§ 6.5.1. Sloping Channel Drop

A.

Approach Apron. A minimum ten (10) foot long riprap apron should be constructed immediately upstream of the drop to protect against the increasing velocities and turbulence which result as the water approaches the sloping portion of the drop structure. The same riprap and bedding design should be used as specified for the portion of the drop structure immediately downstream of the drop.

B.

Chute. The chute shall have roughened faces and shall be no steeper than 2:1. The length, L, of the chute depends upon the hydraulic characteristics of the channel and drop. For a unit discharge, q, of 30 cubic feet per second per foot, L would be about 15 feet, that is, about ½ of the q value. The L should not be less than ten (10) feet, even for low q values.

C.

Downstream Apron. The length of the downstream apron shall be sized according to Table 6-3 and shall be constructed of reinforced concrete or riprap depending on structural requirements.

Table 6-2 Minimum Roughness Coefficients of New or Altered Channels
Type of Channel and Description	Manning's Coefficients
1. Grass lined
a. Bermuda (with regular mowing)	.040
b. St. Augustine (with regular mowing)	.045
c. Native grasses and vegetation not mowed regularly	.060
2. Concrete
a. Concrete lined (rough finish)	.020
b. Concrete lined (smooth finish-culverts)	.015
c. Concrete rip-rap (exposed rubble)	.025
3. Gabion	.035
4. Rock-cut	.025
Source: 1. Chow, V.T. Open Channel Hydraulics . 1959. 2. WRC Engineering, Inc. B oulder County Storm Drainage Criteria Manual . 1984.

 

Table 6-3 Length of Downstream Apron
Maximum Unit Discharge, q (cfs/ft)	Length of Downstream Apron, L B (ft)
0-14	10
15	15
20	20
25	20
30	25
Source: City of Austin, Watershed Engineering Division.