The Rainfall Runoff Relation for a 775 Km2 Basin Under Dry-Soil Conditions Is As Follows

SEV322 Hydraulics and Hydrology

Assignment 2 (Due on 4th May 2011)

Q1

The rainfall–runoff relation for a 775 km2 basin under dry-soil conditions is as follows:

(a)Draw the rainfall–runoff relations for the two storms on a graph sheet.

A storm of 2 hour effective duration occurred over this basin at a time when the soil was dry. The resulting hydrograph was as follows:

(b) Determine the runoff from the basin resulting from this storm in mm. Assume zero base flow.

(c) Approximately how much rain was there in this storm?

(d) What was the average coefficient of runoff (= runoff volume / rainfall volume) during this storm?

(e) Predict the peak flow that would result from a 4-hour storm during which there was 114 mm of rainfall. Once again, assume dry-soil conditions, that is, zero base flow.

Q2.

The 30-min unit hydrograph for a 2.25 km2 urban catchment is given by

(a) Verify that the unit hydrograph is consistent with a 1 cm rainfall excess,

(b) Estimate the runoff hydrograph for a 30 min rainfall excess of 3 cm,

(c) Estimate the runoff hydrograph for a 60 min rainfall excess of 8 cm,

(d) Determine the S-hydrograph from the given 30-min hydrograph,

(e) Calculate the 60 min unit hydrograph from the S-curve and verify that it corresponds to a 1 cm rainfall excess.

Q3.

Annual maximum values of 60 min-duration rainfall are presented in the Table. Develop a model for storm rainfall frequency analysis using “extreme value type I (Gumbel)” distribution and present your results in a graphical form (rainfall values Vs reduced variate, y and AEP). Calculate the maximum values of 60 minute rainfall for ARI values of 5, 10 and 50 years.

60 min rainfall values in mm:

49, 66, 36, 58, 41, 47, 74, 53, 76, 57, 80, 66, 68, 61, 88, 49, 33, 96, 94, 80, 62, 71, 111, 64, 52, 64, 34, 70, 57, 92, 66, 65, 63, 60, 68

Q4.

The annual stream flow peaks (in m3/s) are obtained from the 52 year records at the gauging station on the Tanjil River near Willow Grove in the Latrobe Valley, Victoria. The mean, standard deviation and skewness coefficient were calculated for the 52 recorded values as well as for the log-transformed (base 10) values. These are given below:

Original data Log-transformed data

Mean97.8 (m3/s)1.838

Std. Deviation109.7 (m3/s)0.337

Coeff.of Skewness2.9640.767

1. Calculate the flood magnitudes corresponding to the Annual Exceedance Probabilities (AEP) of 2%, 1% and 0.5 % assuming a normal distribution to the data points.
2. Repeat the above calculations assuming a log-normal (LN2) distribution fits the data points.
3. Repeat calculations in (a) assuming that Gumbel distribution fits the data
4. What would be the flood magnitude for the AEP of 1% if a log-Pearson type III (LP3) distribution is assumed for the data
5. The magnitude of a recent flood event at the same gauging station was estimated at 285.4 m3/s. What is the AEP of this event?
   

Frequency factors of the distribution can be obtained using methods indicated in the lecture slides.

Q5.

Storm water drainage from a residential development of 12,000 m2 is to be connected to the city’s main drainage system. The runoff coefficient for the development is estimated as 0.4 and the Manning’s n for the overland flow as 0.20. The average flow length may be taken as 70 m and the average slope as 0.7%. The city’s drainage system is based on an ARI of 20 years and the rainfall intensity, R (not the rainfall-excess) is estimated using the formula:

R = 3155 / (t0.81 + 6.19)

Where t is in minutes and R is in mm/h.

The time of concentration can be estimated using the kinematic-wave equation;

Where,

tc is the time of concentration in minutes

L is the overland flow length in m

n is the Manning’s roughness

I is the rainfall excess in mm/h

S is the slope

Determine the time of concentration and the peak flow for the design of the main drainage pipe.