Factors affecting the performance and cost-efficiency of sand storage - PDF document

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/280304046 Factors affecting the performance and cost-efficiency of sand storage dams (presentation) Conference Paper · July 2015 CITATIONS READS 0 191 4 authors , including: Josep De Trincheria Walter Leal Filho Technische Universität Hamburg Hochschule für Angewandte Wissenschaften Hamburg 22 PUBLICATIONS 64 CITATIONS 624 PUBLICATIONS 5,790 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: AFHRINET View project ESIDA - Epidemiological Surveillance for Infectious Diseases in Sub-Saharan Africa View project All content following this page was uploaded by Josep De Trincheria on 23 July 2015. The user has requested enhancement of the downloaded file.

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

Josep de Trincheria Josep de Trincheria Josep de Trincheria Factors affecting the performance and cost-efficiency of sand storage dams De Trincheria, Josep, Hamburg University of Technology, Germany josepm.trinxeria@gmail.com Co-authors: Nissen-Petersen, Erik; Leal, Walter; Otterpohl, Ralf Josep de Trincheria 36 th IAHR World Congress The Hague, Netherlands 2 nd 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 Source: Borst and de Haas, 2006

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 out: 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 8,000 7,000 Volume of sand (m3) 6,000 5,000 4,000 3,000 2,000 1,000 0 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 83% presented volumes of sand <1000 m3  2 types of reservoirs: Clogged and graded-bedded 

4.1 Results: Sand storage capacity, clogged reservoirs Josep de Trincheria Josep de Trincheria

4.1 Results: Sand storage capacity, graded-bedded reservoirs Josep de Trincheria

12 4.2 Results: Yearly water yield 2,000 1,800 1,600 1,400 1,200 m3/year 1,000 800 600 400 200 0 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 Yield Yield seepage losses Yield evaporation Yield contribution riverbanks 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 90 80 70 N° households/dry season 60 50 40 30 20 10 0 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 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 

4.4 Results: Cost-efficiency 22,500.00 Actual cost-efficiency evaporation Actual cost-efficiency contribution Costs 20,000.00 17,570 17,500.00 15,000.00 EUR/m3 12,500.00 10,000.00 7,500.00 5,000.00 2,500.00 15 0.00 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 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 of low specific yield  Permeable reservoirs

16 5.2 Structural design  There is a need to avoid spillways built in one- stage of elevated height  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 of 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

17 5.3 Spillways built in one-stage  4 reasons for low performance  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 optimal This is justified by:  Variability of rainfall, sediment and runoff  Forced interbedding  Non-selective scouring  Maturity paradox

18 5.4 One-stage spillways: Variability of the rainfall, runoff and sediment transport Seasonal rainfall variability at Dwa plantation in Kibwezi, 1927-1997 Source: (Gichuki, 2000) Ten-year distribution of good, normal and poor rainfall years in Kibwezi Source: (Gichuki, 2000)

19 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

20 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