BROOKS ON THE IMPROVEMENT OF RIVERS.
Belts, as they are technically called, or sand-banks formed at the mouths of rivers, are among the opprobria of science, and are greatly injurious to the country or district at the embouchures of whose streams they are found. By prevent- ing vessels drawing any considerable depth of water from en- tering the river, they not merely check the extension of coin- mercre, but almost forbid its formation ; for though commodities of little bulk in proportion to value might be carried down in small craft, and then removed into the larger vessels, yet the ships, having to remain at anchor in the open sea, are exposed to great danger in stormy weather. The existence of London as a city is almost, and its prosperity entirely, owing to the Thames : capable of floating vessels of considerable burden to London Bridge, and the very largest to Blackwell or Deptford, its entrance-channel, though beset by sand-banks which render the navigation riskful to strangers, is accessible even at low-water to first-class ships of war ; and, putting danger out of the question, many a weakly stomach has been set at rest when the haven of the Thames has been reached in a gale. As an example of an opposite kind, the mighty Mississippi's enormous volume of water is diffused as it reaches the ocean by six channels, the deepest of which has but twelve feet at its bar, the shallowest only seven feet ; and Texas has not a port or a river embouchure worthy of the name. Few of the great rivers of the globe—such as the Ganges, the Indus, the Niger, the Orinoco—enjoy a navigation commensurate to the vastness of their streams ; the waters being dissipated through several mouths, and obstructions formed by sand-banks ; whilst many smaller rivers in countries much more advanced, are rendered of little utility by a bar at the mouth.
Shoals and sand-banks in a river are less mischievous to a country than a bar, unless they lie so close to its mouth as practically to -stop the navigation. To towns or their adjacent districts they are very injurious, forming frequently the difference between advance- ment and stagnation. Even with some peculiar countervailing ad- vantage in natural products, or manufactures, they impose a check upon the free advance of prosperity, first by the difficulties of transport, and next by the cost incurred in removing them.
Various theories have been broached to account for the formation a bars. The celebrated geographer Major RENNELL attributed their origin to the stream itself. "All rivers," says RENNELL, "preserve to a certain extent of space, which is proportioned to the velocity of their streams, a current of water into the sea beyond the points of land that form their embouchures when by the con- tinued resistance of the sea they at last lose ;heir motion. The mud and sand suspended in these waters during their motion are deposited when that motion ceases ; or rather, they are gradually deposited as the current slackens, according to the gravity of the substances that are suspended. This deposition will then form a bank or shallow in the sea," and so forth. On which it is remarked, that if the theory were true, every river should have a bar. The same objection applies to the theory of Mr. DE LA BECHE, that bars are formed from the sea itself; the action of the waves heaping up detritus ; since, by a parity of reasoning, all rivers ought to have bars more or less in proportion to the violence of the waves. Some attribute bars to the ebb-tide not running in the same channel as the flood, or vice versa ; others to the river discharging its waters at right angles into the sea ; others, again, to a deficiency of back-water : all which theories are centradicted by the facts. Bars are found existing quite independent of the direction of the discharge of the waters ; and if the quantity of back-water prevented bars or banks, both the Orinoco and the Mississippi ought to be free. Colonel EMT, a French engineer, has lately promulgated a theory which attributes the formation of banks, and some other phseuomena, to "ground-waves," arising from pointed rocks at the bottom : but this view Mr. BROOKS con- siders opposed to facts observed by himself on the coast of York- shire ; where the formation of the shore is such that ground-waves and deposites should, according to the theory, be the result, but are not produced. Mr. BROOKS does not state his own theory with much perspi- cuity ; but the facts from which be has deduced it are clear and convincing, supposing them to be true. In rivers without bars, "We shall find," says he, " that from thepaction with the QC„..4,E, a long line of navigable course exists with an extremely gentle fall or slope of its surface, at low-water: the river is in this case in a proper train, its longitudinal section presenting a succession of inclined planes, becoming more and more gentle as they approach the ocean; and the lower course of the river, from the slightness of its fall, approximates to the condition of a frith, or deep inlet of the coast, or to that of one of those large natural or artificial harbours, which, being niece tidal receptacles, wherein the influx and efflux take place in equal times, are necessarily free from bars. " The river, being in this perfect state as regards the slope of its surface at low-water, a consequent attendant upon the latter will be an equal duration, or nearly so, of the period taken np by the flow of the flood-tide with that or the ebb, in the lower reach of the river."
A river with a bar is said to display a perfectly opposite condi, tion- " In lien of presenting a longitudinal section of a succession of inclined planes, described in the preceding description of rivers free from bars, as be- coming more and more gentle in proportion to their proximity to the ocean, it will be often found that the declination or slope of some of the upper reaches is less than those nearer the ocean ; and the fall at low-water in the lower reaches of the river is always so great as to produce a striking difference in the vertical rise of tide, even at a short distance from the sea; and attend- ant upon this defective state of the section presented by the surface of the river St low-water, is a great extension of the duration of the ebb, beyond that of the upward current of the flood-tide."
This is all clear enough. But when Mr. BROOKS comes to enun- ciate his theory, we do not so well follow him- " During the period of the first quarter flood, the current, in lieu of being able to take its natural upward course, as in rivers where no bar exists, is op- posed or effectually checked by the effluent back-water; the declination of the stream in the lower division of the river presenting a head which insures a strong downward current, long after the tide would have been able to maintain an upward course, provided the back-water had had a free discharge. At this period the flood-tide, by reason of its greater specific gravity, occupies the lower stratum of the tide-way, and like a wedge endeavours to force its course up the channel ; which it is unable to effect, but merely elevates the lighter effluent water, the lower strata of which being checked by the opposition of the tidal water, yields to the latter the sand or other materials which it was capable of holding in suspension previously to its encountering the conflicting action of the flood-tide; and where this takes place the bar is formed."
The ideas which these words convey are neither clear nor satis- factory to our apprehension. The principle of Mr. BROOKS, we imagine, is this—that the ebb-tide, running more violently than the flood, effectually resists, for the first quarter of the flood, its up- ward efforts ; duripg which time, the detritus brought down in sus- pension is deposited in one place, from the waters being obstructed by the meeting currents, instead of being carried out to sea and there dissipated. This principle we can perfectly understand as a proposition. But whether the deposite would be limited to so nar- row a space as bars usually occupy, may be matter of question ; as the more natural result would seem to be to raise the bed of the river for some distance from its mouth,—unless, indeed, the velo- city of the ebb-tide should scour this deposite down, to accumulate at the month, where the force of the stream diminishes.
We do not know, however, that this question as to the theory of the bar's formation, has renal to do with the plan which Mr. Bsooirs has promulgated for improving rivers; because this latter rests upon an imitation of nature, and not upon an inter- pretation of her mysteries : and even should his process not suc- ceed in removing, it might probably reduce the bar, as it would most certainly improve the internal navigation of the river. His plan is simply this—to remove the banks or shoals which stretch across the bed of the river like dams, and have the effect of prevent- ing the proper discharge of the back-water during the continuance of the ebb. For example, in the Lune, the rise of the " ordinary spring-tide" at Lancaster is about 10 feet ; and at Masson, only five miles nearer the Irish sea, the rise is 21 feet,"—making a [very enormous] difference of flow or declination of the surface of the river, of 11 feet in that short distance, caused solely by obstructions to the ingress of the tide." An equal discrepancy is also observed in the respective durations of the rise and fall of the tides at the two places. It will not escape observation, that besides the
im- provement to the navigation, which ensues from the greater body of tidal water consequent upon deepening the river, the uninter- rupted flux and reflux of so augmented a body materially scours the liver, reducing shoals or preventing their formation.
Of the expense of such operations as Mr. BROOKS proposes we cannot judge. He states that they are not so great as it might be supposed, and frequently, if made with judgment, comparatively trivial ; and he supports his statement by a reference to facts. The Clyde, before the obstructions were removed by Mr. GOLBOENE to- wards the end of the last century, only allowed barges drawing three feet to reach Glasgow : now vessels are navigated to the Broomielaw quay drawing upwards of twelve feet. Similar im- provements have taken place in the Tees, under the direction of Mr. BROOKS. Nor should it be forgotten, that inefficient or tem- porary improvements, or evasions of difficulties, are attended with great and continual expenses. Corporations are annually laying out money in improvements on rivers, or rather in labours to keep matters from getting worse : a canal from Cheiter to Heswell, recommended by Sir JOHN RENNIE as preferable to attempting the improvement of the Dee, was estimated at 560,0001.: a great ex- pense is continually incurred for dredging in the Thames—and not in what our author calls geological shoals, which no direction of the current can remove, but on deposite shoals, which imme- diately begin to accumulate again. "It is a remarkable fact, that notwithstanding the enormous sum of 125,0001. was employed in dredging the river Thames off Woolwich between the years 1808 and 1816, the riyer us now (1840) in as bad a state as ever, and the mud and silt is accumulating instead of decreasing : in 1816 alone as much as 29,0001. was thus expended, &lithe sum amounts On an average to 16,0001. per annum, to such little purpose."
We have confined ourselves to the leading subject of the Treatise on the Improvement of Rivers ; but there are many subordinate topics handled by Mr. Bitooxs—such as the removal of shoals, which rather impede navigation than obstruct the tidal flow ; the effects producible from contracting the banks to increase the vo- lume of water ; and the best principle and cheapest mode of form- ing permanent or temporary embankments. There are also some useful comments on the present condition of several of our rivers— for instance, the Tyne, the Lune, the Dee, and the Ballyshannon- together with a cursory survey of some of the principal rivers of the globe, preparatory to the exposition of the author's own theory. The volume is worth examination by civil engineers, or any one interested in river-navigation ; whatever conclusion may be arrived at with respect to the theory of Mr. Bitooxs on the formation of bars.
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