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= Aquifer thickness in ft. H du = Draw up in ft. r o = Radius of influence in ft. r w = Radius of well in ft. Diminishing rate of recharge : Based on above discussion it can be said that the recharge will stop when the effective head H eff becomes zero. This will happen due to several reasons – (1) Increasing aquifer hydrostatic pressure H aq (2)Falling recharge water level H R (3) Increasing frictional losses due to choking of filter pit media, screens, rusting of casing pipes, chocking of gravel pack and formation. Assuming all other factors remaining same an important phenomena takes place continuously is that with the recharging of aquifer the hydrostatic pressure of the aquifer keeps increasing if there is no pumping from that aquifer or if the rate of transportation of water from the recharge point to the pumping point in the command area is lesser than the recharge rate. This will cause diminishing rate of recharge whereby the rate of recharge will gradually keep falling upto a point where it equals the rate of transportation of water from the recharge point to the pumping point. At this point an equilibrium state will occur and recharge will continue at this rate if all other conditions remain constant. Therefore for a sustainable recharge rate our site selection must ensure the rate of transportation of water from the recharge point to the point of use as close as possible to the recharge rate. Upper limit of Recharge pressure on aquifer : The maximum pressure of recharge water being injected into the aquifer shall be such that it does not cause fracturing of the formation of the aquifer. If fracturing occurs then there is usually a severe loss in hydraulic conductivity because the bedding planes are disturbed. On the other hand while recharging into massive consolidated rock formation fracturing may increase the rate of recharge (ref. Howard and Fast,1970). The pressure that will cause fracturing varies widely depending upon the nature of formation and the designer must determine this prior to designing the recharge system. Fracturing pressure ranges from as low as 11.3 k Pa/Meter for poorly consolidated coastal plain 368 sediments to 27.1 k Pa/Meter for crystalline rock. (Warner and Lehr,1981). As a general guideline the for most recharge wells in unconsolidated sediments the recharge pressure shall be controlled so that the positive head does not exceed a value equal to 0.2 times the depth from the ground level to the top of the screen or filter pack. (Olsthoorn, 1982). � Design considerations : Following are the major elements of a recharge well – • The Filter pit or collection pit as it is popularly known. • Well casing • Well screens • Filter/Gravel pack Now we will discuss important design considerations while designing these elements- The Filter pit FUNCTION OF THE FILTER PIT • Main function is to reduce turbidity of raw water to reduce chocking of gravel pack • Keep feeding filtered water to Recharge well IMPORTANCE OF FILTER PIT • To sustain the Recharge process • To maintain the Recharge rate DESIGN CONSIDERATION • Filter media should not get clogged • The System to collect percolated water should have following features – 1. Large % open area for minimal frictional head loss. 2. Facility for cleaning or backwash 3. It is preferred to be made from non-corrosive material because during the period of no recharge this will be exposed to atmosphere. DIFFERENT DESIGN FOR FILTER PIT 1. Vertical flow filter pit 2. Horizontal flow filter 1. VERTICAL FLOW FILTER DESIGN
• Layers of gravel/media laid horizontally in filter pit • Some collection system is at bottom • Water percolates vertically downwards thru filter media and goes into the well assembly. DISADVANTAGE / LIMITATIONS OF VERTICAL FLOW FILTER • Highly susceptible to clogging due to vertically settling impurities in water and floating matters which reduces permeability of filter media. • Difficult to clean or back wash. • No water holding facility • Lot of manpower and cost required to clean the media. • Recharge tubewell cannot be used as a pumping well during summer • Monitoring the recharge process is very difficult. • The life and effectiveness of such filter pit is not very long. 2. HORIZONTAL FLOW FILTER DESIGN 369 • Two cylindrical V-wire screen are fitted concentric to each other • Annular space is filled with graded gravel/ coarse sand • The central screen is fitted directly to the assembly of well • The water by its head pressure –flow thru outer screen via the gravel pack and pass thru the fine screen and enter the well assembly ADVANTAGE OF HORIZONTAL FLOW FILTER • The floating matter will float on the water and will not block the screen. • The heavy matter and silt will be settled at bottom of the pit not clogging the filter media and can be easily cleaned. • The outer screen will not allow clogging of the filter media • Easy to clean by back washing or by removing the gravels • The filter media can be changed very fast. • The filter pit has the facility to hold the water. • The recharge process can be measured very easily. • The recharge tube well can be used as a pumping well during summer. • The recharge tube well can be used as a monitor well for getting the details for underground water quality/quantity. Well Casing - The casing shall have smooth inner surface for minimum friction. The casing diameter shall be selected based on the targeted recharge rate so as to keep the down hole velocity within a maximum limit of 1.5 Meter/Sec. Well Screens - This is another important element, which has great bearing on the frictional loss and sustenance of recharge well and its mechanical life. Following are peculiar phenomena in a recharge well as compared to a pumping well which puts more responsibility on the well screens – � In a pumping well the sediment sand that is smaller than the screen slot is pumped out of the aquifer and gravel pack and the well is cleaned after
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Page 7 and 8: DOLASANE , SANGAMNER RANGE I, SANGA
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Page 13 and 14: the determination of specification
Page 15 and 16: Hydrological Parameters (mm) 140 12
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Page 23 and 24: Table 3 : Seasonal water use (mm) i
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Page 29 and 30: the base of the trench to assist dr
Page 31 and 32: SUDS (Sustainable Drainage Systems)
Page 33 and 34: Figure 3 : Cascade model of constru
Page 37: H eff = H R - H aq - H f ……..
Page 43 and 44: 1.3 % annually, and the total energ
Page 45 and 46: Figure 1a. Schematic diagram of sol
Page 47 and 48: Figure 5. Cross-section of partitio
Page 51 and 52: Fig. 1 (a) - (d) : Water Level Soun
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Page 57 and 58: Geology of Area The area is a part
Page 59 and 60: ground water recharge, which includ
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Page 71 and 72: Table 2 : Tolerance limit for metal
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INTRODUCTION Indian water sector is
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REPORTB OF WORLD COMMISION ON DAMS
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