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Ineffective flow areas are used in HEC-RAS to represent areas where flow is not being conveyed. The bridge opening does not affect this cross section. Cross Section 1 represents the channel where the flows are already expanded.The losses associated with the Expansion Reach are represented with the expansion coefficient at Cross Section 2.Represents the area where flow is expanding from the bridge opening back to the channel (and overbanks).The Expansion Reach (from Cross Section 2 to Cross Section 1): Cross Section 4 represents the channel before the flows are contracted.The additional losses associated with the Contraction Reach are represented with the contraction coefficient at Cross Section 3.Represents the area where flow is contracting from the channel (and overbanks) into the bridge opening.The Contraction Reach (from Cross Section 4 to Cross Section 3):
HEC RAS DECK/ROADWAY LOCATION MANUAL
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In riverine systems, bridges typically represent an obstruction to channel and floodplain flow.