Core Walls.—Puddle is the material generally used in the British Isles to form the core wall, and reinforced concrete in America. Puddle is not an absolutely impermeable material, and the thickness of a puddle core wall must be much greater than that of a concrete core wall. The use of concrete core walls has been limited in the British Isles owing to the apprehension that such walls would crack under pressure due to unequal settle ment of the embankment. That such fears are unfounded is proved by the numerous successful examples of such construc tion in America.
The very greatest care must be exercised in the construction of a puddle core wall to prevent the occurrence of any layer through which water could pass owing to the erosion which may take place, causing the formation of a cavity and the failure of the bank; whereas no erosion of the concrete would take place, and a crack would soon be sealed by earth carried in suspension by the water.
Tunnel Outlets.—It is necessary to divert the stream during construction, and for this purpose it is advisable to construct a tunnel round one end of the bank through which the stream may flow, and through which the supply pipe can ultimately be laid from the Valve Tower. A cheaper form of construction is to build a culvert under the bank, but many cases of whole or partial failure of such culverts have occurred due to the varying earth pressure. (See C. J. Wood, "Tunnel Outlets," Proc. Inst. C.E. vol. 59.) Flood Works.—When a reservoir formed by a masonry dam overflows, water passes harmlessly over the top of the masonry into the stream below. It is obvious, however, that water cannot be allowed to overflow an earth embankment, as the material would rapidly erode, leading to the failure of the bank. It is therefore necessary to allow for the escape of flood water in such a way that the water level can never rise to a height that would endanger the bank.
The usual flood escape is provided by a weir cf such a length and placed at such a level below the top of the bank as will ensure that the water in the reservoir can never rise above it, the weir discharging into a masonry channel placed in the hillside at one end of the bank. Another and more economical escape consists of a vertical shaft communicating below with the tunnel through which the stream was diverted during construction and terminating above in a bell-mouthed opening, the periphery of which forms the overflow weir.
The main factor in determining the maximum intensity of a flood must obviously be the amount of rain which fell in .a given period, a condition which varies so widely in different parts of the globe, that records of flood discharge in one country would not be applicable to another.
The curve on fig. 3 shows the relationship between rainfall and period, and is derived from the formula determined by Prof. Talbot as applicable to the Eastern United States for maximum rainfalls during different periods. The small circles show actual observations made in the British Isles, from which it appears that Talbot's curve fits British conditions fairly well. This fact is of importance as indicating that records of floods in the Eastern States are of assistance in arriving at the maximum intensity of discharge from different catchment areas in the British Isles. The larger the catchment, the smaller is the flood discharge per unit of area.
Unfortunately, the estimates of maximum flood discharges from small catchment areas, are scanty and not very reliable. Most reservoirs are placed at comparatively high altitudes, where the catchment area is small, in order to impound water free from pollution and to furnish water to the district of supply by gravity. Failures of such reservoirs, due to insufficient provision for the discharge of flood water, are not infrequent, and demand serious consideration owing to the consequent loss of life and property.