4.17 DIKES, LEVEES, AND IMPOUNDMENTS - SUBPROJECT 17 



Dikes and levees are used in coastal areas to direct water flow into 

 or away from a given area. These structures often cause significant 

 disturbances to vital fish and wildlife resources. Once in place, dikes 

 and levees affect both immediate and long-term changes in lowland 

 habitats and adjacent estuarine waters. Solutions to the problem lie in 

 discouraging those land uses in the area to be diked that are incompatible 

 with natural flooding conditions, and limiting the use of dikes and levees 

 as much as possible. 



4.17.1 Summary 



Dikes and levees are similar types of structures built to hold water 

 either in or out of an area. Constructed levees are linear earthen 

 walls placed most commonly on floodolains on both sides of streams and 

 rivers to prevent high water from flooding the bordering land. Natural 

 berms along riverways are also known as levees. Dikes are embankments con- 

 structed of earth or other materials that are often placed across wetlands 

 and tidal creeks to prevent coastal flooding from storms or even daily 

 tidal incursions. Other incentives for construction of dikes and levees 

 are navigation improvement, protection of developed floodplains, creation 

 of suitable acreage for crops such as rice and salt hay, and biological 

 management (mosquito control and waterfowl impoundments). At OCS 

 facilities, dikes may be constructed to contain spoil from associated 

 channel dredging activities and to contain leakage of oil and refinery 

 products from the facility site. As such they are environmental safe- 

 guards necessary for the prevention of water pollution. 



Major potential disturbances of dikes and levees are obliteration 

 of habitats and the disruption of water flows. These hydrologic changes 

 can trigger profound long-term shifts in salinity levels, nutrient cycles, 

 sediment distribution and, ultimately, the carrying capacity of the 

 system. Dikes used in restoration or enhancement projects may have a 

 net beneficial effect. 



4.17.2 Flood Control and Drainage 



This subsection deals with earthworks that are constructed for 

 flood control or to assist in land drainage. Levees and dikes built 

 for these purposes involve clearing and grubbing, earth borrow, levee 

 construction, spoil discharge onto wetlands, and in some cases, riprap. 



Construction operations involving large earthworks may require 

 extensive excavation of borrow areas, from which fill material is 

 obtained. Borrow areas are generally as close as possible to the site 

 of the levee or dike but they also may be located where there are spe- 

 cific types of fill required for the project. For example, the core 

 area of a levee or of an earthfill dike requires clay so that the struc- 

 ture will be essentially impervious. Since waterborne materials are often 

 deposited in graded sizes, river bluffs, floodplains, and stream bottoms. 



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