WATER HARVESTING



CHAPTER-XI-1 (G.O.119 Contd..)

RAIN WATER HARVESTING STRUCTURES

GENERAL:

Rain water harvesting is the technique of collection and storage of rain water at surface or in sub-surface aquifers, before it is lost as surface run-off. The augmented resource can be harvested in the time of need.

Artificial recharge to ground water is a process by which the ground water reservoir is augmented at rate exceeding that under natural conditions of replenishment.

NEED:

To overcome the inadequacy of water to meet our demands.

To arrest decline in ground water levels.

To enhance availability of ground water at specific place and time and utilize rain water for sustainable development.

To increase infiltration of rain water in the subsoil which has decreased drastically in urban areas due to paving of open area

To improve ground water quality by dilution.

To increase agriculture production.

To improve ecology of the area by increase in vegetation cover, etc.

ADVANTAGES:

Cost of recharge to sub-surface reservoir is lower than surface reservoirs.

The aquifer serves as distribution system also.

No land is wasted for storage purpose and no population displacement is involved.

Ground water is not directly exposed to evaporation and pollution.

Storing water underground is environment friendly.

It increases the productivity of aquifer.

It reduces flood hazards.

Effects rise in ground water levels.

Mitigates the effects of drought.

Reduces soil erosion.

METHODS:

The methods of ground water recharge mainly are:

Urban Areas - Roof Top Rain Water /Storm runoff harvesting through

Recharge Pit

Recharge Trench

Tube well

Recharge Well

Rural Areas - Rain Water Harvesting through

Gully Plug

Contour Bund

Gabion Structure

Percolation tank

Check Dam/ Cement Plug/ Nala Bund

Recharge shaft

Dug well Recharge

Ground Water Dams/Subsurface Dyke

TECHNIQUES:

URBAN AREAS

In urban areas, rain water available from roof tops of buildings, paved and unpaved areas goes waste. This water can be recharged to aquifer and can be utilized gainfully at the time of need. The rain water harvesting system needs to be designed in a way that it does not occupy large space for collection and recharge system. A few techniques of roof top rain water harvesting in urban areas are described below.

ROOF TOP RAIN WATER HARVESTING THROUGH RECHARGE PIT

In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rain water harvesting can be done through recharge pits.

The technique is suitable for buildings having a roof area of 100 sq. m and are constructed for recharging the shallow aquifers.

Recharge Pits may be of any shape and size and are generally constructed 1 to 2m wide and 2 to 3m deep which are back filled with boulders (5-20 cm), gravels (5-10mm) and coarse sand (1.5-2mm) in graded form. Boulders at the bottom, gravels in between and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the coarse sand layer and can easily be removed. For smaller roof area, pit may be filled with broken bricks/ cobbles.

A mesh should be provided at the roof so that leaves or any other solid waste / debris is prevented from entering the pit and a de-silting /collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit.

The top layer of sand should be cleaned periodically to maintain the recharge rate.

By-pass arrangement has to be provided before the collection chamber to reject the first showers.

 

ROOF TOP RAIN WATER HARVESTING THROUGH RECHARGE TRENCH

Recharge trenches are suitable for buildings having roof area of 200-300 sq. m. and where permeable strata are available at shallow depths.

Trench may be 0.5 to 1 m wide, 1 to 1.5m deep and 10 to 20m long depending upon the availability of water to recharge.

These are back filled with boulders (5-20cm), gravel (5-10mm) and coarse sand (1.5-2mm) in graded form – boulders at the bottom, gravel in between and coarse sand at the top so that the silt content that will come with runoff will be coarse sand at the top of the sand layer and can easily be removed.

A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the trenches and a de-silting/collection chamber may also be provided on ground to arrest the flow of finer particles to the trench.

By-pass arrangement be provided before the collection chamber to reject the first showers.

The top layer of sand should be cleaned periodically to maintain the recharge rate.

ROOF TOP RAIN WATER HARVESTING THROUGH EXISTING TUBEWELLS

In areas where the shallow aquifers have dried up and existing tube wells are tapping deeper aquifer, roof to rain water harvesting through existing tube well can be adopted to recharge the deeper aquifers.

PVC pipes of 10cm dia are connected to roof drains to collect rainwater. The first roof runoff is let off through the bottom of drainpipe. After closing the bottom pipe, the rainwater of subsequent rain showers is taken through a T to an online PVC filter. The filter may be provided before water enters the tube wells. The filter is 1–1.2m in length and is made up of PVC pipe. It’s diameter should vary depending on the area of roof, 15cm if roof area is less than 150sq m and 20cm if the roof area is more. The filter is provided with a reducer of 6.25cm on both the sides. Filter is divided into three chambers by PVC screens so that filter material is not mixed up. The first chamber is filled up with gravel (6-10mm), middle chamber with pebbles (12-20mm) and last chamber with bigger pebbles (20-40mm).

If the roof area is more, a filter pit may be provided. Rainwater from roofs is taken to collection/de-silting chambers located on ground. These collection chambers are interconnected as well as connected to the filter pit through pipes having a slope of 1:15. The filter pit may vary in shape and size depending upon available runoff and are back-filled with graded material, boulder at the bottom, gravel in the middle and sand at the top with varying thickness (0.30-0.50m) and may be separated by screen. The pit is divided into two chambers, filter material in one chamber and other chamber is kept empty to accommodate excess filtered water and to monitor the quality of filtered water. A connecting pipe with recharge well is provided at the bottom of the pit for recharging of filtered water through well.

ROOF TOP RAIN WATER HARVESTING THROUGH TRENCH WITH RECHARGE WELL

In areas where the surface soil is impervious and large quantities of roof water or surface runoff is available within a very short period of heavy rainfall, the use of trench/ pits is made to store the water in a filter media and subsequently recharge to ground water through specially constructed recharge wells.

This technique is ideally suited for area where permeable horizon is within 3m below ground level.

Recharge well of 100-300 diameter is constructed to a depth of at least 3 to 5m below the water level. Based on the lithology of the area well assembly is designed with slotted pipe against the shallow and deeper aquifer.

A lateral trench of 1.5 to 3m width and 10 to 30m length, depending upon the availability of water is constructed with the recharge well in the centre.

The number of recharge wells in the trench can be decided on the basis of water availability and local vertical permeability of the rocks.

The trench is backfilled with boulders, gravels and coarse sand to act as a filter media for the recharge wells.

If the aquifer is available at greater depth say more than 20m, a shallow shaft of 2 to 5m diameter and 3-5m deep may be constructed depending upon availability of runoff. Inside the shaft a recharge well of 100-300mm dia is constructed for recharging the available water to the deeper aquifers. At the bottom of the shaft a filter media is provided to avoid choking of recharge well.

Procedure for Obtaining Building Permission for all commercial, public buildings and new buildings on plots of 300sq.m and above:

It is mandatory to construct Rain Water Harvesting Structures in all commercial, public buildings and new buildings on plots of 300sq.m and above. Clearance of plan for the construction of new buildings of the aforesaid categories shall only be given if they have a provision in the building design itself for Rain Water Harvesting Structures.

The applicant shall submit the Building Application duly paying necessary fee and charges along with the documents as prescribed. The detailed site plan and the location plan shall show the details of rain water harvesting structures proposed in the site and also submit the detailed cross sections of the rain water harvesting structures.

The applicant shall submit an undertaking that he shall construct the said structures before applying for the Occupancy Certificate and shall continuously maintain the structures for the purpose for which they are meant.

On submission of the Completion Certificate submitted by the applicant for issue of the Occupancy Certificate, the Commissioner/Person authorized for this purpose shall inspect the site to check the construction of Rain Water Harvesting Structures as per the sanctioned Building Plans.

The indicative provisions for rainwater harvesting by building types shall be as given in the Table below.

TABLE – 35

PROVISIONS FOR RAINWATER HARVESTING BY BUILDING TYPES

Sl. No

Category / Use

Area of the Plot (sq.m)

Provisions to be made

Other conditions

(A)

(B)

(C)

(D)

(E)

1.

 Residential Plotted Houses

 

 New Proposals

200 and above

Construction of Rain Water Harvesting Structure.

 Shall have emphasis on both storage and reuse.

2.

 Group Housing

 

New Proposals

 All Plot Sizes

(i) Construction of Rain Water

Harvesting Structure.          

(ii) Concrete paving to be avoided and permeable  Materials are to be used for all open parking spaces.                       

Should indicate the system of Storm Water Drainage, Rain Water Harvesting Structure and Recharging well.                   

3.

 Public and Semi Public Buildings

 

New Proposals

 All Plot Sizes

(i) Construction of Rain Water Harvesting Structure and storage.

(ii) Shall have Recharge Pits.                       

Shall  have emphasis  on  both storage and reuse.           

4.

 Commercial / Mixed Use

 

All Proposals

 All Plot Sizes

(i)  Construction   of   Rain   Water Harvesting Structure.

(ii)  Soft  landscape  provisions  and open spaces with percolation pits.

(iii) Common treatment plant to be made  part  of  the  integrated development.            

(i) Shall have emphasis on both storage and reuse.

(ii) Should indicate the system of Storm Water  Drainage, Rain Water Harvesting  Structure and Recharging well.                                           

5.

 Industrial

 

All Proposals

 All Plot Sizes

(i) Construction of Rain Water Harvesting Structure.

(ii) Soft landscape  provisions and open spaces with percolation pits.

(iii) Use of abandoned bore well for recharging of ground water.

(iv) Common treatment plant to be made part of the  integrated development.

(i) Should indicate the system of Storm Water Drainage, Rain Water Harvesting Structure and Recharging well.

(ii) Provision  should  be made not to  inject contaminate water

Into recharge structures in industrial areas and care is to be taken to keep such structures away from sewer lines, septic tanks, soak pits, land fill and other sources of contamination

6.

Other Proposals

All Plot Sizes

Similar as above

Similar as above

Payment of fees in case of Rain Water Harvesting Structures:

Necessary fees or charges shall be paid as levied for construction of Rain Water Harvesting Structures along with the Building Application.

Security Deposit:

The applicant shall deposit an amount as prescribed towards the construction of rain water harvesting structures along with the Building Application which is refundable on successful construction of the rain water harvesting structure and submission of detailed photographs and a certificate to that extent while applying for Occupancy Certificate.

In case of failure in construction of rain water harvesting structures the security deposit shall be forfeited and the ULB shall construct the rain water harvesting structures in the said premises duly utilizing the security deposit.

Penalty in case of failure in construction of Rain Water Harvesting Structures by the Owners:

In case of failure in construction of the rain water harvesting structures by the applicant, the ULB shall levy a penalty equivalent to additional 10% of Property Tax till the rain water harvesting structures are constructed and maintained.

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