The open-end conduits used to convey water from one side of the roadway through the embankment to the other side are typically referred to as culverts. A network or system of conduits to carry storm water intercepted by inlets is referred to as a storm drain system. Conduits for culverts and storm drains are available in many different shapes, sizes, and materials, as discussed subsequently. Available shapes include circular, elliptical (horizontal or vertical), pipe-arch, arch, and box shapes. Factors that affect the shape at a particular site include the fill height, construction costs, and potential for clogging by debris. Where the cover over the conduit is limited, pipe-arch, arch, elliptical (horizontal), or box shapes may be more applicable. Where the fill height is great, circular shapes tend to be structurally and economically more favorable. Factors involved in the selection process include hydraulic, structural, construction, maintenance, and durability requirements. (See Art. 5.5 for hydraulic design of culverts.)
A system of closed conduits (storm drains and culverts) to convey the runoff from the inlets to the outfall must be designed starting at the upstream end and proceeding downstream. Each section of pipe that extends from inlet to inlet, or from an inlet to the final outfall, is called a run. Each run requires a separate analysis because of the change in flow at each, and possible changes in slope, pipe size, and type. After all runs are initially sized, the hydraulic grade line is developed (Art. 5.3.4). Unlike the sizing of the conduits, the calculations for this proceed in an upstream direction. In addition to head loss from friction along the length of the culvert, the hydraulic grade line must account for the effects of losses caused by turbulence at junctions and bends. Once the hydraulic grade line is established, it may be compared with the grade line of the system to ensure that it does not exceed an allowable high-water elevation. If it should extend above these allowable elevations, then the initial design must be adjusted.
In addition to system sizing based on hydraulic requirements, conduits should generally not be smaller than 12 to 18 in (300 to 450 mm) in diameter, and should have a minimum velocity of not less than 2.5 ft/s, to reduce the potential for debris clogging. Greater minimum diameters may be appropriate in some cases, particularly under high fills.
Flow in storm drains is assumed to be steady uniform flow. With this assumption, one of two hydraulic design approaches for sizing the run may be used, either open — channel flow or pressure flow. Open-channel flow assumes the flow in the conduit is open to atmospheric pressure; that is, the depth of the flow must be less than the height of the conduit. Pressure flow assumes the conduit is full with the wetted perimeter equal to the complete perimeter of the conduit. In this case, unlike open — channel flow, a pressure head will be above the conduit.
The maximum possible flow in a circular conduit under open-channel flow occurs when the barrel is approximately 95 percent flowing full. This is referred to as just — full capacity or just-full discharge.
Storm drain systems based on open-channel flow will have larger conduits than those based on pressure flow. This allows for a slight factor of safety when there is an unanticipated increase in runoff, which is desirable because the determination of the flow entering the system is not an exact science. However, initial construction costs will be somewhat higher.
If the design is based on pressure flow, the inlet and access hole elevations will be the allowable high-water elevations and should not be exceeded. Additionally, existing systems may need to be analyzed assuming pressure flow in order to accommodate new design flows.
It is common among state departments of transportation to design storm drain systems using both open-channel flow and pressure flow. The system is initially designed for the just-full capacity using a lesser design frequency, say a 5-year or 10-year design frequency. After this initial sizing, the elevation of the hydraulic grade line is checked using the same or greater design frequency. The hydraulic grade line is then compared to critical high-water elevations, which should not be exceeded.
The storm drain system can outfall into a body of water, a stream or river, an existing storm drain system, or a channel. Conformance to National Pollutant Discharge Elimination System (NPDES) and local water quality regulations may be necessary whenever discharging pavement runoff. (See Chap. 1.) Regardless of the type of outfall, the flow line of the outfall should be lower than the elevation of the outlet. The outlet should be positioned so that the flow of the outfall is directed downstream, thus limiting erosion. (See Highway Drainage Guidelines, Vol. IX, AASHTO, 1999; and Design and Construction of Storm and Sanitary Sewers, ASCE, 1986.)
