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into the Van BTbber flood pl~ln unless controls ~re exercised. The developmenT along The three creeks Is mosTly roSidan- ti~ I, a IthOl.lgh !ndustrl~l, corrmercl~l, and publ ic facll i ties a Iso exist. ThemaincC<mlercialareaofArvada is located near the con- flue~e of Ralston and Van Blbber Creaks. Table 3 shows the pQpul~tlons of Arvad~ taken during past censuses. Growth of Arvada began Increasing rapidly In the 1940's as shown by a 60 percent increase in population for that decade. Satween 1950 and 1960, the population Increased 715 percent. PrelIm- inary 1970 census figures show a population increase of t~6 percent from 1960. These rapid i~raases In populatIon confirm that Arvad~ Is still growing. As such, urbanization will contInue up the Ralston, Leyden, and Van 8ibber Creek val Jeys. Respect for the flood haz.ard In futuro planning and design are necessary to insure that future Qxpanslon does not cre~te costly flood problems. 1900 1920 19~0 1950 1960 1970 !! Est.19!lOY Est. 1990 Est. 2000 m 1,482 2,359 19,2~2 47,374 9,306 14,400 30,725 55,687 127,520 255,057 325,000 450,000 600,000 Roads Across tho Streams Ralston Croek has 21 street or highway crossings and one raIlroad crossing. Major roadways 1~lude State Highway 121 On Wadsworth Blvd. and the Colorado and $outhorn Railroad. Seventeen of the roadway structures are culverts and the remainder are brIdges. AI I of the structures, whether culverts or brIdges, are too small to p~ss the Intermediate Reglona! Floods without some of the flows over- topolnga portIon of the roadway "",,bankments. Inscrneinstancesthe lo~est part of the road !s et approaches dIstant from the bridges, The overflo~s occur at these low poInts leaving the bridge decks above the f lood~aters. leydenCreeklscrossed by four streets, ell of which have culverts for the ~aterway openings. Several new rOl;lds are beIng developed In the flood plain but they either do not cross the stream or only have temporary culverts. Van Blbber Creek has 13 stroot crossings, II on bridges and 2 over the storm sewer near the mouth. Many of the bridges are obstructlonstofloodflo... BrIdge data on each of these three streams Is shown In the Reference Point tabulations pages 38 - 52. The relationships of the brldQe~ to the Intermed!ate Reglo~1 and Standard Project Floods Is showngraphicaTly on the Profiles, Table 3 POPULATIONS Year Arvada Populations Jefferson County !!PreliMlrklryl970Census. ?! Projactions taken from pamphlet entitled "Denver SMSA Population," January 1969, pu~llsht>d l.>y th" O."wer R"9;Ondl Counci I of Governments. Obstructions to Flood Flows During large floods a major portion of the floodwater Is carried on the flood plain. Roads across the flood plaln obstruct the flQ"s and CauSa the flood depths upstreaM of the roadways to rise until floodwaters can overtop the roads. HIgh road embankments such asFootHIllsRoad,whlchcrossesRalstonandVanBlbberCreeks, produce large obstructions. The floodwaters upstream of the road may be over 10 feet higher than on the downstreaM sIde of the road. The Increasodstagesusuallyoccuronlya short distance upstream of the bridges boc(luseof tnesteapnessof thastreams, Some of the " "