Chapter 13: Storm Water Management

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Section 1: Introduction

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Storm Water Management and Best Management Practices

Storm water is defined in the Construction General Permit (CGP) as "Rainfall runoff, snow melt runoff, and surface runoff and drainage." For TxDOT purposes, storm water includes overland flow, and flow in ditches and storm drain systems.

Storm water management includes non-structural and structural measures such as the following:

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  • erosion control to minimize erosion and sediment transport
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  • storm water detention and retention systems to reduce peak runoff rates and improve water quality
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  • sedimentation and filtration systems remove debris, suspended solids, and insoluble pollutants
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  • vegetation buffers to reduce transport of pollutants.

Measures intended to mitigate storm water quantity and quality problems are termed “best management practices” (BMPs). These measures include detention and retention ponds which delay storm water flow and trap sediment, rock filter dams for the same reasons, silt fences to trap sediment, various filter materials in socks or tubes, and vegetation to retard flow and trap sediment.

Quantity

Urbanization, which includes transportation activities, increases storm water volume and velocity by increasing the amount of impervious cover. Improved storm drain systems increase the rate of runoff from a location such as a roadway or land development. Recognition is growing that rapid disposal of runoff from developing areas increases the frequency of flooding in downstream areas. The results can increase flooding, soil erosion, sedimentation, stream bank erosion and channel enlargement, and pollution of surface and subsurface waters.

Where developed areas already exist are downstream of more recent development, as is the predominant sequence of development in the United States, flooding reduces property values and may lead to abandonment of property. Massive investments in flood control works are sometimes required to reduce flood damage. The alternative is to provide flood protection by storm water management in the upstream developing areas. Where pollution abatement as well as flood control is an objective, additional or alternative storm water management measures may be necessary to provide source control of storm water pollution.

Water quality problems in surface waters often stem from nonpoint as well as point sources of pollution. A point source is a single identifiable localized source of pollution while a nonpoint source comes from diffuse sources, such as polluted runoff from agricultural areas draining into a river. Water quality goals for surface waters cannot be achieved solely by separation of combined sewers but require abatement of pollution from nonpoint sources as well.

Highway construction, operation, and maintenance contribute a variety of pollutants to surface and subsurface water. Solids, nutrients, heavy metals, oil and grease, pesticides, and bacteria all can be associated with highway runoff. Although the impacts of highway runoff pollution on receiving waters may not be significant, it is generally recognized that responsible agencies may be required by federal and state regulations to apply the BMP available to reduce pollutant loads entering a water body. One of the primary objectives of an Environmental Impact Statement (EIS) is the quantification of possible pollutants emanating from the operation and maintenance of highway and other transportation facilities, so that a sound judgment can be made as to the overall usefulness of the facility. (For more information on EIS, refer to the Environmental Documentation in the Project Development Process Manual.)

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Requirements for Construction Activities

The TxDOT publication Storm Water Management Guidelines for Construction Activities (TxDOT, 2002) details the department’s procedures and recommended BMPs to be included in a Storm water Pollution Prevention Plan (SW3P) for proposed projects. Appropriate BMPs are recommended for all construction projects.

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Storm Drain Systems Requirements

The U.S. Environmental Protection Agency (EPA) National Pollution Discharge Elimination System (NPDES) permit requirements for Municipal Separate Storm Sewer Systems (MS4) are the primary regulations that may affect the extent to which storm water BMPs are necessary. The Division of Environmental Affairs should be consulted to determine the status of the permit and the management plan for the municipality of interest.

In addition to NPDES permit requirements, over the Edwards Aquifer recharge zone, TxDOT is obligated to comply with a memorandum of understanding with the TNRCC that espouses the need for BMPs. Refer to the Division of Environmental Affairs for details of the most current agreement.

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