(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)