Laserfiche WebLink
<br />!'lij')~l~ <br />'_'. {, __..l. <br /> <br />This model has been developed for use in the subalpine zone <br />where runoff occurs principally from melting snow. It <br />simulates on a daily basis: OJ ~!inter snow acctnnulation, <br />(2) the energy balance, (3) snowpack condition, (4) <br />evapotranspiration, (5) sncwnelt, and (6) resultant water <br />available for streamflow on any combination of aspect, slope, <br />elevatiorl, and forest cover con1p0sition ond density. The <br />model simulates the form of precipitation (rain or snow), the <br />melting process, and. snav-pack condition in terms of energy <br />levels and free water requirements. <br /> <br />Table IV-5 summarizes results from the urban runoff analysis <br />for a typical year. These were calculated with the stream <br />gage discharge records and Table III-7 water quality data. <br />Shown are the number of storm events and voltnnes of runoff <br />generated from undisturbed (baseline) and developed areas. <br />Developed area runoff increased much above baseline for April <br />and August (sncwnelt and rain respectively). During May and <br />June developed area runoff is less than be.seline since <br />private land snow would melt off earlier than before <br />develoJ;TOOnt. Total developed area runoff would increase frcrro <br />about 13.4 to 23.6 inches. During sncwnelt runoff, an event <br />occurs each day; whereas in the summer months, events occur <br />only in response to thundershowers and general rainfall. <br /> <br />The loading of each constituent was obtained by multiplying <br />the recommended value in Table IV-4 times the simulated <br />number of events times the quantity of runoff times the acres <br />of developed area. These values are compared with the loads <br />of each constituent observed in the West Fork at the county <br />line during 1985 (stream gage,0934l500). <br /> <br />Cumulative potential discharge of suspended sediment and <br />other urban runoff pollutants into the West Fork of the San <br />Juan River is small relative to the existing aIOOunts (of <br />those parameters) in the river as measured in the West Fork <br />at the county line. <br /> <br />Estimated annual increase in loads to the West Fork varied <br />from 0.17% (suspended sediment) to 14% (BOD). These <br />increases would be higher if the 90th percentile urban site <br />EMC value is used rather than the median site EMC (Table <br />IV-4). <br /> <br />No water quality standard exceedances (Table In-8) are <br />anticipated although an extremely heavy thundershower <br />flushing the base area during a low flow period could result <br />in a tenporary exceedance of one or more of the parameter <br />standards, particularly for nitrogen ond phosphorus. This <br />type of transient exceedance has little biological <br />significance. l>-.s shown in Table IV-5, most of the urban <br />runoff would occur with snowmelt runoff in April and Hay when <br /> <br />171 <br />