Table 1: Comparison of estimated versus actual 

 of Reclamation (2000), with modified falling limb 



during each event were used. 



discharge into the upper Nueces Delta using curves from Bureau 

 curves. Only daily stage values greater than 0.76 m (2.5 ft) msl 



EVENT DATE 



Total 



Duration 



(days) 



Actual 



Discharge 



(acre-ft) 



Estimated 



Discharge 



(acre-ft) 



Deviation 

 from Actual 



Maximum Stage 

 Attained 

 (ft msl) 



1997 



1998 



1999 



TOTAL 



7,740 



8,837 



14.2% 



Note: 1 acre-fl = 1.2336 10' mM ft = 0.3046 m 



number of years in the period (annual frequency). Daily peak values greater than 123 10^ m^ (100 acre-ft) were 

 rounded to the nearest hundred, and those less than this were rounded to the nearest ten. From annual 

 frequency, cvimulative frequency was then determined by incremental summation. The return period of peak 

 flow events for a given magnitude in each period was then calcvilated as the inverse of cumulative frequency. 

 Finally, event timing was used to identify seasonal and annual patterns in flow events, and was determined by 

 Slimming daily discharge values by week for the entire period vinder review. 



RESULTS 



For pvuposes of comparison, the 60-year record under investigation was divided into three separate periods, 

 each corresponding to the construction of a major reservoir in the basin (Table 2). 



Period I extends from January 1, 1940 to April 9, 1958 (approximately 18.3 years). During this period the only 

 major regulating structure in the basin was La Fruta Dam on the Nueces River. The dam's mfluence on larger 

 flood events in the watershed was limited because the storage capacity of the reservoirs capacity was relatively 

 small to begin with, and this decreased significandy overtime due to sedimentation (City of Corpus Christi 

 1990). Given the absence of data prior to this structure. Period I therefore represents approximate "baseline" 

 conditions in the watershed with, minimal influences on stream flow from reservoir construction. 



Period II extends from April 10, 1958, when Wesley E. Seale Dam was closed, to May 17, 1982 (approximately 

 24.1 years). Wesley Seale Dam was also constructed on the Nueces River just downstream of the La Fruta dam 

 site, submerging and replacing it as the City of Corpus Christi's primary water supply. Once completed, the 

 larger size of Wesley Seale Dam enabled it to more significandy affect flood events. Period II therefore 

 represents an intermediate period of reservoir development in the watershed. 



Period III extends from May 18, 1982, when Choke Canyon Dam on the Frio River was declared substantially 

 complete, to December 31, 1999 (approximately 17.6 years). The addition of Choke Canyon Dam's storage 

 capacity to that of Lake Corpus Christi increased the total storage capacity in the basin to over 1,221,165 10' m' 

 (990,000 acre-ft). For die latter part of this period (since June 24, 1997), Lake Corpus Christi was operated at 

 an elevation of only 27.74 m (91.0 ft) msl (effective storage capacity of approximately 229,431 10' m' 

 (186,000 acre-ft)) because of safety concerns. Period III therefore represents the climax period (or present 

 conditions) of reservoir development and operation in the watershed. 



For each of these three periods, data from the largest flood event in that period was not considered in the 

 analysis of flood event magnitude, frequency and duration for two primary reasons. First, these events were 

 considered extra-ordinary, and therefore were not typical of the more frequent flow events of primary interest 



Appendix C ♦ C-13 



