OBJECTIVES 



The objectives of this analysis were to characterize, through analysis of the magnitude, frequency, duration and 

 timing of flow events: 



1) changes in the historic freshwater flow regime of the upper Nueces Delta, and 



2) the potential for restoration of freshwater flow resulting from the Rincon Bayou Demonstration Project. 



BACKGROUND 



Description of the Study Area 



As the Nueces River flows toward Corpus Christi Bay along the south Texas coast, it passes along the southern 

 edge of a large delta located in southern San Patricio County (Figvue 1). This delta is an integral part of the 

 Nueces Estuar)', and is roughly 70 square kilometers (km) in size. The water surface for most of the channels 

 and ponds in the delta is very near sea level, and the low-l^ing flats are intermittendy inundated from the bay by 

 tides and storm surges. The delta is crossed from north to south by numerous buried pipelines and two 

 Missouri-Pacific railroads. One of the most dominant hydraulic features of the upper delta is a broad, tidally- 

 influenced channel known as Rincon Bayou. Between the railroads, the upper delta is separated by a modest 

 ridge of high ground along the southern bank of Rincon Bayou. There is some evidence that this crest is what 

 remains of early diking efforts in the delta, probably for agricultural purposes (US Ejigineer Office 1939). 

 Downstream of the eastern-most railroad, the elevations are more uniform and tidal interactions make a 

 separation of the northern and southern delta less distinct. 



The Nueces River Basin has a total watershed area of approximately 44,224 km^. Occasionally, larger flood 

 flows in the river spill over the northern bank and inundate the delta with freshwater. These events usually 

 occur shortiy after periods of heavy rainfall in the basin, usually coinciding with tropical storm activit)' in early 

 fall or with the passage of frontal systems in late spring. These sporadic flooding events supply freshwater to 

 plant communities, transport detrital materials from the vegetation and sediments to the bay, provide a medium 

 for nutrient exchange and buffer bay salinity. The Nueces Delta marsh is therefore one of the most important 

 sources of nutrient material for the Nueces Estuary system (Texas Department of Water Resources 1981). 



Historical Changes in the Study Area 



Physical Changes in the Nueces Delta 



From a broad perspective, large-scale physical form of the delta has changed Utde during the period under 

 review {i.e., 1940-1999). Through comparisons of current and historic maps, each of the two railroads which 

 cross the delta from north to south were in places by 1940, and most of the larger pools and channels in the 

 delta resembled their present locations, shapes and sizes (US Engineer Office 1939). The two railroad crossings 

 were elevated by fill material for most of their span, with the exception of a few bridged crossmgs over the 

 more significant channels. This construction method undoubtedly changed the fundamental hydraulics of the 

 delta system by isolating, restricting, and channelizing some it's water courses, but these changes had already 

 occurred well before the beginning of the period under consideration {i.e., 1940). 



The road bridge over the Nueces River near Calallen has been in place since before 1930, but has been rebuilt 

 on several occasions (Texas State Highway Department 1931). In 1931, a two-lane tresde bridge was 

 constructed by the State of Texas as part of improvements to what was then called State Highway 9. This 

 structure was removed and replaced with a standard bridge in 1956, and then improved to a four-lane bridge in 

 1959 as part of the upgrade to Interstate Highway 37 flexas State Highway Department 1959). Finally, in 1983, 

 the Interstate Highway 37 (IH 37) bridge was upgraded to its current form. 



Presendy, the flooding threshold for the majority of the north bank of the Nueces River in the upper reaches of 

 the delta is about 2.36 m (7.75 fit) msl, although some lower channels are present. Given the natural effects of 

 scouring and deposition during large flow events, the Nueces River has likely cut and filled a coundess number 

 of depressions in its geologic past, continually altering the flooding threshold with each event. In addition to 

 natural causes, human activity has also contributed to this process of change. As one example, numerous large 

 pieces of concrete rubble and re-bar have been unearthed along the north bank of the river just downstream of 



C-2 ^ Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta 



