Instrumentation 



All data ftom the gauge was measured in 

 1 5-minute intervals, stored by a data collection 

 platform (DCP), and transmitted to USGS offices 

 via GOES (Geosyn-chronous Operational 

 Environmental Satellite) in near real time 

 (Figure 2). Water level ia the channel was 

 measured by a pressure transducer, and the 

 gauged water level {i.e., gauge height) was 

 referenced to mean sea level. Datum of the gauge 

 is at mean sea level. Row velocity was measured 

 by an acoustic velocity meter deployed near the 

 center of the channel (Figure 3). The acoustic 

 meter had a resolution of 0.01 feet per second and 

 measured both positive and negative flow in the 

 channel. Rainfall was measured by a tipping 

 bucket rain gauge. Discharge in the channel was 

 computed as the product of the flow cross- 

 sectional area and the mean chatmel velocity. 



Figure 2: View of the 

 Data Collection Platform, 

 Rincon gauge. 



Figure 3: View of the 

 gauging instrumentation, 

 Rincon gauge. 



At this site, there is no well-defined relation between water level (stage) and discharge due to the effect of tide. 

 The cross-sectional area of flow in the channel is a function of water level in the chaimel. The relation between 

 water level and cross-sectional area was therefore determined from a cross-section elevation survey. 

 Furthermore, the acoustic velocity meter measures flow velocity at a single point in the channel. Because the 

 measured velocity is not necessarily the mean velocity in the channel, manual discharge measurements were 

 made to determine discharge and mean channel velocity. These measurements of mean channel velocity were 

 related to acoustic velocity measurements by a rating developed from the manual discharge measurements. 

 When actual measured discharge (during calibration measurements) were compared with discharge values 

 determined from calibrated gauge readings, the potential error for flows above 1 cfs were usually within about 

 10 percent. 



SUMMARY OF DATA 



Origirjally, the Nueces Overflow Channel was designed to have a controlling bottom elevation of 2.0 ft msl. 

 However, days after construction of the channel was completed in late October 1995, the Corpus Christi area 

 received 10-12 inches of local ratafaU in a 2-day period. The resulting runoff and river discharge scoured the 

 newly cut channel to a new controlling bottom elevation at about mean sea level. The effect of this change was 

 that, in addition to freshwater discharge events through the overflow channel, there was also now the 

 opportunity for regular tidal exchange between the river and the upper delta, even when there was litde or no 

 flow coming down the Nueces River. As a result, flow in the channel regularly occurred in both directions 

 during the study period. Positive flow from the Nueces River into Rincon Bayou typically occurred during 

 periods of high discharge in the Nueces River or during rising tide events which pushed water up the river and 

 through the overflow channel. Negative flow from Rincon Bayou into the Nueces River typically occurred 

 when the water level in the upper delta was relatively high immediately after river discharge events or during 

 falling tide conditions. 



Stage 



As discussed above, the Rincon gauge is tidally influenced. The stage data recorded by the gauge therefore 

 indicate influences by both freshwater flow events {i.e., stage events driven by discharge in the Nueces River) 

 and saltwater inundation events {i.e., stage events driven by tidal or other hydro-meteorological activity). This 

 dual relationship can be best shown by comparing stage values between the Rincon gauge and the Calallen 

 gauge (Station 08211500, Nueces River at Calallen) (Figure 4). Although there is a strong correlation between 

 the two gauges, the variance in daily stage at the Rincon gauge is about 1.5 to 2.0 ft for any given stage value at 

 Calallen. This variance is the result of tidal mfluences on the upper delta and the Nueces River below Calallen, 

 which is present regardless of flow in the river. 



A-2 ^ Technical Notes on the Rincon Gauge and Data 



