Figure 3-13: Visual satellite image showing the 

 location of Hurricane Bret in relation to the Nueces 

 Watershed just before landfall (August 22, 1999). 



Having threatened to lay waste to much of south Texas, 

 the storm made landfall as a Category 3 Hurricane. 

 Meteorologists were surprised, however, by Bret's 

 lackluster performance inland. "We ain't surprised," said 

 one Texas native. "You think something named 'Bret' is 

 gonna do much here? I have breakfast cereal with more 

 hair on its chest." 



Source of Image: National Environmental Satellite, Data and 

 Information Service of the National Oceanic and Atmosptieric 

 Administration. 



DISCUSSION 



Flooding Thresholds for the Upper 

 Nueces Delta 



For over 1 3 years prior to the opening of the Nueces 

 Overflow Channel {i.e., October 26, 1995), the 

 minimum flooding threshold required for the Nueces 

 River to spiU freshwater into the upper Nueces Delta 

 was rarely attained (Irlbeck and Ward 2000). Based 

 upon recent estimates, a flow in the Nueces River at 

 Calallen greater than about 59.5 m'/s (2,100 cfs) was 

 required to breach the lowest portion of the northern 

 river bank (Irlbeck and Ockerman 2000). However, 

 excavation of the overflow channel fundamentally 

 changed this condition. The minimum flooding 

 threshold was thus lowered from 1.64 m (5.4 ft msl) to 

 about 0.0 m msl. As can be observed from the 

 suinmary of hydrographic events in Table 3-3, this 

 change not only allowed more frequent diversions of 



fresh water into Rincon Bayou and the upper delta, but 

 also provided the opportunity for other non-riverine 

 elements like wind and tide to force water exchanges 

 between the upper delta and the Nueces River. As a 

 result, near continual {i.e., daily) exchange between the 

 Nueces River and the upper delta was observed during 

 the demonstration period, whereas before the 

 interaction was limited to only extremely rare river 

 inflow events. 



During the 50-month period when the overflow 

 channels were open, over 8,810 10' m' (7,142 acre-ft) 

 was diverted from the Nueces River into Rincon 

 Bayou and the upper delta. Using the hydraulic model 

 developed by the Bureau of Reclamation (2000), it was 

 estimated that, of this total amount, only about 1,204 

 lO' m' (976 acre-ft) would have been diverted without 

 the demonstration project features. Therefore, during 

 the demonstration period, the total volume of 

 freshwater inflow into the upper Nueces Delta was 

 increased by about 732% from what would have 

 occurred without the project. In a longer-term 

 examination, Irlbeck and Ward (2000) analyzed the 

 inflow patterns of the upper delta assuming that 

 demonstration project features had been in place since 

 the completion of Choke Canyon Dam in 1982 

 through 1999. These authors concluded that the 

 average annual inflow amount to the upper delta 

 during this 17.6-year period would have been increased 

 from about 666 10' m' (540 acre-ft) to approximately 

 4,219 10' m' (3,420 acre-ft), or by over 633% from that 

 which would have occurred without the project 

 (Irlbeck and Ward 2000). This long-term analysis has 

 been included as Appendix C of this Concluding 

 Report. 



Activation and Behavior of 

 Demonstration Project Features 



Nueces Overflow Channel 



Because the controlling (bottom) elevation of the 

 Nueces Overflow Charmel was at or near mean sea 

 level, it was activated (or it allowed the exchange of 

 water) when there was a change in water level in either 

 bay or the river. Throughout most of the 



3-20 V Hydrog'aphy 



