Farm Dams for the Sugar Industry - Appendix A

8/10/2019 Farm Dams for the Sugar Industry - Appendix A

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Farm Dams for t he Sugar Indust ry Dam Opt ions

June 2001 FSA Irrigation Page No. A-1

Appendix A DAMOPTIONS

A.1 Introduction

Farm dams can take many forms depending on site conditions. It is important tounderstand the type of dam required for each site and to understand the efficiencyof water storage.

In terms of construction cost, efficiency is measured either as S:E ratio (storage toexcavation ratio) or as cost per megalitre stored. This chapter will show that thecapital cost (earthworks only) of farm dams can vary from $100/ML to $3000/ML.Obviously this strongly affects the economic variability of farm dam design.

Efficiency can also be gauged in terms of evaporation loss. This depends on local

climate and surface area.


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A.2 Gully Dams

A gully dam is an earth embankment built across a gully, stream or depression (seePhotograph A-1). Despite being the most difficult to design and construct, it is the

most favoured due to its good S:E ratios.

Figure A-1 shows a typical layout for a gully dam. As the embankment crosses adrainage line, provision must be made for excess runoff to pass around theembankment. Usually a wide bywash is used but there are alternatives available (seeChapter 4).

Storage capacity and embankment earthworks for gully dams can be estimated usingFigure A-2 and Figure A-3. For large gully dams it is appropriate to survey the siteand use computer software to accurately calculate earthworks.

PHOTOGRAPH A-1 TYPICAL GULLY DAM


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FIGURE A-1-TYPICAL LAYOUT FOR A GULLY DAM

(Nelson, K.D. 1985, Design and Construction of Small Earth Dams, Inkata Press Pty Ltd,Melbourne.)

TOP WATER SUPPLY LEVEL

FREEBOARD

BERM

CUTOFF EXCAVATION

CREST

EMBANKMENT

BORROW PIT

TOP WATER LEVEL

OUTLET

BATTER 2:1

INLE

UPSTREAM TOE

DOWNSTREAM TOE

BORROW PIT EXCAVATION

(3:1 BATTERS)

BYWASH

BATTER 2:1

CUTOFF EXCAVATION

DESIGNED CREST LEVEL

ELEVATION OF EMBANKMENT

LONGITUDINAL SECTION THROUGH STORAGE

PLAN OF STORAGE

NATURAL SURFACE

BYWASH

CONTOURS

RETURNSLOPE


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FIGURE A-2 ESTIMATION EARTHWORKS FOR A GULLY DAM

Based on 1:3 batters internal and external.(Officers of Rural Water Advisory Services 1995, Gully Dams Estimating earthworksand storage volumes [Online]. Available: http://www.dnr.qld.gov.au [Accessed 20

January 2001]).


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FIGURE A-3 ESTIMATION OF CAPACITY FOR GULLY DAMS

(Officers of Rural Water Advisory Services 1995, Gully Dams Estimating earthworksand storage volumes [Online]. Available: http://www.dnr.qld.gov.au [Accessed 20

January 2001]).


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A.3 Hillside Storages

Hillside storages are built on the side of a hill that has no significant drainage line.They have three-sided or curved banks (see Photograph A-2). Catch drains are

typically needed to ensure that that sufficient runoff enters the storage. These damshave poor storage ratios but can provide a gravity supply during irrigation. Abywash is required for such a dam. Figure A-4 shows a typical layout for a hillsidestorage.

Storage capacity and embankment earthworks for hillside storages can be estimatedusing the same figure for a gully dam with higher k values around 1.2 to 1.6. (SeeFigure A-2 and Figure A-3). For large hillside storages it is appropriate to survey thesite and use computer software to accurately calculate earthworks.

PHOTOGRAPH A-2 TYPICAL HILLSIDE STORAGE


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FIGURE A-4 TYPICAL LAYOUT FOR A HILLSIDE STORAGE

(Fitzsimon, R. 1995, Planning Your Farm Dam [Online]. Available:http://www.dnr.qld.gov.au[Accessed 20 January 2001].

CATCH

CATCH

DRAINDRAIN

STONE PITCHINGPROTECTION AGAINST

EROSION

BYWASH FLOWS MUSTBE RETURNED TO THE

BED OF THE DEPRESSIONBEFORE LEAVING THE

TOPWATER

LEVEL

BYWASH RETURN SLOPEIS TO BE WELL GRASSED

TO PREVENT EROSION

BORROW PIT

EXCAVATION


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A.4 Ring Tanks

A ring tank is built on flat ground and consists of a continuous bank that can be

circular, square or rectangular (see Photograph A-3). They are filled by pumpingwater in from an external source. The bank earthworks are obtained from anexcavation inside the bank or from an external sump. Some stored water is belowthe natural surface level and has to be pumped out when needed.

Ring tanks with high embankments are more expensive to construct due to the largeramount of soil moved. However, with an increased height, the volume of waterstored increases while the surface area of exposed water remains the same. This isdesirable for minimizing evaporation losses in proportion to the volume of waterbeing stored. S:E ratios for ring tanks are discussed in Section A.6. Figure A-5 can beused to estimate earthworks requirements for round ring tanks, while Table A-1 and

Table A-2 give earthworks estimations for rectangular ring tanks.

PHOTOGRAPH A-3 TYPICAL RING TANK (SHOWING ADJACENT WATERCOURSEAND EXTERNAL SUMP ON TWO SIDES)


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FIGURE A-5- EARTHWORKS ESTIMATION FOR ROUND RING TANKS


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TABLE A-1 EARTHWORKS ESTIMATION FOR RECTANGULAR RING TANKS(UP TO 75 ML)

Square Rectangular

Length at TWL = width at TWL Length at TWL = 2.0 x width at TWL

CapacityEmb.Height

FillEarthworks

BorrowDepth

OutsideToe

Length

OutsideToe

Width

FillEarthworks

BorrowDepth

OutsideToe

Length

OutsideToe

Width

(ML) (m) (m) (m) (m) (m) (m) (m) (m) (m)

5 2 - - - - - - - -

5 3 - - - - - - - -

5 4 - - - - - - - -

5 5 - - - - - - - -

5 7 - - - - - - - -

10 2 7300 4.0 89 89 - - - -

10 3 - - - - - - - -10 4 - - - - - - - -

10 5 - - - - - - - -

10 7 - - - - - - - -

15 2 9400 1.7 111 111 9700 2.0 144 85

15 3 - - - - - - - -

15 4 - - - - - - - -

15 5 - - - - - - - -

15 7 - - - - - - - -

20 2 11200 1.2 130 130 11700 1.4 170 98

20 3 13700 13.4 97 97 - - - -

20 4 - - - - - - - -

20 5 - - - - - - - -

20 7 - - - - - - - -

30 2 14300 0.8 161 161 14900 0.9 214 120

30 3 17700 8.7 120 120 - - - -

30 4 - - - - - - - -

30 5 - - - - - - - -

30 7 - - - - - - - -

40 2 16900 0.7 188 188 17700 0.7 252 139

40 3 21000 3.1 138 138 21900 3.8 181 106

40 4 - - - - - - - -

40 5 - - - - - - - -

40 7 - - - - - - - -50 2 19200 0.6 211 211 20100 0.6 285 155

50 3 24000 2.3 155 155 25000 2.6 205 117

50 4 - - - - - - - -

50 5 - - - - - - - -

50 7 - - - - - - - -

Inputs: Crest Width 5m, Internal Batter 1:4, External Batter 1:2, Freeboard 1.0m,Corner Radius 0m, includes 0.5m deep cutoff trenchNotes: capacity includes borrow volume, embankment height is above naturalsurface, assumes flat surface, borrow depth assumes all borrow taken from inside,

No value (-) indicates require external borrow.


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TABLE A-2 EARTHWORKS ESTIMATION FOR RECTANGULAR RING TANKS(75 ML TO 500 ML)

Square Rectangular

Length at TWL = width at TWL Length at TWL = 2.0 x width at TWL

CapacityEmb.Height

FillEarthworks

BorrowDepth

OutsideToe

Length

OutsideToe

Width

FillEarthworks

BorrowDepth

OutsideToe

Length

OutsideToe

Width

(ML) (m) (m) (m) (m) (m) (m) (m) (m) (m)

75 2 24100 0.4 261 261 25300 0.5 355 191

75 3 30300 1.5 190 190 31700 1.6 254 142

75 4 37700 5.7 159 159 - - - -

75 5 - - - - - - - -

75 7 - - - - - - - -

100 2 28200 0.4 303 303 29700 0.4 415 221

100 3 35600 1.2 220 220 37300 1.3 297 163100 4 44600 3.4 184 184 46600 4.1 243 139

100 5 - - - - - - - -

100 7 - - - - - - - -

150 2 35200 0.3 374 374 37000 0.3 515 271

150 3 44500 0.9 270 270 46700 0.9 367 199

150 4 56100 2.2 225 225 58800 2.4 301 168

150 5 68400 6.1 198 198 - - - -

150 7 - - - - - - - -

200 2 41000 0.2 434 434 43200 0.2 600 313

200 3 52000 0.7 313 313 54700 0.8 427 229

200 4 65800 1.7 259 259 69200 1.8 350 192200 5 80700 3.9 228 228 84700 4.6 304 171

200 7 - - - - - - - -

300 2 50900 0.2 534 534 53700 0.2 742 384

300 3 64700 0.5 384 384 68100 0.6 528 279

300 4 82200 1.2 317 317 86500 1.3 432 233

300 5 101300 2.5 278 278 106500 2.8 375 207

300 7 - - - - - - - -

400 2 59200 0.2 619 619 62500 0.2 861 444

400 3 75300 0.5 444 444 79400 0.5 612 321

400 4 96000 1.0 366 366 101100 1.1 501 268

400 5 118600 2.0 320 320 124900 2.2 435 236

400 7 166700 8.1 268 268 - - - -

500 2 66500 0.1 694 694 70200 0.1 967 496

500 3 84700 0.4 496 496 89400 0.4 687 358

500 4 108100 0.9 409 409 114000 0.9 562 298

500 5 133900 1.7 358 358 141100 1.8 488 263

500 7 189200 5.6 298 298 199000 6.9 400 223

Inputs: Crest Width 5m, Internal Batter 1:4, External Batter 1:2, Freeboard 1.0m,Corner Radius 0m, includes 0.5m deep cutoff trenchNotes: capacity includes borrow volume, embankment height is above naturalsurface, assumes flat surface, borrow depth assumes all borrow taken from inside,

No value (-) indicates require external borrow.


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A.5 Excavated Sumps

Excavated sumps are fully cut below the natural surface level (see Figure A-6). The

excavated material is generally not required for the bank but is stockpiled nearby ordisposed of. These sumps are ideally built in clay soils or, where this is not possible,should have a clay lining. When excavated to a reasonable depth, this type ofstorage can have the advantage of relatively low evaporation losses, however the S:Eratio is typically in the order of 0.9-1.0, which is not very efficient. Photograph A-4 isan example of a sump constructed adjacent to a ring tank. In this case the excavatedmaterial can be used for the construction of the ring tanks embankment.

With the proper soil conditions, an excavated sump can also be designed to act as asoakage or seepage tank. This setup is possible when the local water table isrelatively close to the surface and underlain by an impermeable rock layer. The

sump is excavated into pervious soil to a depth below the water table. The sump isthen filled naturally by groundwater flows. In this situation, a clay lining is notrequired and can even prove to be counterproductive.

Freeboard is not usually required unless the sump is intended to intercept significantdrainage from surrounding areas. On a typical flat site, a bywash is usually onlyrequired to divert excess runoff in a desired direction.

Figure A-7 and Table A-4 can be used to estimate the capacities for excavatedsumps.

PHOTOGRAPH A-4EXCAVATED SUMP ADJACENT TO RING TANK


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FIGURE A-6 TYPICAL LAYOUT OF EXCAVATED SUMPS

(Nelson, K.D. 1985, Design and Construction of Small Earth Dams, Inkata Press Pty Ltd,Melbourne.)

1000 m3= 1ML

FIGURE A-7 EARTHWORKS/CAPACITY CALCULATIONS FOR EXCAVATED SUMPS

3


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TABLE A-3 EARTHWORKS ESTIMATION FOR EXCAVATED SUMPS


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TABLE A-4 EARTHWORKS ESTIMATION FOR EXCAVATED SUMPS (CONTINUED)


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A.6 Storage Efficiency

Storage efficiency is measured by the storage to excavation ratio (S:E ratio) or capital

cost/ML.

The S:E ratio is calculated as:

)(mVolumeExcavation

1000x(ML)CapacityStorageratioSE

3=

Capital cost/ML is calculated as:

)($/mRateEarthworksx)(mVolumeExcavation

(ML)CapacityStorage

($/ML)cost/MLCapital 33=

Each type of storage has different S:E ratios. Figure A-8 shows the range in S:E ratiofor various storage types. It shows the S:E ratio is 1 or less for excavated sumps. Forring tanks the S:E ratio can range from below 2 for small volumes (


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FIGURE A-8 S:E RATIOS FOR DIFFERENT DAM TYPES

TABLE A-5 APPROXIMATE COSTS AND S:E RATIOS FOR DIFFERENT DAM TYPES

Dam Type Capacity S:E RatioEarthworksCost $/m3

Cost $/ML

1 ML 0.9 3.00 3300ExcavatedTank 5 ML 0.9 2.00 2200

10 ML 2.0 2.00 1000HillsideStorage 100 ML 4.0 1.50 375

10 ML 3.0 2.00 666Gully Dam

100 ML 6.0 1.50 250

100 ML (Square) 1.9 1.50 830Ring Tank

1000 ML (Round) 5.9 1.00 175

S:E~ 5 to 20

S:E~2 to 15

S:E~ 0.8 to 1

S:E~ 2 to 5

HILLSIDE STORAGEOR


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A.6.1 Embankment Height

Three main factors influence the optimal height of embankment:

Cost of earthworks Value of water lost to evaporation Value of land lost to the storage

Increasing embankment height reduces storage efficiency (S:E ratio) because thevolume of earthworks increase disproportionately with height (ieearthworks ~ height). Also, construction standards are more important for damswith greater embankment heights, and this can increase the cost and risk involved.However, to hold the same volume of water, low banked ring tanks need a greatersurface area than high banked ring tanks and therefore incur greater evaporativelosses. Table A-6 shows the increase in earthworks for a 200 ML ring tank withdifferent embankment heights.

Evaporation is not seen as a significant factor in the Bundaberg/sugar cane case,because harvested water is generally irrigated soon after collection and is not storedfor extended periods (usually less than 4 months of the year). Also, the netevaporation from open water surfaces in the Bundaberg area averages around0.1 m/yr. Based on these figures the evaporation per year from a farm dam could bealmost negligible. However, this might not be the case in the dryer areas west ofBundaberg.

The footprint of a dam reduces as the embankment height increases. Reducing theamount of cropland lost to the storage tank may be a high priority if there is limitedcropping land available. However, this might not be as important as first thought asshown in the analysis below.

The relationship between embankment height and cost has been analysed todetermine the optimum height for the Bundaberg area. The earthworks cost and thecost of lost productive land has been included in the analysis. Evaporation was notseen as a significant cost as outline above.

A square 100 ML ring tank was used for the analysis. Assumptions were also madeas to the earthworks rate ($1.50/m) and cost of land ($6000/ha). The results of theanalysis are shown in Table A-7 and presented graphically in Figure A-9. The results

show the optimum height to be 2.5 m. To be conservative and allow for some effectof evaporation the optimum height could be slightly higher at 3.0 m. This is aninteresting result as the optimum height for ring tanks on the Darling Downs isgenerally thought to be 5 to 7 m.


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TABLE A-6 HEIGHT VS. EARTHWORKS FOR A 200 ML RING TANK

CaseHeight

(m)Earthworks

(m3)S:E ratio

Footprint(ha)

Increase in Cost(referenced to Case 1)

1 3.5 59500 3.5 7.9 -

2 4.5 74600 2.8 5.8 25%

3 5.5 90500 2.3 4.6 52%

4 6.5 106500 1.9 3.8 79%

Note: Based on square shape, crest width = 5 m, inner batter slope = 4:1, outer batterslope = 2:1

TABLE A-7- EMBANKMENT HEIGHT VS COST

Earthworks Cost Lost Land CostEmbankmentHeight

(m)Volume

(m)Rate($/m)

CostArea(ha)

LandValue($/ha)

CostTotal Cost

1.5 16660 1.50 $ 24,990 11.4 6000 $ 68,400 $ 93,390

2.5 27233 1.50 $ 40,849 6.4 6000 $ 38,400 $ 79,249

3.5 40270 1.50 $ 60,405 4.9 6000 $ 29,400 $ 89,805

4.5 55583 1.50 $ 83,374 4.2 6000 $ 25,200 $ 108,574

5.5 73167 1.50 $109,750 3.8 6000 $ 22,800 $ 132,550

Note: Based on square shape, crest width = 4 m, inner batter slope = 4:1, outer batter

slope = 2:1, freeboard = 0.5m

$-

$20,000

$40,000

$60,000

$80,000

$100,000

$120,000

$140,000

0 1 2 3 4 5 6

Embankment Height

C

ost($) Earthworks Cost

Lost Land

Total Cost

FIGURE A-9- EMBANKMENT HEIGHT VS COST


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J 2001 FSA I i i P N A 20

A.6.2 Shape

For a given storage volume, various shapes of ring tanks will require differentearthworks volumes. For example, circular tanks require the minimum amount of

earthworks for a given storage volume. Circular tanks therefore have higher S:Eratios. Since cost generally increases linearly with earthworks, circular tanks are theleast expensive tanks to build. However, most sites do not suit a circular ring tank interms of using the available ground most efficiently. Therefore, the higher cost of asquare or rectangular ring tank can often be offset by the benefit of long-term farmefficiency.

Table A-8 shows the difference in cost for different shaped 200 ML ring tanks.

TABLE A-8 COMPARISON OF DIFFERENT SHAPED 200 ML RING TANKS

ShapeEarthworks

(m3)S:E Ratio

Footprint(ha)

Increase in cost(referenced tocircular tank)

Circular 74 100 2.8 5.1 -

Square 82 400 2.5 5.1 11%

Rectangular # 93 800 2.2 5.1 27%

Note: Based on height = 5.0 m, crest width = 5 m, inner batter slope = 4:1, outer batterslope = 2:1, freeboard 1m#Rectangular: Length at centreline = 3 x width at centreline

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Farm Dams for the Sugar Industry - Appendix A