final critique (pigring creek dam fs)

8/11/2019 Final Critique (Pigring Creek Dam FS)

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SUBMITTED BY:

MAR CHRISTIAN M MARTILLANO

BSCE 5B


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SUBMITTED TO:

ENGR WILLER DAYAHAN

PROJECT NAME:Pigring Creek Dam Feasibility Study

PERFORMED BY:SMEC Australia Pty Ltd.

PROPOSED FOR:Nimmitabel Advancement Group Inc.

DATE:February 8, 2010

SOURCE:http://www.cooma.nsw.gov.au/files/docs/council/public_notices/2010/07/6_smec_

feasibility_study_pigring_creek_feb_10_-_web.pdf

BACKGROUND

This study is about the feasibility of constructing a dam on Pigring Creek. The village of

Nimmitabel has subjected to severe water restrictions initiated by one of the longest droughts on

record. To ensure the drought security and future viability of the village, the Nimmitabel

Advancement Group (NAG) has put forward the option of an on stream storage on Pigring

Creek. They required more information on the feasibility of this option prior to agreeing to it, so

they have engaged to SMEC to perform a feasibility study for the proposed option.

SMEC focused their study on three aspects, the geotechnical, hydrology and environment

for them to design the dam. They have concluded that the site is suited for construction of an

earthfill dam. That there are sufficient quantities of fill material available within 500 m of the

dam and this fill will not negatively affect the structural integrity of the dam.

Cost estimates indicate that a dam about 8 m in height providing 200 ML of storage could

potentially be constructed within the budget available if costs are controlled, and this amount of

stored water would likely be sufficient for drought mitigation and security of the existing
http://www.cooma.nsw.gov.au/files/docs/council/public_notices/2010/07/6_smec_http://www.cooma.nsw.gov.au/files/docs/council/public_notices/2010/07/6_smec_http://www.cooma.nsw.gov.au/files/docs/council/public_notices/2010/07/6_smec_


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Nimmitabel water supply. Field and laboratory testing of on-site surface water and surface soil

did not identify any potential issues relating to future water quality. The construction of the earth

wall dam at Pigring Creek will result in the loss of the current terrestrial habitat bordering the

ephemeral creekline, but this habitat is considered to be marginal habitat for common species, it

has the potential to positively influence the biological diversity of the study area and surrounding

areas by providing a permanent source of water for fauna species

GEOTECHNICAL INVESTIGATION

Their investigation includes review of available geotechnical information, excavation of

ten pits to a maximum of 3.3 m below the existing surface, laboratory testing on selected samples

recovered from the test pits, and preparation of a geotechnical report advising the suitability of

the proposed dam. They used desktop study to review the published geological information

within the study area. They clearly stated the sources of the review which is important in doing a

review. In the fieldwork, they recorded the period of test, the location of the pits using GPS, and

it was performed by a geotechnical engineer to ensure that the study will be done accurately. The

3.3 meter pits were excavated using a 5.5 ton excavator, done by strictly following the safety

precautions implemented. After obtaining the needed information and sample, the test pits were

backfilled by the excavated materials, which was compacted to reinstate the natural surface. The

geotechnical laboratory testing was conducted by the NATA registered laboratories of SMEC

Testing Services. The laboratory test includes: Moisture content, Atterberg limit, Particle size

distribution test, Emerson class and Organic content.

The results of the geotechnical study were thorough described. They first describe the

surface condition of the site; the sloping of the valley and flood plain, the flow of the creek and

also the vegetation of the site. On the subsurface condition, they first described the foundation of


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the proposed dam site and its proposed left and right abutments; the soil and rocks on the

different layers revealed on the test pit and the organic content for them to identify the location

to be stripped and excluded from the embankment. They also identified the potential material

sources for use within the embankment dam. Also they described the condition of the potential

site for the proposed spillway.

SMEC concluded that Geotechnical investigations confirmed that the site is suited to an

earth dam. The depth to rock on the right abutment is shallow and the foundation material in the

left abutment is strong enough to support the maximum height of dam envisaged. There are

sufficient quantities of fill material that are available within 500 m of the dam. The use of

available fill material will not negatively affect the structural integrity of the dam.

HYDROLOGY

One of the critical elements of the feasibility is the determination of a satisfactory size of

the Pigring Creek storage and one of the things this is dependent on is the hydrology of the site.

The decision of the size of the dam will greatly influence the quantity of water that are supplied,

stored, used and lost, so the determination of this is very important. Their analysis included the

runoff, rainfall, evaporation, spillway capacity, the quality of water and even the climate change.

Runoff data was crucial to the analysis of storage yield because this may likely give the

approximate runoff of catchment. They used the nearest stream flow gauge and it was assumed

that this site has a linear relationship to the proposed dam site. For the purpose of drought

analysis, they selected the periods of 1999 to2009 as it demonstrated low recorded runoff data.

They required the analysis of the rainfall to determine the relationship of the data

gathered from the runoff and the selection of the period of analysis was appropriate. They used

the datum gathered from the rainfall gauges near the Pigring Creek catchment. Among them, the


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Riverviews Rainfall Gauge was used to check its relationship the runoff data, since it was the

nearest to the catchment site and it contained the full period of record.

Evaporation was also considered for the purpose of establishing whether a relationship

between evaporation and runoff existed and also to determine the potential losses from the

storage might be. They have two evaporation stations options, and they chose Canberra Airport

since it contained 42 year record. According to them, once water is contained in storage, the

effect of evapotranspiration on total volumes is reduced as the surface area and exposure to wind

is reduced. With this and the gathered data obtained, the evaporation that might occur on the

proposed storage can be estimated.

To determine the appropriate size of the spillway, a peak design outflow for the storage

was required. They used the flood frequency analysis of the 30 year period of runoff. They

estimate the acceptable spillway flood capacity by assuming the peak outflow is equal to the

peak inflow and by considering the hazard categories.

They conducted a preliminary assessment of the degree and extent of actual or potential

issues affecting surface and ground water quality within the proposed dam area. One of the

elements carried out for this component of the study was the desktop review which includes the

hydrogeology, aerial photography, and history of the site. They also performed a site visit to

personally observe any potential contamination or any activities that can contaminate the site and

to get soil and surface water sample. Then, they performed a chemical analysis, following the

health-based guidelines since the proposed site is to be used as town water reservoir.

SMEC included the climate change to their study. They considered any changes that

might happen in the near future due to this. A precise estimate could not be done since it is in the


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developing field of science. They expected that there would be a reduction in sustainable yield in

the future.

In summary of their report in the hydrology of the proposed site, they concluded that

there is sufficient water available on the proposed dam site. There were no observed or tested

indications of contamination or other water quality issues. Field and laboratory testing of on-site

surface water and surface soil did not identify any potential issues relating to future water

quality.

ENVIRONMENT

The aim of their environmental assessment is to identify potential environmental

constraints and opportunities to the development of the Pigring Creek site. It detailed the likely

environmental planning process and associated environmental assessment required for the

approval of the proposal. They presented the preliminary findings on the existing environment

including ecology, water quality and heritage based on the desktop studies and limited site

inspections. The objective of this study is to provide a preliminary assessment of the degree and

extent of actual or potential issues affecting the surface water quality within the proposed dam

area. They stated that they did not provide a complete assessment of the environmental status of

the site and is limited to the scope defined.

Ecological Aspects

The Assessment of the ecological study was confined on a desktop review of the study

area. This assessment determines the potential vegetation within the area, the habitat it provides

and the potential for threatened flora species and vegetation communities to occur. They used

this information to determine the potential impacts of the proposed dam, which will include the

inundation of the natural depression along the creek line, ecological constraints, and to inform


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further assessment. They honestly said that they did not complete the detailed field assessment of

the ecology of the area and they recommended a further study within the area must be

undertaken by an experienced ecologist. On their assessment, they found out that the

construction of the earth wall dam at Pigring Creek will result in the loss of the current terrestrial

habitat bordering the ephemeral creek line this habitat is considered to be marginal habitat for

common species, it has the potential to positively influence the biological diversity of the study

area and surrounding areas by providing a permanent source of water for fauna species. The

installation of an environmental flows outlet would aid in maintaining downstream habitat

health.

Other Ecological Aspects

According to their Environmental Planning and Assessment Act 1979, it is necessary to

take due consideration for all issues that have the potential to impact on the environment,

whereas some of these issues may not be significant, still assessment of them is required. They

enlisted potential environmental issues, their potential impact in short and long term and their

recommendations. They included the construction noise, air quality, traffic, visual amenity, water

quality and hydrology, soil erosion, waste contamination, and even carbon emissions.

Indigenous and European Heritage

Heritage database searches and literature reviews were conducted to gather information

on the historical context. Based on the reviews, they confirmed that there is no aboriginal

heritage items have been recorded on the site, but still they recommended to undertake field


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survey during the environmental impact assessment phase of the proposed work to confirm this

report.

Community Consultation

It is required to give opportunity to all relevant stakeholders to voice any concerns to

ensure that an equitable and sustainable outcome is arrived at. They did not state in their report

what the stakeholder have voiced out, or if they conduct the consultaion.

DAM

This report included the concept design, cost and cost estimate of the dam.

Concept Design

The concept design of the dam is based on current best engineering practice and relevant

ANCOLD Guidelines. They consider in their investigation the alignment, geotechnical, design,

technical constraints and the use of local materials.

The proposed dam has been aligned to lie just upstream of the ridge on the left abutment

so that the force of the water in the reservoir would push the embankment against the ridge

improving the stability of the structure. In addition, a cut off under the central section of the dam

would be founded mostly on weathered material upstream of the ridge avoiding the potential for

extensive treatment of the jointed rock in the ridge.

Geotechnical investigations confirmed that the site is suited to an earth dam. The depth to

rock on the right abutment is shallow and the foundation material in the left abutment is strong

enough to support the maximum height of dam envisaged.

They made three dam design options with varying storage capacity and height which was

based on the recommendations of other studies as well as recommended in this study. The site of


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the proposed dam was found to have adequate material available on site in terms of quantity and

properties for construction of an earthfill dam. The earthfill would be sourced either via a borrow

area or through excavation of the spillway through the left abutment. The site of the spillway

may vary depending on the height of the dam selected.

They also included in their design the other miscellaneous parts of the dam such as the

access road and instrumentation. Allowance has been made for a nominal 350 m long unsealed

access road to the dam. In addition a 4 m wide trafficable crest width has been provided in the

design of the embankment to allow access across the dam. They will also provide

instrumentation for measurement of seepage and surface movement. A V-notch weir would be

installed at the downstream toe of the dam to enable monitoring of seepage flows. Survey

markers would be installed in three locations along the dam crest; at maximum section, and

between the maximum section and each abutment.

On their study, they found no technical constraints that would impact the viability of the

proposed dam. The site is well suited to an earth dam up to 13 meters high utilizing the

topographical formation and the locally available materials.

They are required to utilize local materials in the construction of the proposed earth dam

in no way increases the risk of structural failure. As discussed, the on-site materials are well

suited as a source of earthfill. So, this requirement can be done, hence the construction materials

can be minimized.

Cost

The planned annual operating cost will be spending mostly on the dams maintenance.

The cost includes annual surveillance review by a suitably qualified engineer, monthly visits as

part of normal inspection of the water supply works at Nimmitabel, annual weed control and


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annual road maintenance. Other inspections would be required after large floods and for

unplanned events.

Cost Estimate

They prepared cost estimate for the three dam design options. Quantities were estimated

based on the field survey they have undertaken. Their selection of rates for the construction of

the various components of the work were based from the construction rates of similar projects,

regional rates for placing earthfill, published construction rates and rates adopted by SMEC for

recent asset review projects which include similar sized structures. With this, they can make a

good estimate of the project. Their estimations includes the cost of the principal items of

construction for each option, including an establishment cost of 10% of the construction items

(including temporary works), 20% allowance for minor items, 40% allowance for construction

contingencies, allowance of $100,000 each for investigations, design and construction

management, and 20% allowance for design and construction management contingencies. Their

cost estimates indicate that a dam about 8 m in height providing 200 ML of storage could

potentially be constructed within the budget available if costs are controlled.

In general, for me, SMEC did a good study. They are capable of doing this kind of study

since they have experts in the different fields of this study and they have complete equipment

and laboratories needed for the examinations of the sample. In the technical aspect, they studied

all the different aspects that might affect the proposed structure. They examined it detailed by

detailed, looking for possible constraint to the project. SMEC make sure that there will be a

constant supply of water specially during drought season. But on the financial analysis, they did

not include in their study the rate of return or if the project is profitable. They just include in their


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study the cost of the project and its annual operational cost. They did not include on how, when

or if there is a return of investment. It is not clear on how they will finance operational cost.

final critique (pigring creek dam fs)

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