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I I I I I I I I I I I I I I I I I I I 17 Sections were refined so that flow was allowed to pass only through areas of "effective conveyance." Areas which had neg- ligible "effective conveyance" (space and slope to allow passage of water) were eliminated from the appropriate cross sections. In general it was assumed that the flood water, after passing through a constriction (for example, between two buildings) cannot expand at a rate greater than four to one (4:1). Cross sections downstream from constrictions were modeled in such a way that for each four feet of distance along the channel axis, the width of effective flow would expand only one foot on each side of the axis. If areas of inundation existed outside of the lines of "effective conveyance" drawn as described, they were shown on the FHAD Sheets as flooded, but are modeled to be ineffective in carrying the flow. allowed only so far as they would remove an equal conveyance of floodwater from each side of the flood plain. A detailed, technical appendix of hydraulic calculations is available at the office of the Urban Drainage and Flood Control District. Computations were made and are presented in tabular form of a "floodway" along the channel. The floodway is that portion of the flood plain which carries the majority of the flood flow. It is defined as the remaining channel after a hypothetical en- croachment of land is allowed on each side of the channel which will not cause more than a nominal rise in the water surface over the natural lOO-year flood. The allowed nominal rise in water surface used in these computations is no more than one half foot. Where possible, the hypothetical encroachments were