Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/600

576 RIVER ENGINEERING over which there is a less depth than either above or below it. The lowering of such a bar forms one of the main objects of river improvement, as upon the depth that can be obtained over the bar depend the class of vessels that can enter the river, and, in tidal rivers, the period of time during which the entrance can be navigated. A bar may result from the action of the sea, which tends to form a continuous beach across any inlet, and would obliterate the mouths of rivers if the channels were not maintained by the ebb and flow of the tide and the fresh-water dis- charge ; or it may be formed by the conflict of the sea and river water, which checks the current at the mouth and causes the river to deposit the sediment which it held in suspension. The bars at the mouths of the Mersey, the Liffey, and the Adour are due to the first cause; whilst the bars at the mouths of tideless rivers, such as the Missis- sippi, the Danube, and the Rhone, are mainly due to the second. Improvement of Tidal Rivers. idal Tidal rivers differ greatly in their natural character- ed!- istics, owing to the variety in the different conditions f which affect them. Thus the Mersey, with an extreme tidal rise of 30 feet at its mouth, is only tidal for 46 miles, whilst the Seine, with a rise of 22 feet, is tidal for. 91 miles, and the Scheldt, with a rise of only 13| feet, is tidal up to Ghent, a distance of 105 miles. These differ- ences are mainly due to the different falls of the river- beds, but they are also affected by the facility of entry afforded to the flood tide, and the form of the channel up which it flows. The tidal capacity of a river depends on the rise of the tide and the configuration of the banks. The tidal flow into the Mersey amounts to 710,000,000 cubic yards at a high spring tide, whilst the flow into the Scheldt at Flushing, with less than half the tidal rise, reaches 475,000,000 cubic yards. The tidal capacity of the Seine, together with its estuary, formerly exceeded that of the Mersey, but it has been greatly reduced by the training works which have been carried out on it since 1848. ifluence The tidal ebb and flow passing and returning through tides. the entrance channels of a river twice a day exercise a very important influence on its maintenance. The effect of tidal scour is manifested in the history of the harbours of Calais and Ostend, 1 which in old times possessed deep outlet channels, but were injured by reclamation, and in the deterioration of the outfalls of the Fen rivers as soon as the tidal flow was curtailed by the erection of sluices. The power of the tidal scour necessarily varies with the volume of water producing it, and therefore one of the first principles of tidal river improvement is that the tide should be admitted as far up a river as possible, and all obstructions to its flow removed. If, however, the main- tenance of an estuary depended solely upon the tidal ebb and flow, the estuary would gradually silt up, for the flood tide brings in matter in suspension which it washes from the adjacent shores and sandbanks, especially during rough weather, when the waves stir up the sand and silt. The impetus of the tide running up the rising bed of an estuary is gradually checked, till at last slack water occurs, and the silt begins to deposit, which the ebb tide, enfeebled by the friction of the tidal water in its passage up and down the estuary, would of itself be unable com- pletely to remove. The erection of any obstructions to the tidal flow, such as sluices and weirs, increases the period of slack tide, and consequently not only reduces the volume of ebb and flow but also promotes the deposit of silt, to the further detriment of the estuary. The main- tenance of estuaries is secured by the aid of their fresh- 1 L. F. Vernon-Harcourt, Harbours and Docks, pp. 149 and 155. water discharge, which, being penned up during the flood tide, reinforces the ebb and preserves an equilibrium. The fresh-water discharge of a river, depending upon Value ( the area of the basin and the available rainfall, naturally fres h varies greatly in different rivers being, for instance, greater in the Tyne and the Clyde than in the Mersey, though these rivers have little more than one-thirteenth of the tidal capacity of the Mersey. The Seine, with a drainage area of 30,500 square miles, nearly six times the size of the Thames basin, has a discharge of 28,000,000 cubic yards, on the average, each tide (about twenty-eight times that of the Mersey), though this volume sinks into insigni- ficance when compared with the flow of the Danube with a basin of ten times the size, or still more of the Missis- sippi with a basin forty times as large. A large fresh- water discharge is of great value to the maintenance of an estuary, especially when, as in the case of the Seine, it carries little silt in suspension; whilst a small discharge in proportion to the size of an estuary, of which the Mersey is a notable instance, renders the state of the estuary very delicate, and necessitates great vigilance in maintaining its tidal capacity, to which its existence is almost wholly due. The silt brought down by tidal rivers, instead of being carried to their mouths and there deposited, is met by the incoming tide at points varying daily with the states of the tides ; and, moreover, except during slack tide, it is maintained in constant motion up and down the estuary, till at length it gets to the sea. Accordingly, though the sediment of tidal rivers is more or less deposited wherever the velocity of the current is checked, it does not tend to accumulate in one particular part, as in the case of tideless rivers, and therefore the formation of a bar at the mouth is mainly due to the drift by waves along the beach. A flood also, though more largely charged with silt, by giving additional power to the ebb, scours the channel and lowers the bar. The Humber, whose waters are densely burdened with mud which is readily deposited in still water, is nevertheless free from a bar. The best form of estuary for a tidal river is when it Forms enlarges gradually as it approaches the sea, thus affording estuat an increasing capacity for the admission of the tide, and promoting a regular flow. The estuaries approximating to such a form are generally free from bars as, for instance, the Thames, the Severn, and the Scheldt. When, how- ever, a river expands abruptly into a wide estuary on a sandy coast, it winds through the enlarged estuary in an unstable shallow channel, owing to the reduced velocity of the ebb in expanding out, and the checking of the flood tide on reaching the head of the wide estuary. Thus the Seine, with a deep stable channel from Rouen to La Mailleraye, had formerly a shallow shifting dangerous channel from thence to the sea ; the Ribble, with a good depth at Preston, has a shoal irregular channel towards its mouth ; and the Dee, with a moderate channel at Chester, is almost barred to vessels, except at high tide, below Connah's Quay. These estuaries do not possess a well-defined bar, but their long shallow winding channels offer a still more serious impediment to navigation. The worst form is a very irregular estuary with abrupt expan- sions and contractions, of which the Mersey is a pro- minent example, for, in spite of its large rise of tide, it possesses a shallow, irregular, and shifting channel above Liverpool, and is encumbered by a wide bar below. There are three obstructions to which tidal rivers are Met! subject, namely, a bar, a shifting channel, and inadequacy of in of depth ; and there are three general methods which may be resorted to for their improvement, namely, jetties, training walls, and dredging, in addition to the regulation of their upper portion by longitudinal jetties, or banks, as previously mentioned.