§ 4.4.1. Preliminary Design Considerations

A.

Prepare a drainage map of the entire contributing drainage area to be served by the proposed improvements. Recent topographic maps obtained through the use of aerial photogrammetry having contours every one or two feet are usually sufficient when supplemented by field verification of the drainage patterns.

B.

Identify on a plan the location of inlets that are needed regardless of any contributing drainage area. These would be inlets at (1) low points, (2) immediately upstream of median breaks, entrance/exit ramps, cross walks and street intersections, i.e., at any location where water could flow onto the travelway, (3) immediately upgrade of bridges to prevent drainage from flowing onto the bridge decks, (4) immediately downgrade of bridges to intercept bridge deck drainage, (5) immediately upgrade of pedestrian cross walks, (6) at the ends of channels in cut sections, (7) on side streets immediately upgrade of intersections, (8) behind curbs, shoulders or sidewalks to drain low areas.

C.

Select as a first trial the assumed locations for additional inlets needed to intercept runoff to meet drainage criteria requirements. The location of the additional inlets will be refined in subsection 4.4.2.

D.

Select the design rainfall frequency using design criteria in the City's Drainage Criteria Manual.

E.

Calculate the time of concentration at each design point.

F.

Identify the cross-section applicable to each street within the proposed project limits.

G.

Identify the permissible spread of water on each street within the project limits.

H.

For each subarea, show on the drainage map the subarea identification number, the limits of the subarea, the size of the subarea in acres, the travel path used to calculate the time of concentration, the time of concentration, and the runoff coefficient or runoff curve number.