10 The Drainage of Fens and Low Lands. 



determining the sectional area. The time the sluice is closed 

 by the tide should be ascertained by observation, but it may 

 be found approximately from Table VIII. in the Appendix. 



Depth and Width of Drains. — The depth of a drain 

 must be determined principally by the level of the land with 

 reference to low water in the outfall in floods, and by the depth 

 it is thought desirable to maintain the water in the ditches 

 below the surface of the land. This should not be less, where 

 obtainable, than 3 feet or 3 feet 6 inches, to allow the under 

 drains to run clear. In peat land, two feet below the surface 

 is considered sufficient for the water-level. Taking the depth 

 to be 3 feet 6 inches, and allowing for one foot of water in 

 field ditches, the bottom of these will require to be 4 feet 

 6 inches below the surface. The depth of the main drains 

 and level of the water must be such as to provide for the 

 drainage of the lowest land situated the greatest distance 

 fi-om the outfall, and also to allow for the necessary fall in 

 the surface of the water from this land to the outfall. The 

 mean width of the drains must be determined by dividing 

 the area required by the attainable depth. 



The greater the depth within certain limits in proportion to 

 the width, the better will be the discharging power of the 

 drain. The greater the body of water as compared to the 

 area of rubbing surface, the more free is the water to move. 

 The proportion of rubbing surface of the sides and bottom to 

 the area of the water is termed the hydraulic mean depth, 

 and is found by dividing the latter by the length of the 

 former, or the area by the wetted perimeter. 



The best form of channel for conveying water is when its 

 hydraulic mean depth is at a maximum, and this is attained 

 when the mean width is double the depth, or a semicircle 

 with the diameter for the water-line. Such a form, however, 

 is never attained in land drains, and the width will generally 

 be from four to six times the depth. 



Motion of Water. — The motive power which causes 



