Estimating Freshwater Inflow into the Upper Nueces Delta 



Once a representative daily stage record had been developed for the Nueces River at the point of diversion, two 

 different sets of stage-discharge rating curves were used to estimate daily discharge into the upper delta. Prior 

 to this analysis, the Bureau of Reclamation, as part of their Rincon Bayou Demonstration Project, conducted 

 (1996) and revised (2000) a hydraulic study of the relation between flow in the Nueces River and that in Rincon 

 Bayou. These modeling efforts, which did not address the southern portion of the delta, produced a series of 

 rating curves based on field conditions during March of 1993. Among the scenarios developed were the pre- 

 project (or historical) condition, and the post-project (restored) condition. 



Historical Freshwater Inflow into the Upper Nueces Delta 



Under the without-project (historical) scenario analyzed by the Bureau of Reclamation (2000), a total of five 

 natural depressions in the north bank of the Nueces River were identified tliat would naturally contribute to 

 discharge into the delta at various stages in the river (Figure 4). The lowest of these drainage channels was 

 along the west side of the Missouri-Pacific railroad bridge, which had an effective bottom elevation of about 

 1 .64 m (5.4 ft) msl. The hydraulic characteristics of each of these depressions were combined into one 

 commutative set of rating curves, including both a rising and falling limbs, for daily discharge into the upper 

 Nueces Delta (Figures 8 and 9). The obvious 'Tsend" in the rating curves reveal the natural flooding threshold 

 for the greatest part of the river bank, which is about 2.36 m (7.75 ft) msl. Reclamation's without-project rating 

 curves were used without modification, and estimated daily stage values for the Nueces River at the point of 

 diversion were converted to estimate daily discharge into the upper Nueces Delta. 



Potential for Restored Freshwater Inflow into the Upper Nueces Delta 



Base on data obtained from the Rincon gauge, the Bureau of Reclamation (2000) also constructed a set of rating 

 curves, including both rising and fallin g Umbs, which estimated daily discharge into the upper Nueces Delta 

 with the demonstration project features in place (restored condition) (Figures 8 and 9). Because of the 

 compromising effect of tide on water elevations at the point of diversion, and therefore on discharge estimates, 

 these curves did not estimate discharge into the upper delta when the stage in the Nueces River was 0.76 m 

 (2.50 ft) or below. 



However, when discharge estimates for individual events using the falling limb curve (which represented an 

 average of several observed events) were compared with actual discharge data from the Rincon gauge, the 

 results were unsatisfactorily inconsistent. Upon examination, it was discovered that, although the falling limb 

 curve of each event expressed the same slope when plotted, the beginning point of each curve depended upon 

 the maximum water surface elevation attained by the Nueces River during that particular event. Therefore, a 

 series of fallin g Umb curves were subsequendy constructed by extrapolation of the average curve in 0.1 -ft 

 intervals. Discharge estimates for individual events using the modified, event-specific set of rating curves were 

 then tested for accuracy, this time with acceptable results. For each of the eight (8) major freshwater flow 

 events recorded at the Rincon gauge, the estimated discharge was within about 10% of the actual for five 

 (5) events, and within about 25% of the actual for two (2) others (Table 1). The combined accuracy for all 

 discharge estimates made using the revised rating curves for the restored condition, including all eight events, 

 was about 14% over the actual gauged discharge value. 



Once an acceptable set of rating curves for the "restored" condition was thus developed, the estimated daily 

 stage values for the Nueces River at the point of diversion were converted to estimated daily discharge into the 

 upper Nueces Delta. 



Definition of Flow Regime Parameters 



From the two sets of daily inflow data {i.e., historic and restored conditions), fovir separate flow regime 

 characteristics were analyzed; including, event magnitude, duration, frequency and timing. First of all, event magnitude 

 was used to indicate the amoimt of discharge from the Nueces River into the upper Nueces Delta during the 

 period under consideration, and was determined by separating the estimated daily discharge values by period, 

 and then averaging these by month. Next, event duration was used to express the cumulative length of flow 

 events, and was determined by averaging the total number of days in which the stage of the Nueces River 

 exceeded the flooding threshold (event days), regardless of discharge amounts. Also, e.v&at frequency was used to 

 estimate the return period of peak daily flow events into the delta, and was determined by summing the number 

 of events in each period that attained a given peak daily discharge amount, and then dividing this total by the 



Appendix C ♦ C-11 



