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

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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 scontrols must be designed, installed and maintained to: (1) Control 

stormwater volume and velocity within the site to minimize soil erosion.

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(1) Controlling stormwater volume and velocity within the site to minimize soil erosion

relates to controlling or minimizing the discharge stormwater from the site.

Control of volume and velocity of stormwater in conveyances where concentrated flow occurs 

as well as control of volume and velocity of sheet flow on site are necessary to reduce 

mobilization, transport and discharge of sediment and other pollutants. EPA notes that this 

requirement reflects common practice for water handling on construction sites. The need for 

effective erosion control practices is an important component of stormwater management on 

construction sites and is well-known and described in available references. For example, the 

Virginia Erosion and Sediment Control Handbook (Third Edition, available at 

http://www.dcr.virginia.gov/stormwater_management/e_and_s-ftp.shtml) describes the need for 

appropriate water handling practices. At page II-14, the Handbook states:

“The removal of existing vegetative cover and the resulting increase in impermeable 

surface area during development will increase both the volume and velocity of runoff. 

These increases must be taken into account when providing for erosion control.”

The handbook describes the practices that are appropriate for managing the volume and 

velocity of stormwater.

“Keeping slope lengths short and gradients low and preserving natural vegetative cover 

can keep stormwater velocities low and limit erosion hazards. Runoff from the 

development should be safely conveyed to a stable outlet using storm drains, diversions, 

stable waterways, riprapped channels or similar measures.”

“Conveyance systems should be designed to withstand the velocities of projected peak 

discharges. These practices should be operational as soon as possible after the start of 

construction.”

Examples of appropriate controls to address this provision include management of concentrated 

flows through the use of channel liners or other stabilization measures to minimize erosion 

caused by flowing water in channels, use of pipe slope drains to move water down slopes to 

minimize erosion, use of check dams in channels to reduce flow velocities and minimize 

erosion, and use of sediment basins and traps to provide detention and reduction in peak 

flowrates which minimizes downslope erosion. Examples of practices to reduce volume and 

velocity of stormwater with respect to sheet flow or other non-concentrated flow on site include 

the use of slope breaks such as berms to slow water as it flows down slopes and the use of 

cover materials such as mulches and vegetative stabilization on slopes to reduce the velocity of 

stormwater flowing down the slopes. During construction, the volume and rate of runoff 

increases, which increases the capacity of the stormwater to transport pollutants to receiving 

waters (see Controlling Runoff and Erosion at Urban Construction Sites, University of MissouriColumbia Extension, http://extension.missouri.edu/explorepdf/agguides/agengin/g01509.pdf). 

Erosion of soil particles is caused by both rainfall impact energy as well as the energy of flowing 
Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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water. Water flowing over soil as sheet flow as well as concentrated flow overland and in 

conveyances, such as channels, causes detachment of soil particles and transport of these 

particles downslope. These particles can be discharged from the construction site along with the 

stormwater. While removal of some particles in downslope sediment controls (e.g., sediment 

basins) can be accomplished, these sediment controls are not 100% effective in removing 

entrained soil particles. Therefore, some portion of soil that is mobilized (and the pollutants 

associated with those soil particles) can be discharged from the construction site. Controlling 

stormwater volume and velocity within a construction site reduces the amount of erosion caused 

by flowing water, and therefore can reduce the amount of sediment, turbidity and other 

pollutants discharged from the site. For example, a particular sediment basin may be capable or 

removing all particles above 40 microns in diameter through settling. If the stormwater flowing to 

the sediment basin during a particular storm event contains 1,000 pounds of soil, 80% of which 

is above 40 microns, then the basin would remove 80% (or 800 pounds) of the sediment while 

20% (or 200 pounds) would not be removed and would be discharged. However, if during this 

same storm event upslope controls to reduce erosion due to the volume and velocity of 

stormwater were not implemented, then one would expect a larger quantity of sediment to be 

eroded and transported to the sediment basin. For this example, say the total quantity of 

sediment transported to the basin for this event is twice as much, or 2,000 pounds, because 

upslope volume and velocity controls were not implemented. Again, the distribution of soil 

particle sizes is such that 80% of the particles are above 40 microns while 20% is below 40 

microns in diameter. In this case, the amount of sediment not removed by the basin is 20% of 

2,000 pounds, or 400 pounds. This is twice as much as discharged from the example where 

upslope controls to reduce erosion were implemented. Therefore, reducing the volume and 

velocity of stormwater, which reduces the amount of erosion, can directly reduce the quantity of 

sediment and associated pollutants that are discharged.

As EPA has noted in the preamble (see pg. 63011), prevention of erosion is an important 

pollution prevention consideration at construction sites. This is especially important on sites that 

include fine silts and clays since these particles, once eroded and mobilized by stormwater, are 

difficult if not impossible to remove in conventional sediment controls utilized on construction 

sites. The specification of minimum erosion control and water handling practices is appropriate 

in the effluent guideline. If these erosion control techniques are not utilized, one would expect 

higher amounts of pollutants to be discharged from sites since down-slope sediment controls 

have variable performance and may not remove these smaller particle size fractions (see the 

example provided above). The performance of sediment controls can be improved through the 

use of practices such as applying chemical flocculants or other enhanced sediment removal 

techniques specifically designed to remove all particulate size fractions that are present. Some 

sites may implement chemical treatment or other enhancements to improve sediment control 

performance. Reasons for implementing chemical treatment or other enhanced sediment 

controls include receiving water quality standards, numeric discharge standards, and/or 

benchmarks/action levels specified by the permitting authority. However, even if chemical 

flocculants or other enhanced sediment removal techniques are implemented at a site, this does 

not negate the need for implementing up-slope erosion prevention techniques. This is because 

there may be periods of time (for example, during larger storm events and/or during periods of 

construction where the amount of disturbed acreage is below the established thresholds for 

applicability of a numeric standard or benchmark/action level) when the standard or limitation 

would not apply. In these cases, the BMPs would serve as the only requirements controlling the 

discharge of pollutants. In addition, enhanced sediment controls typically involve first holding the 

stormwater in a pond or basin before treatment. During larger storm events or extended periods 

of precipitation, the storage capacity of these basins may be exceeded. If up-slope erosion 

controls are not implemented, then the stormwater flowing into the basins would have a higher 
Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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concentration of pollutants. Therefore, the quantity of pollutants in these basin overflows would 

be much larger than would have occurred had the up-slope erosion controls been implemented. 

Due to these (and other) reasons, the requirement to control stormwater volume and velocity 

within the site in order to reduce erosion and transport of pollutants is necessary on all sites, 

even those subject to numeric discharge standards or benchmarks/action levels.

(2) What EPA does not mean by this requirement.

EPA does not intend for this requirement to apply once construction has ceased and sites have 

been stabilized. This requirement only applies during the construction phase, and does not 

apply to post-construction conditions.

(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 BMP. 

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 

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 did not suggest any change whatsoever to proposed 450.21(a)(2), which is 

identical to final 450.21(a)(1). Thus, NAHB acknowledged, consistent with its overall comments 

in support of a BMP-only ELG, that controlling stormwater volume and velocity within the site to 

minimize soil erosion 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-OW2008-0465-1360. 

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

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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 implemented this provision by requiring that construction stormwater 

conveyance channels be designed to avoid unstabilized areas on the site and to reduce 

erosion, unless infeasible. The provision requires that erosion of channels and their 

embankments, outlets, adjacent streambanks, slopes, and downstream waters to be minimized 

through the use of erosion controls and velocity dissipation devices along the length of any 

constructed stormwater conveyance channel, and at any outlet to provide a non-erosive flow 

velocity. 

(5) Appropriate time for implementation of this BMP in the construction process. 

The proper time for implementation of controls to control both the total volume and velocity of 

stormwater within the site depends on the nature of the control. Some practices (such as 

sediment basins) should be installed very early in the construction process so that they are 

functioning and able to accept runoff from up-slope disturbed areas. Other practices may be 

installed later in the construction process as they are needed. For example, a sediment basin 

may be designed to accept water from several catchments in a project, all of which may not be 

disturbed at the same time. Prior to disturbance of an area, it may be appropriate to install a 

channel to divert runoff from the disturbed area to the basin. When this channel is installed, the 

need for velocity control measures such as a channel lining or check dams would dictate that 

they be installed when the channel is constructed. The need for specific controls is site-specific, 

and will vary based on the nature of the construction activity.