Present Status of Constructed Percolation...
Transcript of Present Status of Constructed Percolation...
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Chapter – III
Present Status of Constructed Percolation Tanks
The State of Maharashtra is divided into five river basins, namely
Godavari, Krishna, Tapi Narmada, and westerly flowing rivers in the
Konkan Coastal Strip (Fig. 1.8). The total area covered under these basins
is 30.88 million hectares (mha), of which 22.54 mha can be cultivated.
The water availability in these river basins annually on an average is
131.56 Billion Cubic Metre (BCM). To harness the above surface water
potential, various major dams, medium projects and minor tanks have
been constructed. Water from the dams is supplied for the different needs
of agriculture, domestic and industrial. At the strategic level, Water
Resource Development (Erstwhile, Irrigation Department, Government of
Maharashtra) through five Irrigation Development Corporations (IDCs) is
responsible for managing the surface water resources and it allocates
water for irrigation, industrial, drinking and sanitation, purpose. A major
portion of the water is consumed by the irrigation sector in the state of
Maharashtra. It has increased gradually from 13.98 BCM during 2002-03
(FY) to 16.49 BCM during 2006-07 (FY). Minor Irrigation Projects, local
sector of the State completed 32 major dams, 178 medium projects and
approximately 2274 minor tanks till the end of June 2005. Maharashtra
ranks fifth from the top in descending order of ultimate irrigation
potential (Ref. NARAA). It is important to note that Punjab, Haryana,
Tamil Nadu and Karnataka have only about half of the ultimate irrigation
potential of Maharashtra and yet Maharashtra has a lower share of area
under irrigation. This is in some way a clear reflection of the failure of
efforts to tap the available potential as well as the difficult topography of
the state.
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Three significant points emerge from this discussion which are,
1. The potential of minor irrigation covers a larger portion of the
ultimate irrigation potential, hence focus should be directed towards
this source.
2. Despite the numerous committees and commissions and volumes of
work on irrigation, the state could utilise only 39.3 percent of its
ultimate irrigation potential compared with the share of potential
utilised by Tamil Nadu (64.6 per cent), Rajasthan (82.7 per cent) and
Gujarat (51.5 per cent).
In this context and notwithstanding the controversy on potential vis-a-vis
utilisation, a question arises about the intensity of efforts in tapping the
potential. In a comparison of the States across the Nation, Maharashtra
has about 15.8 per cent of the total capacity of live storage of water
created in the country (including proposed), which is the second highest
in the country. Thus Maharashtra does not seem to have fallen short in
creating the water storage or at least the State is comfortably placed at the
top rank as far as the creation of the storage capacity is concerned. But in
terms of achievements, the proportion of cultivated area under irrigation
is less than 15 per cent of gross cropped area. Thus the situation is that,
the State has the highest number of dams, high storage capacity created,
but still can claim only one of the bottom ranks in proportion of
cultivated area under irrigation. In addition to surface water, groundwater
constitutes to be a large part of the water supply, especially in rural areas
of Maharashtra, where over 50% of the total water use comes from
groundwater (NRAA, March 2011).
The groundwater resources are regulated and monitored by the
Groundwater Survey & Development Agency under the Water Supply
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and Sanitation Department, Government of Maharashtra. Groundwater
development and management in hard rock aquifer areas, in India and
many other countries have traditionally played a secondary role compared
to that in the areas having high-yielding unconsolidated or semi-
consolidated sediments and carbonate rocks. This has been due to the
relatively poor groundwater resources in hard rocks, low specific capacity
of wells, erratic variations and discontinuities in the aquifer properties
and the difficulties in exploration and quantitative assessment of the
resource.
The Groundwater development in Maharashtra has been traditionally
neglected with the assumption of the relatively poor groundwater
resources in hard rocks, low to moderate yield of wells, erratic behaviour
and variations in the hydrogeological characters of basaltic aquifers and
the limitations in exploration and quantitative assessment. Groundwater
occurring under the basaltic terrain in Maharashtra resides in the soft
mantle of weathered rock under phreatic condition. Under this soft
mantle, groundwater is mostly in semi-confined state in the fissures,
fractures, cracks, and joints. (Deolankar 1980) In basaltic terrain the lava
flow junctions and weathered red tachylitic basalt pockets sandwiched
between two layers of lava flows, also provide additional porosity. The
ratio of the volume of water stored under semi-confined condition within
the body of the hard rock, to the volume of water in the overlying
phreatic aquifer depends on local geohydrological conditions.
A possible management solution, encouraged by Government agencies
over the recent years, is to intervene on the supply side of the
groundwater balance, i.e. by artificial recharge. Artificial recharge
structures encompasses percolation tanks, check dams and, injection
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wells to recharge around defunct dug wells. In many places, a high
density of tanks exists as they were the traditional water source for many
centuries. Artificial recharge is widely promoted through construction of
percolation tanks. The residence time of water in the basins is thus
increased from a few months to a few years and the percolated water is
available in the wells even during the summer season of a drought year
Irrigation based on groundwater, that is extracted through shallow wells
is one of the age old established practice in most of the semi-arid tropical
regions of India. Natural depressions served as an important source of
groundwater recharge. Ponds refer to a reservoir impounding run-off
water behind earthen/cement bunds and embankments constructed across
the slope to harvest and store water in the rainy season, and to use it for
irrigation through water extraction structures like open and bore well, and
other purposes. Village ponds are historical innovation to address
monsoon irregularities and reduce the risk of uncertainties in water
availability in the dry zones. As per the 4th Minor Irrigation Census that
refers to the year 2006-2007, about 6 lakh tanks and storages are created
in the country with 58.9 lakh ha of irrigation potential out of which 39.31
lakh ha has been utilized.
3.1 Percolation Tank
A percolation tank can be defined as an artificially created surface water
body submerging a land area so that the surface runoff is made to
percolate and recharge the groundwater storage (Fig. 3.1). They are not
provided with sluices or outlets for discharging water from the tank for
irrigation or other purposes, but as a safety measure they are provided
with arrangements for spilling away the surplus water that enters the tank
in case of heavy rain spells, so as to avoid over-topping of the tank bund.
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The purpose of percolation tank is to conserve the surface runoff and
recharge groundwater storage. The objective for the construction of these
tanks is to accumulate water and overcome the difficulty of erratic
monsoon in the changing climate scenario. The water collected in these
tanks shall be useful in recharging the groundwater and if needed it can
be used for protective irrigation in the dry spells. Thus the water
accumulated in the tank after the monsoon is made to percolate at the
earliest, without much loss due to evaporation (Fig. 1.11).
Fig. 3.1 What is a percolation tank?
Percolation tanks are to be normally constructed on second or third order
streams, as the catchment area of such streams would be of optimum size.
The field conditions which are necessarily studied during the
identification of the site for construction of percolation tanks are
described below.
The positioning of the tank and its submergence area should be in
non-arable land and in natural depressions requiring lesser land
acquisition.
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There should be cultivable land in the downstream area of the tank in
its command with a number of wells to ensure maximum benefit by
such efforts.
Steps should be taken to prevent severe soil erosion through
appropriate soil conservation measures in the catchment. This will
keep the tank free from siltation which otherwise reduces the storage
capacity resulting in a decrease in percolation efficiency and life of
the structure.
The initial efficiency of a percolation tank is reduced due to silting of its
bottom by receiving muddy runoff from the watershed. If the watershed
is well-forested and has a cover of grass, bushes and crops, the silting
is minimal. But in an average of 5 to 6 Monsoon seasons the tank bed
accumulates about 0.20 to 1.00 meters of silt. Silt reduces the storage
capacity of the tank and also impedes the rate of vertical flow of
recharge because of its low infiltration rate. The efficiency gets reduced
due to silting hence de-silting of tank bed becomes necessary when it
dries in summer. (Limaye S D. 2010).
3.2 Design of Percolation Tank
A percolation tank, like an irrigation tank, has a structure to impound
rainwater flowing through a watershed, and a waste weir to dispose of the
surplus flow in excess of the storage capacity of the lake created. A
masonry waste weir is also necessary to pass surplus water. A percolation
tank is designed to ensure maximum capacity utilization, long life span,
cost effectiveness and optimum recharge to groundwater. Storage
capacity, waste weir, drainage arrangement and cut off trench (COT) are
the important features of percolation tank which need proper design. The
overall design of the percolation tank is similar to an earthen dam
constructed for minor irrigation without the canal.
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Fig. 3.2 Cross section of a bund for percolation tank
These are the most prevalent structures in India as a measure to recharge
the groundwater reservoir in hard rock formations. The efficacy and
feasibility of these structures are more in hard rock formation where the
rocks are highly fractured and weathered. The catchment yield and basin
configuration drawn from topographic surveys at site, determines the
height of the percolation tank. The top of the dam wall is normally kept
2-3 m wide. Upstream and downstream sides of the dam wall are
normally taken as 2.5:1 and 2:1 respectively.
The hydrogeological condition of the site for percolation tank is of utmost
importance. The subsurface rock strata in the submergence area should
have high permeability. The degree and extent of weathering of rocks
should be uniform and not just localized.
The objective of this study is to compare the contribution of artificial
recharge based on the hydrological characters of the various formations to
the overall recharge and discuss alternative options.
To undertake a comprehensive survey of the existing percolation tank and
study and map their inefficiency if any, a study was carried out and the
information was collected through a simple survey form which is given
below.
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Annexure 3.1
Geo hydrological Studies of poorly permeable
Percolation Tanks in Deccan Basaltic Terrain
Questionnaire to short List Percolation Tanks
Village ………… Tahsil …………… District ……………
Gut No. …………… Name of the Farmer ……………….
Well No. ……………
Location of the well ………… Farm,
Bank of Nala, In the Nala
Location Name ……………
User ………… Personal Community
Year of the Digging ………..
Construction year If yes type …… parapet Ht. ………
Shape ………… Circular Square Depth …………
Water level from ground …………… m.
In rainy season …………… winter ……………
Percolation : Bottom / Lateral
Direction in case of lateral
If the bore is taken in horizontal direction, Length..… m.
and or vertical Depth of vertical bore …………….. m
Location at the bottom …….
Use - Drinking Irrigation ……….. Acre.
Rainy seasons ………………. Acre
Winter seasons ………………. Acre
Summer seasons ………………. Acre
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Type of withdrawals Electric motor Diesel Pump
HP ………………. dia. of outlet ……………….. cm.
Quantity of withdrawals Daily …….. Hr. Seasonal
………………... day
Time require for a full recharge
Rainy season ……….. Hrs. Summer………….. Hrs.
winter …………… Hrs.
Name of the surveyor Signature
A detail analysis was made of the data collected from this well survey.
(Fig. 3.3) Consequently the percolation tanks were shortlisted for the low
performance. After analysis of the primary data collected the reasons for
the lower efficiency of the percolation tanks are discussed as follows.
3.3 Reasons for Lower Efficiency
The reasons that contribute to low recharge efficiency can be identified as
follows.
3.3.1 Inadequate catchment : During this study it was observed that some
of the percolation tanks are constructed not at proper site in spite of the
sufficient availability of catchment from where the surface runoff could
be directed towards the tank. For example the Percolation tank of village
Pathri, block Phulambri of Aurangabad District, constructed towards the
western side of the Aurangabad Sillod road demonstrate this condition. It
hardly receives any water even during good monsoon due to the
insufficient catchment area (Fig. 3.4). It was also observed, that at places,
the catchment is disconnected because of the construction of the road or
urbanisation in the catchment areas. These kinds of tanks become defunct
and hardly collect any amount of water.
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Fig. 3.3 Well survey in progress to
shortlist percolation tanks
Fig. 3.4 Percolation tank at village Pathri, Tq. Phulambri,
hardly receives any water due to insufficient
catchment area
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3.3.2 Inadequate attention to the geohydrology : It was observed that
most of the percolation tanks are constructed as per the suitability of the
site from the impounding point of view. A percolation tank which is
primarily constructed for the recharging the groundwater is completely
ignored towards the geohydrological study, surface geology as well as
subsurface geohydrology, while finalising the location. The percolation
tank constructed without paying due attention to the geohydrology, many
times become a the storage tank (Fig. 3.5).
For example percolation tank of Pokhri located in Jafrabad Taluka of
Jalna district is constructed on a gently sloping topography with the good
catchment area. The south and southeastern part of the village bears few
hillocks which generate run off during the monsoon. The top soil layer
varying from the 0 to 4 m is followed by vesicular basalt up to 9 m. This
is underlain by massive basalt which is partially fractured and devoid of
joints. Therefore the percolation through this tank is restricted up to a
depth of about 10-12 m. This is quite evident from the impounded water
in the summer months also. This tank is getting filled up to the capacity
in the rainy season, but percolation from this tank is restricted due to
adverse geohydrological condition and it almost serves as a storage tank.
It is expected that due to percolation of water the tank should become
empty in the month of February. Out of the surveyed 45 tanks, 30 ( i.e.
65%) tanks have never become empty in the month of February or
summer since the time they were constructed. From this it can be
concluded that the selection of sites of such tanks were not proper from
the geohydrological point of view.
The importance of the geohydrology is discussed in detail and is
presented in this study.
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3.3.3 Incomplete/Faulty construction : The construction of many of the
percolation tanks is incomplete. Even though the gorge is not filled up the
embankments are complete, due to which all the water accumulated in the
percolation tanks flows down through the gorge area. Some of the tanks
are not provided with the proper spillway due to which the water
accumulated in the percolation tank spreads on the surrounding low lying
area and finally finds its route towards the downstream. This also causes
lots of erosion of the soil in the surrounding areas. The percolation tanks
for want of proper attention paid to the strengthening of the Hearting and
Casing, are washed away in the heavy intensity rain spells (Fig. 3.6).
Such tanks without the proper consideration of the geohydrological,
topographic and engineering aspects can be categorised as faulty and
unsuccessful.
Out of the two percolation tanks located in Kerul, Tq. Ashti, District
Beed one was found to be empty by the end of November month due to
heavy leakages from the main wall of the percolation tank. The reason
behind this state of affairs is improper or faulty construction of the dam
wall right at the centre.
3.3.4 Silting of percolation tanks : It was also observed that, the rate of
silting, was more than the expectation which could be attributed to
untreated upper catchments of the tanks. The initial storage capacity and
efficiency of a percolation tank is reduced due to silting of its bottom
through muddy runoff from the watershed. On an average of 5 to 6
Monsoon seasons the tank bed accumulates about 0.20 to 1.00 meters of
silt. Silt reduces the storage capacity of the tank and also impedes the rate
of vertical flow of recharge because of its low permeability causing a loss
to groundwater potential.
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Fig. 3.5 Poorly permeable percolation tank turned into a
storage tank where water stands till the advent of
summer
Fig. 3.6 Casing of percolation tank had been washed away
following spells of heavy rains due to faulty
construction
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The percolation tank located near Pachan wadgaon and Sarvadi located in
Taluka (Tq. ) and Dist. Jalna have become defunct due siltation (Fig. 3.7).
The untreated upper catchments bring in the huge amount of silt in the
water during the rainy season and it gets deposited in the percolation
tank. The silting reduces the infiltration of the water into the subsurface
though geohydrology of the area is favourable.
3.3.5 Poor / lack of repairs and maintenance : Lack of repair and
maintenance of the percolation tank also reduces its efficiency. At some
places it was found that the spillway is either broken or completely
diminished due to which the water cannot be stored in the tank. In a few
of the places it was noticed that out of 45 tanks studied 7 to 8 need to be
repaired. The leakages in the dam wall were also observed in some of the
tanks (Fig. 3.8).
The percolation tank located in village Asai Tq. Jafrabad Dist Jalna was
observed to be leaking from the centre of the dam wall. The leakage was
found to be from the casing of the percolation tank and was in a sorry
state of affair for the want of repair.
3.3.6 Inappropriate site location : The percolation tank is constructed to
serve as a groundwater recharge structure. The site for this structure is
selected so as the open wells or bore wells in the near vicinity, used for
the irrigation should get maximum benefit. It was observed at a few
places that the percolation tanks are constructed at lower elevations where
as the agricultural lands and the open wells located in it are at higher
elevation than that of the percolation tank (Fig. 3.9).
The percolation tank in Chincholi Budruk Tq Phulambri Dist.
Aurangabad is constructed at the lower elevation than that of the village.
In other words the village and its agricultural land; fall in the catchment
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Fig. 3.7 Percolation tank at Panchan Wadaon has become
defunct due to siltation
Fig. 3.8 Leakages in the dam wall owing to lack of
maintenance
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of the percolation tank. Hence the wells located in these agricultural land
do not get any benefit from this percolation tank though the subsurface
strata is favourable for percolation of stored water in the tank.
After ascertaining the failure/success of the percolation tanks the
percolation tanks were shortlisted on the basis of the performance
attributing to the geohydrological condition. The details of the total,
Percolation tanks surveyed and shortlisted for the detail survey are given
in the table below.
The physical dimensions of the percolation tank were also measured to
know the impounding area of the tank.
Table 3.1 Details of the PT surveyed and studied in detail
Sr.
No.
Name of the
district
No. of
PT.
No. of PT. surveyed
for short listing
Detailed
PT. Studied
1. Aurangabad 1393 12 3
2. Jalna 487 10 4
3. Beed 1006 08 2
4. Osmanabad 788 08 3
5. Latur 606 07 2
Total 4280 45 14
To assess the detail geohydrological condition of these tanks, a detail well
inventory of the surrounding wells a detail well inventory was carried out
in the surrounding areas by demarcating representative deep wells. It was
ensured that all the wells in the downstream were below the level of the
bottom of percolation tank and the geological lithologs were drawn was
carried out as per the form given below in Annexure 3.2. The wells which
were studied in detail through well inventory were plotted accurately on
the cadastral map of the respective village.
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Fig. 3.9 Percolation tank constructed at a lower level than
the adjacent agricultural lands and the open wells
Fig. 3.10 Geohydrological study of open well
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Annexure-3.2
Geo hydrological Studies of poorly permeable
Percolation Tanks in Deccan Basaltic Terrain
Well Inventory Form
Date :- ……./……./………
1. Name of the Village………………… .
Taluka…………… District. ………………….
2. Toposheet No. ……………. . Coordinates: …………….
Census Code ……………….
3. Population …………………. Altitude range ……………
Area of village ………………
4. Accessibility to Village and percolation tank.
a) Village from main town ………. km. along road …………….
b) Interior road from ………. direction N/S/E/W……………. km.
c) Percolation tanks from the N/S/E/W Distance : ……………….
d) Near gut No. …… right / left / exactly downstream or ……….
5. Percolation tank study
a) Area of village under influence of percolation tank …………….
b) Total submerged area of percolation tank in September ………
c) Total submerged area of percolation tank in March / April …. 25%
d) Catchment area, degraded / steep / gentle / uneven or …………
e) Construction related leakage from ………. Location ………….
Rate of flow: …………………. Quantity …………………. if any.
……….………………….……………………..….………………
f) Field characters of rock type exposed in submergence and at right
and left banks of Percolation Tanks ………………….……….
g) Siltation Thickness …………… m. type …………..
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6. Geomorphology of the area:
a) Hills / hillocks slope / steep / gentle ………………….
b) Main stream flow N/S/E/W ……………………….….
c) Drainage pattern ……………………………………...
d) Location of Percolation Tanks ……………………….
e) Any other ………………………………………….….
7. Soil formation of the area.
a) Black cotton soil. Area / location ………… depth ……….. m.
b) Sandy / Yellow / coarse soil / canker formation / mimic soil
(gravels) waterlogged soil any other ………………………..
Area / location …………………… depth …………… m
8.Geo-hydrological survey, observations.
a) Surface geology….………………….
i. Traverse along ………………….
ii. Exposures / Flow demarcation ………………….
iii. Type of basalt ………………….
iv. If jointed, closely spaced / broadly spaced : ………………….
v. If weathered, spheroidal / sheet / or any ………………….
Geo-hydrological characters (from well section)
i) Compact basalt
1) Thickness (from G. L.) ……….. m. from ……… to …….m.
2) Jointing pattern– closely spaced/broadly spaced open/closed
Columnar / vertical 3 sets / Interconnected or Not. …………
3) Fresh /weathered. If weathered type – highly / moderately /
Spheroidal / any other. ………………..
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ii) Amygdaloidal basalt
1) Thickness (from G.L.) ………. m. From ……… to …… m
2) Fresh / Weathered / highly / moderately sheeted or any
other…………………
iii) Hydrothermally Altered Basalt.
1) Thickness (from G.L. …….. m., From ……… to ……..
2) Fresh / weathered, highly / moderately or any:
iv) Tachylytic Basalt
1) Thickness from (G.L.) …….. m., From ……… to ……… m.
2) Red / Green / black
Fresh or weathered / highly / moderately / sheeted or any
other…………………….
v) Volcanic Breccia
1) Thickness (from G.L.) ……….. m. from …….. to ……….. m.
2) Fresh / weathered, highly / moderately. Any …………………
3) Matrix of zeolite / lava / tachylitic or any other.
9. Well inventory
a) Total number of wells in the village: ……………….
b) Wells before percolation tank construction: …………………
c) Wells after percolation tank construction : …………………..
d) Total No. of wells getting benefit from percolation tank :……
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10. Classification of wells according to:
a) High yield: Well No.…………. total………….
b) Depth (+ 10 m) :- Well No. ………….total ………….
c) Different section :- Well No. ………….total ………….
d) Perennial well :- Well No. ………….total ………….
e) Seasonal well :- Well No. ………….total ………….
f) Pump 2-3 / hrs (summer) : Well No. ………….total ………….
11. Measurement Details of Percolation Tank
a) Height of Percolation Tanks …………….. .
b) Length of wall ……………...
c) Width of wall at the bottom ……………..
d) Width of wall at the top ……………..
e) Height of spillway – ……………...
f) Length of the spillway ……………..
Various open wells observed and studied during this detail
geohydrological study were selected from the immediate downstream of
the percolation tank (Fig. 3.10). The deeper wells which were exhibiting
distinct geological section were considered. The immediate adjacent
wells and with similar geological structures were not considered. A
comprehensive format was designed for the geohydrological study, which
also includes various details like geographic area of the village,
accessibility to the percolation tank, soil formation and measurement
details of the percolation tank apart from the geological and
geohydrological details.
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During the field study and filling up the information in the ‘well
inventory form’, the village in which the percolation tank is located was
identified and marked on the toposheet accurately and Mean Sea Level
(MSL) of the village was ascertained. A revenue map of the village was
obtained from the revenue department or Gram Panchayat and the
percolation tank under study was marked on that map with respective gat
numbers. The surrounding gat numbers were also collected during the
well inventory so as to confirm the location of the percolation tank on the
village revenue map.
The population of the village and the area of the village was noted which
could lead to the requirement of water for the irrigation.
Accessibility to the village and the shortlisted percolation tank was
assessed and distance and direction for the approach of the percolation
tank was also noted. This was carefully done because in some of the
villages there were more than two percolation tanks located in different
directions and locations. This detail geographical information was also
recorded during the geohydrological study about the road, direction and
distance of the percolation tank from the village. This helped to avoid the
confusion where more than one percolation tank in a village was existing.
Area of the village and the area under the influence of the percolation
tank was noted either from the secondary information available with the
revenue officer of the village or during the group discussion. On the field
visit to the tank the total area under submergence after the monsoon (in
the month of September) and during the post monsoon (in the month of
March) was noted. This was very useful to assess the permeability and
the geohydrological condition of the percolation tanks.
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The nature of catchment plays an important role in bringing the run off to
the percolation tank. Hence, the area of the catchment was also assessed
along with the type of the catchment like degraded and steep or gentle
catchment. During the focused group discussion if the villagers reported
that the percolation tank was not holding the water till the month of
February and also the surrounding area was not getting benefitted, then
the possibility of leakages or faulty construction was assessed. The
leakages if any, were located on the map and approximate quantity of
water along with the rate of flow was also noted. At few places, though
the geohydrological condition and the catchment were favourable but no
impact was seen. In such situation the percolation tank was found to be
filled up with the silt. The thickness of siltation at various locations was
also noted in the form (Fig. 3.11).
Geomorphology of the surrounding area was assessed through the
toposheet as well as on the field. The stream flows drainage pattern with
regard to the position of the percolation tank was critically examined.
Soil layer has an important role while percolating the water to the
subsurface. The impounded water in the percolation tank has to pass
through the soil layer to contribute to the groundwater. The penetration of
the water depends on the soil character. Different soils like black cotton
soil, sandy soil, coarse soil, Kankar formation have different ability to
percolate water through them. The type of soil type was noted along with
the thickness so as to understand their role in groundwater recharge.
In the Geohydrological observations, Surface geology of the area was
ascertained with the surface exposures seen along the Nala sections or
along the hills where ever distinctly visible. On the basis of the
demarcation criteria like irregular top surface, pipe amygdales at the
bottom of the flow etc. demarcation was done and their superimposition
was carefully noted. This could give a brief idea about the geology and
types of flows occurring in the region.
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Fig. 3.11 Percolation tank with excessive silt deposition
Fig. 3.12 Topographic survey of percolation tank
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After identification of the flow and their geohydrological characters were
also noted. The joint spacing in the case of compact basalt flows, whereas
sheet or spheroidal jointing in case of amygdaloidal basalt flows was also
noted. This geohydrological character like jointing pattern, weathering
were very useful to understand the occurrence and availability of the
groundwater in that particular rock formation. The degree and extent of
weathering were useful for understanding the influence of geology in
recharging the shallow aquifer.
After completion of the detailed geohydrological study of the short listed
percolation tanks, a topographic survey was carried out in the command
area of the percolation tanks (Fig. 3.12). The fly levels were taken in the
surrounding area specially marking the representative wells in which the
contact of the two flows was visible. The levels were converted to the
MSL with the help of the toposheet and the Geo-Hydrological sections of
selected percolation tanks were drawn. The detail percolation tank wise
discussion and geohydrology is discussed in the next chapter.
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