on the site-specific mix of soil types (click on the graphic below for a larger view).

The following table displays this information in a tabular format and provides the 24-hour, 1-, 2-, 5-, 10-, and 100-year storm events for Fairfax, Loudoun, and Prince William Counties (all of which are greater than 1.7").

District of Columbia's Draft MS-4 Requirements

In the District of Columbia's (the District's) draft Municipal Separate Storm Sewer System (MS-4) permit (Draft NPDES Permit No. DC0000221 and associated fact sheet), Section 4.1.1.b, "Performance Standard for Federal Facilities," the EPA has used the following language:

"The District shall ensure through requirements for design, construction and maintenance that federal facilities undertaking new or redevelopment of 5,000 square feet or more comply with one of the following:

  • Adopt the design, construction and maintenance of stormwater controls to achieve on-site retention of 1.7" of stormwater from a 24-hour storm with a 72-hour antecedent dry period through evapotranspiration, infiltration and/or stormwater harvesting and use for all new development and redevelopment greater than 5,000 square feet in the District; or

  • Adopt the design, construction and maintenance of stormwater controls to achieve the retention of the predevelopment runoff volume of stormwater from a 24-hour storm with a 72-hour antecedent dry period through evapotranspiration, infiltration and/or stormwater harvesting and use for all new development and redevelopment greater than 5,000 square feet in the District. Determination of the predevelopment runoff volume must be based on a full hydrologic and hydraulic analysis of the site that ensures maintenance of predevelopment hydrographs (volume, rate and duration) for the 1, 2, 10 and 100 year 24-hour storm events. The modeled predevelopment condition must be meadow."
The District's draft MS-4 permit (Sections 4.1.1.a and 4.1.1.b) differentiates between federal and non-federal facilities. For non-federal facilities, the on-site stormwater retention requirement is 1.2", whereas the retention requirement for federal facilities is 1.7" (as it is in EPA's draft guidance). In addition, the EPA has specifically stated that the "modeled predevelopment condition must be meadow," for both cases, rather than the undefined "predevelopment" condition as described in stormwater guidance documents previously discussed. The table below lists CN differences between forest and meadow:

 Soil type

 Curve Number*



A  30   30 
B  55   58 
C  70   71 
D 77 78
* These CNs were taken from TR-55:  Urban Hydrology for Small
Watersheds, Table 2-2c.  Runoff
curve numbers for other agricultural lands. (Soil Conservation Service, 1986)

By including language in the District's MS-4 permit requiring the replication of predevelopment hydrology for the 1-, 2-, 10-, and 100-year 24-hour storm events in lieu of the 1.7"/1.2" retention option, many site developers will likely choose the retention option unless there are downstream flooding issues. It is also likely that EPA will move to include similar language in other MS-4 permits that are currently up for renewal and those that will expire soon.

Implications for Federal Stormwater Regulation

The Section 502 federal guidance and its implementation in MS-4 permits may also provide a mechanism for the implementation of proposed federal stormwater regulations, specifically S. 1816 in the Senate and H.R. 3852 and H.R. 3265 in the House of Representatives. These bills would require:

S. 1816, Section 117(j)(3)(A)(i): The owner or operator of any development or redevelopment project possessing an impervious footprint that exceeds a threshold to be determined by the Administrator through rulemaking, will use site planning, design, construction, and maintenance strategies for the property to maintain or restore, to the maximum extent technically feasible, the predevelopment hydrology of the property with regard to the temperature, rate, volume, and duration of flow.

H.R. 3952 Section 117(i)(3)(B) and H.R. 3265, Section 3(j)(1)(B): REQUIREMENTS- A permit under section 402(p) for a unit of local government within the Chesapeake Bay watershed shall include requirements to ensure that a project to develop land within the jurisdiction of such unit of local government that affects land that is more than 1 acre in size and that is less than 5 percent covered by impervious surfaces prior to the project is carried out in a manner that not less than the volume of the 95th percentile precipitation event shall infiltrate, evapotranspirate from, or be harvested and used on such site after the project is completed.
For more information, please contact Mike Rolband, Jennifer Brophy-Price, Bethany Bezak, Bill Nell, or Frank Graziano.

 Soil Conservation Service, 1986.  Technical Release 55.
 The CN used for soil types A, B, C, and D ("Woods") were respectively 30, 55, 70, and 77 (TR-55; Table 2-2c. Runoff curve numbers for other agricultural lands).