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Flooding on the tributary washes draining from the Book Cliffs and Roan Plateau is caused by intense rainfall from cloudburst storms'. but definitive informationondoud- burot floods on the,e streams is very limited, Predpitation records for the Frultaareashow the following I-day rainfall amounts (actual storm durations unknown): Dale Amoun! (Inches) 1.08 1.42 1,20 Oclober13.1957 Augus131,1963 June24,1969 In the study area, 100-year floods on the Colorado River would be caused by ~nowmelt and S(){)..year floods would be cauood by snowmelt augmented br rain, On the tributary washes, 100- and 500,year floods would result from cloudbur~t ~torms, FLOOD HISTORY The Colorado River has a long history of snowmelt flooding, but detailed information on flood evenl. in the study area is unavailable.Thc earlicst Colorado River flood known in the Fruita arcaoeeurred in June- July 1884. Other floods on that stream were recorded in 1917. 1920,192l,l935,1952,and 1957. In general,the 1884 flood is considered the most severe known in thc Fruitaarca.lt ~ultedfromrapidme1tingofadeepsnow- pack and concurrent heavy rains. Cloudburst storms are known to have occurred over the tributary drainage basins and reportedly causcd the washes to overflow many times. but, duc to the rural nature of the re~ion, definitive data On these flood events are not available. FLOOD DAMAGE PREVENTION Most of the reservoir. in the basins of the Colorado and Gunnison Rivers above Fruita areop"rated for water conservation purposes and eonooqucntly provide only ineidental flood protection to the Pruita area, In the Gunnison River drainage, however, regulations for joint conservation,f100d control operation Qf Paonia Reservoir have been prepared by the CorpsQf Engineers and the Bureau of Reclamation, The conservation operation of Blue Mesa Reser"oir provides substantial flood control benefits, butformaloperatingregulation~for flood control have not been prepared, The incidental storage, Iimitcd flood eontrol spaee, trans-mountain diversions, and irrigation uses have served and will probably continue to serve In reducing peak flows for very large floods in tbestudy reaeh, Flood plain regulations have bcen adopted by Mesa County, Designated flood plains, inc1uding low hazard and floodway zones, are incorporated into county zoninJl: maps. Applications for new dcvelopmcntor revision 'S;m"ltan<o""noodin.o:onallt,.o\rib"t.,;""pr"b~b]ywou]d not<><<",d"""'\oo,mall .".."I."""tord""dbur>l ,torn", , I I I I I I I I I I I I I I I I I I I I I I I I I I I I I . I I I I I I ofexistingde"c1opmentinthe~eareasmustb€ appro,'ed br the Count;' Commission. and sueh new Or rcvised development is subject to pfO\"Isions designed to prohibit: a. StoraJl:e of materials that are floatable Or ma"bcdetrimental to human, animal. Or plant life, b, Disposal of garbage 0" other solid waste material~, c. Residential use, d. Creation or deposition of additional debris. e. Placement 01 fill, installationofstruc- tures. or stornge of materials that could ad"er~ely affect floodflol\' and pos.~ibly cause additionalfloodinl/:, Fruita doeo not have a zoning-ordinance. Theeommunity is Intheproee~sofadopting ~ubdivi~ion regulations, but the regulations under consideration do not address flood hazardsperse. FUTURE FLOODS 100- AND SOO-YEAR FLOODS The loo-yearflood is one witha peak flow magnitude that has a 1 percent ehance of being equalled or exceeded in anrgiven rear, and a frequenc;'ofoccurrenceofaboutoncc in 100yearsonthelong-termaveragl',Similarly, the ::;OO'ycar flood h~s a 0,2 percent chance of being equalled orexCl'ede<l in any given year, and a frequency of OCCllTrenCe of about once in 500 years on the long'term average. As previously indicated, snowmelt runoff from the Colorado River drainage basin and convective type eloudburst storm runoff from the drainage basins 01 the tributarv washes create the most Severe fiood conditions in the study area, The unit hydrographs for Big Salt Wash, Little Salt Wash, and Reed Wash were derived by the S'l<raph method,utilizingan S, graph developed lor Riflc Creek at Rifle. A regional snowmelt flood envelope curve for the CoJorado River was developed using flow, diseharge frequency data, Snowmelt flows at Fruita were developed from frequency eurves for the Colorado and Gunn;son Rivers abovc Grand Junction. Based on a"ailablc data, the 1921 flood was selected as being most representative for combined runoff from the two rivers, and the standard project flood was determined to have a frequencyof25Oyears(50percentlargerthan the 1921 flood). To establish standard project flows On the Colorado River, a 150 percent value o! the 1921 floodflowsatPalisade was determined and then reduced by S,OOO cubic feet per second to reflect the effect of upstream reservoirs, For standard project flows on the Gunnison River, 150 percentQf the 1921 f1oodflow at Grand Junction was establi~h>dandthendividcdintorunofrabov" and below Blue Mesa Reservoir (55 and 45 percent, respectively), Blue Mesa Reservoir WaS completed in 1965. Runoff above the reservoir was computed asa ratio of the 1921 flows and adjusted for present conditions. Reservoir releases were made so that down- stream channcl capacities would not be exceeded and assuming maintenance ofmin- imum power pOOl level. Downstream runoff was then added to arrive at present standard project flow at Grand Junction, Flows in the two rivers were combined for standard project flow at Frulta, The loo-year flood ""'ent was estabJished as an 89 percent value of the standard project event. The resulting f100(1nows arcshowll i"TaLle3, 1'''1/,,6. 5