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) Minimize the

disturbance of steep slopes.

____________________________________________________________________________

(1) Minimizing the disturbance of steep slopes controls or minimizes 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 flatter 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, which 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 slopes 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 erosion 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 considerations 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 than 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.

While 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 

factor (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 RUSLE 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 relationship. 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 slope 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. Likewise, 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 provides 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 establishes 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, disturbance 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 reflecting 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 for 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 discharge 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 nonnumeric effluent limitations 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 adopting 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 construction 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 authorities relying on BMP-based approaches 
Supplemental 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 proposed Option 1, the BMPonly ELG, NAHB suggested 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 NAHB 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 minimum, 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/NSPS, 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 which 

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 limitations 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 event that the permitting authority does not provide any 

additional specificity, it is typical 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 narrative effluent limitations 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 requirement in the final permit for permittees to minimize 

the disturbance of steep slopes. EPA has defined steep slopes as follows: “where a state, Tribe, 

local government, or industry technical manual (e.g., stormwater BMP manual) has defined 

what is to be considered 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 disturbance of steep slopes does not prohibit 

disturbances of steep slopes and that, in fact, for some projects disturbances of steep slopes 

are required for the project, and thus, it would not be practicable to minimize the disturbance of 

steep slopes beyond what is integral to the project (e.g., road cut in mountainous terrain).

(5) Appropriate 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.