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

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Miscellaneous items that NAHB cited that have not been covered in one of the other

supplemental explanation papers 

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450.21(a)(2) Control stormwater discharges, including both peak flowrates and total 

stormwater volume, to minimize erosion at outlets and to minimize downstream channel 

and streambank erosion.

This requirement is intended to minimize scouring and erosion caused by high peak flowrates 

as well as longer durations of discharges. During land development, typically peak discharge 

rates as well as total runoff volumes increase. This is because as vegetation is removed and 

soil is compacted, less precipitation is captured by vegetation and infiltrated. In addition, soil 

compaction and installation of drainage improvements reduces the amount of time it takes 

runoff to travel from the up-gradient areas of the site to the outlet or discharge point of the 

catchment. Typically, sediment basins or traps are placed at or near low points of drainageways 

in order to temporarily detain the stormwater to allow for particulates to settle. These basins and 

traps are also typically designed to reduce the peak rate of discharge to levels at or near the 

predevelopment levels. Reducing peak discharge rates can reduce downstream flooding as well 

as reduce channel erosion. At the point of discharge, which is typically a pipe or channel, 

installation of riprap or other stabilization measures is often necessary because the 

concentrated discharge can cause erosion. This eroded material can then be discharged to the 

receiving water. In addition to peak flowrates, it is also important to consider the duration of 

flows. If discharges are at or near predevelopment rates, but the duration of flows at this rate is 

extended, then this increase in flow duration can cause accelerated erosion of the receiving 

channel. Therefore, consideration of both peak discharge rates as well as flow duration may be 

important in some cases. The need to address flow duration depends on a number of factors, 

including the duration of the construction project, the size of the construction project in relation 

to the size of the receiving stream watershed at the point of discharge, and receiving stream 

geomorphology (slope, bed material, flow regime, etc.). Reducing flow duration impacts can be 

accomplished either by reducing the total volume of stormwater being discharged or by 

providing extended detention to reduce discharge rates to levels below predevelopment 

conditions.

This requirement is not intended to address discharges after construction has been completed 

(i.e., post-construction stormwater), although the same concepts regarding peak flowrates and 

flow duration impacts on receiving stream stability apply in the post-developed condition.

450.21(a)(5) Minimize sediment discharges from the site. The design, installation and 

maintenance of erosion and sediment controls must address factors such as the amount, 

frequency, intensity and duration of precipitation, the nature of resulting stormwater 

runoff, and soil characteristics, including the range of soil particle sizes expected to be 

present on the site.

Factors including precipitation characteristics and soil characteristics are important to consider 

when managing construction site stormwater runoff. Different areas of the country have different 

precipitation patterns. In some areas, precipitation occurs fairly frequently and for long 

durations, but the intensity of precipitation is generally low. In other areas, precipitation occurs 

fairly infrequently, but the intensity of precipitation is very high while the duration of precipitation 

is very low. In some areas, most precipitation occurs during a general rainy season, while in 

other areas precipitation occurs year-round. Some areas are dominated by cellular events (e.g., 
Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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thunderstorms) while in other areas precipitation is primarily associated with frontal systems. 

Therefore, proper consideration of precipitation characteristics is important so that earthdisturbing activities can be planned during periods with lower risk of precipitation and erosion 

and sediment control practices can be designed to convey and manage the precipitation that is 

expected to occur.

Soil characteristics are also important considerations when selecting and designing appropriate 

erosion and sediment controls. Characteristics such as particle size distribution, erosivity and 

cohesiveness are important considerations. A site that has primarily sandy soils in general might 

easily erode, however, the particles can be readily removed in sediment controls. A site that has 

clay soils may not readily erode, however, once mobilized these particles can be difficult to 

manage because once clay particles have been eroded and are in suspension, they are 

generally difficult to remove without the use of a flocculant. Therefore, consideration of soil 

types prior to design and installation of controls and consideration of the appropriate installation 

and maintenance practices of those controls requires an understanding of the soil 

characteristics of the site.

450.21(c) Dewatering. Discharges from dewatering activities, including discharges from 

dewatering of trenches and excavations, are prohibited unless managed by appropriate 

controls.

Dewatering includes pumping of groundwater to lower the local water table so that foundations, 

trenches and other areas can be excavated. Dewatering also includes pumping water that has 

accumulated in foundation excavations and trench excavations, either through precipitation or 

groundwater seepage. Proper management of dewatering discharges requires consideration of 

the nature of the dewatering activity being conducted and the characteristics of the dewatering 

discharge. When pumping of groundwater is done in order to lower a local water table so that 

excavations can occur, generally this water may be fairly clean and free from sediment and 

other pollutants. In these cases, the water may need little or no management prior to being 

discharged. Water that has accumulated in an excavation, on the other hand, may be highly 

turbid and contain a great deal of sediment. In these cases treatment of the water in a sediment 

basin or an alternative control such as a geotextile filter bag may be necessary in order to 

remove the sediment and turbidity prior to discharge. 

450.21(d)(3) Minimize the discharge of pollutants from spills and leaks and implement 

chemical spill and leak prevention and response procedures.

Management of spills and leaks is a fairly straightforward process. There are a number of 

management practices that can be implemented to minimize the occurrence of discharges from 

spills and leaks. This requirement is similar to the provision at 450.21(e)(3), the difference being 

that (e)(3) is related to areas where materials are stored or staged on-site (such as dedicated 

fuel storage and refilling areas and maintenance areas) and is intended to apply to activities 

permittees must take to ensure that these materials are not exposed to precipitation and 

stormwater where they are stored and used. The provision at 450.21(d)(3) applies to proper 

management of accidental or unintentional spills or leaks. Spill prevention is a primary 

consideration for areas such as fuel storage areas. In these areas, the use of secondary 

containment devices and pre-staging of spill management materials (e.g., absorbent materials) 

may be appropriate. For spills and leaks that occur on-site outside of containment areas, 
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measures to contain and manage the leak can include the use of spill kits containing absorbent 

materials and removal of soil contaminated by leaks.

450.21(e)(1) Wastewater from washout of concrete, unless managed by an appropriate 

control.

There are a number of appropriate controls for management of concrete washout. The use of a 

dedicated concrete washout pit, bin or container is a common practice. Concrete washout from 

trucks and other equipment is directed to a lined pit or container where the water can evaporate 

and excess concrete can harden. Once hardened or dried, the material can be disposed of offsite in an appropriate manner. Another practice is the use of a concrete washout treatment 

system. These mobile systems can process and treat the concrete washout, providing 

separation of soilds and reduction of pH.

450.21(f) Surface Outlets. When discharging from basins and impoundments, utilize 

outlet structures that withdraw water from the surface, unless infeasible.

A surface outlet is any device that withdraws water from the surface of a basin or impoundment. 

Examples include skimmers and flashboard risers used on sediment basins and traps and a 

stone weir located on a sediment trap. Generally, a surface outlet should be feasible on the 

majority of construction sites. Exceptions may include areas with extended cold-weather, where 

surface outlets such as a skimmer may not be feasible during certain time periods (although it is 

expected that they would be used during other periods). Where a surface outlet, such as 

skimmer, may not be feasible during extended periods of cold-weather, the use of a flashboard 

riser may be an alternative control that is feasible.