Table 4: Summary of historic annual event duration in the upper 

 Nueces Delta. 



Diuation 



As with event magnitude, event duration under historical conditions also exhibited two seasonal peaks (Figure 

 12). During Period I, the spring peak (June) and fall peak (September) attained about 5 average event days per 

 month (5.0 and 4.8, respectively). During Period II, the spring peak (Jvme) was similar to that of Period I, but 

 attained only 3.0 average event days per month, and the fall peak (October) about 4.2 average event days. In 

 Period III, event duration also showed dramatic decreases, but unlike event magnitude, seasonal peaks were still 

 discemable. The spring peak (May) peaked at about 1 . 1 average event days per month, and the fall peak 

 (October) only about 0.5 average event days. During Period III, an anomalous third peak (Februar)') attained 

 about 0.6 average event days, which reflects a disproportional influence of an (uncommon) winter event in 1992. 



Annual event duration represents the sum of 

 all event days during the period divided by 

 the period's duration in years. For Period I, 

 event duration averaged about 21.1 average 

 event days per year (Table 4). This mean fell 

 to about 16.8 event days during the period 

 after the closure of Wesley Seale Dam, and to 

 about 4.5 event days during the period after 

 the closure of Choke Canyon Dam. 



Frequency 



In general, the historical return period for event peak flows during Period I were slightly shorter than those in 

 Period II (except for the largest values), but not appreciably (Figure 13). Period III, however, did exhibit a 

 dramatic increase in the return period of event peak flows. 



Period 



1940-1958 

 1958-1982 

 1982-1999 



Historic 



Mean Number of Event 



Days per Year 



Percent Change 

 from Period I 



21.1 

 16.8 

 4.5 



-20% 



-79% 



The difference in the return period of a flow 

 event with a daily peak flow of 123.4 10^ m^ 

 (100 acre-ft) between Periods I and II was 

 an increase of less than a month (0.06 years) 

 (Table 5). However, for the same event in 

 Period III, the return period rose by over 

 19 months (1.64 years). There was no 

 return period for events with daily peak 

 flows of greater than 617 lO'* m^ (500 acre- 

 ft) during Period III. 



Timing 



Table 5: Summary of the historic return period for a flow event 

 into the upper Nueces Delta with a daily peak flow of 100 acre-ft. 



Period 



I: 1940-1958 

 II: 1958-1982 

 III: 1982-1999 



Historic 



Return Period 



(years) 



Percent Change 

 from Period I 



0.56 

 0.62 

 2.20 



11% 

 293% 



Note: 1 acre-tt = 1.2335 10^ m^ 



In the three-dimensional presentation of event timing (Figure 14), the x-axis represents the calendar year in 

 weeks, the y-axis represents the year of record and the z-axis represents total event discharge into the upper 

 Nueces Delta. The seasonal peaks (spring and fall) observed in the magnitude and duration analyses were also 

 manifest in event timing under historical conditions. As observed from the x-axis, spring flow events (May-June) 

 were more frequent and smaller than fall flow events (September-October), which were more sporadic but 

 generally larger during Periods I and II (Figure 13). This seasonal pattern, however, was noticeably absent during 

 Period III. Winter (December-January) and svimmer (August) flow events for all periods were very rare. 



From the perspective of the y-axis, "wet" or "dry" periods in the delta were evident over the past 60 years. The 

 more significant "wet" years for each period include 1941, 1942, 1946 and 1957, during Period I; 1958, 1967, 

 1971, 1973 and 1981 during Period II; and 1987 and 1992 during Period III. Significant "dry" years in the delta 

 include 1943, 1947, 1950, 1952, and 1955-56 in Period I; 1962-63, 1966, 1969, 1972 and 1978 in Period II; and 

 all of Period III with the exception of 1987 and 1992. Except for an apparent decrease in the size of spring 

 events during Period II, there was no obvious difference in flow event appearance between Periods I and II. 

 There was, however, a marked difference in the comparison of flow events in Period III with either of the two 

 preceding periods. The only discemable events in Period III were the summer event of 1987 (which would have 

 been a much more significant event had not a large part of the flooding event served to fill and spill a nearly 

 empty Choke Canyon Reservoir), and the late winter and spring events of 1992. Even so, these two events, 

 while the largest in Period III, would be considered extremely small events in either of Periods I or II. 

 Therefore, both the relative number and size of flow events in the delta contrasted starkly with the two 

 preceding periods. 



Appendix C ♦ C-17 



