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the affects of the irrigation canals on the basin hydrology were ignored. The assumption was made that the ditches could carr" aoorQxi~~tely 25 percent more than their decre~ct water right$ , .. . (]2) which are approximately US cfs, 155 cfs, 141 cfs, and 95 cfs for the Pleasant Valley and r~ke Canal, ~ew ~ercer Canal, Larimer County No. 2 Canal, and the Arthur Irrigation Ditch respectively. Based on this assumption the New Mercer and the Larimer County NO. 2 which would be most influential in basin hydrology can only carry approximately 10.0 percent of the 100 year existing storm runoff flows which traverse the ditches. TO further substantiate the assumption that the ditches will have minor impact on future hydrology, the photog~aphs included on pages 11 - 13 of this ~eport were taken on May 13, 1930, and no irrigation water was turned into the ditches until June 2, 1980. The New ~ercer and Ltlrimcr County ~o. 2 appeared to be flowing at approximately two thirds capacity with only minor storm inflow. In addition, a3 the WCGt Vine Drainage BaSin ~ecomes fully developed and the drainage basins between the West Vine Drainage Basin and the Cache La Poudrc Riv(:>r develop, the canals will not have significant capacity, especially during the irrigation season. Also, as the West Vine Basin devolo~s, current policy o( t.he ,]itch companies will discourdge tht'-us" of th.. cdnals for intercepting urban storm runoff. aCcurate and representative of physical characteristics as possible. The individual parameters are dis~ussed as follows: 1. Impervious AJ;;oa rercen~,=- Impervious areas were actually measured for each subbasin from the base maps provided by the City. In many undeveloped agricultural areas, less than 5 percent of the total subbasin area was impervious. However, through mutual agreement with the TeChnical COft',nittee, it was decided that no less than 5 percent of the total area would be used as impervious area, eVen if measure- ments indicated the actual percentage was less. 2. Overland Flow Lengths Overland flow lengthS were determined by measurement on the 100 scale orthophotos. As many lengths as necessary to represent the subbasin wer" scaled from the maps and the average length was used. J. Detention ilnd Pepression Storaqe For all subbasins, th() imoervious Storiiq", wdS assumed to be 0.067 ill. '<'h<" surface storage fo~ ~erviQl1s areilS ranqed from 0.20 in. to 0.30 in. For corr.pletely undeveloped, ayricultural subbasins, 0.30 in. was used for surface storage and f<:>r fully developed, The City of Fort Collins has recently initiated a project to study consolidation of these f6ur irrigation canals. 'Che scope of work will include de!termin.ltiO:l of th() 0ffects of consolidation relating to many fi<c~ors, including protection r>f ,,11 "'x;.Rti,,'J i,.-ri'J"torR IHhl '.1'"""" Rt"r'" rll"off. 'T'h'" f'ln<:,tio1"' of the irrigatLon ditches relative to urban storm runoff could change as a result of this study. 4" Manning's Roughness Coefficient for The Manning's roughness coefficient used for impervious areas was 0.016. subu~ban a"eas, 0.20 In. was USed. (111 Overland Flow generally However, io IV.E. Supbasin PaIan~!~rs As mentioned pr-cvLO\:sly, th" reliilbility of results obtained from s.,.,'.~.M. is ,'!cpcnden~ on the ab:lity to ;Jccurately represent the physical characteristics of the subbasins. ~ethods used to select subbasin parameters assured values were! as suboasins where the llTlp"rV10us arei< basically consisted of rock and/or rack outcrops, a roughness coefficient of 0.020 W,1S used. !-lanninq's roughness coefficient for ~rvious ~re~s ranged frox 0.20 to 0.25. !"or ur.dcveloped, ilgrieultural subbasins 0.20 was ~scd and for lawn arasses 0.25 was used. -17- -13-