Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

1 40 CFR 450.21(a) Erosion and Sediment Controls. Design, install and maintain effective erosion controls and sediment controls to minimize the discharge of pollutants. At a minimum, such controls must be designed, installed and maintained to: ( 4 ) M inimize the

disturbance of steep slopes

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(1) M inimizing the disturbance of steep slopes control s or minimiz

es the discharge of

stormwater from the site

.

Soil erosion rates are a function of a number of parameters, including soil particle size distribution, soil texture, cohesiveness, rainfall energy, slope, slope length, and the type and quantity of cover. Exposed soils are much more prone to erosion than covered or stabilized soils. In addition, all other factors being equal, erosion on steeper sloped areas will be much greater than erosion on flatter areas. This is because gravity will cause water to generally flow faster down a steeper slope than a flatt er slope (other factors, such as surface roughness being held constant). Generally, projects on steeper slopes would generate greater quantities of sediment, turbidity and other pollutants than those on flatter slopes because of the faster flowing
water, w hich can mobilize and transport more sediment

. Since downslope sediment controls are not 100% effective in removing sediment, turbidity and other pollutants (see example in (a)(1) Supplementary Explanation), minimizing both the amount of land on steeper sl opes exposed as well as the duration of exposure on steeper slopes would be expected to reduce the amount of pollutants contained in stormwater discharges. Minimizing the disturbance of steep slopes during construction activity can be accomplished through a number of practices. These include practices related to how much soil is exposed on steep slopes at one time , such as phasing land disturbing activities, and providing timely soil stabilization on slopes, such as through the use of mulches, rolled erosio n control products and vegetation. Permittees should have flexibility to select appropriate controls to minimize disturbance of steep slopes at their individual

sites. Permittees also should have flexibility to schedule and phase construction activities so as to limit the amount of land disturbed at one time and the duration of exposure on steep slopes.

The final rule does not specify what is considered a steep slope. EPA has left this decision up to the permitting authority, as local and regional considera tions such as soil types, soil erodibility and rainfall and climatic factors are important to consider in the context of establishing specific permit language. However, due to the fact that disturbed soils on steep slopes have much higher erosion rates tha n disturbed soils on flatter slopes (all other factors being equal), disturbances on steep slopes are given special consideration in the rule. Permitting authorities may want to consider a number of factors when developing specific criteria for permits.

W hile both slope length and slope steepness are factors that determine soil erosion rates on slopes, soil loss increases more rapidly with slope steepness than it does with slope length. One commonly accepted method of estimating soil loss for exposed areas (including construction sites) is the Revised Universal Soil Loss Equation (RUSLE). In RUSLE, slope steepness and slope length are considered through the use of the slope length factor (L) and slope steepness

facto r (S). The combination of the slope length factor and the slope steepness factor is known as the topographic factor (LS). An example of the influence of both slope length and slope steepness can be found in the RUSLE documentation (K.G. Renard, G.R. Foster , G.A. Weesies, D.K. McCool and D.C. Yoder, Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE), Agricultural Handbook Number 703, United States Department of Agriculture, 1997). The RUSL E manual contains tables of LS factors for different slope lengths and slope steepness. Table 4

- 3 of the manual, which is appropriate for evaluating construction sites where the ratio of rill to interrill erosion is high, can be used to illustrate this rel ationship. For example, the LS factor for a 4%Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

2 slope steepness and a 100

- foot slope length is 0.55. If the slope steepness is held the same (4%) but the slope length is doubled (to 200 feet), the LS factor increases to 0.79, an increase

of 44%. If the slop e steepness is doubled while the slope length is held the same, the increase is the LS factor is much greater. For example, doubling the slope steepness to 8% from 4% with a 100 foot slope length yields an LS factor of 1.1, which is a 100% increase. Likewi se, with a 200 foot slope length, doubling the slope steepness from 4% to 8% yields an LS factor of 1.72, which is a 118% increase. This illustrates how erosion rates can increase considerably as both slope length and slope steepness increase, which provid es a rationale for specific provisions in the final rule addressing disturbance on steep slopes.

(2) What EPA does not mean by this requirement

.

While the rule establish es conditions for minimizing disturbance on steep slopes, the rule does not prevent or prohibit disturbance on steep slopes or creation of a steep slope if such slopes are integral to the project construction

. EPA recognizes that for some projects, disturban ce on steep slopes may be necessary for construction (an example would include a road cut and fill in mountainous terrain). Rather, this provision is intended to provide permitting authorities with the ability to develop specific permit language re flecting such factors as duration of exposure, slope steepness, slope length, time of year when disturbance occurs, and appropriate BMPs for different slope conditions. This provision is also intended to allow permittees flexibility in selecting appropriate BMPs f or different site conditions. For example, permittees may select a number of products for stabilizing steep slopes, including both vegetative and non

- vegetative systems, to meet specific project requirements (consistent with requirements established by the permitting authority). EPA does not intend this requirement to apply to creation of soil stockpiles, although control of runoff from soil stockpiles may be required by other provisions of the rule, such as the provision at 450.21(a) to minimize the discha rge of pollutants and 450.21(a)(3) to minimize the amount of soil exposed.

(3) In its comments on the proposed C&D ELG/NSPS, NAHB expressed support for non

numeric effluent limitati ons in the form of BMPs like this provision

.

In its cover comment letter on the proposed C&D ELG/NSPS, NAHB stated that it “has consistently advocated for an ELG that allows for site

- specific selection of BMPs, appropriate sediment basin sizing criteria and the implementation of pollution prevention practices.” NAHB comment cover letter at 2. NAHB stated that EPA’s proposed Option 1, a BMP

- only ELG, “would build upon the significant environmental progress made to date by ensuring that all construction sites meet a minimum standard of excellence by a dopting those erosion and sediment controls that have evolved and are now accepted across the country.” Id. NAHB urged EPA to broaden “best management practices and pollution prevention to ensure these practices can be implemented at every permitted cons truction site, in all circumstances across the country.” Id. at 3.

In its detailed comments attached to its cover letter, NAHB continued to support a BMP

- only ELG to control stormwater discharges from active construction sites. NAHB urged EPA “to remove any requirements to meet numeric effluent limits and proceed with a BMP

- based ELG to control stormwater discharge.” NAHB detailed comments at 38. NAHB claimed that “[i]n the case of EPA's proposed C&D ELGs, the combination of ELG precedent and the fact that EPA is focusing exclusively on stormwater provides irrefutable support for a BMP

- based ELG approach.” Id. at 42. NAHB concluded that “[t] here is substantial authority for and evidence regarding the success associated with permitting authoritie s relying on BMP

- based approachesSupplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

3 to achieve environmental objectives. EPA also should rely on a BMP

- based approach for the C&D ELG rulemaking.” Id. at 46 (emphasis in original).

In addition, in its redline/strikeout “precisely crafted edits” of EPA’s p roposed Option 1, the BMP

only ELG, NAHB suggest ed a revision to proposed 450.21(a)( 10 ) to make that provision read as follows: “Minimize duration of disturbance of steep slopes.” (EPA had proposed “Minimize disturbance of steep slopes.”) Thus, while NA HB suggested targeting this BMP to minimizing the amount of time that steep slopes are disturbed, it did not suggest deleting the requirement, thereby acknowledging, consistent with its overall comments in support of a BMP

- only ELG, that minimizing, at a m inimum, the duration of disturbance of steep slopes is key to controlling and minimizing the discharge of stormwater from active construction sites. See reference to “precisely crafted edits” on page 144 of NAHB’s detailed comments on the proposed

ELG/NSP S, as well as the attachment to EPA

- HQ

- OW

- 2008

- 0465

- 1360. In addition, NAHB recognized on pages 159

- 60 of its detailed comment letter “the increased pollutant potential associated with disturbed sloped areas” and recommended two items to EPA, neither of w hich is limited to or specifically focused on the “duration” of steep slope disturbance, and both of which are consistent with EPA’s final requirement to “Minimize the disturbance of steep slopes.”

( 4 ) Implementation of this provision in NPDES permits.

The construction stormwater ELG will be implemented in NPDES permits as the applicable technology

- based effluent limitation s in those NPDES permits. The ELG is not directly applicable to any regulated entity . If the permitting authority chooses to copy the ELG verbatim into its draft proposed NPDES permit as the technology

- based effluent limitation, dischargers will have an opportunity to provide comment on the need for more specificity if they are

concerned that the narrative nature of the limit could unfairly subject them to arbitrary

enforcement. Furthermore, in the e vent that the permitting authority does not provide any additional specificity, it is t ypical in stormwater permits for active construction for the permitting

authorities to include a requirement to prepare a storm water pollution prevention plan ( SWPPP ) in the permit. While the provisions of the SWPPP are not effluent limitations themselves, they indicate how the discharger intends to meet the narra tive effluent limitati ons in the permit. As

such

, SWPPPs can be helpful to the regulated entity in demonstrating how they are meeting

their effluent limitations.

In the 2012 CGP

, EPA has included a requir ement in the final permit for permittees to minimize the disturbance of steep slopes. EPA has defined st eep slopes as follows: “ where a state, Tribe, local government, or industry technical manual (e.g., stormwater BMP manual) has defined

what is to be consider ed a “steep slope”, this permit’s definition automatically adopts that definition. Where no such definition exists, steep slopes are automatically defined as those that are 15 percent or greater in grade. ” EPA believes that this definition is responsive to concerns that the proposed definition of 15 percent was not flexible enough. EPA elaborates in the final permit fact sheet by describing types of practices that minimize steep slope disturbances. EPA also clarifies that the requirement to minimize distu rbance of steep slopes d oes not prohibit

disturbances of steep slopes and that

, in fact, f or some projects disturbances of steep slopes are required for the project, and thus, it would not be practicable to minimize the disturbance of steep sl op es beyond w hat is integral to the project (e.g., road cut in mountainous terrain)

.

( 5 ) A ppropriate time for implementation of this BMP in the construction process

.Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

4 The appropriate time for implementing controls on disturbances of steep slopes will be project specific. Generally, permittees may want to consider, during the planning phase of projects, whether project configurations or alignments can be adjusted to avoid disturbing these areas altogether. If disturbance of steep slopes is necessary, construction scheduling and sequencing should consider when steep slopes are to be disturbed

, and disturbed slopes should be

stabilized as soon as feasible or directed by the permitting authority . By phasing construction activities, permittees may be able to limit the area of steep slopes disturbed at one time.