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img023 - GEOG/NRES 401 Watershed Hydrology 2011 Professor M...

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Unformatted text preview: GEOG/NRES 401 Watershed Hydrology 2011 Professor M. Sivapalan TUTORIAL CLASS April 7, 2011 1. An agricultural watershed was urbanized over a period of 20 years; A triangular unit hydrograph was developed for this watershed for an excess rainfall duration of one hour (i.e., one hour unit hydrograph). Before urbanization, the average rate of infiltration and other losses was 7.5 mm/ hr, and the unit hydrograph had a peak discharge of 0.5 m35-1 mrn-1 at 3 hr and a base time of 9 hr. ‘ ‘ ' ’ After urbanization, because of the increase in impervious surfaces, the loss rate dropped to 3.75 mm/ hr, the peak discharge of the unit hydrograph was increased to 0.75 m3s-1 mm-1, occurring at 1 hr, and the base time was reduced to 6 hr. For a two-hour storm in which 25 mm of rainfall fell in the first hour 12.5 mm of rainfall in the second hour, determine the direct runoff hydrographs before and after urbanization. What is the effect of urbanization on the flood peaks? 2. You are given the unit hydrograph for a 1 mm storm of 2 hours duration. It is tabulated below: t (hours) u2(t) m3/s t (hours) u2(t) m3/s O 0 12 5.4 2 3.7 14 3.1 4 10.8 16 1.7 6 13.4 . 18 0.9 8 11.7 20 0 10 8.4 51.} “AS/e 7/ ‘ M ” . (b) Determine the unit hydrograph for a torm of 4 hours duration. 2 76 O /i\ we . 1M 41 my ,/ (c) Using the appropriate unit hydrograph, caEulate the quicltjlow hydrograph for this catchment that would result from the following storm. What is the peak flow, time to peak, and the time base of the quickflow? (a) What is the catchment area? time Glours) 0-4 4—8 8-12 effective rain intensity (mm/hr) 2.0 5.0 3.0 at): HAHN 20U4H—é3 t W, (73-52) ...
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