structures of adequate drainage capacity; 3) operation of water control structures to maximize gravity 

 drainage discharge (e.g., removing all stop-logs); and 4) using periods of low precipitation and low tides 

 to facilitate gravity drainage. Larger semi-impoundments were slower to dewater than smaller semi- 

 impoundments. For example, semi-impoundment 8 (10 ha) could be completely dewatered in two days 

 of exceptionally low tides. Under the same tidal conditions, however, only 5.1-10.2 cm of water could 

 be drained off the larger semi-impoundments. These observations suggest that larger semi-impoundments 

 may need proportionally more drainage capability than smaller semi-impoundments. For this reason, 

 improvements at LPWMA often involved the installation of additional flap-gated, variable -crested culverts 

 in the larger semi-impoundments in order to increase drainage capacity and drawdown success (See Table 

 1). Total drainage capability per marsh semi-impoundment at LPWMA averaged 215% greater than 

 that recommended by current U.S. Soil Conservation Service guidelines (See Table 2); however, gravity 

 drainage capacity on larger semi-impoundments was found to be only adequate. 



When drawdowns were attempted at LPWMA, the goal was to dry and crack the pond bottoms within 

 that semi-impoundment. This was necessary to consolidate bottom sediments and promote the 

 proliferation of submerged aquatic plants (Joanen and Glasgow 1965). Although pond bottoms had to 

 be dewatered to stimulate the germination of annuals, extensive drying was not usually necessary or 

 desirable. Miller and Arend (1960), Burgess (1969), and Prevost (1987) found that annuals germinated 

 better on moist soil than dry soil. After initial drying and germination, annuals appeared to fare better 

 when occasional showers or residual water kept the pond bottoms moist. Other researchers have found 



Table 2. Comparison of drainage/exchange capacity of semi-impoundments at Little Pecan 

 Wildlife Management Area, Cameron Parish, LA., versus that recommended by current Soil 

 Conservation Service guidelines. 



Average difference 15 215% 



a Semi-impounded lake 

 Calculation excluded Little Pecan Lake and those semi-impoundments (units 5 and 6) where 



the installation of the desired type and size water control structure was not permitted by the 



regulatory agencies. 



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