42 
MIDDLE LIAS OF NORTHAMPTONSHIRE. 
Feb., 1889. 
nevertheless, it would prevent the smaller floods, and mitigate 
the larger, allowing them to do the maximum amount of 
good, and less harm than they do now. Below is a summary 
of the advantages likely to result from the application of this 
method:— 
1. —The river gravel of the Nen, with its cap of alluvium, 
naturally represents the former extent of the river when in 
flood, and now approximately coincides, in superficial extent 
and position, with the area subject to floods. The average 
depth of the gravel is almost certainly greater than the 
average depth of flood-water, therefore, if the flood-water at 
any tune present on the ground had free access to it, and 
found it empty, such water would be wholly or nearly 
disposed of, and the whole flood greatly reduced in intensity. 
The dumb-wells, constructed for other purposes, would tend 
to keep the river gravel empty, and access to the gravel 
would, be greatly facilitated by the various devices already 
proposed, and so a great quantity of water completely 
prevented from doing liaim, particularly if the river were 
connected with the gravel. Not only would the river gravel 
act as a reservoir, but it would be continually emptying and 
making room for more water, to the extent the dumb-wells 
are able to receive such water. 
There need be no fear of the gravel being over-taxed as a 
filter, for within the area dealt with, there would be almost 
as many square miles of filtering material as acres would be 
required for the assumed maximum capacity of the dumb- 
wells, at the rate of square yards for each 1,000 gallons in 
twenty-four hours, or a descent of about 6 inch per hour. 
2. —The water disposed of would not be lost to the 
district, and if used as this scheme suggests, it would be 
returned to the river in a pretty regular volume, after having 
served some useful purpose. I find that the utilisation of 
the river gravel as a temporary reservoir for flood-water was 
suggested some years ago by Professor Prestwicli.* The 
suggestion arose out of a proposal to construct impounding 
reservoirs along the Thames 'Valley for the same purpose. 
Professor Prestwich’s remarks were about as follows:—The 
Thames and Clierwell were liable to floods of such magnitude 
that, however useful storage reservoirs might be in providing 
additional water in times of drought, no practicable extent 
of storage conld prevent floods. Large reservoirs, in fact, 
already existed, compared with which any artificial reservoirs 
* “ Kainfall and Evaporation,” by Symons, Greaves, and Evans. 
Discussion on the Papers. Proceedings of the Institute of Civil 
Engineers, 1876. 
