Renewal time. It is instructive to evaluate how 

 much each basin depends on runoff to supply fresh- 

 water and to estimate roughly how often the waters 

 in each basin are replaced or renewed. Renewal time 

 is an important concept in water quality management, 

 and has been modified by management practices in 

 the Chenier Plain basins (table 3.48). First, the annual 

 rain surplus decreases from east to west across the 

 Chenier Plain, from 60 cm (23.6 in) in the Vermilion 

 Basin to only 20 cm (7.9 in) in the East Bay Basin. 

 This surplus multiplied by the surface area of each 

 basin gives the total volume of freshwater generated 

 within that basin and available for runoff or ground- 

 water recharge. The large surplus from local runoff 

 explains why the Chenier Plain region is so fresh in 

 spite of its proximity to the coast. The volume of 

 water entering each basin from upstream is shown for 

 comparison. The freshwater budgets for the Calcasieu 

 and Sabine basins are dominated by riverine input; 

 the other basins depend primarily on local rainfall. 

 Estimates of the renewal time of water for each basin 

 listed in table 3.48 are based on the assumption that 

 the freshwater surplus is the sole agent of water re- 

 newal. Since the tidal prism is a significant percentage 

 of the mean water depth of these shallow estuaries, 

 the renewal time would be different than indicated in 

 table 3.48. However, the Gulf tidal waters tend to 

 move in on flood tides and then recede again on ebb 

 tides, with mixing only where they interface with 

 estuarine waters, so that the net exchange is probably 

 quite small (Happ et al. 1977). Wind-generated cur- 

 rents can also increase renewal time as discussed in 

 part 3.3.5. In table 3.48, another calculation is made 

 for the large inland lakes only, with the assumption 

 that all runoff in a basin empties into these lakes. 

 Calculations for the Vermilion and East Bay basins 

 are misleading because they are parts of larger basin 

 systems outside the Chenier Plain boundaries. How- 

 ever, the relatively long renewal time for East Bay is 

 probably correct in a qualitative sense, and the sys- 

 tem as a whole would be expected to have more ma- 

 rine influence than other basins. Vermilion Basin is 

 probably fresher than indicated by the renewal time 

 because the Atchafalaya River to the east depresses 

 salinity throughout the Atchafalaya/Vermilion Bay 

 systems. 



The rapid renewal times for Calcasieu and Sabine 

 basins result from the large riverine input. This rapid 

 flushing of basin waters suggests a capacity to sustain 

 higher nutrient loadings than poorly flushed systems. 



An increase in the depth and width of tidal inlets 

 should result in a faster renewal time. Small canals in 

 the interior basin act the same way. They offer less 

 resistance to water than natural, shallow, sinuous 

 channels allowing water to move in and out more rap- 

 idly. They also change circulation patterns and de- 

 crease sheetflow across wetlands. Excessive diversion 

 of water from upstream for agriculture and industry 

 increases the renewal time (days) of the basin. Since 

 the renewal time is related to the total nutrient load a 

 body of water can assimilate, its modification can in- 

 fluence the eutrophic state of the water body. 



3.3.5 SUBUNITS DOMINATED BY WIND-DRIVEN 

 CURRENTS 



The large, shallow lakes of the Chenier Plain are 

 affected by tides, but wind is often the dominant 

 mechanism controlling currents, water height, and 

 flushing rates. Dominant winds along the coast are 

 either from a southerly direction (usually in sununer) 

 or from the north (in winter), as previously indicated 

 in figure 2-6. In large lakes, especially ones which are 

 oriented north to south, such as Calcasieu and Sabine, 

 these winds, acting over a long fetch, generate currents 

 that can reach up to 3% of the wind velocity (Murray 

 1975). 



The effect of wind on water levels is often dra- 

 matic, and waterflows generated by the buildup of a 

 hydrauUc pressure gradient across small inlets and 

 narrow channels connecting lakes can be large. The 

 initial response of the water surface to a wind change 

 is rather rapid, usually occurring in less than 24 hr. 

 Sustained winds blowing across a water surface tend 

 to push the water in the direction of the wind, piling 

 it up against the shore, until an equilibrium is reached 

 between the wind stress in one direction and the op- 

 posing water slope created by the water buildup. This 

 wind setup reaches a maximum value rather rapidly, 

 depending on windspeed and the open water fetch 

 (table 3.49). When tide is phased with winds, their 

 combined action can change water levels several meters 

 in a matter of hours. After the initial, predictable and 

 rapid response, sustained winds have unpredictable ef- 

 fects (Wax 1977). For instance, the response to dif- 

 ferent weather types is shown at Hackberry about 

 halfway up the western shore of Calcasieu Lake near 

 the ship channel (fig. 3-21). Synoptic Weather Type 

 1 represents initiation of a typical cold front with 

 northerly winds. It always results in a lowering of the 

 water level, whether surplus water is available or not. 

 However, Weather Type 3, typically weather following 

 a cold front and representing northerly air flow sus- 

 tained over several days, showed variable effects on 

 water level. To explain the unpredictable results in 

 Calcasieu Lake , Wax (1977) suggests that river runoff 

 into the upper basin generated by the same weather 

 conditions might arrive at the lake several days after 

 cold front passage, or a long-term setup could occur 

 as winds pile water at the outlet at the southern end 

 of the lake causing a bottleneck in the flow and raising 

 water levels at the Hackberry gage. 



As shown in figure 3-2 1 , Weather Types 1 , 2, and 

 3 associated with northerly winds all tend to depress 

 water levels, whereas easterly winds (Types 4 and 5) 

 and south- southeastedy winds (Types 6 and 7) tend 

 to increase water levels by forcing water into the in- 

 shore estuaries against the slight surface slope. 



These weather events influence flushing times, 

 turbidity, and wefland flooding. When turbulence and 

 currents increase and water levels are abruptly changed 

 by combinations of tide and winds, mixing of tidal 

 and estuarine waters is increased. This mixing and the 

 magnified flows through tidal inlets significantly in- 

 crease the rate of flushing of the estuary. In shallow 

 bays and lakes, wind-driven waters stir up bottom 

 sediments and increase turbidity. 



68 



