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<br />the affects of the irrigation canals on the basin hydrology
<br />were ignored, The assumption was made that the ditches could
<br />carr" aoorQxi~~tely 25 percent more than their decre~ct water right$
<br />, .. . (]2)
<br />which are approximately US cfs, 155 cfs, 141 cfs, and 95 cfs
<br />for the Pleasant Valley and r~ke Canal, ~ew ~ercer Canal,
<br />Larimer County No, 2 Canal, and the Arthur Irrigation Ditch
<br />respectively. Based on this assumption the New Mercer and the
<br />Larimer County NO. 2 which would be most influential in basin
<br />hydrology can only carry approximately 10.0 percent of the 100
<br />year existing storm runoff flows which traverse the ditches,
<br />TO further substantiate the assumption that the ditches will have
<br />minor impact on future hydrology, the photog~aphs included on pages
<br />11 - 13 of this ~eport were taken on May 13, 1930, and no
<br />irrigation water was turned into the ditches until June 2, 1980,
<br />The New ~ercer and Ltlrimcr County ~o. 2 appeared to be flowing
<br />at approximately two thirds capacity with only minor storm inflow,
<br />In addition, a3 the WCGt Vine Drainage BaSin ~ecomes fully
<br />developed and the drainage basins between the West Vine Drainage
<br />Basin and the Cache La Poudro Riv(:>r develop, the canals will
<br />not have significant capacity, especially during the irrigation
<br />season, Also, as the West Vine Basin devolo~s, current policy
<br />o( t.he ,]itch companies will discourdge tht'-us" of t.h.. cdnals
<br />for intercepting urban storm runoff,
<br />
<br />aCcurate and representative of physical characteristics as
<br />possible. The individual parameters are dis~ussed as follows:
<br />1. Imp€rviol.ls AJ;;ea rercen~,=-
<br />Impervious areas were actually measured for each
<br />subbasin from the base maps provided by the City.
<br />In many undeveloped agricultural areas, less
<br />than 5 percent of the total subbasin area was
<br />impervious, However, through mutual agreement
<br />with the TeChnical COtt',nittee, it was decided
<br />that no less than 5 percent of the total area
<br />would be used as impervious area, eVen if measure-
<br />ments indicated the actual percentage was less,
<br />2. Overland Flow Lengths
<br />Overland flow lengths were determined by
<br />measurement on the 100 scale orthophotos,
<br />As many lengths as necessary to represent the
<br />subbasin wer" scaled from the maps and the
<br />average length was used.
<br />J, Detention and Pepression Storaqe
<br />For all subbasins, th() lmoervious Storiiq", wdS
<br />assumed to be 0,067 in. '<'h", surface storage for
<br />~erviol1s areilS ranqed from 0,20 in, to 0,30 in,
<br />For cornpletely undeveloped, ayricultural subbasins,
<br />0,30 in, was used for surface storage "od f<:>r
<br />fully developed,
<br />
<br />The City of Fort Collins has recently initiated a project to
<br />study consolidation of these four irrigation canals. 'Che
<br />scope of work will include de!termin.ltio:l of th() 0ffects of
<br />consolidation relating to many filC~OrS, including protection
<br />"f ,,11 "'xi.sti...'] i1"ri'}i'ltorR IHhl '.1'"""" Rt"r'" rll"off. 'T'h'" f'ln<:,tiol'
<br />of the irrigatLon ditches relative to urban storm r~noff could
<br />change as a result of this study.
<br />
<br />4,
<br />
<br />Manning's Roughness Coefficient for
<br />The Manning's roughness coefficient
<br />used for impcrviou5 areaS was 0.016.
<br />
<br />suburban a"ea~, 0.20 1n, was used.
<br />(111
<br />Overland Flow
<br />
<br />gener(llly
<br />
<br />However,
<br />
<br />io
<br />
<br />IV.E, Supbasin PaIan~!~rs
<br />As mentioned pr-cvLo\:sly, th" r",tiilbitity of results obtained
<br />from s".,'.~,M, is ,'!cpcnden~ on the "b:lity to ;Jccurat('ly
<br />represent the physical characteri~tics of the subbasins. ~cthods
<br />used to select subbasin par"meters assured values were! as
<br />
<br />subo"sins where the 1lTlp"rV10us areil b"sically
<br />consisted of rock and/or rOck outcrops, " roughness
<br />coefficient of 0,020 W,1S used. ~"nning's roughness
<br />coefficient for ~rvious ~re~s ranged frox 0,20
<br />to 0,25. "or ur.dcveloped, agrieultural subbasins
<br />0.20 was ~scd and for lawn Grasses 0,25 was used,
<br />
<br />-17- -13-
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