TRANSACTIONS OF SECTION G. 695 



ing a water supply from distant parts, towns have materially altered the normal 

 condition of the discharge of the rainfall off the area they occupy, and during 

 heavy rains send a large body of water into an already overcharged stream, and 

 thus in a great measure contribute towards the flooding, while the withdrawal of 

 the water necessary for the town takes away the supply from the springs which 

 feed the upper part of the rivers. 



2ifin the Discharge of Sewage in Tidal Rivers* By Henry Law, M. Inst. C.E. 



A tidal river may be looked upon as a reservoir of a very elongated form, sub- 

 ject to the following conditions, namely : — 



1st. That it is supplied with water of three different qualities from three dif- 

 ferent sources, that is to say — 



The water constantly draining off the surface of the basin forming the water- 

 shed of the river, and that derived from the land springs which find vent in its bed ; 

 this we will designate river water. 



The water entering the mouth of the river from the sea under tidal influence, 

 which we will distinguish as sea water. 



The polluted water discharged from the sewers, which we will term sewage. 



2nd. That the actual and relative quantities of these are not constant, but vary 

 within certain limits. 



3rd. That the supply of sea water is not constant, but intermittent, being poured 

 into the reservoir for a certain number of hours ; and then for a certain period the 

 reservoir being allowed to discharge a proportion of its contents. 



Now the subject of our inquiry is, what, under the conditions assumed above, 

 will be the mean or average composition of the water contained in the reservoir or 

 river ? 



In order to obtain a practical result, let us investigate this question, adopting 

 the mean values for the several quantities which apply in the case of the river 

 Thames. 



First, then, as to the extent and capacity of the reservoir. The tidal portion of 

 the river Thames extends from Yantlet Creek to Teddington Lock, a total distance 

 of about 60^ miles ; its breadth varies from about 200 feet to 22,800 feet, or about 

 4^ miles, at its mouth ; its superficial area at high water is 720,594,410 square 

 feet, or nearly 26 square miles. 



The mean range of the tide at the mouth — that is, at Yantlet Creek — is 14 feet, 

 at London Bridge 17 feet 4 inches, and at Teddington Lock 3 feet. 



The mean tidal capacity of the river, that is to say, the difference in the 

 quantity of the water which is contained by the river at high water and at low 

 water, with the above-stated mean range of tide, is 14,179,538,300 cubic feet. 



Now, as has been already stated, this body of water is derived from three sources — 

 namely, the sea, the land drainage, and the sewage ; and it is necessary in the next 

 place to ascertain the relative quantities furnished from each of these sources. 



The downward flow of the Thames, or, in other words, the mean daily dis- 

 charge from the drainage of the Thames Valley, may be estimated at 1,923,626,000 

 gallons, a quantity which, we may incidentally remark, is about one-third of the 

 rainfall. 



From the above, however, must be deducted 100,000,000 gallons which is daily 

 abstracted from the river above Teddington Weir for the supply of water to the 

 metropolis ; leaving a total quantity of 1,823,626,000 gallons, or 291,780,160 cubic 

 feet, for the mean daily discharge, being 145,890,080 cubic feet as the mean quan- 

 tity of river water contributed each tide. 



The mean quantity of the sewage discharged into the Thames from the two 

 outfalls at Barking and Crossness maybe taken at 120,000,000 gallons daily, equiva- 

 lent to 9,600,000 cubic feet every tide, making with the river water a total of 

 155,490,080 cubic feet, which being deducted from the mean quantity already 



* This paper was published in extenso in ' Engineering,' for August 30, and in 

 the ' Journal of Gas Lighting,' for September 3, 1878. 



