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Bridge Design <br /> With the flow under the bridge being open channel flow, <br /> the Mannings Equation was used to design the bridge <br /> demensions. Because of available prestressed concrete <br /> bridge structures , a 50-foot span was selected . <br /> Allowing one foot for the vertical abutments at each <br /> end of the bridge , the free span is 48 feet. A <br /> practical clearance , using existing road elevations , <br /> was determined to be 14 feet above the river . For the <br /> open channel flow calculations , a 13 .5-foot clearance <br /> was first used. This resulted in an estimated flow of <br /> nearly 20 ,000 cfs of water for the maximum flow under <br /> the structure . The design flow of 15 ,700 cfs of water <br /> is estimated to be only 11 .5 feet deep thus allowing <br /> 2.5 feet of clear space under the bridge during high <br /> flow to allow for a trash load . This will result in a <br /> flow area of 550 square feet . The Highway department <br /> estimate of area required to pass the flow was was 600 <br /> sq ft . The total bridge flow area at 14 feet high and <br /> 48 feet wide is 672 sq ft , again affording a margin of <br /> safety.Hydrologic Design - Supplimental Information <br />