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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2, Drainageway E - Simms Street storm sewer improvements. <br />3. Drainageway F - 6th Avenue West detention pond. <br />4. Drainageway G - culvert improvements at old Golden Road, <br />5. Drainageway K-l - culvert at Jewell Avenue and Welch Way, <br />Each culvert and/or bridge in the drainageway was rated. The "capacity" <br />of the culvert or bridge was defined as the discharge at which street over- <br />topping first occurs, The capacity of a detention facility was defined <br />as the discharge capacity of the spillway. The capacity of each <br />facility is presented in Table 7. <br />C. Flood Plain Determination <br />The flood profile for a majority of the channel in the study area <br />was determined using a computerized backwater program. The program computes <br />the flood profile by the Standard Step Method for normal flow conditions <br />and substitutes the critical depth profile in reaches of rapid (super critical) <br />flood flow. The program is t itl ed "HEC-2 Wa ter Surface Profil es" (Reference <br />16). The computational procedure is similar to "Method 1, Backwater Curves <br />in River Channels". Backwater computations were made for the 10 and 100 <br />year discharges based on future development and existing channel conditions. <br />The profiles were plotted on the 1" = 100', 2 foot contour interval mapping <br />and the 100 year flood plains were outlined. Due to the steepness of the <br />channel banks in most of the study area, minimal difference occurs between <br />the areal extent of the flood plains for the 10 and 100 year floods, <br />The floodway was defined along with the flood plain. The floodway is <br />that portion of the flood plain where the depth is greater than 2 feet, the <br />velocities are greater than 4 feet per second or where encroachment on the <br /> <br />portion of flood outside of the floodway will cause depths to increase less <br />than 0.5 feet. The program has the capability to calculate the floodway <br />information using several different encroachment methods. The floodway for <br />the Green Mountain study was calculated USing Method 4 with a maximum allow- <br />able water surface rise of 0.5 feet. The 100 year flood plain and floodway <br />data are contained in Table B. <br /> <br />9 <br />