Owing to the fact that earth for the construction of most of the huts in a bazaar has been excavated from the immediate vicinity of the buildings, it is a matter of much consideration to determine the most suitable levels the surface drains should start at, as it is of importance that these should be as shallow as circumstances admit of in order that they may obtain the full benefit of purification by sun and air. By adopting as high a level as possible at the head of the drain it enables better gradients, or falls, to be given which aids much in self-cleansing. The greatest care should be taken not to lose, or waste what little fall there is in the plains, as this is simply invaluable. Deep drains rapidly become foul at the sides and bottom, are difficult to flush owing to the quantity of water required to do so effectively, and great temptation is also given to adjacent house-holders to bridge them over with wide platforms, the consequence being that, sooner or later, the storm-water drain, designed as an open one, and for which it may originally have been more or less suitable, soon changes its character, and becomes a badly designed sewer, which imprisons and concentrates noxious effluvia. When a drain runs beneath a road, provision should be made for a part of it being easily uncovered to admit of examination and cleaning. All drains should open into others at acute, and not at right angles, and must join at top to top, and not at base level; where necessary the difference of level can be made up by falls.

The surface drainage of small roads and paths in a bazaar is best provided for by the construction of a central drain down the middle, to which the ground is made to slightly slope from each side; this prevents the accumulation of filth in the so-called side drains, which are generally merely long pits; and, provided a slight fall be given, they are self-cleaning at every shower of rain.

In paths or gullies a small concrete saucer drain can be constructed, at a cost not exceeding four annas a lineal foot, into which the house connections can be made,—the paths being paved with bricks, set flat, not costing more than Rs. 2–8 per 100 square feet. These should be laid at a good slope to the drain, and as only foot traffic need be provided for, is amply strong enough. In many cases it will be found that adjacent house-holders are quite willing to pay for this work, when once a commencement is made, and the advantage is obvious to them.

In kutcha roadside drains care must be taken that, in the process of cleaning, which generally consists in the removal of the bed, they do not in time become permanent roadside trenches without fall; it is much better to have no drains at all than this; mere depressions which, when dry, can be swept, and which will be washed clean after a heavy shower of rain, are much preferable. The proper bed-level of a kutcha drain should be permanently marked by wooden pegs driven well into the earth, and built into a masonry pillar, 1 foot 10 inches square, or by masonry profiles, at intervals of 100 feet apart; this shows at once if the bed-level or section has been unduly lowered during the process of cleaning out.

When designing a drainage system for a town it is essential that provision for flushing be made at the same time, otherwise the drains become receptacles for filth for about two-thirds of the year. This can be arranged by the construction of flushing tanks, which, when full automatically discharge into the drains, by water mains laid underground discharging into the head or summits of the drains; and also by the drains being divided up into convenient sections by stops or sluices, which are lifted automatically, or by manual labour, when the section is full of water, thus ensuring an effective flush.

A type design of an automatic flushing tank is shown in Fig. 1. These cost about Rs. 100 each complete, the syphon pipe alone representing some Rs. 30. Where sufficient head is not unavailable for this type, Adams’ Patent Adamic Flusher may be suitable. These cost about Rs. 70 for a 4 pipe size, and works with a head of 10 inches.

Where sullage water is discharged into storm-water drains this should be treated in sullage filters at convenient intervals. See Fig. 18, page 36.

The most suitable form for surface drains is the semi-circular base, with side slopes of 1 to 2·4, as the discharge is only slightly less than that of the ovoid section, and the drains are more easy to construct. They can be easily swept clean, or run through with a wooden board made to fit the section, and pushed along by a boy.

The Dacca type rectangular drain, as shown in Fig. 2, page 5, is very suitable for narrow lanes. The dimensions can be altered to suit local conditions.

Much useful information as to the preparation of drainage projects, and tables of discharges of different sections, will be found in Practical Instructions in Surface Drainage, by Mr. H. A. Gubbay, Executive Engineer, Public Works Department, published by the Government of Bengal.

In most cases, when designing a system of drainage, it is advisable merely to take the general surface level of the bazaar as the level to be drained, leaving artificially caused depressions to be filled up with the dÉbris of old buildings, and any available suitable material as opportunity occurs. It is also generally unnecessary to provide for a very heavy rainfall. The usual provision in this part of India is for a run off due to ½ an inch of rainfall per hour from densely built over, and ¼th of an inch from suburban areas.

It is more scientific to design the drains with reference to the possible flushing power and facilities available, rather than that of the maximum rainfall.

The importance of proper drainage, especially in connection with checking the spread of malarial fever by anopheles mosquitoes has, owing to recent researches, been fully recognised. These are found to breed most extensively in the earth-lined drains alongside streets. Where brick-lined, the current should be strong enough to wash away the larvÆ, but it is quite otherwise on the numerous kutcha ones in every bazaar. It is also essential, in Bengal, to make use of the powers conferred by the Municipal Act (Section 195) to compel owners to fill up small depressions which, during the rains, form extensive and numerous breeding-grounds. Every attempt should be made each year to brick-line a section of the roadside drains as money is available, where funds do not admit of much being done. Grass and weeds in the earthen drains must be cleared out at regular intervals during the rains, and the oftener the better. Mosquitoes of the culex tribe cannot also be disregarded, as these have been proved to convey elephantiasis and other diseases, and stegomyia, which abound in Lower Bengal, are the hosts of yellow fever.