Autobiography of Sir John Rennie, F.R.S 63

Autobiography of Sir John Rennie, F.R.S 63

Again, if it be the making of a harbour, the student must first

thoroughly examine the nature of the locality, that is, its geographical

position and geological character. As regards the former, whether the

harbour is to be at the mouth of a river, whether that river discharges

its waters into a bay, or through a projecting exposed line of coast

where the main tidal currents run continuously and rapidly past it. With

regard to the latter, whether the adjacent coasts be flat and alluvial;

or elevated, but still composed of soft alluvial or sandy and calcareous

soil, easily abraded or worn away by the passing currents; or whether

they be composed of the harder or primary rocks. He must also carefully

consider the strength and the direction of the currents. All these

various conditions must be carefully weighed before coming to a decision.

 

In constructing close harbours, the same observations must be made. Each

of these cases requires a totally different kind of treatment, and the

correct method can only be ascertained by a thorough investigation and

knowledge of the local circumstances, such as winds, tides, currents,

coasts, &c., so that the harbour when constructed may afford every

facility for ingress and egress, safety when within, and not be liable to

any deposit.

 

In order to give the requisite supply of water to canals it is imperative

that sufficient reservoirs should be established chiefly at the high

level if possible, also at each intervening ascent and descent; but

it is most desirable that there should be only one high level, and

generally speaking this may obtained; but when, from particular local

circumstances, this cannot be done, then the high levels, even at

considerable extra expense, should be reduced to as few as practicable.

The same may be said with regard to railways, but in the case of canals

it is always absolutely necessary that there should be reservoir space

to supply the greatest amount of lockage that may be required during

the season when there is the least quantity of rainfall. The rainfall in

any given district may always be ascertained by proper rain gauges; and

whenever it has been found that there is no probability of obtaining a

sufficiency of water to pass the amount of trade that may be expected

over any given length of canal, then the high level must be lowered

sufficiently to obtain the required supply. When, from peculiar local

circumstances, this cannot be done, then it will become necessary to

erect steam engines of the requisite power to pump back the water from

the lower to the higher levels. But as a rule it will be found, that by

laying out a canal properly, and by storing sufficient water to answer

all the required lock supply at proper places, pumping back will only be

necessary in extreme cases. This, however, is a question of detail that

will be governed by the local circumstances of each particular case.

With regard to the construction of canals, that must be regulated by

the quantity of trade to be passed, and the charges that it will bear;

but, within certain limits, the larger the canal the better. In the case

of ship canals for seaborne vessels, it is advisable to construct them

wherever they can be made at a reasonable cost, and there is traffic

enough to pay a fair interest upon the capital.

 

In the drainage of extensive districts of lowlands, whether bordering

upon rivers or otherwise, it is the better plan, with some exceptions,

to divide the lowland from the highland waters, and to discharge them by

separate outfalls; because if they are both discharged by one outfall,

the highland water, coming from a higher level, and naturally having the

greatest velocity, will force its way first to the outfall, and until

it is discharged the lowland water cannot get off, but will accumulate

upon and inundate the adjacent lands. Again, if only one outfall be

provided, a much more extensive system of main and interior drains

will be required, as these latter must serve as reservoirs to contain

both waters until they can be discharged by the common outfall; but by

keeping them distinct from each other, the highland water may readily be

discharged into the upper part of the rivers or watercourses, whilst the

lowland water may be made to discharge itself at the lowest point the

outfall will admit of, and will get off before the highland water can

reach it. Moreover, the highland water, being discharged so much higher

up the watercourses or rivers, will scour out their channels as well as

the outfall, prevent them from filling up, and preserve them in the best

state both for drainage and navigation. These catchwater drains for the

highland waters will also be found very useful for supplying the lowland

districts with fresh water for cattle, domestic purposes, and irrigation

during the summer and dry seasons, when fresh water is so much needed for

the lowlands. This system was first introduced by my father, in 1805, in

the drainage of the extensive district of lowlands bordering upon the

river Witham, between Boston and Lincoln, amounting to about 150,000

acres.

 

Generally speaking, before attempting to improve the interior drainage of

any lowland district, it is necessary, in the first place, to examine the

state of the outfall, and how far it is capable of improvement; before

this is ascertained it is impossible to lay down any effectual plan. In

order to make the outfall effective it should be improved to the greatest

extent practicable, so that the low-water line or level may be reduced

to the lowest point. Having done this, the interior drainage may be laid

out accordingly. When this is combined with the catchwater system above

described, the drainage may be rendered as complete as possible, as far

as it can be upon the natural principle of gravitation. When the water

cannot be discharged from the outfall at all times by gravitation, we

must enlarge the main and tributary drains, so that they may serve as

reservoirs to contain the drainage water during the time that the outfall

sluice is closed in consequence of the water in the river or the sea,

where the outfall sluice may be placed, being higher than the level of

the water in the main and interior drains. No land can be considered as

properly drained unless the surface of the water in the adjacent drains

can be kept from 2 to 3 feet below the surface of the adjacent lands

at all times. There must be no stagnation of water; at the same time

there must always be the means, as far as practicable, of supplying the

land with that proper degree of moisture necessary for nourishing the

soil, either from the direct rainfall or from the water discharged into

the catchwater drains from the adjacent highlands; and if these be not

sufficient, then they may be supplemented by reservoirs of the proper

dimensions attached to them. The best mode of arranging this is, of

course, a matter of detail, keeping always in view the great principle of

a thorough drainage and an ample supply of fresh water. The system that I

have above explained is based upon the soundest principles of theory and

practice, and therefore I feel no hesitation in recommending it.

 

With regard to the sewerage and drainage of towns, the same principle

may be adopted, modified according to local circumstances. The drains

here will require greater fall or inclination. The sewage should not

be discharged into the watercourses, but into separate depôts at a

proper distance from the dwellings. These depôts should be thoroughly

ventilated, and the sewage deodorized by mixing it with earth, or some

other suitable substance, that will not impair its value, and then it may

be sold for manure; and thus instead of becoming a nuisance it may be

turned to profitable account.

 

All rivers in densely populated countries should have their flood waters

stored in capacious reservoirs, with proper sluices, in the main or

adjacent subsidiary valleys, so that during the dry seasons there may

be always an ample supply of good water for domestic and agricultural

purposes, irrigation, and navigation. The reservoirs will also be

advantageous in preventing the too frequent inundations and consequent

devastation caused by floods.

 

In waterworks gravitation should be adopted wherever practicable, so

that the source of supply shall be placed at such an elevation that it

may command the highest part of the buildings to be supplied, thus all

artificial power for pumping will be avoided. But in most cases, except

where natural lakes can be found, it will be necessary to make settling

or filtering reservoirs, from which the water when sufficiently pure

may be delivered into the supply reservoirs, and both of these should

be capacious enough to contain a sufficient supply for a month, more

or less, according to the particular local circumstances. Last, but

not least, the quality of the water for the proposed supply should be

thoroughly tested chemically, in order to ascertain its purity; it should

be as soft as possible, and be free from vegetable as well as all other

matter prejudicial _to health_; and it must be obtained in sufficient

quantity to guarantee a supply of thirty gallons a day to each inhabitant

of the town, with the means of augmenting the supply at the same rate for

any increase of inhabitants. The conduit which is to supply the service

reservoir should be covered throughout, as well as the service reservoir,

which of course should be occasionally cleansed; the other, or settling

reservoir, near the fountain head, need not be covered if made large

enough; that also should be cleansed as often as is necessary.

 

Where the water cannot be supplied by means of gravitation, then the

artificial method of pumping by steam engines or water-wheels, or other

means, must be adopted; but in this case also settling, filtering,

and service reservoirs must be employed, as already described. It is

unnecessary to remark that in all cases the reservoirs and conduits

should be made thoroughly water-tight and impervious to any drainage

water from the adjacent districts.

 

Docks may be divided into two classes, viz. floating and dry docks; the

former may be designated as enclosed spaces filled with water, penned up

to such depth as may be required for floating vessels of all classes.

These docks or basins must be rendered water-tight, and in most cases it

is necessary to surround them with nearly vertical walls, to economize

space and to enable vessels to come alongside and discharge and receive

cargoes.

 

With regard to the situation of these docks and designing the plans for

them, this depends upon the local circumstances and the requirements of

the particular class of vessels that they are to accommodate, and the

trade that is to be carried on in them. Without a thorough knowledge of

all these circumstances it is impossible to give anything like a correct

opinion as to their dimensions, mode of designing them, or any other

particulars. I may say generally, however, that as these docks are always

situated contiguous to some river or harbour, either with or without the

tidal ebb and flow, the position and direction of the entrances to the

docks become of the greatest importance, in order that they may not be

too much exposed, and that vessels may be enabled to enter and depart

with the greatest facility; and in such part of the river or harbour

where there is the greatest depth of water and the best channel outwards

and inwards. There should also, as far as possible, be the means of

supplying the basins with clear water, in order to diminish the amount

of deposit within; there should also be a smaller or entrance basin

adjoining the outer lock, the level of water in which can be more readily

adjusted with that of the adjacent river or harbour, so that vessels may

be taken into the docks with the greatest despatch out of the reach of