Question

1- A 20-cm well penetrates 25 m below the static water table. After 24 hours of pumping out at the rate of 800 lpm, the water level in a test well at 80 m from the pumping well is lowered by 0.53 m and in a test well 20 m away 2.11 m. Find the coefficient of transmissibility of the aquifer.

2-What is the nature of ground water flow? State the law governing the flow with limitations, if any.

3-A well penetrates into an unconfined aquifer having a saturated depth of 50 m. The discharge is 250 1pm at 8 m drawdown. What would be the discharge at 10 m drawdown. The radius of influence in both the cases may be taken as same.

4-Describe the method of construction of open wells:
(i) in a soil where a clayey stratum is encountered.
(ii) in rocky sub-strata.

5 -A tubwell penetrates fully an unconfined aquifer. Calculate the discharge from the well in lpm from the following data:

Diameter of the well 30 cm

Drawdown in the well 3 m

Effective length of the strainer under the above drawdown 10 m

Coefficient of permeability of the aquifer 40 m/day

Radius of zero drawdown 300 m

Answer 1

Answer 2 :

Ground water flow nature : Groundwater flows from regions of higher hydraulic head to regions of lower hydraulic head.The change in hydraulic head along a groundwater flow path is termed as hydraulic gradient.Groundwater flows faster where hydraulic gradient and hydraulic conductivty are larger.Groundwater flow rates are commonly much smaller than surface flow.Groundwater flow path and directions are highly modified during glaciations.

Law governing the groundwater flow :  

Darcy's Law --Darcy defined how water moves through a saturated porus medium with analogy of a cylinder fitted with inflow and outflow pipes.He showed that velocity was a function of difference in head over a finite distance l as

   velocity= -k(dh/dl) ,where (dh/dl) is hydraulic gradient & k is hydraulic conductivity.

Limitations Of darcy law:

1. Darcy Law does not hold on scale smaller than REV(Represntive element scale)scale.
2. Dracy law only applies to parallel or laminar flow.
3. Darcy law is valid when reynold number is less than 1.

Answer 3: Since the radius of influence is same for both the drawdown ,the discharge at diffrent drawdown will also be same. Hence the discharge at 10 m drawdown will be 250 lpm.