isns.] 



if A R M l<: RS' REGISTER. 



33 



is, in many situations, ol" considtirable extent), 

 fruni this opemiioii of iIh* current on the edfre of 

 the bunks. (»>= has heiMi explMined in .set'linn 1.). 

 the surlliff of the vviiole fieiils, ihu.=' situated, is 

 e.\|)ose(l to inun(hition. when the river is swollen 

 by tiie rain, and the torrents whiih descend from 

 the iieii'liborinir hei<>lits: often sweepins? away the 

 crop, and deterioratinif tiie soil, by iiie lodgincr of 

 Siind and irravel at one place, and the carryinff 

 Hway the mould at another. A snddi^n and rapid 

 flood has akso the eiiect of chnntjina' the course of 

 a river; makiiiir it abandon the old, and taice a 

 new channel, throu>rh part of the crround. perhaps 

 more valuable, by \ts being better cultivated, and 

 on that account more loose, and easily acted upon 

 by the current. In this case, the lo.ss is a<jsra- 

 vated, by the old channel being- left a useless 

 waste, in addition to the space occupied by tlie 

 new course which the water has tak-en. 



Accounts of liavoc, committed by such inunda- 

 tions, abound in all the county reports, and im- 

 periously enforce the necessity of adopt intj meas- 

 ures to prevent them. From the following; detail 

 of the evils committed m one district, a judgment 

 may be formed of the general loss sustained by 

 the country at large: — "The haughs of Isla," says 

 the surveyor of Perthshire, "Irom Ruthven bank 

 to the mouth of that river, adis'ance often miles, 

 are exposed to irmndations which are great and 

 destructive. In tiie glens lying north of the Stor- 

 mont, floods do uireat havoc among the hay and 

 other crops. In Athol, at Bun-rannoch, in Glen- 

 dociiart and Gleiilochy, torrents from the moun- 

 tains swell the rivers so suddenly, that they spread 

 far and wide, in many places beyond their banks, 

 and trequenlly sweep ofl" almost the whole labors 

 of the year. The Tay and the Earn, the Devon, 

 the Allan, and almost every river within low 

 banks, which takes its course through flat land, 

 brings desolation on the finest fields, which lie on 

 its sides." 



In the northern counties of Scotland, inunda- 

 tions are not so frerjuent as in the southern. The 

 reason is, that where a channel is rocky, which is 

 the case with most ot' the highland rivers, it is 

 generally deep, and the river seldom overflows its 

 banks; but where the intersected strata is of a soft 

 and iiravelly kind, and the expanse of the flat or 

 haugh-land of greater extent, as in the low coun- 

 try, the channel is invariably shallow, quickly 

 filled, and soon overflowed. 



The manner in which inundations proceed, de- 

 serves particular attention, because, from observ- 

 ing this, we will be the belter able to discover the 

 proper remedv. "'When a river swells," says the 

 learned Button, "its celerity uniformly increases, 

 till it begins to overflow the banks. Fri>m that 

 moment its rapidity is checked, which is the rea- 

 son why inundations always continue several 

 days; fur, though the r|uantity of water should be 

 diminished after the commencement of the itiun- 

 dation, u would, notwilhsanding, continue to over- 

 flow; because this circumstance depends more on 

 the celerity than the quantity of water. If it 

 were otherwise, rivers would often overflow their 

 banks for an hour or two, and then retire to their 

 channels, which never does happen. An inunda- 

 tion, on the contrary, always lasts some days, sup- 

 posing the rains have ceased; and l<;ss water runs 

 in the river, because the overflowing of wafers di- 

 minishes their celerity; and, consequently, al- 

 Vol. IV— 5 



though the same quantity of water arrives not in 



the same time as lijrmeriy, the effect is the same 

 as if a larger qnaniiiy had been brought down." 



Farlher, it may be observed on the motion of 

 water, that, mathematically speaking, if a river 

 runs on a bottom havinii-an equal descent, (he ve- 

 locity of the water will increase, the fiirther it 

 runs. Thus, suppose the descent to be one fliot 

 per mile, after the first mile the water will have 

 acquired a velocity of eight fi^et per second; after 

 four miles, its velocity would be sixteen feet per 

 second; and at the end of sixteen miles, it would 

 run at the rate of thirty-two feet per second — its 

 velocity at every point being the same proportion- 

 ally, as would be acquired by a f)ody filling the 

 same perpendicular height. These rules, with re- 

 gard to the motion of rivers, cannot however be 

 so accuiately observed, on account of the perpetu- 

 al obstruction the water meets with against the 

 sides and bottom of the i hannel it flows in, which 

 counteracts the power of gravity, and reduces the 

 water to an uniform motion, where the declivity 

 of the bottom and sides are regular. Supposing a 

 given quantity of water is to be carried off', the 

 smaller the descent, the width and depth, or sec- 

 tion of the river, must be the greater; for the 

 water, running in a large body, and slower, meets 

 with a less proportional obstruction from the sides 

 and bottom. Also, supposing the same quan ity 

 to be discharged, the larger the body it runs in, and 

 the slower the motion, the more liable its course ia 

 to be obstructed by stones, mud, weeds, &c. 



Large and deep rivers run sufficiently swift, and 

 discharge vast quantities of water, with a descent 

 of one foot per mile. 



Small rivers and large burns require about two 

 feet per mile. Small burns scarcely keep a free 

 course under four feet per mile. 



When a cut or channel, therefore, is to be made 

 ibr a river or stream of water, through a tract of 

 level ground, it is preferable to make it deeper at 

 the lower extremity, in order to give a greater de- 

 scent upon the bottom all the way, than to make 

 a broad and less deep channel all the way upon a 

 less descent; for the former method is attended 

 with less expense of digging, and is better calcu- 

 lated to keep an open course. 



From this explanation, it is obvious that the 

 leading principle of all operations, to prevent a 

 river from overflovving, should be to increase and 

 preserve its celerity. 



For this purpose, if may be first observed, that 

 it is much better to deepen than to widen a chan- 

 nel. A contrary advice is given by some writers,* 

 but, it is apprehended, without a sufficient atten- 

 tion to circumstances. If a river were always 

 equally full, there can be no doubt that to widen 

 its channel would prove a most eflTectual method 

 to confine if within its banks. But as the quantity 

 of water is constantly fluctuating, the widening of 

 the channel would produce the following bad ef- 

 fects: — 1. That when the stream happened to be 

 small, if would scoop out a winding bed for itself 

 in thetniddle of the channel; and upon swelling, 

 would follow the direction of this bed, and thereby 

 strike with violence against the banks. 2. That 

 by lessening the depth of the river, its celerity 

 would be diminished, and its liability to overflow 

 of course increased; for it is an established fact, 



* Beatson, Marshall, he. 



