RIVER THAMES
 it
the works are designed with much care. Experiments made with pneumatic erosion point to that expedient as a probable means of disturbing and throwing into suspension troublesome shoals, so that the force of the ebb may be enabled to carry them seawards. Mechanical erosion on the same principle has been applied successfully.
   When remodelling the route of a tortuous river one matter of importance to be studied is the variation of geological formation along its course. In laying out channel curves full advantage should be taken of the comparative resistances of the strata to the impact of flow of tidal water. Broad principles only can be predicated in relation to the design of such schemes, as each problem has to be evolved on its individual merits. The motion of flowing water under the force of gravitation follows a rotative path, and its particles ricochet on meeting an obstruction. The resultant of these combined motions in the deeper areas of a stream is a downward, boring action tending to erode soft soil and redeposit it in slacker water.
   The dynamic force of a stream is particularly active on its concave surfaces by reason of the transverse swirling action set up as it swings round a bend. The water gradient thus created results in a raking effect across the bed of the river, producing a deepening of the inner area of concavity and a shoaling on its outer area on or near the opposite shore. Plate II shows the cross-section of the Thames in a straight run at Long Reach. It will be noted that on both shores the batter of the river banks is coincident and the river bed fairly uniform in depth. Plate II also shows a section of the river through St. Clement’s Reach, Greenhithe. The currents on the concave (Essex) shore have in this section produced a steep gradient and excavated the river bed to great depths. On the Kentish (convex) shore the gradient of the river bank is relatively flat and the river bed shoal. The navigation channel, or thalweg, for large vessels round such bends naturally follows the deeper water or the track of concavity. If the river section shown in this plate had been unduly contracted at the bend its lack of symmetry would have been relatively increased both on its shoal and scoured sides.