Estuary of the River Mersey : The effect of the discharge of crude sewage into the estuary of the River Mersey on the amount and hardness of the deposit in the estuary.

  • Great Britain. Water Pollution Research Board
Date:
1938
Catalogue details

Licence: Public Domain Mark

Credit: Estuary of the River Mersey : The effect of the discharge of crude sewage into the estuary of the River Mersey on the amount and hardness of the deposit in the estuary. Source: Wellcome Collection.

    72/386 (page 44)
    Admiralty. In this method, the tidal streams, which are assumed to be equal in strength and opposite in direction on the flood and ebb, are separated from the residual current on which they are superimposed and the average strength and direction of this current is computed. The currents at the five stations, with those at two additional stations for which data were supplied by the Hydrographic Department of the Admiralty, are shown in Fig. 12. The currents have a general set to the north-west, that is, out of Liverpool Bay, and there is a drift of approximately 0-3 knot at the Bar Light Vessel. This drift would represent a travel of 7 nautical miles per day. From float measurements it was calculated that the average excess distance travelled seaward in the Narrows was about 2 miles per day, and it is difficult to account for the much greater current found at the Bar. The Bar Light Vessel is anchored in deep water some miles seaward of the channel which runs between sand banks through the Bay, and at this position the current should not be unduly influenced by the presence of the channel. The currents at the Formby and Crosby Light Vessels, however, are controlled by the shape of the channel, the currents setting towards the concave side. If, however, the resultant of the currents at the two positions is computed, it is found to have almost exactly the same direction and nearly the same speed as the current at the Bar Light Ship. It is possible that the high current drift observed at the Bar may be due in part to the influence of the River Dee. It is possible also that there may be a circulatory movement of water in the Bay, although this has not been detected during the present investigation. Such a system might, for example, be set up if there is a current drift to the eastward inshore along the Welsh coast. From the open nature of Liverpool Bay, it must be a matter of considerable difficulty to estimate the rate at which substances discharged into it would pass out into the open sea. It seems probable, however, that this rate may be considerably higher than the rate of travel downstream of water in the enclosed upper basin of the Estuary. SUMMARY From determinations of salinity at high and low water in the Estuary, the mean proportion of fresh water at different positions has been calculated. The proportion of the total volume of fresh water entering the Estuary which is discharged in the form of sewage has also been estimated and the concentration of sewage in the Estuary water under average tidal conditions has been computed. At high water the average concentration of sewage in 99 per cent. of the Estuary water is estimated not to exceed 0:8 per cent., and at low water not to exceed 1-3 per cent. in 98 per cent. of the total volume. In these calculations the organic matter discharged in the form of industrial wastes has not been considered. Estimates of the time taken by sewage to pass through the Upper Estuary to Liverpool Bay have been made by two methods. In the first method, based on the volume of fresh water in the Estuary and the volume of fresh water entering the Estuary daily, the estimated time taken for material to travel from Warrington to Rock Light was 17 to 18 days. In the second method, based on observations of free-drifting floats, the seaward rate of travel of material discharged in the Narrows was estimated at rather more than 2 nautical miles per day. If this rate of travel were constant throughout the Estuary, the time taken by material to travel from Warrington to Rock Light would be about 11 days. Current observations in Liverpool Bay suggest that water passing into the Bay from the Upper Estuary is rapidly replaced by new sea water. REFERENCES {) ALEXANDER, W. B., SOUTHGATE, B. A., and BASSINDALE, R. Survey of the River Tees. Part II, The Estuary—Chemical and Biological. Department of Scientific and Industrial Research, Water Pollution Research Technical Paper No. 5. H.M. Stationery Office, 1935. ®) Instructions for reducing and analysing tidal stream observations. H.D. 290. Published for the Hydrographic Department, Admiralty, by H.M. Stationery Office [1929].