(A) Culvert hydraulics.
(1) Culvert hydraulics shall be evaluated using the procedures established by the Federal Highway Administration (FHWA) as presented within the publication entitled Hydraulic Design of Highway Culverts (1985), often referred to as “HDS-5". Culverts shall be evaluated for both inlet control and outlet control, to ensure that the correct headwater elevation is determined. Use of computer programs, such as the FHWA “HY-8" (1999) program or equivalents, is also acceptable for culvert analysis and selection.
(2) In order to expedite review and approval of the hydraulic design of culverts by the Public Works Engineer, the Culvert Design Form within “HDS-5" should be used. This form is also provided as Figure 7.1. The computer reports produced by the “HY-8" or equivalent programs are also acceptable for presenting culvert analysis results.
(B) Debris grates.
(1) As part of the culvert design process, the engineer shall consider whether or not the upstream watershed would yield sufficient naturally produced or man-made debris to pose a potential blockage problem. If debris is considered a problem, then an appropriate grate shall be considered, or the culvert shall be enlarged to account for blockage. Because of the large number of combinations of culverts and types of debris possible, there is no single standard grate design. Rather, the engineer is advised to review the Federal Highway Administration manual entitled Debris-Control Structures (1971) to help aid in selecting an appropriate debris grate.
(2) It is the policy of the Public Works Department that debris grates on culverts be used only where necessary. The recommended method of accounting for expected debris problems is to increase the size of the culvert, whenever possible.
(C) Outlet protection.
(1) Outlet protection shall be evaluated for all culverts. The following guidelines, adapted from the Arizona Department of Transportation, are suggested for determining what type of outlet protection is required. Outlet protection shall be required as shown in Table 7.1 where erodible soil conditions exist in the downstream channel bed and/or banks.
Table 7.1: Culvert Outlet Protection Requirements
Culvert Outlet Velocity | Required Outlet Protection |
Less than 4 fps | No protection required |
More than 4 fps and less than 10 fps | Dumped rock riprap (See § 152.72(B)(2)) |
More than 10 fps and less than 15 fps | Wire tied or grouted rock riprap |
Greater than 15 fps | Energy dissipater |
(2) For culverts with outlet velocities greater than 15 fps, an energy dissipater shall be considered. The objective of an energy dissipater is to return the flow to a condition that approximates the existing flow width, depth and velocity. The engineer designing energy dissipaters shall refer to the FHWA publication entitled Hydraulic Design of Energy Dissipater for Culverts and Channels, HEC No. 14, (1983).
(Res. 1637, passed 2-28-02)