APPENDIX D 

 DESIGN OF PITS 



There are two basic designs to consider when mining floodplain gravel by 

 pit excavation: pit not connected, or pit connected to an active channel. A 

 properly designed unconnected pit can provide waterfowl, shorebird, and amphib- 

 ious mammal habitat. If the pit is connected to the active channel, the pit 

 can also provide fish habitat. The outlet channel of the connected pit allows 

 fish that become trapped in the pit during high water to emigrate from the pit 

 at any time. If the pit is unconnected, it should be protected from the 20- 

 year flood. Fish trapped during these floods are considered lost from the 

 r i ver popu I at i on. 



SHAPE AND DEPTH 



The desired configuration for a gravel pit excavated in an inactive 

 floodplain or terrace is long and narrow, in the shape of a channel, with a 

 variety of depths (Figure D- I ) . If the pit is connected to the river or fish 

 are to be stocked in the pit, the mean depth should be greater than 2.5 m to 

 allow fish survival during winter. For a pit with a configuration as shown in 

 Figure D- I , the following are two examples of depth regimes that will result 

 in a mean depth of 2.5 m: 



A. For a minimum mean depth with a minimum of littoral area 



Mean of depth Percent of 



interva I (m) pit area 



25 

 10 

 10 

 50 

 5 



AAaximum depth: 5 m 

 Mean depth: 2.5 m 



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