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Factors affecting the performance and cost-efficiency of sand storage dams (presentation)

Conference Paper · July 2015

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www.iahr2015.info 36th IAHR World Congress The Hague, Netherlands 2nd July 2015

Factors affecting the performance and cost-efficiency

• f sand storage dams

De Trincheria, Josep, Hamburg University of Technology, Germany josepm.trinxeria@gmail.com Co-authors: Nissen-Petersen, Erik; Leal, Walter; Otterpohl, Ralf 36th IAHR World Congress The Hague, Netherlands 2nd July 2015

Outline

• 1. Sand storage dams
• 2. Objectives
• 3. Methodology
• 4. Real-life performance and cost-efficiency
• 5. Key driving factors
• 6. Practical recommendations
• 7. References
• 8. Further remarks

• 1. Sand storage dams (I)
• Small-scale hydraulic retention structures, which are built

across seasonal streams in rural arid and semi-arid areas of low-income economies, especially sub-Saharan Africa, Brasil and India.

• They are sited and designed to create an artificial reservoir

made of coarse/medium sandy alluvium sediments. During the wet period, this reservoir is expected to be filled up with water and yield and supply water during the dry season.

Sketch of a sand storage dam

• 1. Sand storage dams (II)
• In order to achieve this, it is basic and crucial to site the sand

dams in catchment areas with adequate production of coarse/medium sand sediments.

• It is also vital to avoid siltation and accumulation of fine sand

sediments in order to optimise the yield and supply of the reservoirs.

• General lack of data and capacity to conduct continuous

monitoring and assessment of relevant hydrogeological variables.

• 2. Objectives
• 1. To identify and analyse the factors which may

affect the performance and cost-efficiency of sand storage dams.

• 2. To recommend simple practical actions to optimise

performance and cost-efficiency in as many different biophysical conditions taking into account the generalised absence of data of relevant hydrogeological variables.

• 3. Methodology
• Based on an extensive physical survey of 30 SDs by direct measurement of the

depth of alluvium sediments (De Trincheria et al. 2015, submitted). For each SD, 3 probing points across the width along 20 sub-sections of 300 m were carried

• ut: 60/SD; 1800 in total
• Sand storage capacity  The accuracy of the formula [1,2,3,4] was improved: high

number of probing points; specific capacity for each probing point; real throwback; geometrical shape

• Yield and supply capacity  The accuracy of the calculation was improved:

bimodal rainfall season; seepage and evaporation losses; potential contribution from the riverbanks and sediments upstream, most representative specific yield; length of the dry periods; consumption of local communities

• Construction costs and cost-efficiency  On-the-ground measurements
• Review of scientific literature and technical guidelines to further identify and

analyse the performance and cost-efficiency factors

4.1 Results: Sand storage capacity

• 83% presented volumes of sand <1000 m3
• 2 types of reservoirs: Clogged and graded-bedded

1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 25 7 13 14 22 12 29 30 3 5 2 4 1 8 6 9 11 10 15 17 19 24 27 16 18 20 21 26 28 31 Volume of sand (m3)

4.1 Results: Sand storage capacity, clogged reservoirs

4.1 Results: Sand storage capacity, graded-bedded reservoirs

12

200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 25 7 13 14 22 12 29 30 3 5 2 4 1 8 6 9 11 10 15 17 19 24 27 16 18 20 21 26 28 31

m3/year Yield Yield seepage losses Yield evaporation Yield contribution riverbanks

4.2 Results: Yearly water yield

• Average specific yield 6.9% → 7 [3,12]% is the specific yield for silty

and sandy clay alluvium sediments

• The average yields were 112 m3/year
• 2000 m3 is the minimum satisfactory yearly yield of a sand dam [5]

4.3 Results: Water supply capacity

10 20 30 40 50 60 70 80 90 25 7 13 14 22 12 29 30 3 5 2 4 1 8 6 9 11 10 15 17 19 24 27 16 18 20 21 26 28 31

N° households/dry season

Actual Evap H Actual Evap C Actual Riv H Actual Riv C

• Total aggregated supply capacity for the 30 SDs was 64 and 39 households
• 90 is the average number of households per village in rural area
• This is equivalent to 320 and 195 individuals
• 17,000 inhabitants in the entire study area
• 660 inhabitants is the typical village size

15 17,570

0.00 2,500.00 5,000.00 7,500.00 10,000.00 12,500.00 15,000.00 17,500.00 20,000.00 22,500.00 25 7 14 13 12 29 3 5 2 4 8 1 11 9 6 17 10 19 15 24 27 18 20 21 31 16 28 26 EUR/m3

Actual cost-efficiency evaporation Actual cost-efficiency contribution Costs

4.4 Results: Cost-efficiency

• Total/Average costs EUR 241,899/ EUR 8,639/SD
• Average yield cost-efficiency: 5,635 EUR/m3
• EUR 134,830 were invested in SDs producing yearly yields lower than 1

m3/year

• Average supply cost-efficiency: 7,312 EUR/household
• Between EUR 167,715 and 190,425 did not supply water to any household

• 5. Key driving factors

5.1 Siting procedure  There is a need to avoid:

• Reservoirs with fine sand
• Reservoirs with silty and/or clayey sediments
• Reservoirs with other fine grain-size alluvium sediments
• f low specific yield
• Permeable reservoirs

• Design principle: Maximise the accumulation of coarse sand and avoid

siltation (low specific yield of the reservoir)

• Height of the spillway is a key parameter  adequate flow velocities

which optimise the deposition of coarse sand sediments and scouring

• f finer grain-size sediments [7]

in as many different catchment areas, and rainfall, runoff and sediment transport conditions as possible

• As higher the stage height, higher the probability to block the

suspended load and vulnerability to variability, and lower the effective replicability in different catchment areas with different geological conditions

16

5.2 Structural design  There is a need to avoid spillways built in one- stage of elevated height

• There are sand dams built in one stage with no apparent siltation problems [6]

 Catchment areas producing large volumes of sand sediments and/or time- specific rainfall, runoff and sediment transport conditions which have allowed the accumulation of sand sediments

• 2 key remarks:
• It should not be assumed that these highly specific conditions are

applicable to other areas or different time-periods.

• There is a high probability that the performance and cost-efficiency is not
• ptimal

This is justified by:

• Variability of rainfall, sediment and runoff
• Forced interbedding
• Non-selective scouring
• Maturity paradox

17

5.3 Spillways built in one-stage  4 reasons for low performance

5.4 One-stage spillways: Variability of the rainfall, runoff and sediment transport

Ten-year distribution of good, normal and poor rainfall years in Kibwezi

18

Seasonal rainfall variability at Dwa plantation in Kibwezi, 1927-1997

5.5 One-stage spillways: Forced interbedding

• Construction of one-stage high spillways  Higher probability to block

the bedload and suspended load

•  Production of graded-bedded reservoirs
• The expected water yield of the sand dam may be assumed to be much

lower than the real-life water yield of the reservoir

19

5.6 One-stage spillways: Non-selective scouring of fine grain-size sediments

• Subsequent floods should not be assumed to systematically wash away fine

sediments and leave the coarsest ones

• The energy of the flow is highly variable  inherent extreme temporal and

spatial variability of rainfall, flood and sediment transport

• The depth of the fine grain-size sediments accumulated causes that the energy

required to effectively scour is higher

• As higher the stage height, the lower the probability that the river flow will have

the required energy to effectively scour silty and clayey sediments

20

5.7 One-stage spillways: Maturity paradox

• Towards the end of the filling up of the SDs built as one-stage high spillways
• Spillway height will always have the adequate height to only retain the coarsest

bedload sediments [7]

• Independently of the actual distribution of sediments in the reservoir, it will

always appear to be homogenously filled up with sandy alluvium sediments

• Reservoir will be wrongly assumed to be mature and homogenously filled of

coarse/medium sand sediments

• The real-life specific yield may be significantly lower as expected  low

performance and cost-efficiency

21

• 6. Practical recommendations

6.1 Systematic siting procedure which allows the selection

• f sub-sections of the riverbed which receive adequate

volumes of coarse and/or medium sandy alluvium sediments and are impermeable

6.2 Correct structural design: Build the spillway by stages of reduced height in order to maximise the probability to block the bedload in as many different conditions as possible

6.3 Practical recommendations: Height of the stages

• Without an adequate evaluation of the bedload and suspended load

characteristics, the precautionary height of 18-50 cm

• 18 cm [8]  high proportions of fine alluvium sediments
• 50 cm [9]  high proportions of coarse sandy alluvium sediments.
• The height of the stage should be adapted to the minimum height of

the bedload layer

• Only after an evaluation of the rainfall, runoff and sediment transport,

coupled with conversations with local communities, the implementation of higher stage heights may be possible with a higher probability to avoid siltation

24

Thank you for your attention!

• 7. References (I)
• [1] Hudson, N. (1975). Field engineering for agricultural
• development. Oxford, U.K.: Clarendon Press
• [2] Lawrence, P. & Lo Cascio, A. (2004). Sedimentation in small

dams: Hydrology and drawdown computations. United Kingdom: HR Wallingford Ltd and Department for International Development

• [3] Sawunyama, T., Senzanje, A., & Mhizha, A. (2006).

Estimation of small reservoir storage capacities in Limpopo river basin using geographical information systems (GIS) and remotely sensed surface areas: Case of Mzingwane catchment. Physics and Chemistry of the Earth, Parts A/B/C, 31(15), 935- 943

• [4] Stephens, T. (2010). Manual on small earth dams: A guide to

siting, design and construction. Rome, Italy: Food and Agriculture Organization of the United Nations.

• 7. References (II)
• [5] Lasage, R., Aerts, J. C., Verburg, P. H. & Sileshi, A. S. (2013).

The role of small scale sand dams in securing water supply under climate change in Ethiopia. Mitigation and Adaptation Strategies for Global Change 20 (2) 317-339

• [6] Ertsen, M., Hut, I. R. & van de Giesen, N. (2006).

Understanding hydrological processes around groundwater dams in Kenya. Geophysical Research Abstracts, 8(00942).

• [7] Wipplinger, O. (1958). Storage of water in sand. South West

Africa Administration Water Affair Branch. Windhoek, Namibia.

• [8] Ochieng, G. M., Otieno, F. A., Shitote, S. M. & Sitters, C. C.

(2008). Investigating the effect of step height increment, channel slope, and flow rate on specific yield of sand dams.

• [9] Nissen-Petersen, E. (2006). Water from Dry Riverbeds. How dry

and sandy riverbeds can be turned into water sources by hand-dug wells, subsurface dams, weirs and sand dams. ASAL Consultants Limited for the Danish International Development Assistance.

Factors affecting the performance and cost-efficiency

• f sand storage dams

De Trincheria, Josep, Hamburg University of Technology, Germany josepm.trinxeria@gmail.com Co-authors: Nissen-Petersen, Erik; Leal, Walter; Otterpohl, Ralf 36th IAHR World Congress The Hague, Netherlands 2nd July 2015

www.iahr2015.info 36th IAHR World Congress The Hague, Netherlands 2nd July 2015

Further remarks

3.1 Study Area

3.2 Example of study site

5.1 Correct siting: Reservoir collecting sandy alluvium sediments

5.1 Incorrect siting: Reservoirs with silty and/or clayey sediments

5.5 One-stage spillways: Forced interbedding

www.outreach.canterbury.ac.nz

5.8 Incorrect design: Seepage losses

36

Stage 1. Spillway is 50 cm above the sand level Stage 2. Flood has brought sand to the level of the spillway Stage 3. Spillway is raised to 50 cm above new sand level Stage 4. Flood has deposited sand to the new level of the spillway Stage 5. Spillway is raised to 50 cm above new sand level Stage 6. This procedure is repeated until the spillway is fully closed

6.2 Build the spillway by stages of reduced height in order to maximise the probability to block the bedload in as many different conditions as possible

• Even by stages, 100% perfection cannot be achieved  with smallest floods the

velocity of flow may always be low enough for the deposition of fine grain-size sediments of reduced specific yield [7].

• Spillways by stages according to the most probable flood  SDs accumulating

silty and clayey alluvium sediments during poor rainfall years and droughts.

• High spillways in one stage  systematic accumulation of large volumes of fine

grain-size sediments  low performance and cost-efficiency

38

6.4 Practical recommendations: Multi-stage construction

• Specific yield analysis of representative sediments in the reservoir

should always be systematically carried out

• In the absence of those:
• It is recommended to use 20-25% as the reference maximum

specific yield of coarse sand reservoirs

39

6.5 Practical recommendations: Real-life specific yield

40

6.6 Other practical recommendations

• To select naturally deep layers of sand sediments and/or alluvial

aquifers

• To take into account the slope of the riverbed and its effect on the

throwback, as they can maximise performance and cost-efficiency

Take-home messages

1. SDs evaluated showed significant low performance and cost-efficiency. This was mainly due to:  Incorrect siting: not adequate production of volumes of coarse/medium sediments  incorrect design: siltation was not effectively prevented (even in the cases of satisfactory production of sand sediments) because the spillways were built in one stage of elevated height (1-5 m) 2. Siting SDs in impermeable sub-sections of the riverbed with adequate production of coarse and medium sandy alluvium sediments is always strictly required 3. Constructing the spillway by stages [0.18-0.50 m] maximises the probability that the performance and cost-efficiency will be optimal (if SD has been adequately sited)

Way forward

1. Reformulation of currently available technical guidelines to improve the robustness of the performance and cost-efficiency in any biophysical conditions (geology, rainfall, runoff, sediment transport) 2. Further performance evaluations in other areas with different biophysical conditions 3. Further research on the stage height in different biophysical conditions 4. Continuous monitoring of the performance of current and new SDs, and relevant hydrogeological variables: rainfall, runoff and sediment transport