In the first place, care should be taken in determining the site of an embankment across an estuary that there shall be no concentration or alteration of the general direction of the currents, or scouring action will be created; for the erosive and other deleterious effects of wave action upon a shore are influenced by the angle at which they are impelled against it. A prudent course to adopt is to carefully preserve the usual channels by means of bridges, particularly in ground of a loose character, such as is usually found in partly landlocked waters; for if the velocity is increased, the earth which has been deposited by the original current being reduced or impeded resulting in the suspended matter in the water falling to the bottom, will be subject to a force that will again disturb and cause it to return to its previous suspensory condition; and any disturbance of the normal currents may destroy the equilibrium of stability and alter the flow, and when they are affected it may be most difficult to restore them to their original state, for water will always endeavour to obtain the easiest channel. It is essential to know the heaviest flood discharge of any river that may flow into an estuary, the greatest depth and velocity of the river, the normal and flood channels, their sectional area and direction, and the extent and shape of the catchment area, so as to establish the required opening to give the natural waterway; for it is important not to interfere with the tidal capacity of an estuary or the volume or flow of any upland waters into the sea, as a navigable channel may become filled with silt, especially when the shore is flat or sandbanks exist; and upon a sandy coast an embankment across an estuary with openings for a navigable channel will probably cause it to become difficult to navigate and, perhaps, impossible, without constant dredging and other works of maintenance, as an embankment may obstruct and deflect the currents and prevent them carrying away the suspended matter. An open viaduct instead of an embankment is almost always to be preferred, and may be necessary; for the power of a current to scour or move particles is greatly augmented by a small increase of the velocity, and the earth may be in such a delicately balanced condition that any increase of scouring action may destroy the seat of an embankment. To prevent leakage and scour of the base of an embankment near a river resting upon loose soil, curtain walls are sometimes inserted upon both sides extending to a considerable depth, thereby affording security against an embankment merely resting upon a mound which may gradually erode, with the result that it must finally slip and be destroyed; and it may happen, unless the foundations are carried down into impervious soil, that water may escape underneath and undermine it; such action is obstructed by carrying the slopes a few feet below the ground so as to prevent through surface percolation.