FLOOD09429

• County: Statewide
• Basin: Statewide
• Title: Hydrologic Engineering Methods for Water Resources Development Volume 6
• Date: 7/1/1975
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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A stream having a top width B flows past a bridge having an

opening b between abutments. Both approaches are earth fills. All

flow passes through b. Cross section 3 describes the contraction by

starting alpng the top of the road, following along the ground sur-

face across b, and returning to the top of the road. Cross section 1

must be located far enough upstream to describe i;he approaching flow,

and a distance equal to b is usually adequate. Cross section 4 must

be located far enough downstream to describe the expanding flow.

Topographic maps can be used to determine the paths that water will

follow. Such paths may be difficult to locate as water may even flow

down the borrow ditch beside the road. If no information is available,

it may be reasonable to assume that the boundary of expanding flow
downstream from the bridge is at an angle of 150 with the main flow
direction.

The long dasheJ lines downstream from the road in fig. 4.01 rep-
resent flow boundaries between the main current and eddies (denoted
as "L" and "R"). The short dashed lines show how tile digitized geo-
metric model. as described by cross sections 1, 3, and 4, would fit
the estimated flow boundaries.

It is important to locate section 4 as close to the road as

reasonable since the friction loss between sections 3 and 4 is cal-

culated using the average of the conveyances of the two sections.

A good test of the adequacy of the completed model is to reproduce

an experienced event.

If the flow rate increases to the point of inundating the entire

road way, the problem of specifying the ineffective flow area becomes

4.05