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Stormwater Management

Proper and practical Stormwater Management is a sensitive and critical element in the Land Development Industry; as it is with many CMT projects.

In simple terms, ‘Stormwater Management’ is commonly referred to managing the quantity and quality of stormwater runoff from a project site.

Stormwater is of concern for two main issues: one related to the volume and timing of runoff water (flood control and water supplies) and the other related to potential contaminants that the water is carrying, i.e. water pollution.

The term Best Management Practice (BMP) is often used to refer to both structural or engineered control devices and systems (e.g. retention ponds) to treat polluted stormwater, as well as operational or procedural practices.  There are many forms of stormwater management and BMPs, including:

Managing Water Quality:

In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA). The goal of the CWA is to restore all “Waters of the United States” to their “fishable” and “swimmable” conditions.

Under the CWA, point source discharges to “Waters of the United States” require National Pollution Discharge Elimination System (NPDES) permits. To address the nationwide problem of stormwater pollution, Congress broadened the CWA definition of “point source” in 1987 to include industrial stormwater discharges and municipal separate storm sewer systems (“MS4″).  These facilities were required to obtain NPDES permits. This 1987 expansion was promulgated in two phases: Phase I and Phase II.

Due to regulations, proximity to surrounding water bodies, or other site considerations, permanent water quality systems may be incorporated into a project.  Typical design considerations include retention ponds, proprietary stormwater treatment and filtering systems, bioswales, or other structural BMPs.

Managing Water Quantity:

Typical developments increase the impervious area on a site due to the addition of pavements, buildings, etc.  In turn, the stormwater runoff directly increases due to the decreased area for stormwater to soak into the ground, as it did prior to development.  To compensate for the increase in impervious area caused by development, a detention facility is a typical design solution.

A detention facility is typically a detention or retention basin/pond that temporarily hold-back stormwater release to account for the increase runoff.  Detention/retention ponds provide general flood protection and can also control large storm events and flooding conditions. The ponds help manage the excess runoff generated by newly-constructed impervious surfaces such as roads, parking lots, and rooftops.

A detention basin is a stormwater management facility that is designed to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of a time. These basins are also called “dry ponds”, “holding ponds” or “dry detention basins” if no permanent pool of water exists.

A retention basin is used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. Sometimes called a ‘wet pond’ or ‘wet detention basin’, it is an artificial lake with vegetation around the perimeter, and includes a permanent pool of water in its design.

A retentiion pond is distinguished from a detention basin, which temporarily stores water after a storm, but eventually empties out at a controlled rate to a downstream water body. It also differs from an infiltration basin which is designed to direct stormwater to groundwater through permeable soils.

Wet ponds are frequently used for water quality improvement, groundwater recharge, flood protection, aesthetic improvement or any combination of these.  Sometimes they act as a replacement for the natural absorption of a forest or other natural process that was lost when an area is developed.  These facilities are designed to blend into neighborhoods and viewed as an aesthetically-benficial feature.

Storm water is typically channeled to a basin through street and/or parking lot storm drains and a network of storm sewer channels and pipes. The basins are designed to allow relatively large flows of water to enter, but discharges to receiving waters are limited by outlet structures that are designed to manage the release rate yet accommodate large storm events.

In regards to drainage and managing stormwater, many states and local governments have enacted their own stormwater management laws and ordinances, and some have published stormwater treatment design manuals. Some of these state and local requirements have expanded coverage beyond the federal requirements.

CMT has extensive experience with a vast array of stormwater management requirements and we custom-design our projects based upon the governing regulations and client needs.

For more information on construction stormwater pollution, NPDES permits, and Stormwater Pollution Prevention Plans (SWPPP), visit: SWPPP Compliance.