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<br />d. Other Considerations~-Impact on the environment, fish passage <br />requirements, importance of roadway in terms of traffic volume, use by <br />emergency vehicles and school buses, length of detour, land use, and <br />appearance or aesthetics are examples of other considerations which may <br />influence culvert selection. <br /> <br />3-1.1 Culvert Shapes <br /> <br />A wide variety of standard shapes and sizes are available for most culvert <br />materials. Since equivalent openings can be provided by a number of standard <br />shapes, the selection of shape may not be critical in terms of hydraulic <br />performance. Shape selection is often governed by factors such as depth of <br />cover or limited headwater elevation. In such cases a low profile shape may <br />be needed. Other factors such as the potential for clogging by debris, the <br />need for a natural stream bottom, or structural and hydraulic requirements may <br />influence the selection of culvert shape. Each of the common culvert shapes <br />are discussed in the following paragraphs. <br /> <br />a. Circular--The circular shape is the most common shape manufactured for <br />pipe culverts. It is hydraulically and structurally efficient under most <br />conditions. Possible hydraulic drawbacks are that circular pipe generally <br />causes some reduction in stream width during low flows. It may also be more <br />prone to clogging than some other shapes due to the diminishing free surface <br />as the pipe fills beyond the midpoint. With very large diameter corrugated <br />metal pipes, the flexibility of the sidewalls dictates that special care be <br />taken during backfill construction to maintain uniform curvature. <br /> <br />b. Pipe Arch and Elliptical Shapes--Pipe arch and elliptical shapes are <br />often used instead of circular pipe when the distance from channel insert to <br />pavement surface is limited or when a wider section is desirable for low flow <br />levels. These shapes may also be prone to clogging as the depth of flow <br />increases and the free surface diminishes. Pipe arch and elliptical shapes <br />are not as structurally efficient as a circular shape. <br /> <br />c. Arches--Arch culverts offer less of an obstruction to the waterway than <br />pipe arches and can be used to provide a natural stream bottom where the <br />stream bottom is naturally erosion resistant. Foundation conditions must be <br />adequate to support the footings. Riprap is frequently used for scour <br />protection. <br /> <br />d. Box Sections--Rectangular cross-section culverts are easily adaptable to <br />a wide range of site conditions including sites which require low profile <br />structures. Due to the flat sides and top, rectangular shapes are not as <br />structurally efficient as other culvert shapes. <br /> <br />e. Multiple Barrels--Multiple barrels are used to obtain adequate hydraulic <br />capacity under low embankments or for wide waterways. In some locations they <br />may be prone to clogging as the area between the barrels tends to catch debris <br />and sediment. When a channel is artificially widened, multiple barrels placed <br />beyond the dominant channel are subject to excessive sedimentation. The span <br />or opening length of multiple barrel culverts includes the distance between <br /> <br />17 <br />