8 The Drainage of Fens and Low Lands. 



which it falls, the quantity depending on its nature. If the 

 higher lands consist of either sandstone or limestone, or 

 especially if of chalk, the absorption, after heavy rain, will be 

 very great, and the discharge off the surface to the drain very 

 small. In some chalk districts the absoi-ption is so great that 

 there is an entire absence of streams ; in others, the discharge 

 is so small, that the waterway of the bridges and culverts is 

 not an eighth of that required in clay districts. 



A large fall of rain at any particular period does not 

 necessarily produce a flood in flat districts. When rainfall 

 succeeds a season of dry weather, it takes some time to 

 saturate the land ; and owing to the large capacity of the 

 arterial drains, their small declivity, and the level character 

 of the land, it occupies some time before these become fully 

 charged. On the other hand, if rain falls after a continuance 

 of wet weather, the land, together with the drains, is already 

 charged, and any exceptionally heavy rain, although even for 

 a short duration, is at once succeeded by a flood, as the drains 

 cannot carry off the excess in their surcharged state. 



In the fen districts of Lincolnshire the average rainfall of 

 recent wet years has been 32*39 inches, of which 17 '52 inches 

 was due to the six winter months, September ,to February, 

 which, spread over this period, gives sRti average daily rainfall 

 of o ' 097. Taking the periods of excessive rain which occurred 

 during the same time, extending over six to thirty successive 

 days, the greatest average fall per day has been 0*41 inch for 

 fourteen days in October 1883 and November 1885, the next 

 highest being 0*29 for six days in February 1883. The 

 average mean rainfall during the twenty-one floods since 1852 

 was o * 26 for seventeen days. 



The quantity of drainage which was provided for by the 

 old Fen engineers was that due to the water arising from a 

 continuous rainfall of a quarter of an inch of rain in twenty- 

 four hours, making no deductions for soakage or evaporation. 

 This calculation was also adopted by Sir John Hawkshaw 



