204 Oti the Construction of Dams, d-c. 



Should it be decided to raise the \Yatcr so as to inundate the 

 hanks on each side, the embankment to prevent the water 

 thus raised finding its way round the flanks of the dam, — 

 these flanks should bo completed before the dam itself is 

 closed. This embankment may be formed of earth; its 

 section may be as in figure 7, about three feet thick at the 

 top, which should be about one foot above the highest water 

 line, the upstream slope at least 2 of base to 1 of height ; 

 the down stream 1 of base to 1 of height : in case the soil 

 is light and porous, it will be necessary to excavate a trench 

 in the line of the embankment, about two feet into the 

 ground, and about two feet wide ; to puddle this well with 

 clay, and to form a wall of the same through the centre of 

 the embankment, till above the water line, as in figure 7, to 

 render it water-tight. Where a current can act upon it, the 

 base may be protected by stones, and by planks or fascines 

 pinned down parallel to its direction. In all cases ample 

 provision should be made for the passage of the waste water ; 

 when it is not allowed to pass over the dam, waste channels 

 should be made, and the passage of the water through these, 

 regulated by sluices, either self-acting (which is the safest 

 plan), or worked by meu. Great care must be taken that 

 the action of the water through these sluices does not tear 

 up and wash away the ground below to an extent to endanger 

 the structure. Aprons (constructed as before described) 

 must be laid in rear of the sluices, except when these are 

 fixed upon rock, and must be canied down to a distance 

 proportional to the body of water discharged, and to the 

 full, also taking into consideration the nature of the soil.* 



* Bridges may often be converted into temporary dams by blocking up the 

 archway, taking care that the mass thus formed is sufiicient to support the 

 accumulated body of water, which must not be taken for granted with most 

 bridges. 



