University of Colorado



University of Colorado

Department of Civil, Environmental and Architectural Engineering

Engineering Hydrology

CVEN-4333 Spring 2005

Finals (Take Home)

Date: 04/30/2005

Due: 05/01/2005 – 11AM

50 points + 20 points for the make up question

Total 70 points

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Please write the steps clearly so that points can be awarded even when the numerical answers are incorrect. You can use textbook/class notes, calculators/Excel. Refrain from consulting with each other – I trust your conscience and honor system.

Make Up Question

1. The Great Salt Lake (in Utah) surface is at an elevation 1500m. A parcel of air at temperature 12oC; 840mb pressure and relative humidity 60% picks up moisture from the lake and ascends the Wasatch mountain ranges. The mountain top is at 1200m from the lake surface. The water temperature 15oC and the wind speed at 2m above the water surface is 3 m/s (water surface roughness is 0.03cm).

a) Estimate the elevation above which precipitation occurs.

b) Assuming that water falls as precipitation as soon as it condenses, estimate the average precipitation rate between the precipitation level and the top of the mountains.

c) Estimate the snow level.

d) What is the evaporation from the lake [5, 5, 2, 8]

Infiltration

2. Consider the following storm:

Time (h) 0-2 2-4

Rainfall Intensity (cm/h) 0.5 2

Philip's equation is applicable with Kp = 0.2 cm/h and Sp = 3 cm/h0.5.

Determine the infiltration and excess rainfall in each two hour increment. What is the time that ponding first occurs in this storm? [6,4]

[Hint: Assume the two rainfall pulses to be independent – i.e. analyze them individually]

3.Show that the ponding time under rainfall of intensity i for a soil described by Horton’s equation with parameters fo, fc and k is given by

Tp = (1/i*k) [fo – i + fc* [ln( (fo – fc) / (i – fc) ) ]

Where Tp is the time to ponding

[Following the three steps outlined in the class which are - from the infiltration rate equation re-arranged to get an expression for time ‘t’; plug this into the cumulative infiltration equation; and set this equal to cumulative infiltration at ponding (i Tp); re-arrange to get the required expression]

(b) Calculate the cumulative infiltration and the infiltration rate after one hour of rainfall at 3cm / h on a clay loam with a 25 percent initial effective saturation. What is the time when the cumulative infiltration will be twice as much as it is at the end of the first hour. [4,3,3]

Unit Hydrograph / Runoff

4. Measured total hourly discharge rates (cfs) from a 2.48 square miles drainage basin are tabulated below. The hydrograph was produced by a rainstorm having a uniform intensity of 2.6 in/hr starting at 9 A.M and ending at 11 A.M. The base flow for the entire period was a constant 100 cfs

Time 8AM 9 10 11 12 1PM 2 3

Total discharge (cfs) 100 100 300 450 300 150 100 100

(a) At what time did direct runoff begin?

(b) Determine the excess rainfall depth (the total rainfall is of course, 2.6 x 2 = 5.2 inches).

( c ) Derive a 2-hr Unit hydrograph

[1, 5, 4]

5. You are in-charge of delineating the flood plain for the South Boulder Creek. To do this you have to estimate the peak discharge corresponding to a 100-year 1-hour design storm, which will be passed through a flood plain model for delineation. The peak discharge is obtained from the outflow hydrograph – which in turn is obtained by convoluting the excess rainfall with the unit hydrograph. The following information is provided:

(i) The 100-year design rainfall for 1-hour duration is 3.0 inches

(ii) The area of the watershed is 132 square miles

(iii) The time of concentration (Tc) for this watershed is 1.5 hours

(iv) Assume antecedent moisture condition to be wet (i.e. AMC III). The SCS curve number for normal antecedent moisture condition (AMC II) is 84.

You have to obtain (a) Excess Rainfall (b) the Unit Hydrograh and ( c ) the peak discharge by convolution [5, 4,6]

Course Synthesis

6. You are now the team leader to design a reservoir in the Pacific Northwest. Assume that you have access to monthly flow data at a nearby gauge on the river and no other data. Having been exposed to a wide ranging aspects of the hydrologic cycle and large-scale climate in the Hydrology course – outline the steps in your design process. [5]

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