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<br />(2) The size, shape, and slope of a culvert's drainage area influence the <br />amount of runoff that may be collected and the speed with which it <br />will reach the culvert. The amount of time required for water to <br />flow to the culvert from the most remote part of a drainage area is <br />referred to as the time of concentration. Changes in the drainage <br />area may influence the time of concentration. <br /> <br />straightening or enclosing streams and eliminating temporary storage <br />by replacing undersized upstream pipes are examples of changes which <br />may decrease time of concentration. Land use changes may also <br />decrease time of concentration since water will flow more quickly <br />over paved surfaces. Since higher rainfall intensities occur for <br />shorter storm durations, changes in time of concentration can have a <br />significant impact on runoff. Drainage areas are sometimes altered <br />and flow diverted from one watershed to another. <br /> <br />3-2.2 Hydraulic Capacity. <br /> <br />The factors affecting capacity may include headwater depth, tai1water depth, <br />inlet geometry, the slope of the culvert barrel, and the roughness of the <br />culvert barrel. These factors are illustrated in exhibit 17. The various <br />combinations of the factors affecting flow can be grouped into two types of <br />conditions in culverts: inlet control and outlet control. <br /> <br />SHAPE AND AREA <br />OF BARREL <br /> <br />HEAD WATER <br />DEPTH <br />(H:J <br /> <br /> <br /> <br />TAIL WATER <br />DEPTH <br />_(TW) <br />--- <br /> <br />INLET EDGE <br /> <br />SLOPE -1 <br />(SO) LENGTH (L) <br /> <br />(N) <br />ROUGHNESS <br /> <br />-I <br /> <br />Source: Adapted from Concrete Pipe Handbook. <br />American Concrete Pipe Association <br /> <br />Exhibit 17. Factors affecting culvert discharge. <br /> <br />28 <br />