ON THE DRAINAGE-AREA ETC. OF THE RIVER AVON. 179 



bed, lines of the ordinary run of low water, and lines of spring and neap 

 tides. The crest of the dam at Netham is 19-78 feet above Ordnance datum, 

 and is the level at which the floating harbour of liristol is maintained. All 

 tides above this level flow over the dam up towards Hanham and Kej-nsham. 



Another diagram of tidal observations, taken simultaneously for a fort- 

 night at the mouth of the Avon and at Cumberland Basin, shows the relative 

 heights and times of the tides at these stations through a complete range of 

 springs and neaps. It will be seen that the level of high water at Cumber- 

 land Basin is, on an average, about 7 inches higher than at the mouth of the 

 river. We also find that, as regards time, high water at Cumberland Basin 

 is about the same as at Kingroad. High-water equinoctial springs is, how- 

 ever, at Netham about a quarter of an hour, and at Hanham half an hour 

 later than at Cumberland Basin. 



Connected with this part of the subject is the duration of flood and ebb, 

 and the rate of rise and fall of tide at Kingroad. The general result of our 

 observations shows that, at extreme low neaps, the flood is longer than the 

 ebb by about one hour ; but that, as the tides increase in range, the duration 

 of ebb becomes progressively longer than flood, till at the highest equinoctial 

 springs the tide rises from low to high water in 4 hours 45 minutes, and takes 

 about 7 hours 30 minutes to ebb. A reference to the diagram (Plate VII. 

 fig. 2) will show the rate, hour by hour, of rise and fall at the mouth of the 

 Avon for a low neap and a high equinoctial spring-tide. The rapid rate of 

 rise of the spring- tide is remarkable, being 11 feet 11 inches in the second 

 hour, and 12 feet 4 inches in the third hour of flood. 



The velocity of run of tide is not great in the Avon, the highest rate, from 

 observations taken at spring-tides in the river 1| mile below Cumberland 

 Basin, hardly reaching 3 miles an hour. In the Severn at Kingroad the velo- 

 city on half-flood at high spring-tides comes up to about 6 miles, and at half- 

 ebb to about 4| miles an hour. 



Amongst other diagrams connected with the tides will be found some which 

 show simultaneous observations of a low neap, and the highest spring this 

 year at Cardiff, Portishead, Avonmouth, Bristol, and Sharpness, kindly taken 

 by the engineers of the docks at these several places (fig. 3). 



One important though unwelcome feature connected with the tides of the 

 Avon is the enormous quantity of mud held in suspension in the water. 

 With the exception of the Humber, there is probably no river in England 

 that in this respect will compare with it. This part of the subject is one not 

 merely of scientific interest, but of practical economical importance ; for it is 

 necessary that its efifect should be taken into consideration in all questions 

 of dock construction or maintenance in this district. From many obsei-va- 

 tions made to ascertain the average amount of mud held in suspension in the 

 water in the river, it is found that, from any given volume of the tide-water, 

 there will be a deposit of about g^th part of mud, which becomes, under super- 

 imposed layers, soon converted into stifii" silt. In the Severn the quantity, 

 though very considerable, is less than in the Avon. 



The general character of this mud is somewhat different from that of the 

 alluvial deposit which forms the banks of the river Avon and the adjacent 

 flat lands bordering on the Severn. This alluvium is generally found to con- 

 sist of several feet of stift' brown clay, or brick-earth, underneath the top soil, 

 below which is a thick bed of bluish silt, containing much very fine quick- 

 sand and with but little clay. Below this again is almost invariably found a 

 bed of coarse gravel, with frequent fossil remains of red deer, horse, and 



n2 



