FLOOD09697 - Laserfiche WebLink

Home Browse Search

C. Eslimate the percentage impervious cover for each unit. 1. Use a rotameter to estimate road lenglh and multiply by the aver- age road width to estimate the area of roadways. 2. Estimate parking lot areas by directly measuring their dimen- sions or by using a planimeter. 3. Estimate Ihe impervious area conne cled with dwelling by counting the number of homes and multiplying by the average impervious area for a single home. Add to this total in. dividual estimates for larger structures such as induslrial planls, hospitals, and churches. 4. Estimale sidewalk area from a measurement of dimensions. III. The characleristic impervious length fac- tor is eslimaled as shown by Figure 2.5. Drainage palhs usually can be predicled by the conjunctive use of contour and streel maps. In this study, only a few field observations of flow at street com- ers were needed. Summary of Calculated Urban Parameters The impervious cover and characteristic imper- vious length factors for the specific urban area of lhis study were all measured mostly from aerial photo. Boundary of catchment area /' )"" ImpervIOUs / oreo,ol- I / / / . Impervious areas / /' /,/ poin' (II = ImlHlfviou9 area In! = Toto! impervious ,~ 1:0111 Lm=~ L,=~ L Figure 2.5. Sketch illustrating the characteristic imper- vious length. Lt, for a given catchment. graphs dated 1975. Raw data for each unit shown by Figure 2.4 were inpul to a computer program (Ap- pendix B) 10 estimate the following: I) the impervious cover by categories; 2) the characteristic impervious length factor; and 3) the fraction of impervious cover. The estimates for items (2) and (3) for the subzones of Figure 2.2 are summarized by Table 2.2. The figure of 2400 square feet of impervious area for an average urban dwelling was derived by sampling 21 residenlial blocks in lwo urban watersheds. For each block, mean areas were calculated for the driveway and for Ihe dwelling. By averaging for the entire study area, a mean residence area of 1833.2 square feet and a mean driveway area of 553.6 square feet (for a lotal of 2386.8 square feel) were obtamed. Confidence limils of95 percenl were computed for residences as between 1716.0 square feet and 1949.4 square feet, and for driveways as belween 476.6 square feet and 630.0 square feet. Hydrologic Characteristics of the Study Area The Mill, and Big and Little Cottonwood Creek walersheds are frequently subject to slorm runoff wpJch exceeds the capacity of the stonn drainage sys- tem and which, therefore, produces flood damages. Most of the climatologic, hydrologic, and geologic data pertaining to the area are published in agency annual reports or other public files, and therefore, were available for this study. In addilion, aerial photo- graphs taken in June and July of 1975 were obtained from Ihe U. S. Department of Agriculture. Climale Runoff originates as precipitation. and precipi- tation patterns, as modified by snow storage, cause flooding conditions. The influence of the Wasatch Mountain Range on 100 general precipitation pattern throughout the easterly portion of Salt Lake County is shown by Figure 2.6 (u. S. Wealher Bureau, 1963; Kaliser, 1973). More than two.thirds of the precipi- tation along the Was'lch Front occurs during the win- ter months, mostly in Ihe form of snow that fall dur- ing orographic lifting as air currents pass from west to east over the mountain front. In summer, the uneven heating of the ground surface creales vertical lifting, leading to high intensity convective storms of short durations and of small aerial exlent. The Weather Bureau (National Wealher Service) has maintained continuous precipitation records at Salt Lake City for more than 85 years. Temperatures The wannesl temperatures of spring induce new leaf and vegetative growth. Wann summer lempera- 18