ON THE DRAINAGE-AREA ETC. OF THE RIVKR AVON. 177 



elevation will, however, be found in the Frome (Wiltshire) district, -s^liere 

 the Old Red Sandstone, at Downliead Common (Mendips), reaches 1078 feet ; 

 the Inferior Oolite, at East Cranmore, 814 feet ; the Mountain Limestone, 

 at Leigh-upon-Mendip, 800 feet ; and the Coal-measures above Mells 

 about 770 feet. The Inferior Oolite at Lansdown, near Bath, also rises to 

 about 7i30 feet, and at Dundry, south of Bristol, to about 750 feet. 



The hilly and non-absorbent character of the soils of a great portion of tlie 

 district causes the rain which falls to be carried off rapidly ; and heavy floods 

 are sometimes experienced in Bristol, especially when the discharge of flood- 

 waters, through tlie floating harbour, is impeded for a time by the rise of high 

 spring-tides. The mean average annual rainfall in Bristol is about 32^ inches. 

 This amount is increased on the slopes of the Cotswold and Mendip Hills, on 

 the latter of which it averages about 47 inches ; but the mean of the whole 

 district would perhaps not be greatly different from that at Bristol, when the 

 lessened quantity, faUing on the eastern part of the drainage-area, is taken 

 into calculation. 



In considering the tides of the Avon, it may be desirable for a moment to 

 refer to the special tidal phenomena of the Bristol Channel and Severn estuary. 

 The crest of the free tidal wave of the ocean, which in the deep waters of the 

 Atlantic roUs forward the high-water line at a rate of probably not less than 

 about 500 miles an liour, enters the English and Irish t!hannels with a gra- 

 dually decreasing velocity, owing to the resistance from the seas becoming 

 more shoal ; and this retardation is further increased in the Bristol Channel 

 by the converging lines of its shores. The whole of the appreciable tide in 

 the estuary of tlie Severn is due to the momentum of the wave originated in 

 the deep waters of the open ocean. One important result of these conditions 

 is, that as increased resistance is met, so the wave is forced higher and the 

 tidal range magnified ; and while the rate of progress of the crest of the wave 

 is much diminished, the actual movement of the particles of water to and 

 fro, in flood and ebb, becomes more rapid owing to the greater rise and fall. 

 "We have thus a great range of tide in the Severn and the Avon with a con- 

 siderable velocity, especially in the narrow and deep parts of the former 

 river. 



As evidence of the retarded advance of the crest of the tide-wave and the 

 increased range of tide spoken of in the Bristol Channel, we may take the 

 case of an ordinary spring-tide, which, advancing in fi'om the Atlantic, brings 

 higli water off the Scilly Islands at 4'' 30'» o'clock, with a rise of tide of 

 IG feet above mean low-water springs at that point. This crest of high water 

 will reach Lundy, a distance of 140 miles, at 5'' 15'", where the rise will be 

 27 feet ; Nash Point, 49 miles from Lundy, at 6" 25'", with a rise of 33 feet ; 

 Cardiff, 24 miles from jSTash Point, at 6'' 56", with a rise of 37| feet ; King- 

 road (mouth of the Avon), 16 miles from Cardiff, at 7'' 13'", with a rise of 

 40 feet ; and Sharpness, 18^ miles above Kingroad, at 7'' 58'", with a rise of 

 25 feet, this latter above Ordnance datum, or an absolute height of about 2 feet 

 4 inches above high water at Kingroad, the total range at Sharpness being less 

 than at Kingroad, on account of the great slope of the bed of the river. At 

 Eramilode, about 13 miles above Sharpness, the effect of the gorging up of the 

 tide has attained its maximum, and the tide flowing up the remaining distance 

 to Gloucester is due entirely to the acquired momentum. At Gloucester the 

 further flow upwards is stopped at ordinary tides by weirs recently erected, 

 although the top of equinoctial springs flows over them. 



The total range of tides at the mouth of the Avon, and the great difference 

 1875. « 



