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**Well Hydraulics**

**Specific yield (S**_{y})

The specific yield of an unconfined aquifer is the ratio of volume of water which will flow under saturated condition due to gravity effect to the total volume of aquifer (v).

S_{y}= V_{wy}/V where, V_{wy}= Volume of water yielded under gravity effect and V = total volume of water.**Specific retention:**The specific retention of an unconfined aquifer is the ratio of volume of water retained against gravity effect to the total volume of aquifer (v).

S_{R}= V_{WR}/V where, V_{WR}= Volume of water retained under gravity effect.

**Coefficient of transmissibility**

T = kH where, H = Thickness

k = Coefficient of permeability

**Unconfined Aquifer**

**Theims Theory**

h_{1}+ s_{1}= h_{2}+ s_{2}

where, q = Rate of flow in m^{3}/s

h_{1 }= Height of water table of 1^{st}observation well

h_{2}= Height of water table of 2^{nd}observation well

s_{1}= Drawdown of 1^{st }test well

s_{2}= Drawdown of 2^{nd}test well.

r_{1}= and r_{2}are radius of 1^{st}and 2^{nd}observation wells respectively.**Dupits Theory**

R = 3000.S√K and S = H - h

Where, S = Drawdown in the well

k = Permeability coefficient in m/s.

R = Radius of influence in 'm'

150m ≤ R ≤ 300m

r = Radius of tes well in 'm'.

Results of dupits theory are not accurate because 'R' is based on empirical relation.

**Confined Aquifer**

- Theims theory where, b = width
- Dupits theory

**Spherical flow through well**

q_{s = K.2πr.s} _{where, r = Radius of well}

S = Drawdown

q_{s} = Rate of flow through spherical well in m^{3}/s q_{s }= 1/30.qradial flow

**Pumping-In-Test**

**Open end test**

K = q/5.5rh where, r = Radius of pipe

h = Head of water above the base of pipe, it may include gravity head and pressure head.**Tacker test**

K = q/(2πLh)log_{10}(L/r) ... when L > 10r

where, L = Length of perforated section of pipe

k = q/(2πLh)sin - 1(L/2r) ... when L < 10 r

r = Radius of pipe

h = Head of which water is added.

**Open well (Recuperation test)**

q = (C/A).Volume

Where, C/A = (2.303/T)log_{10}(h_{1}/h_{2})

C/A = Specific yield or specific capacity of an open well.

T = Time in 'sec'

h_{1} = Position of water table of t = 0

h_{2} = Position of water table of t = T

**Value of Permeability**

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