Hydrology & Water Resources Engineering Subject Code:150602

Ground Water Ground Water: Ground water may be defined as the underground water that occurs in the saturated zone of variable thickness and depth, below the earth’s surface. Ground Water hydrology: Ground water hydrology is the science of occurance, distribution and movement of water below the surface of earth.

Terms related to Ground Water 1) Aquifer: An aquifer may be defined as a geological formation that contains sufficient permeable materials which permits storage as well as movement of water through it under ordinary field conditions. Ex. Sands, Gravels 2) Aquiclude: An aquiclude may be defined as a geological formation of relatively impermeable material which permits storage of water but it is not capable of transmitting water in sufficient quantity. Ex. Clay

3) Aquitard: A geological formation that has poor permeability, but through which seepage is possible and hence, it does not yield water freely to well. It may transmit vertically appreciable quantities of water to or from adjacent aquifers. Ex. Sandy Clay 4) Aquifuge: An aquifuge may be defined as a geological formation of relatively impermeable material which neither contains nor transmits water. Ex. Solid rocks

Porosity: Porosity of soil or rock may be defined as the ration of the volume of voids to the total volume of the material. Permeability: Is defined as the property of a rock or unconsolidated sediment, to permit flow of water through it.

Specific Yield (Sy): It is the ratio of volume of water that after saturation, can be drained by gravity to its own volume. Specific Retention (Sr): It is the ratio of volume of water it will retain after saturation against the force of gravity to its own volume.

Storage coefficient (S): The water yielding capacity of a confined aquifer can be expressed in terms of its storage coefficient. -Storage coefficient is defined as the volume of water that an aquifer releases from or takes into storage per unit surface area of aquifer per unit change in the component of head normal to that surface.

Coefficient of Permeability (K): The coefficient of permibility is defined as the velocity of flow which will occur through the total cross-sectional area of the soil under a unit hydraulic gradient..

Coefficient of Transmissibility (T): Coefficient of transmissibility is defined as the rate of flow of water in (m3/day) through vertical strip of aquifer of unit width (1 m) and extending the full saturation height under unit hydraulic gradient. Where, T = Transmissibility (m3/day) K = Coefficient of permeability (m/day) B = Thickness of aquifer (m)

Water Table (W.T.): Water table is the surface of water level in an unconfined aquifer at atmospheric pressure.

Division of sub-surface water: Zone of aeration Zone of saturation

Darcy’s Law Laminar flow through saturated soil mass, the discharge per unit time is proportional to the hydraulic gradient.

Where, Q = Discharge per unit time A = Total c/s area of soil mass i = Hydraulic gradient = h/L k = Darcy’s coefficient of permeability V = Velocity of flow.

Assumptions of Darcy’s Law: The soil is saturated The flow through soil is laminar The flow is continuous and steady the total cross sectional area of soil mass is considered The temperature at the time of testing is 27o C (Atmospheric).

Types of Aquifers: Unconfined Aquifer Unconfined aquifer is the one in which water table forms the upper Surface of the zone of saturation. It is also known as water table aquifer.

Confined Aquifer: A confined aquifer is the one in which ground water is confined under pressure greater than atmospheric pressure by overlaying relatively impermeable strata. It is also known as artesian aquifer or pressure aquifer.

Section II

Open Wells Well Hydrology Types of Wells: Open wells Based on depth Shallow wells Deep Well Based on type of well Kachha wells Well with impervious lining Well with Pervious lining

Based on depth Shallow wells: Shallow wells are those which rest in the top water bearing strata and draw their supplies from the surrounding material. Deep wells: A deep well is a well which rest on impervious layer and draw their supplies from the previous formation lying below the motar layer.

2) Tube wells:

Open Well Tube Well Diameter of well is large (1 to 10 m) Diameter of well is small ( 5 cm to 30 cm) Depth is small (2 to 20 m) Depth is large (20 to 500 m) The walls of well are made of pervious or impervious materials like brick, Stone A steel pipe with perforations and strainer is used as tube well Discharge is about 0.005 m3/s Discharge is less as compared to open well The cross sectional area of flow is large The cross sectional area of flow is small

Well Hydraulics – “Dupuits theory”

Assumptions in Dupuit – Thiem theory: The aquifer is homogeneous, isotropic and of uniform thickness The aquifer has an infinite lateral extent The well fully penetrates the aquifer and receives water from the entire thickness The flow lines are radial. The flow is horizontal and uniformly distributed in the vertical direction along the thickness of the aquifer. Flow is laminar and Darcy’s law is applicable The pumping had been continued for a sufficiently long period at a uniform rate to attain an equilibrium stage or the steady state The slope of the hydraulic gradient line is small, and it can be taken equal to tanθ instead of sinθ. The well is of small size and has negligible storage and therefore all the pumped water comes out from the aquifer. The water is released from the storage in aquifers as soon as the water table falls

Cone of depression: Before starting the pumping, the water level in the wells is stationary. As the pumping is started, the water level in the wells drops. The radial flow occurs into wells, and the water table (or piezometric surface) assumes the shape of an inverted cone around the well as the pumping is continued call as cone of depression

Radius of influence: The area influenced by the cone of depression is called the area of influence and its radius is known as the radius of influence.

Drawdown: The drop in the water table level at any point from the original static level is called drawdown. The maximum drawdown occurs at the well, and as the radial distance from the well increase, the drawdown decrease.

Steady state: If the pumping is continued for a long time, a stage is reached when The cone of depression becomes stationary. At this stage, the rate of inflow into the well is exactly equal to the rate of outflow from the Well. This stage is know as equilibrium stage or steady state. Steady state flow to wells in unconfined aquifer Steady state flow to wells in confined aquifer

Steady state flow to wells in unconfined aquifer Let, r = radius of the well R = radius of zero drawdown H = Thickness of the aquifer from the impermeable layer to the initial water table level. s = drawdown at well h = depth of water measured in the well above impermeable layer

(1) Dupuit’s formula (2) Thiem’s formula

Ex. A pumping test was made in a medium sand and gravel to a depth of 15 m where a bed of clay was encountered. The normal ground water level was at the surface. Observation holes were located at distances of 3 m and 7.5 m from the pumped well. At a discharge of 3.6 lit/sec from the pumping well, a steady state was attained in about 25 hr. The drawdown at 3 m was 1.65 m and at 7.5 m was 0.36 m. Compute the coefficient of permeability. Ans: k = 2.917 x 10-5 m/s or 0.00291 cm/s

Ex. A tube well penetrates fully an unconfined aquifer. Calculate the discharge from the tube well under the following conditions: Diameter of the well = 30 cm Drawdown = 2 m Effective length of strainer under drawdown = 10 m Coefficient of permeability of aquifer = 0.05 cm/s Radius of zero drawdown = 300 m Ans: Q = 9.06 x 10-3 m3/s

Steady state flow to wells in confined aquifer: Where, Ax = cross-section area of flow b = thickness of confined aquifer ix = dy/dx

Where, T = coefficient of transmissibility = bk s = drawdown at the well

Ex. A fully penetrating well of dia Ex. A fully penetrating well of dia. 0.3 m draws water from a confined aquifer of permeability 0.001 m/s and thickness 15 m. if steady satate discharge is found to be 1/30 m3/s compute the drawdown at points 10 m and 40 m from the centre of the well. Take radius of influence of well 1000 m. Ans: S1= 1.63 m, S2= 1.14 m

Ex. A tube well of 30 cm diameter completelly penetrates a confined aquifer of permeability 45 m/day. The length of strainer is 20 m. Under steady state of pumping the drawdown at the well was found to be 3 m and radius of influence was 300 m. Calculate the discharge. Ans: Q = 0.0257 m3/s

Ex. Design a tube well for the following data. Yield required = 0.10 m3/s Radius of circle of influence = 200 m Coefficient of permeability = 60 m/day Drawdown = 6 m Thickness of confined aquifer = 30 m Ans: r = 79.8 mm

Ex. A tube well fully penetrates a confined aquifer of thickness 30 m and coefficient of permeability 38 m/day. Determine the radius of the well if the yield required is 40 lit/sec under a drawdown of 4 m. Use radius of circle of influence as recommended by Sichardt. Sichardt Ans: r = 66 mm

Yield of an open well: a) Constant level pumping test

b) Recuperation test

Ex. Design an open well in fine sand to give a discharge of 0 Ex. Design an open well in fine sand to give a discharge of 0.003 cumecs when worked under a depression head of 2.5 m. Ans: A = 8.64 m2, d = 3.32 m

Ex. During a recuperation test, the water in an open well was depressed by pumping by 2.5 m and it recuperated 1.8 m in 80 min. Find a) Yield from a well of 4 m dia. Under a depression head of 3 m b) the dia. Of the well to yield 8 lit/Sec under a depression head of 2 m. Ans: K/A = 0.955, Q = 36 m3/hr, d = 4.38 m

Prepared by, Dr Dhruvesh Patel www.drdhruveshpatel.com Image Source: www.google.com