The roadway surface water can be removed by a series of drains that carry the water into a collection and disposal system. The curb, gutter, and inlet design must keep flooding within the parameters established in roadway drainage guidelines. The hydraulic efficiency of inlets is related to the roadway grade, the cross-grade, the inlet geometry, and the design of the curb and gutters.
Curbs are divided into two classes: barrier and mountable. Barrier curbs are steepfaced and generally 6 to 8 in (150 to 200 mm) high. Mountable curbs are generally 6 in (150 mm) high or less with relatively flat sloping faces to allow vehicles to cross them when required. Neither barrier curbs nor mountable curbs should be used on highspeed roadways. (See Chap. 6, Safety Systems.)
Gutters begin at the bottom of the curb and extend toward the roadway a varying distance, usually 1 to 6 ft (300 to 1800 mm). They may or may not be constructed with the same material as the roadway.
The longitudinal grade of the gutter is controlled by the highway grade line. For drainage purposes, it is important to maintain some minimum longitudinal slope to
ensure that runoff does not accumulate in ponds. Gutter cross-slopes of 5 to 8 percent should be maintained for a distance of 2 to 3 ft (600 to 900 mm) for that portion of the gutter adjacent to the curb.
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The following modification of Manning’s equation may be used to determine the spread of the gutter flow as well as the maximum depth at the curb face. This applies to a section with a single cross-slope. (For additional information, nomographs, and flow solutions for gutters with composite cross-slopes, see Urban Drainage Design Manual, HEC 22, FHWA.)
where Q = rate of discharge, ft3/s
K = 0.56 for U. S. Customary units (0.376 for SI units) n = Manning’s coefficient of roughness SX = cross-slope S = longitudinal slope
T = spread or top width of flow in gutter = d/Sx, ft d = depth of flow at face of curb, ft
Example: Gutter Flow Spread and Depth. A concrete gutter for a roadway with a grade of 0.05 and a cross-slope of 0.04 must accommodate a flow of 1.4 ft3/s. Determine the spread of the flow and its depth at the curb face. Assume n = 0.15. Substitute in Eq. (5.19) and solve for the spread T as follows:
1.4 = ( KK )(0.04)1 67(0.05)05T267
T267 = 36.23
T = 3.84 ft (1.17 m)
It follows that the depth at the curb is d = TSX = 3.84 X 0.04 = 0.15 ft (46 mm).
