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<br />PURPOSE OF STUDY <br /> <br />The City of Arvada requests a revision to the Flood Insurance Rate <br />Map for newly annexed property on lower Ralston Creek in what used <br />to be unincorporated Jefferson County. The Jefferson County panel <br />numbers for the property in question are 080087- 0160B and 0155B. <br />The City of Arvada panel number for the same property is 085072 <br />0001 B. The property is to be annexed by the City for construction <br />of a municipal golf course and residential development. <br /> <br />LOCATION & DESCRIPTION <br /> <br />The site is located within Sections 1 and 2, Township 3 South, <br />Range 70 West of the 6th Principal Meridian, County of Jefferson, <br />State of Colorado. Quaker Street traverses the site from north to <br />south between 72nd and 64th Avenues. Approximately 1 mile of <br />Ralston Creek traverse the si te from west to east. The west <br />property line crosses the creek approximately 700' east of Church <br />Ditch and the east property line crosses the creek about 200' west <br />of Croke Canal. <br /> <br />The tributary area for the lower Ralston Creek basin between Blunn <br />Reservoir and Indiana St. remains mainly rural. Some <br />commercial/industrial development exists along the creek at Indiana <br />St. but the proposed golf course site and the parcels immediately <br />upstream and downstream remain presently undeveloped. The Ralston <br />Creek corridor in this area is primarily used as agricultural land. <br /> <br />The existing terrain along the Ralston Creek corridor associated <br />with this study consists of open fields of natural grasses and <br />weeds. There are several large cottonwood tre~s scattered along the <br />creek bank in isolated locations. The floodplain slopes slightly <br />from west to east at about 1 to 3%. The existing creek bed is from <br />3 to 10 feet wide with an eroded bank depth of 6 to 24 inches. The <br />average slope of the creek profile is 1.2% with a mean low flow of <br />5 cfs. <br /> <br />The lower Ralston Creek Basin is located on the western flank of <br />the Denver Basin. This basin is a large asymetrical structural <br />downwarp just east of the front range. Based on test borings done <br />in 1988, subsoil conditions are highly variable. The average <br />topsoil depth of 8" overlays sandy silt, sandy clay, silty sand, <br />sands, gravels, claystone and sandstone bedrock. The SCS classifies <br />this basin as a hydrologic soil group C. <br /> <br />'-' <br />