about land subsidence ppt

1. The different concepts that will be covered
includes
− Artificial recharge techniques
− Surface spreading techniques
− Subsurface techniques
− Groundwater conservation techniques
− Rooftop rainwater collection and recharge of groundwater.

2. Artificial groundwater recharge technology
• The artificial groundwater
recharge technology can
be broadly categorized as
follows

3. • Except above, water conservation structures like dams, sub-surface dykes
(or locally termed as Bandharas) are entirely prevalent to capture sub-
surface flows.
• Similarly, in hard surface areas rock fracturing strategies such as
sectional blasting of boreholes with suitable techniques has been
operated to inter-connect the fractures and gear up the groundwater
recharge. Cement sealing of fractures, through specially built borewell
has been utilized in the state of Maharashtra to preserve sub-surface flow
and increase borewell yield.

4. ARTIFICIAL RECHARGE TECHNIQUES
Artificial recharge techniques consists of direct methods and indirect
methods.
Direct methods: involve techniques such as surface spreading
techniques and sub surfacetechniques.
Surface spreading techniques: involves methods where water is
spread on the surface. Thetechniques include flooding, ditch and
furrows, recharge basins, runoff conservation structures and stream
modification or augmentation. Bench terracing, contour bonds, gully
plugs, nalah bunds, check dams percolation ponds etc. constitutes the
various runoffconservation structures.

5. Subsurface techniques: involves processes where water is put in
underground wells. It consists of structures such as Injection wells
(Recharge wells), Gravity head recharge wells,Recharge pits and
shafts.
The different recharge techniques for increasingly deep permeable
materials involves
− The surface basin which involves surface spreading
− Excavated basin
− Trenches
− Shafts or vadose zone well which is deeper and
− Aquifer well – which takes the recharge water at a lower level to reach
the level of aquifers

6. Indirect methods: Direct methods involve increasing the amount of
water at a particular location whereas, in indirect methods, water is
drawn from other locations by creating a hydraulic gradient by
pumping.
If water is pumped from an area continuously, a hydraulic gradient will
get created and induce water to flow from surrounding areas through the
soil pores. This method is called Induced recharge from surface water
sources by pumping.
There are other indirect methods such as Aquifer modification bore
blasting or hydrofracturing is employed such that multiple aquifers are
connected and water can be transferred from one to another. Yield is
increased in this method.

7. Combined techniques: involves the use of both the surface
spreading technique as well as the subsurface techniques.
Example – Trench in an excavated basin
Groundwater conservation techniques: groundwater is
retained with different structures such as subsurface dykes
(similar to dams). Such underground structures are known as
bandharas.
Another groundwater conservation technique involves the
fracture sealing cementation techniques which involve the
sealing of fractures and preventing subsurface flows.

8. • These are designed such that the groundwater is retained for longer
periods in a particular basin or a watershed by arresting the subsurface
flow. Some of such traditional structures include groundwater dams or
subsurface dykes or underground bandharas. Fracture sealing
cementation techniques can also be employed.
• These structures can be constructed particularly across streams or
valleys of 150 to 200 meters wide and to create storage reservoirs with
suitable recharge conditions and low seepage losses. The wall has to
be constructed such that its bottom connects to the impermeable rock
so that water does not leak through below.
Trench depth:
− 5 m deep the bottom width 2 m.
− 15-20 m deep bottom width 5 m.

9. Various combinations of surface and sub-surface recharge methods may
be used in conjunction under favorable hydrogeological conditions for
optimum recharge of ground water reservoirs.
The selection of methods to be combined in such cases is site-specific.
Commonly adopted combination methods include recharge basins with
shafts, percolation ponds with recharge pits or shafts and induced
recharge with wells tapping multiple aquifers permitting water to flow
from upper to lower aquifer zones through the annular space between the
walls and casing (connector wells) etc.
Combination Methods

10. The water artificially recharged into an aquifer is immediately governed
by natural groundwater flow regime.It is necessary to adopt
groundwater conservation measures so that the recharged water remains
available when needed.
Groundwater Dams / Underground Barriers: A groundwater dam is a
sub-surface barrier across stream that retards the natural groundwater
flow of the system and stores water below ground surface to meet the
demands during the period of greatest need.
The main purpose of groundwater dam is to arrest the flow of
groundwater out of the sub-basin and increase the storage within the
aquifer.

11. Fracture-Sealing Cementation Technique:
• Fracture-sealing cementation is a suitable water conservation measure in dry
situations.
• The boreholes located on such zones prove productive but due to dissipation of
the limited storage along preferred flow planes, in case of adverse
topographical situation, these become dry by the end of winter or summer.
• This measure can also be used to prevent ingress of saline or polluted water
from a known source.
• Various combinations of surface and sub-surface recharge methods may be
used in conjunction under favourable hydrogeological conditions for optimum
recharge of groundwater reservoirs.

12. Groundwater Conservation Techniques:
 Ground water conservation techniques are intended to retain the ground water
for longer periods in the basin/watershed by arresting the sub-surface flow.
 The known techniques of ground water conservation are
a) Ground water dams/ sub-surface dykes/ Underground ‘Bandharas’ and
b) Fracture sealing Cementation techniques.
A sub-surface dyke / ground water dam is a sub-surface barrier constructed across
a streamchannel for arresting/retarding the ground water flow and increase the
ground water storage. At favorable locations, such dams can also be constructed
not only across streams, but in large areas of the valley as well for conserving
ground water.

13. The figure shows the concrete wall constructed to prevent the flow from the upper areas toward the stream
to allow for percolation. Subsurface dyke or groundwater dam acts as a barrier and arrests or retards the
groundwater flow and helps in increasing the groundwater storage in this particular area.

14. Sub-surface dyke
Sub-surface dyke is a sub-surface barrier/hidden check dam across a
stream or river which retards the natural subsurface/ground water flow
of the system and stores water below ground surface to meet the
demand during the period of need.
The main purpose of groundwater dam is to arrest the base flow and it
helps to maintain groundwater level in the sub-basin. This underground
structure arrest or minimize the base flow of water in the sand bed near
the valley portion so as to build-up the ground water storage in the
upstream side of the subsurface dyke.

15. Ground water has become the most important source of domestic,
industrial and irrigation supply in many regions.
Although the total amount of water on earth is generally assumed to
have remained virtually constant, the rapid growth in population,
development in agriculture and industries putting more stress on the
quality and quantity aspects of natural system.
The most suitable location for a sub-surface dyke is proposed should
have shallow impervious layer with wide valley and narrow outlet
having limited thickness of loose soil or porous rock on the top with
massive or impervious rock below.

16. Subsurface dykes
It is a sub-surface barrier across a stream which slows down the natural
sub_x0002_surface /groundwater flow of the system and capture water
beneath ground surface to meet the water demand.
• The main cause of groundwater dam is to capture the flow of
groundwater out of the sub-basin and increase the storage capacity of
the aquifer.
• Suitable in hard rocks or alluvium forested area.

17. Subsurface dyke for surface water harvesting

18. Artificial groundwater recharge structure at Rajgarh district in MP.

19. Ground Water Dams Or Sub-Surface Dykes Or Underground
Bandharas (UGB):
These are basically ground water conservation structures and are
effective to provide sustainability to groundwater structures by arresting
sub surface flow.
A ground water dam is a sub-surface barrier across stream which retards
the natural ground water flow of the system and stores water below
ground surface to meet the demands during the period of need.
The main purpose of ground water dam is to arrest the flow of ground
water out of the sub-basin and increase the storage within the aquifer.
By dowing so the water levels in upstream part of ground water dam
rises saturating the otherwise dry part of aquifer.

20.  Since the water is stored within the aquifer, submergence of land can
be avoided and land above reservoir can be utilised even after the
construction of the dam.
No evaporation loss from the reservoir takes place.
No siltation in the reservoir takes place.
The potential disaster like collapse of dams can be avoided
The aquifer to be replenished is generally one which is already over
exploited by tube well pumpage and the declining trend of water levels
in the aquifer has set in. Because of the confining layers of low
permeability the aquifer can not get natural replenishment from the
surface and needs direct injection through recharge wells.
The underground dam has following advantages: -

21. Artificial recharge of aquifers by injection well is also done in coastal
regions to arrest the ingress of sea water and to combat the problems of
land subsidence in areas where confined aquifers are heavily pumped.
In alluvial areas injection well recharging a single aquifer or multiple
aquifers can be constructed in a fashion similar to normal gravel packed
pumping well.
The only difference is that cement sealing of the upper section of the
well is done in order to prevent the injection pressures from forcing
leakage of water through the annular space of bore hole and well
assembly.
In hard rock areas casing and well screens may not be required. An
injection pipe with opening against the aquifer to be recharged may be
sufficient.

22. However, in case of number of permeable horizons separated by
impervious rocks like vesicular basalts or cavernous limestones, a
properly designed injection well may be constructed with slotted pipe
against the aquifer to be recharged.
In practice the injection rates are limited by the physical characteristics
of the aquifer. In the vicinity of well, the speed of groundwater flow
may increase to the point that the aquifer is eroded, specially if it is
made up of unconsolidated or semi-consolidated rocks.
In confined aquifer confining layers may fail if too great pressure is
created under them. If this occurs, the aquifer will become clogged in
the vicinity of the borehole and/or may collapse.

23. Advantages & Disadvantages:
Advantages:
I. The basic principle of the subsurface dyke is this: instead of storing the
water in the surface reservoirs, water can be injected and stored in
underground. It is composed of a cut-off wall by which the base flow is
dammed (or intrusion of the seawater is prevented),
II. And facilitates (like wells, intake, shaft and infiltration wells) that yield
good water which is stored in sand bed.
III. Water storage in subsurface dyke offers as a major advantage that
evaporation losses are much less for water stored underground.
IV. Since the subsurface dyke suffers virtually no loss of stored water from
evaporation, it is more advantageous than the surface dam in dry regions.

24. V. Further, risk of contamination of the stored water from the surface is reduced
because parasites cannot breed in underground water. Contamination of the
water by insects and animals cannot take place because the water is not visible
on the sand surface. Health hazards such as mosquito breeding are avoided.
VI. The technology is preferred by the community for several reasons: it
increases the capacity of traditional wells, it is simple and less expensive to
construct, replicable and easily maintainable by the community.
VII. Even unskilled workers also can take-up this work.
Disadvantages:
I. Since the utilization of stored groundwater in a subsurface dam requires
pumping, operating costs are higher than those of a surface dam.
II. The size of the voids between the solids of sand determines the capacity of
the basin and base flow. When the particle size is small, the water stored in the
sand bed will be reduced,
III. Survey and design require trained persons to avoid possible failures.

25. Suitability of Artificial Recharge Structures
under Combination of Factors:
Based on the discussions regarding various artificial recharge methods
and structures, an attempt has been made to prescribe structures
suitable for different slope categories, aquifer types and amount of
precipitation received.
A matrix (Table 1) has been developed for easy visualization of these
combinations and their possible variations. Three broad columns
represent three distinct hydrogeologic settings normally encountered
in nature.

26. Each of these columns is split further to represent areas based on the
adequacy of rainfall received. Areas receiving annual precipitation of
less than 1000 mm and not having access to any surface inflow source
are taken as areas with limited source water availability.
Four different slope categories have been considered in the matrix,
representing runoff zone, piedmont zone, transition zone and storage
zone. Indirectly, this classification also takes into account the status of
ground water flow in the aquifer. Within each row the upper box
represents the unconfined aquifers and the lower one represents for the
leaky confined and confined aquifers.

29. The matrix thus tries to separate out 48 different combinations, all of which may
not be relevant or suitable for effecting artificial recharge. Further, it is to be
remembered that in a natural situation there are smooth transitions of conditions
stipulated from one column or row to the other. Hence this tabulation will serve
the purpose of broadly identifying recommended method or structure. The final
choice should be governed by actual relevance of factors at a given site.

30. The important advantages of artificial recharge are:
It has no adverse social impacts such as displacement of population, loss of scarce agricultural land etc.
The technology is appropriate and generally well understood by both the technologists and the general
population.
It is environment friendly, controls soil erosion and flood and provides sufficient soil moisture even
during summer months.
Groundwater recharge stores water during the wet season for use in the dry season when demand is the
highest.
The quality of the aquifer water can be improved by recharging with high-quality injected water.
Recharge can significantly increase the sustainable yield of an aquifer.
Recharge methods are environmentally attractive, particularly in arid regions.
Most aquifer recharge systems are easy to operate.
In many river basins, control of surface_x0002_water run-off to provide aquifer recharge reduces
sedimentation problems.
Results in energy saving due to reduction in suction and delivery head as a result of rise in water levels.

31. Disadvantages of artificial recharge are:
There are a number of problems associated with the use of artificial recharge
techniques.
These include disadvantages related to aspects such as recovery efficiency
(e.g., not all of the added water may be recoverable),
cost effectiveness,
contamination risks due to injection of recharge water of poor quality,
clogging of aquifers, and a lack of knowledge about the long term
implications of the recharge process.
Hence, careful consideration should be given to the selection of an appropriate
site for artificial recharge in a specific area.
Thus, there is a need for further research and development of artificial recharge
techniques for a variety of conditions. In addition, the economic, managerial
and institutional aspects of artificial recharge projects need to be studied
further.

32. Benefits of groundwater recharge
 There are following advantages of artificial recharging of groundwater aquifers:
 Subsurface storage space is available free of cost and inundation is avoided
Evaporation losses are negligible and temperature variations are minimum
Quality improvement by infiltration through the permeable media
It has no adverse social impacts such as displacement of population, loss of scarce
agricultural land etc.
It is a environment friendly technology that controls soil erosion and flood like situations, and
provides sufficient soil moisture during dry spell or water deficit conditions.
Water stored in soil profile is relatively immune to natural and man-made catastrophes.