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**Chapter 4:****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 effects to the total volume of aquifer (V).

where, V

**SPECIFIC RETENTION:**

The specific retention of an unconfined aquifer is the ration of volume of water retained against gravity effect to the total volume of aquifer (V)

where V_{wr} = Volume of water retained under gravity effect

**Remember:-** **S _{y }+S_{R}=n where, n = porostiy**

**COEFFICIENT OF TRANSMISSIBILITY:****T=kH**

where,

H = Thickness

K = Coefficient of permeability

**UNCONFINED AQUIFER:**

**(a) Theims Theory**

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

s1 = Drawdown of 1st test well

s2 = Drawdown of 2nd test well.**h _{1} + s_{1} = h_{2 }+ s_{2}**

S

S

r

**(b) Dupits Theory**:

where, S = Drawdown in the well

K = Permeability coefficient in m/s.

R = Radius of influence in ‘m’

150m ≤ R ≤ 300m

r = Radius of test well in ‘m’.

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

**CONFINED AQUIFER:**

**(i) Theims theory**:**(ii) Dupits theory**

**SPHERICAL FLOW THROUGH WELL**

q_{s}=k2πrs, R = Radius of well

S = Drawdown

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

**PUMPING-IN-TEST****(a) Open end test**

where, r = radius of pipe, h = head of water above the base of pipe, it may include gravity head and pressure head.

**(b) Tacker test**

where, L = Length of perforated section of pipe

r = Radius of pipe

h = head of which water is added

** OPEN WELL (RCUPERATION TEST):**

where,

Volume = A. H

A = Area of well

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

**VALUES OF PERMEABILITY**:

SOIL | K (cm/sec) | Degree ofpermeability | |

1. | Coarse Gravel | >1 | High |

2. | Fine gravel-Fine sand | 1 to 10^{-2} | medium |

3. | Silt-Sand admixtures,loose silts, rock flour and loess | 10^{- 2} to 10^{-4} | Low |

4. | Dense slit, clay silt admixutures,non-homogenous clays | 10^{-4} to 10^{-6} | Very low |

5. | Homogenous clays | < 10^{-6} | Impervious |

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