Because of the adverse effects caused by marsh alteration, impoundment 

 is normally used only in situations where open-marsh management is not 

 feasible. Required ditches and dikes should be properly designed 

 (Figure 48). Impoundments for salt-marsh mosquito control should also be 



Figure 48. Schematic of ditch, dike, and impoundment cross sections 

 for mosquito control (Source: Reference 151). 



Marsh Surface 



SPOIL 

 PILE 



Berm 



18" Water Surface 



DITCH 



-10'- 



A. DITCH CROSS SECTION 



Grass - 



Crown 



B. DIKE CROSS SECTION 



designed for closure only during breeding (warm) months and opened to 

 drying or tidal action during the rest of the year [152]. 



Although open-marsh water management is generally preferred to 

 impoundments, exceptions occur because tidal conditions may be insufficient 

 to perform the function, or the multiple resource management functions (e.g. 

 waterfowl impoundments) may be planned. Impoundment may be the only 

 environmentally sound option where there is insufficient tidal amplitude 

 (e.g., in the southeastern states) to energize the flow of water through 

 the connector channels of the open-marsh system. Depending on site, 

 design, and management characteristics, an impoundment may enhance the 



179 



