A small town (area = 0.74 square mile) discharges storm runoff into an adjacent

Question

A small town (area = 0.74 square mile) discharges storm runoff into an adjacent river. The mean annual low flow in the river is 20 cfs, and the suspended-solids concentration upstream of the river is 100 mg/L. The town engineer needs to assess the effect of a 6-hr storm with average runoff rate of 1.2 cfs on the suspended-solids load. The estimated mean annual rainfall is 37 inches and the mean minimum January temperature is 25 degree F. Use Driver and Tasker (1998) equation to estimate the load of suspended solids in lb_m.

Explanation / Answer

3. Common hydrologic units a. Precipitation - inches (in) b. Precipitation rate or intensityinches per hour (in/hr) c. Runoff-inches (in) d. Runoff volume - watershed inches (in); acre-feet (ac-ft); cubic feet (ft2 ) e. Runoff rate - cubic feet per second (cfs) f. Evaporation - inches (in) g. Interception -inches (in) h. Infiltration – inches/hour (in/hr) i. i Percolation - inches/hour (in/hr) j. Storage - cubic feet (ft3 acre feet (ac-ft); watershed inches 4. Common definitions a. Time of concentration (T) - The time it takes water to move from the hydraulically most distant point in a watershed to a watershed outlet, usually expressed in hours. It is used to estimate peak discharge or to develop a hydrograph. b. Hydrograph - A graph showing the discharge, velocity, or other property of water with respect to time. Usually we work with discharge hydrographs, where the points on the hydrograph are expressed in cfs. c. Peak discharge (q ) - The maximum discharge rate for a given hydrograph or flood event, usually expressed in efs, d. Runoff (Q) - That portion of precipitation which appears as streamflow, usually expressed in inches. e. Velocity (V) - The distance traveled divided by the time required to travel that distance, usually expressed in feet per second (ft/s) f. Flood routing - Determining the changes in a hydrograph as it moves downstream through a valley or through a reservoir (then sometimes called reservoir routing). g. Initial abstraction 0) - The portion of precipitation occurring before surface runoffbegins, usually expressed in inches. It consists mainly of interception, infiltration, and depression storage. h. Runoff curve number (CN) - An index of the runoff potential from a specific combination of cover, land use, and soil. i. Watershed (w/s)~The area contained within a divide above a specified point on a stream. It can also be called drainage area, subarea, basin or catchment area. j. Frequency - An expression or measure of how often a hydrologic event of given size or magnitude should, on the average, be equaled or exceeded. It is usually expressed in years. For example, a 50-year frequency flood will be equaled or exceeded in size, on the average, once in 50 years. k. Travel time - The average time for water to flow through a portion of a stream, usually expressed as hours. 5. Common conversions a. 1 cfs-day = 1.9835 or approximately 2 acre-feet (2 ac-ft = 1 cfs-day) 1ft3 /s(day)*86,400s/1day*1ac/43,560 ft2 =1.9835 ac-ft b. 1 acre-foot = 12.1 cfs-hour (12.1 cfs-hr = 1 ac-ft) 1ac-ft * 1cfs-day/1.9835 ac-ft * 24hrs/day =12.0998cfs-hr c. 1 acre-inch per hour =1.0083 or approximately 1 cfs (1 cfs = 1 ac-in/hr) (1 ac-in/hr )* (12.1 cfs-hr/1ac-ft) *(1ft/12in) = 1.0083 cfs d. 1cfs= 448.8 gallons per minute (448.8 gpm= 1cfs) =448.8 gpm e. 1 inch per hour= 1.008cfs per acre (1cfs/ac=1in/hr) 1 in/hr * 1 ft/12in *1hr/3600s *43,560 ft2 /1ac = 1.008 cfs/ac 1ft3 =7.48 gallons S 1) 2 ac-ft = 1 cfs-day ummary: 2) 12.1 cfs-hr = 1 ac-ft 3) 1 cfs = 1 ac-in/hr 4) 448.8 gpm = 1 cfs 5) 1 in/hr = 1 cfs/ac 6) 1 ft3 = 7.48 gallons

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