But a dredged channel is necessarily a narrow one (see [Fig. 8]), and ships will have to negotiate the sharp bends in a narrow channel and against a stronger tide, and also to swing at anchor, for which a wide area is necessary.
Objections against Dockisation.
Although this proposal has been mooted for some time past, scarcely any valid objection has been brought forward, but such as have been mentioned are mostly based on misconceptions.
One writer thought the river would become stagnant. As a matter of fact the sources of stagnation would be carried down the river by the fresh-water flow continuously, and there is no more reason to anticipate stagnation in the lower river than the upper river, where it has for ages been held up in the same way by numerous dams.
Another writer talks of the “cleansing power of the tides,” and it is a pity to see greater authorities, who ought to know better, speaking also in this way. It has been abundantly proved that the tides—as far as a clean river is concerned—are wholly detrimental. They back up twice daily the natural drainage of the river for five hours, and keep it in solution and circulation for forty-five days before removing it, the effect being exactly similar to backing up in a sewer.
THE POOL BELOW TOWER BRIDGE.
It has also been suggested that the sewage effluents discharged into the river at Crossness and Barking may cause the river below to become foul. Here again is misconception. The effluents—after precipitation of the solids, which is chemically effected, and the carrying out to sea of the resulting sludge to the amount of two million tons annually—contain very little impurity (only seven grains per gallon), and it has been proved by Dr. Dupré that 9/10ths of this becomes oxidised and absorbed in the large volume of water between the discharge and Gravesend. It is well known that in the case of “sewage effluents poured into a sufficiently large volume of otherwise comparatively pure water, the dissolved organic matter contained in it disappears with remarkable rapidity” (Sir Alex. Binnie).
Another critic suggests that the lower river will soon silt up under the new conditions. Most persons—seeing the filthy state of the water—naturally think there must be a large deposit from it. But it has been shown that this suspended matter is the result of tidal currents keeping the mud stirred up everlastingly. An examination of the affluents of the Thames shows that they contain very little suspended matter, and therefore when the locked Thames has deposited its charge of suspended matter any future soilage must come from its affluents—that is, from the upland waters and the sewage effluents, which latter will only affect it below the point of their discharge.
A calculation from official data of the quantities actually now passing into the Thames, from all sources, gives less than 1/10th of an inch annually over the river bottom; so that in ten years the deposit will not exceed 1 in., even without any improvement in the prevention of pollution. It has been estimated by Dibdin that the sewage outfalls could be removed to Gravesend, below the barrage, for the sum of £4,000,000.