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(a) Roadway Culverts Criteria. A culvert is designed to carry water from one (1) side of the road to the other. The size and shape of the culvert should be such that it will carry a predetermined design peak discharge without the depth of water at the entrance or the velocity at outlet exceeding allowable limits.

The culvert design procedure recommended for use is Hydraulic Engineering Circular No. 5. or industry recognized computer programs. This circular can be obtained from the Superintendent of Documents, U.S. Government Printing Office.

Single span culverts, including concrete box or slab top, should always be considered in lieu of multiple cell pipe culverts when they are the only structures that will meet the physical requirements introduced by rigid headwater controls.

The plan for each culvert shall have the drainage area in acres and the estimated runoff or design discharge in cubic feet per second shown.

The culvert inlet flowline elevation should be set such that it will be deep enough to provide an adequate outlet for future storm sewer improvements upstream.

(b) Roadway Culverts Information.

(1) Design Storm Frequency.

a. Ten (10) year storm minimum.

b. Twenty-five (25) year storm for arterial streets.

(2) Design Flow.

a. Areas under two hundred (200) acres use Rational Method Q = CiA or industry recognized computer programs.

b. Areas between two hundred (200) and three hundred (300) acres transition between Rational Method and Technical Release 55 or industry recognized computer programs.

c. Areas over three hundred (300) acres use Technical Release 55 or industry recognized computer programs.

(3) Runoff Coefficient. Based on Table No. 1 (see elsewhere in this article).

(4) Maximum Allowable Headwater.

a. Eighteen (18) inches below top of curb.

b. Twelve (12) inches below edge of pavement.

c. One and two-tenths (1.2) times diameter.

d. Diameter or rise plus four (4) feet or 2D, whichever is lower, in deep ravines.

(5) Manning’s “n” Value.

a. Box culvert: 0.011;

b. Slab top culvert: 0.03 to 0.05;

c. Concrete pipe: 0.012;

d. Corrugated metal: 0.019 to 0.032;

e. Polyvinyl chloride (PVC): 0.01.

(6) Entrance Loss Coefficient.

a. Box culvert and slab top culvert: 0.2 to 0.5;

b. Concrete pipe: 0.2;

c. Corrugated metal: 0.2 to 0.9;

d. Polyvinyl chloride (PVC): 0.2.

(7) Minimum Cover. Desirable, thirty (30) inches to pavement subgrade.

(8) Maximum Cover. The structural design criteria for culverts will be the same as that required by the Indiana Department of Transportation.

(9) Maximum Allowable Outlet Velocity.

a. Bare earth channel: 6 F.P.S.

b. Rock protection: 18 F.P.S.

c. Stilling basin: 18 F.P.S.

(10) End protection:

a. Full height headwall with flared wings.

b. Other special type headwalls must be approved before use.

c. Flared-end section may be approved by Town Engineer.

d. Multiple pipe culverts must have a headwall.

(Ord. No. 1707, T. 6, § 4, 7-22-20)