Drainage by Gravitation, 13 



a coefficient varying with the nature of the stream, and 

 determined originally by experiment checked by practice, 

 V = (\/R X 2 F) C. V equals mean velocity in feet per 

 second ; R equals hydraulic mean depth in feet ; F, fall of 

 surface in one mile in feet ; C, a constant, varying from 0*90 

 in rivers and large streams with considerable depth of water 

 to 0*60 for small drains in good order. The surface velocity 

 in the centre of the stream, as found by floats or by a current- 

 meter, must be multiplied by 0*83 to find the mean velocity 

 of ordinary drains. 



The amount of retardation caused by friction, is an element 

 that can only be determined by experiment. Authorities 

 place this figure as low as 4 per cent, for streams discharging 

 a large volume of water, say from 2000 to 3000 cubic feet a 

 second, and as high as 40 per cent, for small streams of one- 

 hundredth this capacity. From observations deduced by 

 experience and comparison of the various formula given by 

 Beardmore, Stevenson, Neville, and others, the following may 

 be taken as the factors by which the theoretical discharge is to 

 be multiplied to allow for friction, and as generally applicable 

 to drains in low flat districts. 



Depth. Mean Width p ^ 

 feet. feet. 



Mam drams ........ 6 30 'So 



Secondary drains 4 20 * 70 



Small drains 2 10 '60 



Weeds. — If weeds are allowed to grow in the drains con- 

 siderable deduction must be made from the theoi-'etical calcula- 

 tion, not only from the fact that they abstract from the area of 

 the drain, but necessarily retard the flow of the water. The 

 actual amount of deduction will vary with the quantity and 

 nature of the weeds, but a rule is given by Neville which 

 provides that the hydraulic mean depth of the drain should 

 be multiplied by i • 70. 



The following is an illustration of the way in which weeds 

 hold up the water. In the river Hull in Yorkshire, over a 

 distance of 5f miles between Hempholme Lock and the 



