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

1 Miscellaneous items that NAHB c ited 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 o f 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 lo w 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 pea k 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 discharg e 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 durat ion 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.

Th is 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

- d eveloped 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 natur e 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 con struction 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, preci pitation 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

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

2 thunderstorms) while in other areas precipitation is primarily associated with frontal systems. Therefore, proper consideration of precipitation characteristics is important so that earth

disturbing ac tivities 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 prim arily 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 part icles 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 unle ss 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 excavat ions 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 prio r 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 filte r 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 implement ed to minimize the occurrence of discharges from spills and leaks. This requirement is similar to the provision at 450.21(e)(3), the diffe rence 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 a re 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 sto rage 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,Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

3 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 exce ss concrete can harden. Once hardened or dried, the material can be disposed of off

site 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, providin g 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

skimme r, may not be feasible during extended periods of cold

- weather

, the use of a flashboard riser may be an alternative control that is feasible.