Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers / Comprising the history of his professional life, together with reminiscences dating from the commencement of the century to the present time.

by Sir John Rennie

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.