water surface. As the front approaches the coastline, 

 onshore wind flow is increased, setting up water levels 

 along the coastline. With the frontal passage, winds 

 turn abrupdy to the northern quadrant, reversing the 

 direction of stress. Ward (1997) differentiates two 

 classes of fronts: the relatively short-lived low-energy 

 "equinoctial" frontal passages that do not force a 

 response in the large water body of the Gulf, and the 

 large-scale, longer duration "outbreak" fronts that 

 result in exchange between the Gulf and the interior 

 bays. 



The response of the Gulf of Mexico during frontal 

 passages is the single most important factor 

 determining the total response of the interior bay 

 systems. For Nueces Bay there is a two-step response, 

 the response of the bay to setdown of Corpus Christi 

 Bay, and the response of Corpus Christi Bay to 

 setdown in the Gulf of Mexico. The cross-bay 

 transports are about the same magnitude for both 

 equinoctial and polar-outbreak fronts. However, the 

 volume of water exchanged is much greater for the 

 polar-outbreak fronts since a response to the Gulf is 

 involved (Ward 1997). The cross-bay transports, on 

 the other hand, occur much more quickly but entail 

 smaller water level changes and smaller volumes 

 (generally on the order of 1% of the volume of the 

 bay). 



Another short-period force affecting water levels in the 

 Nueces Estuary and Delta is the sea breeze cycle. A 

 sea breeze is a solenoidal circulation produced by the 

 diurnal variation in density of the lower atmosphere 

 resulting from the surface temperature differential of 

 the land and sea (Haltiner and Martin 1957). It is 

 ultimately caused by the difference in thermodynamics 

 of sea water and land surface and is most pronounced 

 along their boundary. As the sea breeze circulation 

 begins to develop, it imposes an organized circulation 

 in the lower atmosphere that spreads inland and 

 increases the wind speed. The reverse circulation 

 develops in the evening as a land breeze, spreading out 

 to sea from the coastline. In the coastal zone itself, the 

 sea breeze is manifested as a diurnal variation in wind 

 velocity superposed on the normal onshore flow from 

 the Gulf of Mexico. The familiar freshening of winds 

 in the afternoon and the increase of short-crested 



wind-waves (chop) are well-known features of summer 

 hydrography in these bays attending the sea breeze. 



The sea breeze is a relatively weak circulation, and its 

 importance depends on other factors affecting wind. 

 The effect of the sea breeze on water levels in the 

 estuary may be minimized by more dynamic 

 atmospheric processes, such as airmass replacement or 

 interception of radiation by clouds and can be masked 

 even by the prevailing onshore flow. The sea breeze is 

 therefore best developed during conditions when these 

 other influences are uncommon, which is typically 

 during summer. 



Finally, the most extreme water level responses of the 

 Nueces Estuary to meteorological forcing are 

 associated with the storm surges of tropical 

 depressions. These intense organized tropical systems, 

 the most extreme representative being the hurricane, 

 are characterized by a cyclonic circulation with intense 

 swirhng winds around a center of extremely low 

 pressure. The low pressure center and the circulating 

 winds combine to create an elevated mound of water 

 (storm surge) that moves with the depression, but the 

 wind stress on the water's surface is the more 

 important determinant of the magnitude of the surge. 



As the cyclone and associated storm surge make 

 landfall, the volume of water in the surge behaves as 

 any long-period shallow-water wave, slowing due to 

 shoaling water depths and steepening. As the surge 

 propagates into bays and estuaries, it is subjected to 

 various local physiographic modifications. In some 

 regions, this can lead to further amplification of the 

 surge height, and some of the highest recorded surges 

 on the Texas coast have occurred on the inland side of 

 the bays, the largest being Hurricane Carla surge 

 measured at Port Lavaca in excess of 6.7 m. While a 

 hurricane can inflict damage through high winds, 

 tornadoes and intense rainfall, it is the surge, perhaps 

 in concert with wave attack, that is responsible for 

 most of hurricane-related impacts on the Texas coast. 



In extreme instances, tropical storms may also have a 

 secondary effect upon water levels in the estuary due to 

 heavy precipitation. Such was the case with Hurricane 

 Beulah, which made landfall in the autumn of 1967. 



Chapter Tm ♦ 2-5 



