DRAINING. 



65 



Draining. Table of the Fall or Slope, and Mature of Ike Boltom of 

 V """"Y~" 1 "' Rivers and Streams. 



FaU of 



rivers and 



Fall. 



1 in 10 



30 



50 



70 



100 



200 



400 



600 



1000 



2000 



Bottom. 



Torrents. 



streams. 



Rock 



Large stones 

 Smaller stones 

 Pebbles 

 Small Pebbles 

 Very small do. 

 Fine gravel 

 Coarse sand 

 Common sand 

 Muddy sand and clay 



("Mud and soft clav, some river") ,,. , 

 1 weeds ,.t bottom f Dull and 



6000 Ooze and many weeds j sluggish. 



j 



| Rapid 



> Gentle streams. 



i000 



The above Table is calculated for small rivers and 

 brooks, as already observed. Rivers would require less, 

 and small streamlets and drains more descent, to have 

 the same velocity and description of channel. Straight 

 and even channels also permit the water to flow much 

 swifter than where it has, with the same slope, to wind 

 round the different sinuosities of the natural bed, to 

 pass over shoals, or through narrows, &c. 



It may be observed, that a bottom of fibrous bog will 

 bear a greater velocity than any loose earth or gravel. 

 The internetting of the fibres prevents them from being 

 torn away. Accordingly, bog diains stand well, even 

 upon great slopes, if they are not cut to the under stra- 

 tum ; but where that is the case, as it is generally found 

 to be gravelly or sandy clay, the bottom speedily gives 

 way with the stream, and the drains choke, or blow up if 

 they are hollow. It appears, therefore, not only advisa- 

 ble, but even highly necessary, to draw in sloping ground 

 the drains across the slope, so as to be nearly on a level, 

 since a fall of 1 in fr:0, the greatest that is absolutely 

 IK (< -- TV, is not sensibly very different from the hori- 

 zontal. This disposition of the drains at the same time 

 enables them to catch and collect the water as it de- 

 scends throat,--!! the soil down the slope, and prevents 

 the same water from successively moistening lower and 

 lower pieces of ground. This principle, however, is by 

 no means generally understood by common far. 

 among whom, in many cases, the only idea seems to be 

 to carry away the water as fast as they can ; and this 

 they attempt by perpendicular parallel drains running 

 down the slope, or by many branches collecting rapidly 

 into one main down the hollows. They do not ob- 

 serve, that, in these cases, the water which falls on the 

 field, unless it hapj>ens just to fall over one of the drains, 

 is not in the smallest degree assisted in its passage 

 away ; for it will most probably descend through the 

 loose arable soil, until it comes to the firm bottom, and 

 then, as it can have no tendency to proceed sidewise in- 

 to any of the drains, it must continue to trickle down 

 the slope until it arrive at the bottom, in which situa- 

 tion a drain, though it may perhaps afford it a more un- 

 obstructed passage, can give no more fall than original- 

 ly existed on the surface of the field. And hence, by 

 the way, the propriety of rounding the ridges, that there 

 may be a fall either way into the furrow from the 

 crown ; and upon which point it may be noted, that if 

 the ridges are laid to come right down the slope, there 

 can be very little use for furrows at all. In such cases, 

 if it is not otherwise inconvenient, the furrows should 

 be drawn obliquely across the slope, and, acting as 

 catchwaters, would greatly diminish the number of hoi. 



VOL. VIII. PART I. 



low drains that would be requisite in an arable field. Draining. 

 But more of this by and by. ^"^"Y"^^ 



The dimensions of open drains will chiefly depend D' men - 

 on the nature of the soil, and quantity of water to be slons f . 

 conveyed by them. The best form for facilitating the pcn 

 passage of the water, is to make the slope on each side 

 1C inches of base to a foot of perpendicular height ; the 

 breadth at bottom may be two-thirds of the depth of 

 water, unless particular circumstances make it desirable 

 to widen the channel so as to lower the surface. As to 

 water-way, we have usually found the channels of ri- 

 vers and streams running about two feet per second to 

 contain about a square foot area of section for each 

 hundred acres of drainage ; but this will vary accord- 

 ing to soil and climate. The usual size of the smaller 

 ditches running through farms are three or four feet 

 top, one foot bottom, and about three feet deep ; but it 

 is difficult to preserve the sides so steep. The drain- 

 age of every kind of ground is effectual, when the wa- 

 ter is two feet within soil. 



When the site is liable to be injured by foreign wa- 

 ters descending into it from the conterminous upper 

 grounds, it is most advisable to cut them off at higher 

 levels, before they descend into the flats, which, by this 

 means, will have only their own waters to discharge. 

 For that purpose, the catchwater drain must be resort- 

 ed to, being drawn along the boundary of the upper 

 grounds, so as to intercept the springs and streams 

 flowing from them, until a convenient opportunity be 

 found of discharging them into the receiving stream. 

 It frequently happens that the water which is thus pro- 

 cured affords sufficient power to work hydraulic ma- 

 chinery, by which the neighbouring low lands may be 

 freed from their proper waters, even when they have 

 no natural outfall. , 



When the natural channel of the foreign waters lies Embank- 

 through the site to be drained, as is the case with most incuts. 

 rivers which overflow their low grounds, it is common- 

 ly necessary to confine them with embankments on ei- 

 ther side, to prevent inundation in time of floods. 

 The size and dimensions of the embankment must de- 

 pend on the depth of water and reach of the waves to 

 which it may be exposed. They must not only be high 

 enough to keep out floods equal to the highest previ- 

 ously known, but also those which may occur after the 

 space over which they used to spread has been dimi- 

 nished by the embankment. The floods of all rivers 

 increase after embankment, since they are not only de- 

 prived thereby of a certain portion of channel, but also 

 of those natural basons or reservoirs which used to re- 

 ceive the surplus waters of sudden swells, and of course 

 regulate the supply to the lower readies of the river. 



Where long-continued floods keep the water for ma- 

 ny days above the level of the neighbouring lands, it 

 becomes advisable to puddle the embankment, or to 

 carry up a wall or partition of impervious stuff in its 

 middle, to prevent the transpiration of the waters. It 

 is necessary that the puddle wall be inserted a foot or 

 more under the original surface, as the waters in that 

 case, having a greater space to pass through under the 

 foundation ere they can rise again on the inside, are 

 likely to choke up the several pores by their sullage more 

 effectually. 



The exterior slope of embankments should be at least 

 twice the height. Even three times or upwards is ad- 

 visable, when a reach of wave is to be resisted ; but 

 these slopes are seldom, given, and hence the frequent 

 failure of such structures. 



The thickness at the top of an embankment may,thee 



