STORM WATER DRAINAGE NETWORK IN DIGHA

Absence of a properly implemented drainage system is causing adverse effect on the hygienic condition in the town of Digha. The drains are too few and were constructed mainly to flush out excess water during high tides. More importantly, the drains also are carrying sullage water and the polluted water is discharged into the sea causing appreciable deterioration in the quality of sea water.

It is a fact that the area lying near the sea coast is mainly of silty sand allowing high rate of percolation of surface run off to ground for which widespread water logging is not in evidence in the area till now, yet with the town getting more populated day by day, incidence of tourists visit looking up along with hotels and institutional buildings getting concentrated in localized area, the need for commissioning an engineered drainage system has been badly felt to discharge the water efficiently into the sea without causing any harmful effect on the environment.

Such a drainage system will exclude any chance of water logging and at the same time will offer a clean and pleasant look for the town. Covered surface drains, to be built with brick masonry and concrete cover, have been designed and planned for implementation at Digha only to collect the surface run off and carry the water by gravity flow.

Sankarpur, being sparsely populated and having a very few hotels / lodges all surrounded by vast expanse of agricultural lands, has been left out of such drainage facility at present.

Construction of storm water drains is an allowed item under CRZ regulation.

To take care of the septic waste separate scheme under PHED has been developed for setting up of STP and development of underground sewerage lines.

For the storm water drainage scheme under DSDA, the project area is basically divided into 2 parts, one the seaside, and the other landside of the foreshore road. The seaside being sloped towards the sea, so it was advisable to follow the outfall at “Jatra Nala” the major outfall of this area.

But the seaside of the old Digha is too far to drain it to this Nala. Considering the proposed landscape of the beach front it was not advisable to drain it to the sea. It will be diverted towards the landside along the main road leading to the major outfall of the landside that is the canal along the newly constructed bye-pass. Most of the landside portions naturally tend to slope towards the drainage canal of the bypass making it an easy and natural drainage system without the help of any mechanical means.

As per the available data, an average intensity of rainfall of 50 mm/hr has been considered. The design has been done as per the rainfall data collected, time of concentration, the graph of the CPHEEO manual and the empirical formulas developed for the most towns situated in the Indian plains.                       

As per CPHEEO manual the residential, commercial and greenery has 75%, 90% and 20% respectively of imperviousness.

Considering the proposed development according to the CRZ rules, it was assumed that the project area will be divided into different zones.

·         Zone 1 : Already built to an average of 30-40% and will remain same.

·         Zone 2 : Already built to an average of 50-60% but considered to be 80% in future.

·         Zone 3 : Very less built form, with public buildings and so may develop to 30% built form in future.

·         Zone 4 : Existing open and green area on sea which will remain to be green (20%).

·         Zone 5 : Heavily built new digha area of average 50-60% but considered to be 90% in future.

·         Zone 6 : Existing open and green area on sea which will remain to be green (20%).

·         Zone 7 : Village area, but most prospective area for controlled development (30%).

Masonry covered drains has been considered for the storm water drainage of the project area. Considering the size of the drains, widths of the road, it has been proposed to cover the drain with precast perforated RCC slab. Cast iron gratings have been avoided considering theft, the cost aspect as well as the weathering effect of the sea. The size, number, orientation of the drains has been done absolutely as per Individual road conditions.

The different conditions of the drains are as follows.

·         Narrow roads will have one side drain with adequate RCC slab to take normal load.

·         RCC underground box drains to cater to the localized thumps of the road due to land form.

·         Totally RCC culvert type drains with flared ends at major road crossings.

·         Drop drains at steep fall in level of the road and invert level beyond 60 cm to maintain the self-cleansing velocity.

·         Ground levels, probable type of future development considered in designing the size and level of proposed drains.

·         No lifting / pumping station has been proposed.

·         The maximum ratio of 1:2 (width : depth) of the drain has been maintained all throughout the area for easy maintenance, construction and to avoid blocking of drains.

·         The major roads will have both side drains to facilitate better inlet of rainwater, as well as easy connection to individual plots.

The storm water drain have been designed with sufficient capacity to avoid frequent flooding of the drainage area and not designed for the peak flow of rare run of such as once in 50 years or more. There may be flooding when any natural calamity occurs or when the precipitation exceeds the design value, which has to be permitted. Though such flooding causes inconvenience, it may have to be accepted once in a while considering the cost economy.

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