Generally, stormwater pollution can be categorized as Point Source (PS) and Non-Point Source (NPS) pollutants. PS pollutants are defined as any source of pollution that enters the surface water of the U.S. through pipes, ditches, channels, etc. Typical examples of PS pollution include industrial and municipal wastewater discharges. NPS pollution is pollutant that comes from overland runoff from agriculture and urban areas. A typical examples of the NPS pollution is fertilizer nutrients that washed off farmlands, golf courses and lawns.

Due to its various types of activities, the N. C. Department of Transportation generates both PS and NPS pollutants. Examples of sources of PS pollutants are maintenance yards, equipment shops, storage facilities (such as salt, fuel, herbicide, fertilizer, etc.), ferry operations and highway stormwater drainage systems.

An Example of NPS pollutant is stormwater runoff from highways with only vegetative shoulders, embankments, and ditches. The pollutants can be generated from various highway activities, which include clearing and grubbing on construction sites, accidental spills, application of, deicing agents, fertilizers, herbicides, and paints. The major constituents of stormwater runoff pollutant from highway runoff are; oil, grease, nitrates, phosphorus, chromium, cadmium, lead, zinc, iron, manganese, copper, chlorides, sulfates and particulates.

The goal of this chapter is to provide a method for evaluation of potential impact of proposed actions, and a procedure for development and implementation of stormwater management practices to protect the quality of the receiving surface waters in the planning, design, construction, and maintenance of a multi-functional transportation system.

Federal Laws

In 1977 the U. S. Congress amended the Federal Pollution Control Act to regulate the discharge of pollution into waters of the U.S. and it was officially designated the Clean Water Act (CWA). It serves as the cornerstone of Federal law for all water quality programs. It directs the Environmental Protection Agency (EPA) and other regulatory agencies to establish standards of water quality for states to follow.

In 1987 Congress passed a further amendment to the act which added stormwater permits to the NPDES program under Section 402. Section 404 of the Act defines navigable waters of the United states and requires permit authorization for the discharge of dredge or fill materials into these waters. A new section ( Section 319) addresses nonpoint source pollution. Section 319 requires each state to better integrate the Coastal Nonpoint Program and the Statewide Nonpoint Program. Pursuant to Section 401 of the Act, issuance of permits under any of the above Sections of the CWA is contingent on receipt of water quality certification by the State in which the discharge originates.

State Laws and Programs

A State Sedimentation Pollution Control Act was adopted in 1973. This promulgated rules and regulations to control accelerated erosion and sediment resulting from land disturbing activities. The Department of Transportation has been delegated the authority to administer an erosion and sedimentprogram within the department. Guidance for the Hydraulics Engineers’ responsibilities in this activity is provided in Chapter XI of these guidelines.

In 1988 the EMC adopted Coastal Stormwater Rules to regulate development activities in the states 20 coastal counties. The rules require developers obtaining CAMA permits to include stringent sediment and erosion control and stormwater management plans. The rules provide low-density and high-density development options. The low-density option allows construction area up to 25% of the lot for sites within one-half (1/2) mile and draining to Class SA (Shellfish) waters and 30% for other coastal areas. The high-density option requires on-site stormwater control measures, such as retention and detention basins.

Highway projects were considered to be exempt from obtaining individual action approval under subparagraph (a)(6), " otherwise meets the provisions of the rule and has boat ramps, public roads and bridges which minimize impervious surfaces, diverts stormwater away from surface waters as much as possible and employs best management practices to minimize water quality impacts".

This act was amended and enacted on Dec. 1 , 1995 expanding requirements to include development activities; draining to Outstanding Resource Waters, and those within one mile of and draining to High Quality Waters. A general permit for NCDOT roadway development activities was issued concurrent with the NPDES stormwater permit.

In 1989 the Water Supply Protection Act to protect drinking water supplies was passed by the State Legislature. It directed the Environmental Management Commission (EMC) to adopt regulations and implement the programs. It also classified the waters of the state based on their quality and significance to the municipalities.

Highway projects were addressed under Section (m) of the final adopted rules on February 13, 1992. "The construction of new roads and bridges and non-residential development should minimize built-upon areas, divert stormwater away from surface water supply as much as possible, and employ best management practices (BMPs) to minimize water quality impacts. To the extent practicable, the construction of new roads in the critical area should be avoided. The Department of Transportation shall use BMPs as outlined in their document entitled,"Best Management Practices for the Protection of Surface Waters"(23).

An NPDES permit for the NCDOT was issued on June 8, 1998.  Requirements contained in the permit address a broad range of DOT activities. Included is a requirement for development of a procedure to document newly constructed stormwater outfalls and add them to a stormwater system inventory of existing facilities. This documentation process will include the development of project stormwater management plans described later in this chapter.

On December 11, 1997, the Environmental Management Commission (EMC) approved the Neuse River Nutrient Sensitive Waters (NSW) Management Strategy. This strategy establishes a goal to reduce annual nitrogen delivery to the Neuse River Estuary from point and nonpoint sources by a minimum of thirty percent (30%). Mandates have been proposed for point source discharge, urban stormwater management, animal operations, riparian buffers, and nutrient management. A temporary riparian rule became effective in January 1998, the entire package of rules is to go into effect August 1, 1999.

The riparian rule requires the protection and maintenance of existing forested buffers on each side (50 feet) of surface waters in the Neuse River Basin intermittent streams, perennial streams, lakes, ponds, and estuaries) as indicated on the most recent version of United States Geological Survey (1:24,000) topographical maps. Certain permitted uses and exemptions that affect highway activities are:

• Ditches- existing ditches through the riparian area may be maintained. New ditches can not be cut through the riparian area and flows must be dispersed into sheet flow before entering the buffer.

• Road crossings- Road crossings through the riparian area are allowed, provided they show that no practical alternative exists. They are designed, constructed and maintained to minimize disturbance and protect water quality. Application for this exemption must be made to DWQ Wetland/401 Unit.

Impact on DOT project development activity from the remaining sections of the final rule are anticipated to be limited to urban stormwater management. Required project measures are expected to be the same as those contained in the stormwater rules for coastal, high quality and outstanding resource waters.

 

Best Management Practices (BMPs)

In March 1997 NCDOT published a handbook entitled "Best Management Practices for Protection of Surface Waters"(20). BMPs are defined as activities, practices and procedures undertaken to prevent or reduce water pollution. They are categorized as preventive and control measures .Preventive measures, also referred to as Non-Structural BMPs, are conceptual management or design practices which eliminate or reduce pollutants at the sources. Control measures, also called Structural BMPs, are engineered means to remove or reduce the concentration of pollutants from the runoff before they enter the receiving streams. The BMP document serves as a compendium covering both preventive and control measures that are implemented in NCDOT's various activities. These activities include general maintenance operations and facilities, construction operations including temporary erosion and sediment control, as well as project planning and design.

Many non-structural BMPs should be considered in the project planning process and initial establishment of general criteria for design to lessen potential for pollutant impact on the receiving streams. Some examples are listed below. Further reference should be made to NCDOT, "Best Management Practices For Protection of Surface Waters"(20).

• Chose alternatives such as widening the existing roadways over new location.

• Use design alternative such as grass medians and shoulders in lieu of impervious materials.

• Select roadways options with shoulder sections over curb and gutter sections.

More site specific BMP usage, including structural BMPs is discussed in the stormwater plan preparation section.

Stormwater Management Plans

The Hydraulics design engineer must develop a Stormwater Management Plan (SMP) as well as drainage recommendations. The SMP will be in a report type format, which reflects the following sequence of development.

• Identify Project Involvement

• Evaluate Potential Impact

• Select and Implement BMPs

• Prepare Design Details

Identify Project Involvement

Evaluate Potential Impact

The Hydraulics Engineer will perform a preliminary evaluation of the potential impact of the proposed project on the receiving stream at each individual site. The evaluation at this point will be somewhat subjective, but will be based on sound judgment and experience. The following parameters should be considered in the evaluation process.

• The proximity of the discharge point to the receiving stream. Is this a direct discharge or is there sufficient area for dilution?

• The volume and type of traffic. Is the volume in excess of 30,000 ADT? Is there heavy truck and/or high potential pollutant traffic?

• The ratio of the impervious surface of contributing highway area to the total watershed of the receiving stream.

• Preventive BMPs that are employed.

• Value of the water resource.

• Site highway geometry and potential for accidental spill.

If this preliminary evaluation suggests that the proposed roadway poses a low risk to the receiving streams, the hydraulic design engineer may document the assessment and conclude that standard BMPs are sufficient for protection of the receiving waters and that no special control measures will be required.

If the preliminary evaluation suggests that the proposed roadway may pose a risk to the receiving streams, the hydraulic design engineer should proceed as follows:

1. Define the target pollutants to be evaluated at the site of interest.

2. Determine the loading of the major pollutants from the proposed highway, based on; traffic counts and types, roadway types, drainage areas, etc. Reference Chapter 3 of FHWA, "Evaluation and Management of Highway Runoff Water Quality,"(17).

 

Select and Implement BMPs

In order to effectively reduce the pollutants from highway runoff, the design engineer should investigate both non-structural and structural BMPs applicable to each point source of interest. The investigation should proceed as follows:

1. Evaluate potential BMP control measures for the site, based on the land, topography, soil and roadway types.
2. Investigate the pollutant removal capabilities of these BMP control measures. For design details reference Chapters 3 and 5 of FHWA, "Evaluation and Management of Highway Runoff Water Quality"(24).
3. Conduct cost and risk assessment for each BMP control measure. Cost analysis shall include land, structure, construction and maintenance.
4. Select the most feasible BMP control measure for the site.

For the design details of these control measures, the design engineer may reference the following publications:

1. "Evaluation and Management of Highway Runoff Water Quality, FHWA-PD- 96-032, 1996(21).
2. "Stormwater Best Management Practices", Division of Water Quality of NCDENR, 1995(25).
3. AASHTO Drainage Guidelines, Volume 12,"Stormwater Management",(1).

 

Prepare Design Details

To complete the drainage and stormwater design, the design engineer should summarize all the recommended control measures in a "report" type format. The report should first include the overview of the project and scope of the stormwater management plans. It should then identify the names, locations, and classifications of the receiving streams at the outlet of each system. At each outlet, all BMP preventive and control measures should be listed and described in details. Design details should be provided on separate sheets for large structures, such as: detention and retention basins, infiltration basins, and constructed wetlands.

The report should also include major drainage structures, such as bridges, culverts, etc., which are located within environmentally sensitive areas. Any direct discharge from these structures such as deck and approach drains, or direct connection of storm drain systems to the culvert, etc. should be avoided and documented.

 

Stream Crossings

As highways cross unique streams, such as trout and anadromous fish streams, special design considerations are required in selecting drainage structures and roadway facilities. It is the goal of the hydraulic design engineers to develop engineering plans which provide favorable aquatic habitats and also are hydraulically feasible and cost effective. In development of the crossing design consideration must be given to the following general guidelines:

1. Flow conditions at normal and bank-full discharges should be thoroughly investigated to ensure that the structures will not impede fish passage.
2. The slope of the replacement culverts should be compatible to that of the existing channel.
3. The bottom slab of culverts should be buried 1 foot below the bed and covered with natural bed materials.
4. Baffles can be installed inside the culvert to promote the establishment of a natural substrate.
5. It is desirable to maintain a normal velocity in the culvert comparable to that of the existing channel
6. In cobble bed stream material comparable to the natural bed material should be placed in the structure.
7. Channel modifications at the inlet and outlet of culverts should follow guidelines presented in the following section.

 

Stream Relocation

As the result of highway improvement activities, such as construction of new roads and widening of existing roads, natural streams sometimes are unavoidably filled or encroached upon by the proposed embankment. Unless the natural streams are properly realigned, it may result in an adverse impact on the fish habitats, bank erosion, channel degradation, and flooding problems. The hydraulic design engineers should thoroughly review the physical and dynamic characteristics of the natural streams and develop replacement channels that are ecological, geomorphic and hydraulically compatible. Reference channel section of Chapter X.

 

Anadromous Fish

Anadromous fish are a unique and valuable resource. Streams utilized by anadromous fish have been identified on Environmental Sensitivity Maps. While most of the anadromous fish are found east of I-95, they migrate in the Neuse and Cape Fear Rivers as far as Wake and Harnett counties. When a proposed highway crosses anadromous fish streams, the hydraulic design engineer should develop the most practical drainage plans, which will least adversely impact their movement and habitats .Design guidance is present in the department’s "Stream crossing Guidelines for Anadromous Fish Passage" (Appendix N).

 

Hazardous Spill Basins

Hazardous Spill Basins are provided in new highway construction major improvement projects at strategic locations along arterial system highways to aid in containment and clean up of accidental spills. The determination of these strategic locations is based on concentrated truck usage areas such as parking sites at rest areas, weight stations, and runaway ramps, as well as for highway segments in close proximity to particularly sensitive waters such as outstanding resource waters and water supply sources. For guidance in the design and selection of location for these devices the engineer should reference the department’s, "Guidelines For The Location And Design Of Hazardous Spill Basins"(Appendix O) .