134 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[April, 



But to obtain that degree of regularity or parallelism which is required, 

 certain excrescences in the area must be enclosed, by which it will become 

 reduced. It is to the consequences of such a measure that the numerous 

 opinions before adverted to were directed, and which liavc now to be con- 

 sidered. 



It has been asserted that the open broad areas of the river at a considerable 

 distance above Liverpool, are necessary for the maintenance of deep water 

 towards tlic river's month ; and it is thence inferred that if the area of the 

 river towards the extremity of the tideway were diminished, great injur)' 

 would be sustained towards the outfall. 



The shoals arc said to accumulate, and the depth of the channel diminish ; 

 and a great proportion of such effects have been attributed to the enclosures 

 that have been made from the river in the upper part of the tideway. 



In order to investigate the subject in question fairly, it is indispensable 

 that the ."iottrcf frmii whence the accumulations are derived be ascertaiiied^ 

 (i. e.) it must be known whether the materials which constitute the accumu- 

 lations for the most part arc derived from the sea shore, os whether they are 

 brought down by the rains from tlic surface of the uplands.' 



That matter is brought from the interior and carried towards the sea, is 

 a fact too well known to require more than allusion to it. But that the 

 quautity so brought down and de]Jositcd in the bed of the river is scarcely 

 perceptible, and that it docs not produce any sensible injury, may, I think, 

 be demonstrated in a sati^factory manner. It must then follow, that the 

 accumulations comiilained of arc xvpplied from the sea. I have confidence 

 in being en.ihlcd to prove that the (/rent ejcpanses in the area of the upper 

 part lift lie river, are not only not beneficial to the outfall, but that they are 

 injurious to it. 



If the accunuilations were derived from the uplands in any sensildc degree, 

 the quantities deposited from time to time might be expected to bear some 

 proportion to the quantities of rain fallen at different periods, because the 

 quautity of matter brought down and conveyed through the u])per division 

 of the river to the tideway, must lie regulated by the quantity of water which 

 conveys it. But it is a fact long ascertained and known beyond doubt, that 

 the accumidation of the sands in the vicinity of Kimcoru (above and below 

 that place) is greatest when there is least n-ater descetidiiiy from the uplands. 

 Such is the amount of accumulation in one dry season, that it is felt by those 

 ■who navigate the upper part of the tideway. It is then to be observed, that 

 the accinnulations progressively increase until the arrival of a laud flood, on 

 which occasion the excess that bad become deposited is removed. The fact 

 therefore is, that the quantities of accumul.itions in the river are inversely in 

 proportion to the quantities of rain ; and hence there is less deposit upon the 

 bed of tlie river in the tideway when the gi'eatest quantity of silt is brought 

 down from tlic uplands. From this reasoning we may infer, that if there 

 were no descending land stream, and if the whole area of tideway were a 

 mere bay, the same would gradually silt up, and become dry land. Such 

 would he the fact, and it will be shown that however extensive the recep- 

 tacles for the mere tidal waters, they do not contribute to the preservation of 

 tlie outfall. 



The cleansing of the outfall is admitted by all to be dependant upon the 

 force of the outward motion of the water. It must therefore follow, that the 

 inward motion of the same (/. e. the flowing tides) will act in a similar man- 

 ner, and bring with them such quantities of sand as they are capable of 

 moving. The question then refers to the comparison of the inward with 

 the outward forces. If the force of the ebbing tide do not exceed that of 

 the flowing tide, it is evident that no greater quantity of sand can be carried 

 out by the former than that which is brought in by the latter. If the ebbing 

 water have an excess of power over that which flows, it is certain that a 

 greater quantity of sand will be carried out than is brought in, and conse- 

 quently the depth must gradually increase. But such, however, is not the 

 fact, clthouyh the ebbiny tides are assisted by the waters from the uplands. 



From what has now been stated, I trust it will appear manifest, that the 

 effect of the flowing tides in raising the bed of the river, exceeds that of the 

 ebbing tides, and hence we may conclude, that the depth if the channel is 

 entirely and exclusively dependant upon tlie water derived from the uplantU. 



Although I cannot imagine a doubt upon the fact just mentioned, the sub- 

 ject is of so much importance, that I must beg permission to make use of 

 another argument. 



If the deposits in the tideway were derived from the uplands, we surely 

 ought to detect the fact by reference to the substance of which they are com- 

 posed. I have obtained specimens of the bed of the river from various parts 

 of it, and have found that the substance in the higher part of the tideway 

 corresponds with that taken from below Liverpool. I have also found that 

 the loose matter in the bed of the river above the tideway, has a different 

 character. It is true that the strata of the district through which the river 

 passes from its source is silicious, and, therefore, the debris partakes of that 

 character ; but in form it difl'ers, and, as may be supposed, is mixed with 

 various other substances, of whicli coal dust and soot may be taken as pro- 

 minent ingredients. Now, the diflfercnee in colour of the general mass of 

 specimens taken from the higher jiart of the river, especially that near Man- 

 chester, and that of the specimens taken from the neighbourhood of Runcorn 

 and Liverpool, is such, that no doubt remains of their being derived from 

 different sources. 



Considering the character of the district through whicli the river passes, 

 the immense consumption of coal on both its banks, and the prodigious 

 quantity of loose coloured matter that must necessarily be washed into its 



stream, I certainly did expect to find some appearance of such matter in the 

 sands in the vicinity of Liverpool, but although I employed a very high mag- 

 nifying power, no such particles could be detected. 



We have also abundance of examples, which prove most obviously, that 

 with tidal rivers the raising of their beds is produced by the flowing tides, 

 while the products of the land waters are not observable until the tides fiave 

 elevated the surface to nearly the heiyht to which they rise. The dimensions 

 of the particles, a descending stream is capable of carrying, depends upon the 

 velocity with which the water moves, and that velocity is determined by the 

 slope of the bed. Most rivers appear to be progressively diminishing in 

 depth, and hence we may safely infer, that their depths towards their outfalls 

 were greater in proportion to the remoteness of the periods ; their slopes 

 must therefore have been greater, and the masses brought down proportion- 

 ally so, and the debris derived from the uplands and deposited in the rivers 

 must increase in dimension in proportion to the depth at which is is found. 

 Although the common velocity of a river may be insufficient for the removal 

 of gross particles, (say coarse sand,) it may be sufficient for carrying matter 

 of a lighter description, and it is probable that all such light matter as ar- 

 rives in the tideway of the Mersey, during the ebb-tide, is actually carried 

 out to sea ; but such as may arrive during the flood-tide, wliich at higli water 

 does not happen to be deposited on those parts of the bed over which a cur- 

 rent ))asses when the tide returns, will remain where it falls. Now this can 

 only happen where the sands have accumulated to a considerable height from 

 another source before described, and it seems that the deposits from the 

 uplands in the process now going on in the Mersey, are for the most part of 

 the lightest description, and they are to be found only under the circum- 

 stances mentioned. All this reasoning is suificiently supported by an exami- 

 nation of the soil of which the upper portion of the marsh land is composed, 

 and may therefore be safely relied on. 



The coasts of Surry, Kent, Suffolk, and a portion of Norfolk, are bordered 

 by beaches of shingle, which are kept in perpetual motion by the action of 

 the sea, and the component parts are continually seeking a place of shelter, 

 and hence they enter and accumulate about the mouths of all inlets which 

 have not the advantage of an opposing force, derived from a never-failing 

 stream from the uplands. The direction of their prevailing course is deter- 

 mined by that of the most frequent or prevailing action of the waves, or 

 breakers of the sea, and although a land stream be sufficiently powerful to 

 maintain a passage to the ocean, yet such is the action upon the loose sub- 

 stances which compose the shingle, that their motion cannot be prevented, 

 and the outfalls of the rivers become diverted into a direction parallel with 

 the shore, unless such an efl'ect be opposed by artificial means. 



If, then, notwithstanding the existence of the constant aid of a land stream, 

 it be difficult to retain an unencumbered outfall, much less can it be expected 

 that a clear opening shall be jireserved where such assistance is not available. 



Leaving the operations of nature entirely free from control, it does appear 

 that all inlets upon a coast invested by a shingle beach, and which are not 

 preserved by the discharge of a stream from the land, must gradually dimi- 

 nish. The accumulating process is abundantly exhibited on the coasts al- 

 luded to. Dovor, Folkestone, Rye, and Shoreham, afford excellent instruction 

 upon the subject. 



Nothing is more common than to assign, as the cause of decay in harbours, 

 the enclosure of spaces which previously received the tidal waters, while the 

 ordinary processes of nature are totally unheeded. I have never yet heard 

 any reasoning which explains in what manner the abstraction of the tidal 

 space can or does produce the effects complained of. If the flowing and 

 ebbing forces be equal, the latter can only remove from a harbour the same 

 quantity of matter the foimer may have deposited. 



But upon careful examination of all the actions contained in the process, 

 it will be seen that the flowing forces are the greatest, and hence we need 

 seek no further for causes that produce the effects which we obser>'e and 

 lament. 



But an approach to parallelism in the banks, is useful in another way : the 

 tapering form of the opposite sides is known to contribute to the advancement 

 of the tides towards the extreme points of their access. The spring tides at 

 Runcorn do note rise to a higher level than the high water mark of the same 

 at Liverpool, while some neap tides, if opposed by the wind, will not reach 

 that place. In the latter case the tides at Liverpool return before the whole 

 estuary has been filled, which would not occur if the area were to be diminished 

 to its best proportion, and the sides properly regulated. 



Vei-y remarkable and interesting evidence on this branch of the subject is 

 to be found in the Severn and Wye. The channel of the Severn is funnel- 

 shaped, and the height to which the water rises increases with the distance 

 reached ; thus — 



At Swansea a spring tide rises 30 feet 

 At the mouth of the Avon . . 40 „ 



At the New Passage 50 „ 



At the mouth of the Wye . . 60 „ 

 At Chepstow 70 „ 



Some portions of the rise at Cheptow may, however, be ascribed to the 

 quantity of water descending from the mountains. These facts I have per- 

 sonally ascertained. 



Now although the Mersey is of a different form from the Severn, yet it 

 may readily be conceived that the momentum of this great body of water in 

 the river below Runcorn, mut, where the space is suddenly contracted, as it 



