26 The Drainage of Fens and Low Lands. 



500 acres, the quantity coming in at the middle of the lateral 

 drains 5.25 cubic feet per second. These drains are taken as 

 running the whole 24 hours. The field ditches to have I'D 

 bottom, slopes | to I, mean depth of running water i foot 

 Supposing that in floods, during the time the sluice doors were 

 closed by the tide, the water rose i foot above the mean level, 

 as before given, and fell at low water to i foot below this 

 level, the drains would hold, between low and high water, a 

 quantity equal to the rainfall of five hours. Supposing, in 

 anticipation of a flood, and before the water had swollen in 

 the outfall, the water in the main drain was run off within 

 3 feet 6 inches of the bottom, the drains would hold up to 

 3 feet below the surface of the land a quantity equal to 

 about one day's rainfall. The area of land occupied by these 

 drains would be 282 acres, equal to ^ of the whole area. 

 If the water had to be pumped, and the pumps worked night 

 and day, the area of the drains would be proportionately less, 

 as would also be the case if the sluice was situated sufficiently 

 above the outfall to allow of a longer discharge than 14 hours. 



The sluice for this district would i-equire to have three open- 

 ings of 14 feet each. The water would approach with a velo- 

 city of I • 078 feet a second, and as the piers of this sluice would 

 have pointed ends, it may be assumed that it would pass 

 through at the same rate, the velocity of approach being 

 sufficient to overcome the friction due to the sides and bottom 

 of the openings. The depth of the water on the sill being 

 8 feet, the same as that in the main drain, the area of the 

 waterway would be 3 x 14'Ox 8 •0 = 336*0 X i '0816 = 363 -42 

 cubic feet per second 



Sluices. — Low fen and marsh lands being almost invariably 

 below the level of the tides, their outfall drains must be pro- 

 tected at the point where they discharge into the sea or tidal 

 stream by doors constructed to keep out the tide when it rises 

 above the level of the water in the drain. 



These doors are generally made self-acting, so that they auto- 



