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I I I I I I I I I I I I I I I I I I I IV-5 FLOODPLAINS AND FLOODWAYS Floodplain delineation was performed using the methodologies described earlier in this section, The IOO-year floodplain for the existing channel configuration with future basin development and Leyden Lake in pl ace is depicted on the drawings in Appendix B. In addition, the residual flood- plain, after proposed improvements, is also shown on these drawings. various agencies are in a position to consistently regulate floodplain development. The tables can also be used by developers or property owners in evaluating the hazards and constraints associated with sites along Lower Ralston, Leyden and Van Bibber Creeks. Two different conditions were evaluated to determine the floodway 1 imits, The first condition was a one-half foot rise in base flood energy grade 1 ine elevation as a result of encroachment. This condition was evaluated through all study reaches, This is the standard for Urban Drainage and Flood Control District projects. The City of Arvada requested, in addition to the one-half foot rise floodway, that a one foot rise floodway also be determined which is consistent with their municipal floodplain management ordinance. This evaluation was performed for all reaches currently within the City of Arvad a or whi ch may be incorporated in the City in the near future. In addition to Table IV-2, Attachment B of this report contains valuable information on floodplains and floodways within the City of Arvada. A spec i al Flood Hazard Area Del i neat i on Study was prepared for Ral ston Creek upstream of Leyden Creek. This was performed by the FEMA contractor, Greiner Engineering Sciences, Inc" at the request of the Urban Drainage & Flood Control District to update their analysis using future basin flow rates. Table IV-2 presents the findings of the floodplain and floodway evalua- tions. The table is divided into four general sections; the first of which is general information such as cross section numbers, stationing, stream thalweg, and the lOO-year discharge, The second portion of the table pre- sents information related to the IOO-year floodplain. This information includes the IOO-year water surface elevation, the top width of the flood- plain and the average velocity of flow in the channel. The floodway por- tions of this table consists of the floodway water surface elevation, the top width of the floodway and the floodway width both right and left of the center 1 ine and the velocity of the floodway. This information is pre- sented for both the one-half and the one-foot floodways. Table IV-2 is intended to provide regulatory agencies with the information necessary to apply floodplain regulations. By providing floodplain eleva- tions, delineating the floodplain and establishing floodway limits, the