demonstration period, bi-directional flow occurred 

 almost daily. Changes in the physical condition of the 

 Nueces Overflow Channel included the encroachment 

 of vegetation along the water's edge, and a slight 

 narrowing of the bottom of the channel due to erosion 

 from its banks (Figure 3-14). 



Since the primary purpose of the demonstration 

 project was to divert a portion of the flow in the 

 Nueces River through the diversion channel, a logical 

 inquiry was the proportion of such a flood so diverted. 

 Upon examination of the relation between the total 

 event flow volume in the Nueces River and in the 

 Nueces Overflow Channel, it was determined that the 

 volume diverted into Rincon Bayou increased generally 



Figure 3-14: The Nueces Overflow Channel ioo,„.i9 

 southwest. Note the changes in vegetation and channel 

 characteristics from October 1995, immediately after 

 construction (above), to June 1999, towards the end of the 

 demonstration period (below). 



Photos courtesy of the Bureau of Reclamation. 



with the flow in the river, and the actual proportion of 

 the flow amount diverted was on the order of 2% of 

 that in the river (Ward 2000). The actual rate of 

 discharge, however, varied considerably between 

 events depending upon the water level in Nueces Bay 

 and Rincon Bayou. 



That the relation between Nueces River event volume 

 and the volume transported through Rincon Bayou 

 should depend upon water level was not unexpected, 

 based upon hydraulic considerations. Unlike a river 

 channel system in which the head gradient and the 

 water level (stage) are closely related, there is no direct 

 relation between water level and flow in the Nueces 

 River below Calallen Diversion Dam because of the 

 corrupting effect of tidal and meteorological 

 water-level variations. For the events observed, the 

 Nueces River hydraulic head was superposed on 

 whatever water level was present in Nueces Bay, which 

 affected how the river head could drive flow through 

 the overflow channel. Deeper water made available a 

 greater cross-section area of the channel and lowered 

 the frictional resistance. Therefore, a given hydraulic 

 head in the Nueces River drove a greater flow through 

 the diversion channel when the Nueces Bay water level 

 was higher. 



Rincon Overflow Channel 



In addition to increased inflow, the demonstration 

 project features also increased the distribution of 

 diverted fresh water within the tidal flats of the upper 

 delta. The controlling elevation of the Rincon 

 Overflow Channel was about 1.14 m (3.75 ft) msl. 

 When water levels in Rincon Bayou exceeded this 

 threshold, flow would pass through the chaimel and 

 across the tidal flats to the northeast. Without this 

 overflow channel, total diversions through the 

 demonstration project would have been lower, and 

 most of the freshwater diverted would have remained 

 channelized in the upper delta. 



Although no direct gauging data were available to 

 determine exactiy when and to what degree the Rincon 

 Overflow Channel was activated during the 

 demonstration period, it is certain that, on at least two 

 occasions, the channel passed a significant amount of 



Ch^ter Three ♦ 3-21 



