Psda Ts Air Tanah-10

Kuliah Oleh : Dr.. Eng. Sigit Sutikno, ST.,MT

PSDA

(Jurusan Teknik Sipil)

TTM-10

Groundwater Movement

Kuliah Oleh : Dr. Eng. Sigit Sutikno, ST.,MT

GROUNDWATER MOVEMENT

1. Introduction

2. General Concept

3. Flow Net

4. Darcy’s Law

5. Hydraulic Conductivity Test

6. Distance-drawdown Analysis

7. Groundwater Overdraft

8. Groundwater Contamination

• Movement of ground water

through pores and fractures is

relatively slow (meters/day)

compared to flow

of water in surface streams

• Flow velocity depends upon:

– Slope of the water table

– Permeability of the rock or

sediment

1. Introduction



The water table is actually a sloping surface.

Slope (gradient) is determined by the difference in water table

elevation (h) over a specified distance (L).

Direction of flow is downslope.

Flow rate depends on the gradient and the properties of the aquifer.

2. General concept

• Hydraulic gradient for an

unconfined aquifer =

approximately the slope of the

water table.

•Hydraulic conductivity = ability of material to allow water to move

through it, expressed in terms of m/day (distance/time). It is a

function of the size and shape of particles, and the size, shape, and

connectivity of pore spaces.

S. Hughes, 2003

Movement of groundwater depends on rock and sediment properties

and the groundwater’s flow potential. Porosity, permeability, specific

yield and specific retention are important components of hydraulic

conductivity.

K = length/time (m/day)

2. General concept


Water table contour lines are similar to topographic lines on a map.

They essentially represent "elevations" in the subsurface. These

elevations are the hydraulic head mentioned above.

S. Hughes, 2003

3. Flow Net


Water table contour lines can be used to

determine the direction groundwater

will flow in a given region.

S. Hughes, 2003

Water table contours (called

equipotential lines) are constructed to

join areas of equal head.

A map of groundwater contour lines

with groundwater flow lines is called a

flow net.

Remember: groundwater always

moves from an area of higher hydraulic

head to an area of lower hydraulic

head.

3. Flow Net

Aquitard (granite)

Qal 100 50

Qal

WT

A simple flow net

Cross-profile view

well

Aquitard

Qal

3. Flow Net


4. Darcy’s Law


Henry Darcy, 1856, studied water flowing through porous material.

His equation describes groundwater flow.

Darcy’s experiment:

• Water is applied under

pressure through end A, flows

through the pipe, and

discharges at end B.

• Water pressure is measured

using piezometer tubes

4. Darcy’s Law


Hydraulic head = dh (change in height between A and B)

Flow length = dL (distance between the two tubes)

Hydraulic gradient (I) = dh / dL

The velocity of groundwater is

based on

a) hydraulic conductivity (K),

b) hydraulic gradient (I).

4. Darcy’s Law


Q = Discharge = volumetric

flow rate, volume of water

flowing through an aquifer per

unit time (m3/day)

A = Area through which the

groundwater is flowing, cross-

sectional area of flow (aquifer

width x thickness, in m2)

The equation is

Q = KIA

5. Hydraulic Conductivity Test


6. Distance-Drawdown Analysis




Persamaan debit tunak

sumur nirtekan



Persamaan debit tunak

sumur tekan

6. Distance-Drawdown Analysis (Home Work)

Sebuah Sumur A dengan diameter (49+N) cm menembus suatu

akuifer seragam nirtekan mencapai lapisan kedap air yang terletak

(39+N) meter di bawah elevasi muka air tanah. Setelah diadakan

pemompaan dengan debit konstan sebesar 1 m3/detik, ternyata

penurunan elevasi muka air tanah dalam keadaan tunak pada

sumur-sumur yang berjarak (20+N) meter dan (50+N) meter dari

sumur A masing-masing adalah 3 m dan 2 m.

Dimana, N : Digit terakhir NIM

Pertanyaan:

a) Berapa nilai konduktivitas hidraulis dari akuifer? (20%)

b) Berapa penurunan muka air yang terjadi pada sumur artesis

A (20%)

c) Jika sumur anda berjarak (150+N) m dari sumur A, berapa

penurunan muka air yang terjadi? (20%)



•In many locations groundwater withdrawal exceeds

natural recharge rates. This is known as overdraft.

What happens when a new well here is heavily pumped?

Flow direction can change



Overpumping will have two effects:

1. Changes the groundwater flow

direction.

2. Lowers the water table, making it

necessary to dig a deeper well.

• Original land users and land owners

often spend lots of money to drill new,

deeper wells.

• Streams become permanently dry.

Dissolved contamination travels with ground water flow

Contamination can be transported to water supply aquifers down flow

Pumping will draw contamination into water supply



Leaking Gasoline

Floats on water table

Dissolves in ground water

Transported by ground water

Contaminates shallow aquifers


Effects of pumping

Accelerates ground water flow

toward well

Captures contamination within

cone of depression

May reverse ground water flow

Will cause saltwater intrusion


Infiltrating water may bring

contaminants down to the water table,

including (but not limited to):

– Pharmaceuticals

– Pesticides/herbicides

– Fertilizers

– Mercury and gold mining

– Landfill pollutants

– Heavy metals

– Bacteria, viruses and parasites from sewage

– Industrial chemicals (PCBs, TCE)

– Acid mine drainage

– Radioactive waste

– Oil and gasoline


Kuliah Oleh : Dr.. Eng. Sigit Sutikno, ST.,MT

That’s all for this class today,

any questions..???

Psda Ts Air Tanah-10

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