The following article will guide you about how to design storm water and surface drains.
Design of Storm Water Drains:
In a separate sewerage system storm water (or rain water) is not mixed with the domestic or sanitary sewage but it is carried separately through storm water drains and disposed of into a large water body such as stream, lake or ocean.
The storm water drains are usually open drains constructed below the ground surface. The small drains are normally provided along either side of the roads. The water collected in the small drains is discharged into main drains which finally discharge it into the water body.
The F.S.L. of the storm drains has to be lower than the N.S.L. of the area drained by them for proper drainage. Further in order to permit gravity discharge the F.S.L. of the storm water drain should be higher than the F.S.L. or H.F.L. of water body into which the flow is discharged. However, if the F.S.L. of the storm water drain is lower than the F.S.L. or H.F.L. of the water body into which the flow is to be discharged, gravity flow cannot take place. In such circumstances the drain discharge has to be disposed of by pumping.
ADVERTISEMENTS:
For designing the city’s storm water drainage system, on the contour map of the area the positions of the drains and the water body are located. After deciding the alignments of the different drains, the catchment areas which will be drained by each drain are marked. The peak discharge expected in the drain (reach by reach) is then worked out for each drain by a suitable method.
An L-section of the drain is drawn and the F.S.L. of the drain is fixed keeping in view the natural surface level (N.S.L.) of the area to be drained and the permissible outfall full supply level (F.S.L.). The F.S.L. line at no place along the length of the drain should go above the N.S.L. line, as this may result in accumulation of storm water (or rain water) in that low lying area. However, sometimes, some encroachment above N.S.L. may be allowed, and in that case, the water from such low lying pockets will be removed by pumping into the gravity drain carrying the major discharge of the catchment.
After fixing the F.S.L. line and thus the slope of the water surface, the bed line (i.e., depth of the drain) has to be fixed.
The depth of the drain is decided from various considerations as indicated below:
ADVERTISEMENTS:
(i) The bed of the considered drain at the outfall point should not go below the bed of the water body into which it is discharged.
(ii) The depth will sometimes be guided by the availability of land for the construction of the drain. When available land width is less, the width of the drain has to be kept less and thus necessitating the construction of deeper drains.
Fixation of Depth of Large Open Drains:
Large open drains are either rectangular or trapezoidal in section. The depth of these drains is selected on the basis of certain empirical formulae and curves based on experimental results.
ADVERTISEMENTS:
These are given below:
(1) For drains having discharge up to 15 cumec-
y = 0.5 √B … (4.59)
Where
ADVERTISEMENTS:
y is depth of drain; and
B is width of drain
(2) For drains having discharge between 15 and 300 cumec-
The values of depth given in the following table for different values of discharges may be adopted.
(3) Central Water and Power Commission-
New-Delhi has recommended Bly ratio for unlined drains carrying discharges ranging from 0.3 to 300 cumec. These are given in the form of a graph between Bly ratio and discharge, as shown in Fig. 4.8.
Further the drains should be economical and the velocities of flow should be such that the sediment present in the flow does not settle and also the bed and sides of the drain are not eroded. In other words the velocities of flow in the drains should be both self-cleansing as well as non-scouring. The values of maximum permissible velocities in unlined as well as lined drains are given in Table 4.11.
The drain section can be designed by using Manning’s formula. Further a free board as suggested in Table 4.12 should be provided above the F.S.L of the drain, which fixes the bank levels on both sides of the drain.
An outfall structure is also required to be constructed for energy dissipation at the place where any drain discharges into another major drain. Further a regulator is also sometimes required to be provided at the outfall of the main drain so as to avoid back flow into the drain when the river or stream, into which it is discharging, is in high floods, and the drain cannot function.
Design of Surface Drains:
These are small size open drains which are sometimes used to carry away sullage and storm water (or rain water) from houses and discharge into the lateral or public sewer through screening chambers built at suitable junction points. The screening chambers prevent the escape of heavy floating matter, rags, waste paper, sticks, etc., into underground sewer.
The surface drains are normally laid along either side of the streets facing boundary walls of houses and buildings and provide a cheap and economical arrangement of collecting surface drainage excluding foul discharge from houses.
These drains have, therefore, been adopted by certain municipalities and local bodies having limited finances. These are also popular in rural areas and villages. However, these drains are open and exposed and are less hygienic. These drains also require more frequent cleaning as bigger size particles are not removed due to the difficulty in ensuring proper self-cleansing velocity.
The surface drains of following four cross-sectional shapes shown in Fig. 4.9 are commonly used:
(1) Rectangular surface drains
(2) Semi-circular surface drains
(3) U-shaped surface drains
(4) V-shaped surface drains
A brief description of each of these four types of surface drains is given below:
(1) Rectangular Surface Drains:
These drains are suitable for carrying large discharge. However, when depth of flow is small the required self-cleansing velocity is not developed in these drains, and hence these get easily clogged.
(2) Semi-Circular Surface Drains:
These drains can be easily constructed. The drains can be also formed by using readymade semi-circular sections of stone- ware or concrete or asbestos cement pipes. These drains are found to be suitable for small streets where the discharge to be carried is of small quantity.
(3) U-Shaped Surface Drains:
These drains are also easy to construct and they combine the advantages of semi-circular surface drains and rectangular surface drains.
(4) V-Shaped Surface Drains:
These drains possess better hydraulic properties, but these drains are difficult to construct. These drains will carry the fluctuating discharge without depositing solids at any point because of the fact that during fair weather, the less volume of sullage will occupy only the bottom portion of the drain and even then, these drains will be capable of producing self-cleansing velocity.
For efficient working, the surface drains should fulfill the following requirements:
(i) The inner surface of the drain should be made smooth. This is usually achieved by applying plaster to the inner surface of drain.
(ii) The joints of such drains should be properly and neatly finished.
(iii) These drains should be laid at such a gradient that self-cleansing velocity is developed.
(iv) These drains should have sufficient carrying capacity, and should have reasonable free board.
(v) These drains should be laid on easy curves.