SALT MARSH DRAINAGE 



147 



THE TIDE-GATE AND SLUICE-BOX 



At points where streams or large ditches cross the dike, tide- 

 gates are required. There are several types of these devices, 

 each being applicable to certain conditions. The type most 

 commonly used includes a sluice-box and a swinging gate. 



The sluice-box ordinarily corresponds to a culvert and lies 

 at right angles to the dike and underneath it. The length is 

 determined by the height of the fill and the slope of the filling 

 material. The cross-section is determined, as for culverts, by 

 the drainage area, rain-fall and run-off. 



The swinging gate is a gate or shutter attached by hinges to 

 the outer end of the sluice-box and so arranged that it will readily 

 open with the falling tide and readily close with the rising tide. 



The purpose of the sluice-box and gate is to discharge water 

 at low tide and close up at high tide in such manner as to prevent 

 the tide from entering. In good construction, the water in the 

 ditches and streams should be maintained virtually at the eleva- 

 tion of low tide. A heavy rain may result in the accumulation 

 of several inches of water, but this will be discharged at the next 

 low tide. 



CONSTRUCTION OF SLUICES 



When the outlet of the ditch or stream is deep enough to pre- 

 vent the tide forcing back accumulations of mud, etc., into the 

 sluice-box, it is a good plan to submerge the sluice-box below 

 mean low tide, in order to prevent floating debris from interfering 



Top of £>iAc 



Mean Lev Tide 



3lolce - 

 box. 



Mean lens Tide _ 



. 



P,l,nj 



5lutce -bo* 



Fig. 103. — Installation of a sluice-box. 



with operation of the tide-gate and also to avoid periodical 

 wetting and drying, which greatly shortens its life. If it is not 

 submerged, it must be screened with heavy material. 



Where excavation for the sluice-box is necessary, a rude coffer- 

 dam may be constructed, using earth for the ends, the water 

 being bailed or pumped out. The box may be set directly on 



