Well Hydraulics - GATE Civil Foundation Engineering Free MCQ Test

The specific yield S_γ of an aquifer is defined as the ratio of volume of water drained by gravity V_wy to the total volume V. It is expressed as percentage. The specific yield S_γ is an indication of the water yielding capacity of an unconfined aquifer.

The specific yield S_γ is given by,
S_γ=(V_wy/V)*100. The specific yield S_γ is proportional to the volume of water drained. This implies that the value of specific yield depends upon the shape of the pores.

The specific retention SR of an aquifer is defined as the ratio of volume of water retained VwR to the total volume V. It is expressed as percentage.

The porosity n, specific retention S_R, specific yield S_γ is given by,
S_γ = (V_wγ/V)*100 and S_R = (V_wR/V)*100
V_w = V_wγ+ V_wR
But n = (V_w/V)*100
∴ n = S_R+S_γ.

The storage coefficient is defined as the volume of water that an aquifer releases per unit surface area per unit change in the component of head normal to that surface.

For a fully saturated soil mass of unit width extending to depth infinity, the discharge through it is known as the coefficient of transmissibility when the hydraulic gradient is unity.

The coefficient of transmissibility T equals the coefficient of permeability k multiplied by the aquifer thickness b. T = bk.

The permeability can be determined by pumping out test. Pumping must continue at a uniform rate for a sufficient time to approach a steady state condition.