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Total event flow in Nueces River (acre-ft) 



Figure 3: Total flow volume carried in the Nueces River versus total flow volume diverted 



through the Nueces Overflow Channel. Only events that met criteria for a flow event in the 



Nueces River were used. The values plotted for Rincon flow volume are the total net exchange, or 



the integrated positive values. 



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



That the relation between Nueces River event volume and the volume transported through the Nueces 

 Overflow Charmel should depend upon water level was not unexpected, based upon hydrauHc 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. The Nueces River hydraulic head is 

 superposed on whatever water level is present in Nueces Bay. However, this water level does affect how the 

 liver head can drive flow through the overflow charmel, because the deeper the water, the greater the cross- 

 section area of the channel and upper delta, and the lower the frictional resistance. Therefore, a given hydraulic 

 head in the Nueces River drives a greater flow through the diversion channel when die Nueces Bay water levels 

 are higher. 



The surprising aspect of Figure 3 was the apparent constancy of the volume diverted versus river flow volume 

 for a given class of water levels. For this there are two possible explications. The first is that this observation 

 was an artifact due to die way that a hydrographic "event" was defined (which includes the entire period in 

 which all of die variables respond, dien return to their pre-event values or, in die case of salinity, to a stable 

 value). Thus the diuation over which Nueces River flow was computed is generally longer dian die duration of 

 the flow event in the diversion channel. The rebuttal to this thought is that there is flow in the Nueces River 

 that occurs when there is not flow in the overflow channel, so it was legitimate to integrate over die full 

 hydrograph in die river channel. The second is diat this observation was a manifestation of die phenomenon 

 of hydraulic capadt}', suggesting diat die Nueces Overflow Channel and upper Rincon Bayou very quickly reach 

 their hydraulic capacity shordy after a flood event begins. The result is diat die volume diverted through the 

 overflow channel becomes substantially constant, even as flow in the Nueces River increases. The present 

 writers are inclined to this second view. If this constancy of volume is a valid inference, it would imply tiiat the 

 2% proportion of flow in the Nueces River diverted into Rincon Bayou is itself an artifact of data points 

 corresponding to different Nueces Bay water levels. 



Appendix^ ♦ B-9 



