Hydrologic Evaluation of Landfill Performance

8/12/2019 Hydrologic Evaluation of Landfill Performance

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CVEN90027 Geotechnical Applications

Year: 2012

Assignment 1

The Hydrologic Evalation o! "and!ill

#er!ormance $Help%

&in 'eng (2))(1

Hao *ing 29011)

Hoc+ T,n Gan (2)1-.


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Abstract

This report outlines the performance analysis of a landfill containment system using the

Hydrologic Evaluation of Landfill Performance (isual HELP! soft"are# arious scenarios

are applied$ including different locations and particular design specifications of thecontainment system# There are % parts to this report:

Part 1 : The containment system sho"n in &ig# 1 is modelled on isual HELP for three

different locations' el)ourne$ *ris)ane and Perth# The performance of the system is

measured in terms of its capacity to prevent "ater into the cell and also out to the

surrounding environment$ and to collect leachate timely for treatment+disposal#

Part 2 : , sensitivity analysis is carried out on the containment system )y considering

the effect of having a different - i! hydraulic conductivity of the )ase liner soil$ ii! Leaf

,rea .nde/$ "hich is a measure of the amount of surface vegetation$ and iii! slope for

the leachate collection layer#

Part % : , guideline regarding the cap performance provided )y EP, ictoria is

considered for the containment system in &ig# 1# , slightly modified cap design as

sho"n in &ig# 2 is modelled on isual HELP and its performance assessed in the

same "ay as in Part 1#

Fall

Fall

Topsoil

Sand

Clay

aste

Gravel

Leachate

Collection

System

Final

Cap

300mm

20, 000mm

150mm dia . slotted

pipes @ 10m pipe

spacing

500mm

200mm

Compacted Clay

H!"Geomem#rane

Composite

iner$00mm

%n&sit' Soil

300mm

/igre 1 "and!ill containment system design !or #art 1


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Topsoil

Sand

Clay

aste

Gravel

Leachate

Collection

System

Final

Cap

300mm

20, 000mm

150mm dia . slottedpipes @ 10m pipe

spacing

500mm

200mm

Compacted Clay

H!" Geomem#rane

Composite

Liner$00mm

%n&sit' Soil

300mmFall

Fall

Geomem#raneH!"

/igre 2 odi!ied containment system !or #art (


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Part 1

1.1 CAPLEAKAGE

The first performance criterion is the capacity of the cap to prevent "ater from entering the

cell#

/igre ( 3ater lea+age throgh the top clay layer $cap%

&igure % sho"s the annual leaage volume going through the top clay layer# ,s evident$

*ris)ane has the largest amount of leaage$ ranging from 1%00m%to 2000m%# Perth rans

the second$ ranging from 20m%to 100m%# el)ourne has the smallest amount$ from 0 to

00m%#

This is due to the different latitudinal positions and thus the climate of the three sites#

*ris)ane has a humid su)tropical climate$ "ith large amounts of rainfall$ especially from

3ovem)er to arch# Perth has more of a editerranean climate$also "ith seasonal rainfall

)ut moderate compared to *ris)ane$ "hile el)ourne has a moderate oceanic climate "hich

is unpredicta)le relative to the t"o other cities$ )ut "ith significantly less rainfall throughout

the year#

&igure 4$ 5 sho" the same information$ )ut in percentages of total rainfall in that area#

The percentage values might not )e legi)le on the graphs$ )ut for all % cities$ the annual

leaages through the cap are in the order of 10647$ "ith el)ourne recording ma/imum

values of around % / 10647$ Perth "ith / 10647$ and *ris)ane "ith 8 / 10647#

Even though the amount of leaage into the *ris)ane and Perth cell is considera)ly more

than in el)ourne in terms of total volume$ they are not too dissimilar in terms of percentage

of rainfall# Evidently$ from the cap performance alone$ it seems that the design is much more

suita)le for el)ourne than it is for *ris)ane or Perth$ )ut considering the significantly higher

amount of rainfall in those cities$ the cap performance is still 9uite encouraging#
http://en.wikipedia.org/wiki/Mediterranean_climatehttp://en.wikipedia.org/wiki/Mediterranean_climate


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/igre ) &ris4ane 5 lea+age throgh cap in 6 o! rain!all

/igre - #erth 5 lea+age throgh cap in 6 o! rain!all


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/igre . el4orne 5 lea+age throgh cap in 6 o! rain!all

1.2 LEACHATECOLLECTION

The second criterion is the capacity of the leachate collection system (ravel drainage layer$pipes and geomem)rane! to drain leachate a"ay so as to reduce the hydraulic head on the

underlying )arrier soil layer#

/igre 7 Volme o! leachate collected at the 4ottom drainage layer


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&igure 8 sho"s the amount of leachate )eing collected )y the drainage system$ "hich

accounts for virtually all of the leaage through the cap for all three cities# This indicates that

the performance of the leachate collection system is e/cellent#

1.3 BASELINERLEAKAGE

/igre "ea+age throgh the 4ase liner

,s &igure ; sho"s and as previously suggested$ the amount of percolation through the )ase

liner is very small$ "ith most of the leachate )eing drained a"ay )y the collection system#

The el)ourne values range from 0 to 0#02 m%$ Perth from 0#01 to 0#04 m%(aside from a

0#08 reading in one year!$ and *ris)ane from 0#08 to 0#11 m %#

,ccording to Ta)le #1 of EP, ic# *est Practice guidelines$ a liner system similar to the one

in this model ()ut "ith additional geote/tiles enclosing the drainage layer! is used to control

seepages to amounts not e/ceeding 10 L+ha+day# That is e9uivalent to $48 L+year of

leaage at a 1# ha site# To put our model into perspective$ the ma/imum annual leaage

(from *ris)ane! is 0#11 m%"hich is e9ual to 110 L# That is still significantly less than the EP,

estimate for the similar type of liner# 3ote that the EP, value 9uoted is not the regulation

ma/imum leaage allo"ed )ut


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=verall$ the design of the containment system is encouraging$ and it suggests that only

minor modifications to the liner design are re9uired to increase the performance to the

desired level# >ome recommendations to improve the performance of the containment cell

are as follo"s:

1# Reduce hydraulic conductivity of clay in cap and base liner - )y selectingmaterials as such#

2# Install a geomembrane liner in cap layer - to allo" more time for "ater to

drain through the sand and reduce hydraulic head on the underlying clay

layer$ thuse reducing the leaage through the cap#

%# Increase density of vegetation at the ground - to increase the amount of

"ater lost through evapotranspiration$ "hich already accounts for a ma


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The hydraulic conductivity of the )arrier soil material used in the )ase liner is increased from

the initial value of 1 / 106?m+s to 1 / 106;m+s$ and the change in the leaage through the

liner is o)served#

/igre 9 "ea+age throgh the 4ase liner 8ith higher hydralic condctivity

&igure ? sho"s that "ith the hydraulic conductivity increased tenfold to 1 / 10 6;m+s$ the

leaage through the )ase liner dramatically increases )y roughly a factor of # Possi)le

reasons as to "hy the hydraulic conductivity is increased could )e the lac of suita)le clay

material or unsatisfactory 9uality control during the compaction of the clay layer#

.t is 9uite evident that the hydraulic conductivity is a sensitive and vulnera)le parameter$

especially considering the relative ease for situations lie the lac in suita)le materials or

9uality control to occur on site# &or this reason$ in >ection #%#2 of the EP, *est Practices

guideline$ the minimum hydraulic conductivity of the material used for the liner is specified to

)e at lo"er than 1 / 106?m+s#

2.2 INCREASE!EGETATION

To represent an increase in the amount of surface vegetation$ the Leaf ,rea .nde/ (L,.! is

increased from the initial value of 2 to #


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/igre 10 "ea+age throgh the 4ase liner 8ith higher "A

,s sho"n in &igure 10$ the impact of increasing the L,. from 2 to is generally insignificant$

"ith the amount of leaage staying almost the same and in some years$ deacreasing and

increasing#

/igre 11 Total amont o! percolation throgh liner in m(

&igure 11 sho"s the total amount of percolation through the )ottom layer# The total amount

actually decreases slightly "ith the increase in vegetation$ though inspection of the

evapotranspiration volume in &igure 12 suggests that this decrease in percolation is

pro)a)ly due to the different "eather data generated in the simulation$ and not the effect of

increased vegetation#

"ea! inde:2

"ea! inde:-


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Evapotranspiration$m(%

22018#22 21?80#?4

;no!! $m(% 44#0; %4#0

/igre 12 Comparison o! evapotranspiration and rno!! amonts

&igure 12 also sho"s that the amount of runoff is reduced "hen the L,. is set to # , densersurface vegetation provides more o)stacles for "ater running off the surface and thus

decreases runoff$ )ut the amount of runoff is 9uite insignificant to start "ith#

.n summary$ increasing the amount of vegetation seems to )e ineffective at improving the

performance of the containment system#

2.3 INCREASERAINAGESLOPE

The drainage slope in the leachate collection layer is increased from 2 to 7#

/igre 1( "ea+age throgh 4ase liner 8ith steeper drainage slope

Evidently$ increasing the slope results in less percolation through the )ase liner# The steeper

slope causes the leachate collected in the drainage layer to drain out more 9uicly and thus

reduce the amount of hydraulic head on the underlying clay liner# .ncreasing the slope )y afactor of 2# (from 2 to 7! resulted in the leaage decreasing )y roughly 07#

, side effect of increasing the drainage slope "hich is not sho"n in the graph a)ove$ is that

the collection pipes are less liely$ or "ill tae longer$ to get clogged up "ith sediments# This

can increase the useful life6span of the leachate collection system#

.n general$ increasing the slope seems to )e a via)le option to improve the performance of

the leachate collection system#

2.4 SUMMARYO"SENSITI!ITY ANALYSIS


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/igre 1)


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>ection ;#1# and Ta)le ;#1 in the EP, *est Practices guideline states that the re9uired cap

performance has to )e such that @the design seepage rate of the cap does not e/ceed 87

of the design seepage rate of the landfill ()asal! linerA#

/igre 1- Comparison o! cap seepage and 4asal liner seepage !or &ris4ane

/igre 1. Comparison o! cap seepage and 4asal liner seepage !or #erth


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/igre 17 Comparison o! cap seepage and 4asal liner seepage !or el4orne

&rom the above figures, it can be seen that the design seepage rate for the top layer is

much greater than the seepage rate of landfill liner, much less 75% of it. This means that all

three sites are not meeting the required performance from the Best ractice guideline. !ost

of the "ater that lea#s through the cap is drained a"ay by the leachate collection layer,

therefore the lea#age through the base liner is very small compared to the cap lea#age.

.n this landfill model "here the ground "ater is "ell )elo" the )ase liner$ the only "ater input

to the cell is from precipitation$ and that has to )e input through the cap# Even if the leachate

collection system completely fails$ the base liner seepage is equal to the cap seepage, the

cap seepage then "ill only go as lo" as &&% of the base liner seepage and not any lo"er.

Therefore in this landfill system design, it is not 'ust impractical but impossible to fulfil the

requirement set by the () regarding cap performance. erhaps this requirement "as

intended for landfill systems "here the ground"ater is above the base liner.

3.2 MOI"IECAPESIGN


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, different cap design is used this time (refer to &ig# 2!$ "here there is an additional

geomem)rane layer at the cap# The same analysis as in Part 1 "as run to this ne" design#

/igre 1 Comparison o! cap seepage and 4asal liner seepage !or &ris4ane (modified)

/igre 19: Comparison of cap seepage and basal liner seepage for Perth (modified)


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/igre 20: Comparison of cap seepage and basal liner seepage for Melbourne (modified)

,s e/pected$ for the modified model, the seepage rate of the cap still e*ceeds 75% of the

design seepage rate of the landfill liner. +o"ever the seepage through the cap has

dramatically decreased due to the addition of the geomembrane.

&igures 21 to 2% )elo" sho"s the percolation through the cap from the original model and

compares it "ith the modified model#(vidently, the added geomembrane decreased the cap

seepage by to - orders of magnitude. hat is even more interesting is that "ith the ne"

design, the seepage through the cap for Brisbane is less than erth and !elbourne, despite

e*periencing much more rainfall. This confirms the earlier prediction that for the originaldesign, only minor modifications had to be made to significantly improve the performance of

the landfill containment system.

,nother advantage to having an e/tra geomem)rane at the cap is that it can also reduce

any potential landfill gas leaage through the cap#


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/igre 21: Comparison of cap percolation between original and modified model (Brisbane).

/igre 22: Comparison of cap percolation between original and modified model (Perth).

/igre 2(: Comparison of cap percolation between original and modified model (Melbourne).

Hydrologic Evaluation of Landfill Performance

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