CE 374K Hydrology Homework Assignment #8

Flood Frequency Analysis


The attached spreadsheet guadpeak.xls and comma separated file guadpeak.csv contain the annual maximum discharges of the Guadalupe River at Victoria for the period 1935 2000. These data are also obtainable in the LRC in /class/maidment/ce374k00. Actually these data are for water years, which begin on October 1 of the previous year and continue to September 30 of the current year. These data were obtained from the Peak Flow file at http://waterdata.usgs.gov/tx/nwis/peak for USGS Station 08176500. The data for 1935 to 1998 are plotted below. In October 1998, an enormous flood occurred on the Guadalupe River (in the 1999 water year) for which the discharge has been estimated by the USGS as being 466,000 cfs. This is not shown on the graph below to avoid making all the other floods seem very small.


(1) Plot a graph of all the data, including the flows in 1999 and 2000.


(2) Estimate the return period of annual maximum discharges which exceed 30,000, 40,000, 50,000, and 60,000 cfs directly from the data by looking at the recurrence interval between flows exceeding those magnitudes in the data from 1935-2000.


(3) Take the logs to base 10 of the data and calculate their mean, standard deviation and coefficient of skewness for the data from 1935 1998, and also including the last two years of data, i.e. 1935-2000.


(4) Estimate the 100 year flood on the Guadalupe River using the Log normal (if coefficient of skewness ~ 0) or Log Pearson III method (if coefficient of skewness is significant). By how much does the inclusion of the October 1998 flood increase the 100 year flood estimate?


This assignment is due on Tuesday April 10