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<br />18 <br /> <br />19 <br /> <br />analysis and alternative planning utilized the future condition flow rate~ due to the small <br />difference between future and existing flowtates. <br /> <br />IV. HYDRAULIC ANALYSIS AND FLOODPLAIN DESCRIPTION <br />A hydraulic analysis was conducted to ascertain the conveyance capacity of hydraulic <br />structures along the major drainageways of Dirty Woman and Crystal creeks. Field verifications <br />of major roadway crossing and channel improvemems were conducted and the general physical <br />condition of the stnlcture(s) noted. A hydraulic analysis was conducted using the U. S. Anny <br />Corps of Engineers (CDE) HEC-2 waler surface profiles program, Cross section dala for the <br />analysis was obrained by using the two-fOOl contour interval planimetric !opographic mapping <br />referenced earlier in tbis repon. The capacity of the major roadway crossing structures were <br />estimated using the HEC-2 water surface profile data, supplemented by HYDRAtN calculations. <br /> <br />Reach Delineation and Descrintions <br />Reaches were delineated for various segmellls of Dirty Woman Creek and Crystal Creek. <br />The reaches were determined based upon the existing physical condition of the low flow, <br />floodplain, and overbanks along the drainageways. The reach limits are shown on Figure 4-1. <br />Selected information for each reach is presented on Table 4--1. Descriptions of the <br />environmental setting of each reach are contained in Section V of this repon. Field visits were <br />conducted to ascertain more site specific information related 10 the existing drainageway <br />conditions. <br /> <br />Rood Histol)' <br />The flood history of the twO hasins within the study area is not known. Newspaper <br />articles of past flood evelllS aiong either of the ~-reeks could !tot In: founJ, There are no clear <br />high watermarks along the drainageways, <br />The potential for flash flooding will nO! increase significantly as urbanization continues. <br />This is because of the numerous natural and incidental storage areas which occur along both <br />creeks, Tn addition to the natural floodplain storage, the future land use will n.m cause a <br />significant rise in the flood discharges. A constant base flow does exist in Diny Woman Creek. <br />Much of this base flow is transmined to Lake Woodmoor via a gravity pipe system and <br />infiltration galleries situated along the creek low flow channel. Lake Woodmoor is water supply <br />reservoir operated by the Woodmoor Water and Sanitation District. During the hydrology <br />analysis, the storage in Lake Woodmoor betwecn the ma"imum operating level and the <br />emergency spillway was assumed to be available for flood water storage. <br /> <br />HydralllicSrmcturelnvenmr\l <br />As pan of the field investigation, the existing drainage facilities were verified and <br />invemoried, The size, type, and condition was recorded for all the bridges, culverts. channels, <br />