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This chapter provides policies and criteria for the analysis and design of culverts and storm drain systems. Analysis methodologies are provided mainly by reference to widely accepted and available design manuals that have been prepared by the Federal Highway Administration and other government agencies.
(Res. 1637, passed 2-28-02)
(A) All natural drainages crossing roadways will be culverted, unless otherwise approved by the Public Works Engineer.
(B) Street crossings shall be designed, as a minimum, to convey the post-development or future condition 10-year peak discharge under the road. The 24-hour duration storm shall be used for estimation of peak discharges for all watersheds greater than 160 acres in area. The peak discharges from the PDMP shall be used where available. Regardless of the size of the culvert, street crossings are to be designed to convey the post-development or future condition 100-year peak discharge under and/or over the road to an area downstream of the crossing to which the flow would have gone in the absence of the crossing. 100-year flow depths over the roadway shall not exceed 1 foot in depth. Flows up to or including the 100-year frequency shall not cause increased flooding of private land, developable lands or buildings, unless a drainage easement is obtained for those areas. The ponded headwater elevation shall be delineated on the site topography map, or delineated by field survey, as required.
(C) The minimum size for culverts draining roadways is 24 inches in diameter or arch equivalent, and for driveways is 18 inches in diameter, unless otherwise approved by the Pubic Works Engineer.
(D) Culverts shall have adequate end treatment at both ends.
(E) Outlet protection shall be evaluated for all culverts as described in this chapter.
(F) All culverts shall be placed in the natural flow line and channel whenever possible. A detail showing the proposed culvert(s) is required. The detail will include but shall not be limited to, invert elevations, top of road elevations, headwalls, inflow and outflow channel geometry, skew angle, and erosion protection.
(G) Minimum cover of fill over culverts must be provided to maintain the structural integrity of the pipe under anticipated loading conditions. Culvert manufacturers provide minimum cover requirements for prefabricated pipe. All street crossing culverts shall have a minimum of one foot of cover. Minimum cover shall be measured from the top of subgrade, which is the bottom of the pavement structural section.
(H) Storm drains shall be designed such that the flow depth during a 10-year 24-hour event shall not exceed the top of curb. Storm drains shall be designed such that the peak discharge during a 100-year 24-hour event is contained within the town right-of-way.
(I) The minimum pipe diameter allowable for public storm-drain systems is 18 inches, unless otherwise approved by the Public Works Engineer. In general main-line storm drains should be at least 24 inches in diameter.
(J) Public storm-drain systems should be designed for gravity flow whenever possible.
(K) The minimum flow velocity in a storm drain is three feet-per-second, for purposes of self- cleaning.
(L) The minimum allowable storm-drain slope for concrete or smooth metal pipe shall be 0.1%. However a minimum slope at 0.3% is desirable, whenever possible.
(M) Manholes shall be located at storm drain junctions, changes in pipe size, sharp curves, angle points in excess of ten degrees and at abrupt changes in grade. Manholes shall also be located at regular intervals as follows:
(1) 300 feet: Pipe diameter 30"
(2) 400 feet: 30" < Pipe diameter 48"
(3) 500 feet: Pipe Diameter > 48"
(Res. 1637, passed 2-28-02)
(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)
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