Ch 2 WS Sources of Water Supply

8/18/2019 Ch 2 WS Sources of Water Supply

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DBU Cha ter two Sources of Water Su

CHAPTER TWO:

Sources of Water Supply

DBU, Institute of Technology

College of Engineering

Department of Civil Engineering

Abraham Atnafu


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Nature of water source determines the components of thewater supply system

Factors to be considered to select source:

Quantity

Quality

Reliability

Safety of sourceWater rights

Environmental impacts…

Introduction


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Hydrologic Cycle


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The water cycle, also known as the hydrologic cycle, describes the continuous

movement of water on,above and below the earth surface. The sun, which

drives the water cycle, radiates solar energy on the oceans and land.Key Hydrological Processes

Precipitation : Condensed water vapor that falls to the earth surface. Most

precipitation occurs as rain, but also includes snow, hail, fog drip, sleet, etc

Runoff: The variety of ways by which water moves across the land. Thisincludes both surface runoff and channel runoff. As it flows, the water may

infiltrate into the ground, evaporate into the air, become stored in lakes or

reservoirs, or be extracted for agricultural or other human uses.

Infiltration: The flow of water from the ground surface into the ground.

Once infiltrated, the water becomes soil moisture or groundwater.

Hydrological Cycle


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Subsurface Flow: The flow of water underground, in the vadose zone and

aquifers. Subsurface water may return to the surface (e.g. as a spring or by

being pumped) or eventually seep into the oceans. Water returns to the

land surface at lower elevation than where it infiltrated, under the force of

gravity or gravity induced pressures. Groundwater tends to move

slowly, and is replenished slowly, so it can remain in aquifers for thousand

of years.

Evaporation and transpiration: The transformation of water from liquid to

gas phases as it moves from the ground or bodies of water into the

overlying atmosphere. The source of energy for evaporation is primarilysolar radiation. Evaporation often implicitly includes transpiration from

plants, though together they are specifically referred to as

evapotranspiration.


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Types of water supply sources

Surface Water Sources

Rain Water

Lakes and reservoirs

River Water

Sea water

Ground Water Sources

Spring Water

Wells

Infiltration galleries


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Rain water might contain dust, smoke, bacteria, carb

dioxide… as falling from high altitude

RW Harvesting- roofs are most effective and can be integra

with tanks

Rain water



Advantages of rainwater collection:◦ Quality of RW is high

◦ Independent

◦ Local materials can be used for collection

◦ No energy costs

◦ Easy to maintain◦ Time saving and convenient

Disadvantages◦ High initial cost (i.e. for a family)

◦ Quantity of water is dependent on the roof area and rainy seasons

◦ Flat taste

Rain water…


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A A i T ,

Z e r i h u n

A l e m a y e h

u

Lakes and reservoirs

Lake Tana

Store water in wet seasons for usage in dry seasons

It is a standing water; because of this:

◦ Quality is very low: turbidity, bacteria and pollutants

◦Thermal stratification i.e. for deep lakes/reservoirs)



River water

Abay river



River water

Abay river

A stream or river is a body of running water othe surface of the earth, from higher to lowe

ground.

Their capacity is dependent on minimum flow peday

Development of rivers requires :

submerged intake structuresmall diversion dams (i.e. for small streams)



Groundwater source

Aquifer



Advantages :It is likely to be free of pathogenic bacteria

free from turbidity and colour

It can be used without further treatmentIt can be found in the close vicinity

It is economical to obtain and distribute

The water-bearing stratum provides a naturalstorage at the point of intake.

Groundwater sources



Disadvantages

often have high in mineral content;

It usually requires pumping.

CATIONS: calcium, magnesium, iron and manganese

ANIONS: bicarbonate, carbonate, and chloride

Groundwater sources…


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SpringsSpring water is a groundwater that

outcrops from ground due to impervious

base that prevents percolation.

Mostly found from sand or gravel aquife


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Gravity springs◦ GW flows over an impervious stratum onto the ground

surface◦

The yield varies with the position of the water table◦ May dry up during or immediately after a dry season

Gravity overflow spring Gravity depression spring


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Artesian springs◦ High quality water due to confinement◦ High discharge due to high pressure in the confinement

◦ Yield is likely uniform and nearly constant over the seasof the year

Artesian depression spring Artesian fissure spring


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Horizontal wells that collect water over practically thei

entire lengths. Simple means of obtaining naturally filtered water

Infiltration gallery


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Replenishment (filling) of aquifers is known as recharge Unconfined aquifers are recharged by precipitation

percolating down from the land’s surface

Confined aquifers are generally recharged where the

aquifer materials are exposed at the land’s surface -called

an outcrop.

Recharge of aquifers


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Recharge of aquifers


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When surface water loses water to the adjacent

aquifer, the stream is called a losing stream.

water flows from the ground water to the stream, it is

called a gaining stream.

Recharge of aquifers…

Aquifer

Impervious layer

Groundwatertable


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To evaluate and classify raw water quality physical, chemical, and bacteriological parameters

raw water can be classified as having poor, fair, and good quality.

To identify sources of pollution

Surface water: urban runoff, agricultural runoff, industrialdischarge, and leachate from landfills;

Groundwater: infiltration from pit-latrines and septic tanks, landfill

leachate, and infiltration from polluted areas.

To assess the treatment required for beneficial uses level of treatment and unit process required are dependent on the

raw water quality

Water quality considerations

Water quality considerations of sources are required for the following purposes.


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Treatment types

Well Aeration Disinfection Fluoridation

WellRapid sand

filtrationAeration Disinfection Fluoridation

For groundwater having excellent quality

For groundwater having moderate quality


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Surface water sources◦ Safe water yield during the drought years

◦ Urbanization and land development in the watershed

◦ Proposed impoundments on tributaries

◦

Water quality◦ Assessment of reliability

◦ Requirements for construction of water supply system components

◦ Economics of the project

◦

Environmental impacts of the project◦ Water rights

Source selection


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Ground water sources Aquifer characteristics (depth, geology)

Safe aquifer yield

Permissible drawdown

Water quality

Source of contamination(gasoline, oil, chemicals)

Saltwater intrusion(areas near to seas or oceans)

Type and extent of recharge area

Rate of recharge

Water rights

Source selection…

R

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An artificial lake formed by the

construction of a dam across a valley

o Contain dam to hold water

o A spillway to allow excess water to flow

o A gate chamber with valves to regulate flo

Reservoirs

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The area of land draining to the dam site is called a

catchment or watershed.

Reservoirs

Outlet


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The following investigations are required for reservoir

planning:

A. Topographic surveying- to produce a topo-map which will

be used as a base for

preparing water surface area vs. elevation curve

plotting storage volume vs. elevation

indicating man-made and natural features that may be

affected

Reservoirs


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B. Geologic investigationsWater tightness of the reservoir basin

Suitability of foundations for the dam

Geological and structural features, such as faults, fissures, etc

Type and depth of overburden

Location of permeable and soluble rocks if any

Ground water conditions in the region

Location and quantity of materials for the dam construction

Reservoirs


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C. Hydrological investigationsdetermination of rainfall, runoff, seepage, and

evaporation in the reservoir catchment from long

years of data.

These information are essential for estimating the reservoir

capacity and design of spill way.

Reservoirs…


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Catchment geology- minimum percolation losses and high runoffpotential

Dam site- strong foundation with minimum seepage loss under thedam.

Narrow valley- sites that resulting lesser dam length

Topography- should be such that large area and valuableproperties are not submerged

Site that creates deep reservoirs- this has the advantages ofminimizing the evaporation loss and submerged area whencompared to shallow reservoirs

Sites that ensure good water quality- avoid sites that are

downstream of waste discharges and tributaries with high silt loads

Criteria for Selecting reservoir sites


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Yield is the amount of water that can be supplied from areservoir within a specified interval of time.

Safe yield or firm yield: is the maximum quantity of

water that can be guarantied during a critical dry

period. the safe yield from a reservoir > maximum day demand

Methods to determine the storage volume of reservoirs:

◦

mass curve method(Rippl) Method.◦ analytical method.

Volume of reservoirs

M h d


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reservoir capacity is determined from accumulated mass inflow andaccumulated demand curves.

Net Inflow= total Inflow-outflow (evaporation, seepage, d/s flow)

Procedure

Prepare accumulated mass inflow curve from the stream hydrograph Prepare the accumulated demand curve on the same scale

Draw tangent lines that are parallel to the accumulated demand curve

at the high points of the accumulated mass curve (P1, P2, P3, etc)

Measure the vertical distances between the tangent lines and the mass

inflow curve (V1, V2, V3, etc.)

Determine the required reservoir storage capacity as the largest of the

vertical distances (V1, V2, V3, etc.)

Mass curve method

M h d


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Mass curve method

Time, Year (month)

Volume

(m3)

V1

V2

Accumulated demand

Accumulated inflow

Spill

Analytic method


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Calculate the net inflow from the given hydrological data Calculate the deficiency (demand – net inflow)

Compute the cumulative deficiency. If the cumulative

deficiency is negative, take the cumulative deficiency as zero

Determine the required reservoir capacity as the maximumcumulative deficiency

Analytic method

(1) (2) (3)= (2)-(1) (4) if CF is – ve, take 0

Net inflow

(m3)

Demand (m3) Deficiency

(m3)

Cumulative deficiency

(m3)

I1 D1 F1 CF1 = F1I2 D2 F2 CF2 = CF1 + F2I3 D3 F3 CF3 = CF2 + F3

E l 2 1Inflo


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Compute the storage requirement

needed for an impounding reservoir for

a constant draft of 23 ML/km2/months

of 30.4 days with the given monthly net

river inflow for a critical year.

Example 2.1 MonthInflo

(ML/

1 94

2 12

3 454 5

5 5

6 2

7 0

8 29 16

10 7

11 72

12 92

13 214 55

15 33

A l ti l S l ti


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Month inflow

draft/

demand

Cummulative

Demand Cummulative inflow Defficiency

cummulative

Deficiency

1 94 23 23 94 -71 0

2 122 23 46 216 -99 0

3 45 23 69 261 -22 0

4 5 23 92 266 18 18

5 5 23 115 271 18 36

6 2 23 138 273 21 577 0 23 161 273 23 80

8 2 23 184 275 21 101

9 16 23 207 291 7 108

10 7 23 230 298 16 124

11 72 23 253 370 -49 75

12 92 23 276 462 -69 613 21 23 299 483 2 8

14 55 23 322 538 -32 0

15 33 23 345 571 -10 0

Analytical Solution

Graphical Solution


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0

100

200

300

400

500

600

0 2 4 6 8 10 12 14

C u m m u l a t i v e Q

Month

Cummulative Demand

Cummulative inflow

419-294=124

Graphical Solution

R e s e

r v o i r f u l l

Start of

Dry period

End of

Dry period

Res. Draw down period

Depletion of Res. Replenishment

of Res.

R e s .

f u l l

Exercise


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Exercise

Safe yield from a given reservoir capacity


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Prepare the mass inflow curve.

From the apices A1, A2, A3 etc. of the mass curve, draw

tangents in such a way that their maximum departure fro

the mass curve does not exceed the specified reservo

capacity. The ordinates E1D

1, E

2D

2, E

3D

3, etc. are all equa

to the reservoir capacity (say 1500 ha.m)

Measure the slopes of each of these tangents. The slope

indicate the yield which can be attained in each year fro

the reservoir of given capacity. The slope of the flattedemand line is the firm yield.

Safe yield from a given reservoir capacity


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Cross section of Dam


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Cross section of Dam

Types of Dams


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Types of Dams

Impoundments: Embankment dams


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Embankment dam-A dam constructed from natural materialsexcavated or obtained nearby. The natural fill materials are placed

and compacted without the addition of any binding agent. Two types-

Earth fill dam ( if compacted soil constitutes over 50% of the dam

volume) and

Rock fill dam (over 50% of the material is coarse-grained material ocrushed rock with impervious membrane).

Advantages: For sites in wide valleys and steep-sided gorges

Adaptability to a broad range of foundation conditions

Use of locally available natural materials

Impoundments: Embankment dams

Impoundments: Concrete Dam


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Disadvantages: Easily damaged or destructed by overflow

leakage and internal erosion

They are gravity dam, arch dam, buttress dam, etc.

Gravity dam

Dependent upon its own mass for stability.

For gorges with very steep side slopes

It can be constructed by Masonry(stone or brick)

Shape: straight or curved Dam height: can be very high for sound foundation

Impoundments: Concrete Dam

Impoundments: Concrete Dams...


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Arch dam Functions structurally as a horizontal arch, transmitting the

major portion of the water load to the abutments or

valley sides rather than to the floor of the valley.

structurally more efficient, needs less concrete volume

Buttress dam

consists of a continuous upstream face supported at

regular intervals by downstream buttresses.

Impoundments: Concrete Dams...

Catchment protection


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Activities that take place within the reservoir catchment have impacts on the quality and quantity

of water stored behind the dam. Catchment protection primarily focuses on maintaining thewater quality and capacity of the reservoir. It involves activities that include

Minimization of diffuse pollution from urban runoff

Minimization of agricultural diffuse pollution

Controlling discharges from point sources such aswastewater treatment plant, industries, etc

Limitation of soil erosion through soil conservation

measures, such as afforestation, etc.

Providing corridors along tributary streams, rivers, and thereservoir

Catchment protection


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Ground water

hydraulics


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Groundwater


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Groundwater is the water beneath the ground surface contained in void spaces (pore spaces between

rock and soil particles, or bedrock fractures).

Basic Terms

Aquifer

An aquifer is an underground layer of water-bearing permeable rock or unconsolidated

materials(gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water

well. Water table

The water table is the level at which the groundwater pressure is equal to atmospheric pressure.

Aquitard

An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another.

Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity.

Groundwater


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Unconfined aquifer

It is an aquifer with the water table as its upper boundary. Because the aquifer is not under pressure the

water level in a well is the same as the water table outside the well.

Confined aquifer

It is an aquifer found between two relatively impermeable layers.

Artesian aquifer/well

It is a confined aquifer containing groundwater that will flow upward through a well, called an artesian

well, without the need for pumping. Water may even reach the ground surface if the natural pressure is

high enough, in which case the well is called a flowing artesian well.

Water well

A water well is an excavation or structure created in the ground by digging, driving, boring or drilling

to access groundwater in underground aquifers.


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Borehole

It is a narrow shaft drilled in the ground as part of a groundwater site assessment.

Piezometric surface

The imaginary surface that everywhere coincides with the piezometric head of the water in the aquifer.In

areas of artesian ground water, it is above the land surface.

Base flow

Base flow is the portion of stream flow that comes from groundwater. It sustains flows in a river

during the dry periods between rainstorms.

Groundwater Recharge

The natural or intentional infiltration (percolation) of surface water into the groundwater system.

Fossil water

Fossil water is groundwater that has remained in an aquifer for thousands or even millions of years.

When geologic changes seal the aquifer off from further recharging, the water becomes trapped inside.


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Aquifer parameters


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(The water yield capacity of aquifers depends on different parameters)

Aquifer parameters


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Storage coefficient (S) : the volume of water that an aquifer

releases from or takes into storage per unit surface area of theaquifer per unit change in head

Hydraulic gradient (dh/dx ): the slope of the piezometric surface

or water table line in m/m. The magnitude of the head

determines the pressure on the groundwater to move and its

velocity.

q p

Examples


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p

Aquifer parameters


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Hydraulic conductivity(K): ratio of velocity to hydraulic

gradient, indicating permeability of porous media.

Transmissivity: the capacity of an aquifer to transmit watermeasure of how easily water in a confined aquifer can flow through

the porous media.

q p

T = Kb, b = saturated thickness

K =QdL

Adh

Aquifer Types


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Unconfined and confined aquifers(An unconfined aquifer does not have confining unit and is defined by water-table. Confined aquifer is overlain by a confining unit that has a lower hydraulic conductivity.)

q yp

Groundwater flow


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Groundwater flows in the direction of decreasing head. equipotential lines lines showing points having equal

pressure.

Flow direction is perpendicular to equipotential lines

Aquifer boundary130 m

135 m

140 m

145 m150 m

155 m

Equipotential lineFlow direction

Velocity of GW


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Velocity can be determined by Darcy’s law V = kS

Darcy law :Q through porous media is proportional to the head loss and inverselyproportional to the length of the flow path.

K = hydraulic conductivity and h is the head loss

Porous medium

Q

h

L

Area = A

elementsmallfor very;K V

or

L

ΔhK

A

QV

dLdh

Determination of K


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Determination of K


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Laboratory methods

Constant head permeameter

V = volume water flowing in time t through of area A, length L, and withconstant head h.

Variable head permeameter

r = radius of the column in which the water level drops

rc = radius of the sample

h1, h2 are heads at times t1 and t2, respectivelyt = t2 – t1

AthVL K

2

12

2

lnhh

t r Lr K

c

Problems


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Determination of K...


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Field Methods

Pumping test: constant removal of water from a single well

and observations of water level declines at several adjacent

wells.

This is the most accurate way

For anisotropic aquifers, the combined horizontal hydraulic

conductivity:

Where, Ki = K in layer i; Zi = thickness of layer I

i

ii

Z

Z K K

Example


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Determination of K...


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ield Methods…

Slug test or piezometer test the simplest method

some volume of water is taken out from the piezometer and the

subsequent rise of the water back to its original position is recorded

in time.

ri-inside radius,

L- the length of the screen section,

ro-the outside radius to- characteristic time interval

Hydraulics of water wells


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Well: hydraulic structure utilized to access water-bearing aquifers

Allows estimation of aquifer hydraulic properties

Provides direct access to ground water conditions

1) Sampling

2) Testing3) Resource Extraction

4) Environmental Restoration

Hydraulics of water wells


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Aquifer test : studies involving analyzing the change, with

time, in water levels in an aquifer caused by withdrawalsthrough wells.

Drawdown/cone of depression : is the difference between the

water level at any time during the test and the original

position. Ground

Cone of depression Drawdown

Well

Original GWT

Impervious

stratum

Radius of influence of steady state pumping

wells


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wells

Piezometric head


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Steady state Radial flow to a wellassumptions:


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• Cone of depression remains in equilibrium

• The water table is only slightly inclined

• Flow direction is horizontal

• Slopes of the water table and the hydraulic gradient are equal• Aquifer: isotropic, homogeneous and infinite extent

• Well fully penetrating the aquifer

Steady Radial Flow to a Well-Confined


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dr

dhrT

dr

dhrbK Q 22

In a confined aquifer, the drawdown curve or cone of depression varies with distance from a

pumping well

For horizontal flow, Q at any radius r equals, from Darcy’s law,

Steady Radial Flow to a Well-Confined


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Integrating after separation of variables, with h = h w at r = r w at the well, yields Thiem Equation.

Near the well, transmissivity, T , may be estimated by observingheads h1 and h2 at two adjacent observation wells located at r1and r2, respectively, from the pumping well.

)(2

ln

12

1

2

hhr

r

QT

w

w

r

r

hhT Q

ln

2

Steady Radial Flow to a Well-Unconfined


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radial flow in an unconfined, homogeneous, isotropic, and

horizontal aquifer yields:dr dH rHK Q 2

Steady Radial Flow to a Well-Unconfined


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1

2

2

1

2

2

ln

)(

r

r

hh K Q

Integrating, the flow rate in a unconfined aquifer from 2 to 1

Solving for K,

1

2

2

1

2

2

ln)( r

r

hh

Q K

Example


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A 0.5 m well fully penetrates an unconfined aquifer of 30 m

depth. Two observation well located 30 and 70 m from the

pumped well have drawdowns of 7 m and 6.4 m, respectively. If

the flow is steady and K = 74 m/d.

what would be the discharge

Estimate the drawdown at the well

Solution


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For unconfined well Q is given as

h1= 30-7 = 23 m, and h2 = 30 – 6.4 = 23.6 m

r1 = 30 m and r2 = 70 m

1

2

2

1

2

2

ln

)(

r

r

hh K Q

daymQ /54.7671

30

70

ln

)236.23(74

322

Solution


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Drawdown at the well,

using the h1=23 m and r w=0.5m/2=.25 m, we have hw

daymh

r

r

hh K Q w

w

w /54.7671

25.0

30ln

)23(74

ln

)( 322

1

22

1

Solving for hw, we have h w = 19.26 m

So the drawdown would be 30.0 – 19.26 = 10.74 m


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Solution


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For confined aquifer Q is given by

Taking between the well and at the radius of influence(R) we

have

h - h w = 6 m

b = 25 m

R = 250 m

w

w

r

r

hh

bK Q ln2

wr

m

day

daymmmQ

250ln

6

sec/86400

/70252sec/1.0 3

Solving for r w, we get r w = 0.12 m or 12 cm

Thus, diameter of the well is 24 cm or 25 cm

Exercise


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A well 60cm in diameter in a confined aquifer was pumped at asteady rate of 0.0311 m3/s.When the well level remained

constant at 85.48 m, the observation well level at a distance of

10.4 m was 86.52 m. Calculate the transmissivity.

Solution


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Interference of wells


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The combined drawdown at a point is equal to the sum of the

drawdowns caused by individual wells.

Reduced yield for each of the wells.

Resultant drawdown


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A A i T ,

Z e r i h u n

A l e m

a y e h u

Pumping and recharging wells


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T

Well construction


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Well construction depends

on

the flow rate,

depth to groundwater,

geologic condition,

casing material, and

economic factors

Shallow and deep well

construction

Shallow well construction


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Shallow wells are less than 30 m deep

constructed by

digging,

boring,

driving, or jetting methods.



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Dug wells: excavated by hand and

are vertical wells.

diameter > 0.5 m and depth < 15 m.

Lining and casing :concrete or brick.

D i ll i f i l th d i ti ll



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Driven wells: a series of pipe lengths driven vertically

downward by repeated impacts into the ground. diameters 25 – 75 mm

Length below 15 m.



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Bored wells : constructed with

hand-operated or power-drivenaugers.

Diameters of 25 to 900 mm

depths up to 30 m



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Jetted wells: a high-velocity stream

of water directed verticallydownward, while the casing that is

lowered into the hole conducts the

water and cuttings to the surface.

Small-diameter holes, up to 10

depths up to 15 m

useful for observation wells and well-

point systems for dewatering

purposes.


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Percussion drilling


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Pulley

Casing pipe

Rope

Tripod


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Deep well construction


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Rotary method: consists of drilling with a hollow, rotating

bit, with drilling mud or water used to increase efficiency. Nocasing is required with drilling mud because the mud forms a

clay lining on the wall of the well. Drilling mud consists of a

suspension of water, bentonite clay, and various organic

additives. A rapid method for drilling in unconsolidated formations

Air rotary methods use compressed air in place of drilling mud and

are convenient for consolidated formations.

Drilling depths can exceed 150 m

Hydraulic rotary drilling


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Mud to settling tank

Tripod

Drilling mud

http://www.dando.co.uk/images/uploads/rotary.htm


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http://www.dando.co.uk/images/uploads/rotary.htm


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END of

chapter 2:

By:Abraham

Atnafu

Ch 2 WS Sources of Water Supply

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