Apart from its cost and the grossly unfair policy of financing and running the docks against the wharfingers, it is evident that this scheme is based upon the possibility of dredging the river to the depth required. Fig. 8 is an actual section of the river, showing the proposed dredged channel as compared with a dockised river.

It seems incomprehensible that any expert authorities should have advised the Government that the river can be effectually dredged. The fact is that it is quite impossible to dredge it to the required depth of about 15 ft. below the present bottom, because experience has shown that with such a river and scouring current the channel will fill up again nearly as fast as it is dredged, the material coming from the foreshores and the estuary. This will give rise to dangerous slipping in of river banks and walls. The estimates of the cost of this dredging (£2,500,000) are therefore entirely misleading.

The present bottom is formed and stands at the natural angle of repose for its present volume, width and currents, and any great interference with this contour such as is proposed—with slopes of 7 to 1—will not stand, the general slope of its bottom now being from 20 to 50 to 1. The Port Trust that undertakes this will find itself spending enormous sums annually in continuous dredging and repairing banks and in compensating owners; all, of course, added to the annual cost of maintenance and to the dues, or charged to the ratepayers.

Glasgow and the Clyde have been instanced as examples of what can be done by dredging. But the Clyde below Glasgow is not a river comparable with the Thames below Gravesend, but an estuary with a very moderate current and tidal range of from about 4 ft. to 10 ft., and the dredging has merely made and kept open a channel in this estuary. The Thames, on the other hand, is a narrow river with a strong scouring current and a range of tide of from 16 ft. to 21 ft. Further than this, Glasgow has spent seven millions in this work, and has to pay large sums to keep the channel open, dredging nearly a million cubic yards every year.

But there are other difficulties. When the river has been deepened as proposed, the tidal volume will be increased about one-third, and therefore its current strengthened and increased, probably two knots per hour. What is worse, the tidal range will be increased proportionately, which means that the high tides will be higher—probably 3 ft. or more—and the low tides lower, by a similar amount, than now. Spring tides may be expected to run the river nearly dry at low water above London Bridge. Results—frequent inundations of waterside districts, more grounding at low water, and more dangerous navigation. Such results have always followed increased tidal volume.

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.