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| ISBN-13: | 9781853390500 |
|---|---|
| Publisher: | Practical Action Publishing |
| Publication date: | 12/01/1988 |
| Pages: | 64 |
| Product dimensions: | 6.25(w) x 9.25(h) x 0.30(d) |
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CHAPTER 1
INTRODUCTION
Supplying safe water and sanitation to all by 1990 is the target of the International Drinking Water Supply and Sanitation Decade. It is clear that this will not be reached, and the work will continue for several decades to come. Experience has shown that there has to be an orientation away from expensive, sophisticated techniques towards appropriate, low-cost and socially acceptable techniques that are adapted to local conditions.
Many developing countries are located in climatic regions where rainfall is seasonal and highly erratic. Supplying water in such regions is to a large extent a matter of storing water from the rainy season to the dry season, and from years with high rainfall to dry years. Using groundwater is a way of overcoming the seasonal shortages, but in some areas even the groundwater resources are depleted towards the end of the dry season and in many areas there are no aquifers available, or they would require deep-drilled wells and pumps for development, a fact that makes this alternative less suitable in certain socio-economic environments.
A method of storing water which has received considerable attention during the last few years is the use of groundwater dams. Damming groundwater for conservation purposes is not a new concept. Groundwater dams were constructed on the island of Sardinia in Roman times and structures in Tunisia show that damming of groundwater was practised by old civilizations in North Africa. There is a report of a sand-storage dam built in Arizona in the eighteenth century. More recently, various small-scale groundwater damming techniques have been developed and applied in many parts of the world, notably in India, southern and East Africa, and Brazil.
1. Definitions
A conventional dam for water storage is usually built across a river or a stream, and· collects surface water which is stored in the open reservoir upstream of the dam. A groundwater dam obstructs the flow of groundwater and stores water below the ground surface. It may also serve as a collecting structure that diverts groundwater flow, for instance to recharge adjacent aquifers, or it can raise the groundwater table in an aquifer with a limited flow of groundwater, which is thus made accessible for pumping.
Groundwater dams may be of two types, sub-surface dams and sand-storage dams. A sub-surface dam is constructed below ground level and arrests the flow in a natural aquifer, whereas a sand-storage dam impounds water in sediments caused to accumulate by the dam itself.
The general principle of a sub-surface dam is shown in Figure 1.1. An aquifer consisting of permeable alluvial sediments in a small valley supplies water to a village by means of a shallow dug well. The area has a monsoon climate, and due to consumption and the natural groundwater flow, the aquifer used to be drained out during the dry season and consequently the well dried up. To prevent this, a trench has been dug across the valley, reaching down to bedrock. An impervious wall has been constructed in the trench, which has then been refilled with the excavated material. The reservoir will be recharged during the monsoon period and the stored water can be used during the dry season. Excess groundwater will flow above the dam crest and recharge downstream aquifers.
The general principle of a sand-storage dam is shown by the example in Figure 1.2. The villagers used to collect their water from the small non-perennial stream at times when it carried water, or from holes dug in the shallow river bed for a short period after the rains. The quantity of water stored was not sufficient to supply water to the village during the entire dry period. By the construction of a weir of suitable height across the stream bed, sand carried by heavy flows during the rains has been deposited and the reservoir has filled up with sand. This artificial aquifer will be replenished each year during the rains, and water will be stored for use during the dry season.
By using this method it is often possible to extract water by gravity from the reservoir by using a pipe through the dam wall, thus avoiding the construction of a well and the problem of pump installation and maintenance.
A groundwater dam can also be a combination of the two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the dam wall rise over the surface, thus causing additional accumulation of sediments. Similarly, when a sand-storage dam is constructed it is usually necessary to excavate a trench in the sand bed in order to reach bedrock or a stable, impervious layer.
2. Advantages
The advantages of using groundwater dams instead of common surface storage are many. Evaporation losses are reduced or even completely avoided and once constructed, the designed storage will be available for a very long time and not, as in the case of surface reservoirs, subject to reduction caused by siltation and vegetal growth.
The water stored is less susceptible to pollution, and health hazards such as mosquito breeding and spreading of snail fever are avoided.
When conventional surface storage is used, it means that land is occupied for the reservoir; in the case of groundwater dams the land above the stored water can be used for other purposes.
3. Experience
This study presents the result of a literature survey combined with field visits to some sites in Africa and India where groundwater dams have been constructed and proposed. Most of the collected literature consists of specific reports from isolated projects. Two references describe more general studies presenting experience from Namibia and French-speaking Africa. The present text does not, although it covers experience from many parts of the world, pretend to be exhaustive in any respect. It presents general conclusions in the introductory chapters and finally some of the field projects are described. For details the reader is referred to the original documents which are all included in the list of references. The main subject is the storage of groundwater for small-scale water supply in developing countries. Groundwater dams for the protection of aquifers or sub-surface works, or such technical solutions that are developed and especially adapted for climatic and technological conditions typical for industrialized countries, are not treated in any depth.
Figure 1.3 shows a map of the world where all groundwater dam construction sites identified under this study have been marked. Most of the sites and areas have been described in the following text, and notably in Chapter 6.
The interest in using groundwater dams for water supply has increased during the last few years in connection with rural development projects, and several research projects are planned or have already started in Asia, Africa and South America.
The Central Ground Water Board of India has sited and constructed a number of sub-surface dams in Kerala, and regional suitability plans for an area in South India have been prepared under a research project at the Royal Institute of Technology, Stockholm (Ahnfors, 1980; Destouni and Johansson, 1987; Nilsson, 1987). The research project has also tested and developed simple techniques for the hydrogeological investigations that should preceed the construction of groundwater dams. Using groundwater dams for water storage is now a widely accepted technique in South India; several dams have already been constructed by state irrigation and forest departments, and large-scale application is planned in connection with present water harvesting and drought relief programmes.
Several dams have been built during the last few years in Ethiopia by government departments with technical and financial support from Sweden (Hansson and Nilsson, 1986). UNESCO has supported the construction of some dams in Africa and a research project is being started by the Lund University of Science and Technology, Sweden, in cooperation with authorities in Zimbabwe (UNESCO, 1984; Bjelm et.al., 1986). Sand-storage dam schemes in Namibia have been studied and described by Professor O. Wipplinger and others (Wipplinger 1958, 1961, 1965 and 1982; Aubroeck, 1971; Beaumont and Kluger, 1973; Stengel, 1968).
Comprehensive studies and field applications in Brazil have been made by Institute de Pesquisas Tecnológicas do Estado de Sao Pãulo (IPT, 1981 and 1982; Oliveira and Leife, 1984).
Generally it can be concluded that the present experience is positive; if properly sited and built, groundwater dams definitely serve their purpose. Some words of caution are needed, however. Before constructing a dam, the hydrogeological conditions at the site have to be known and proper investigations should be carried out. At the same time, the total costs have to be kept to an absolute minimum and consequently the investigations should be done with simple and inexpensive methods. Generalisations within areas with similar conditions should be used whenever possible. Caution is also needed during construction; it has to be properly planned according to seasonal conditions, appropriate materials should be used, and basic engineering practices should be followed.
It is also important to stress that groundwater damming is not a universally applicable method for water supply. It can be applied only if certain physical conditions are at hand and it should be looked upon more as an alternative when water supply cannot be arranged more easily with conventional methods.
CHAPTER 2
PHYSICAL CONDITIONS
1. Climate
The need to dam groundwater for water supply purposes is caused basically by the irregularity of rainfall. In arid areas every drop of water is valuable and should be saved. In monsoon-climate areas the total amount of rainfall would generally be sufficient to cater to the needs of people and agriculture, but here the seasonality means that during some parts of the year water is not available. Damming groundwater is thus a means of bridging over the seasonal dry periods. In addition, quite often the monsoon rains also fail and this can have a disastrous effect on the water-supply situation.
Damming of groundwater may also be done in climatic regions where water is available throughout the year, but where one wishes to increase the quantity or raise the groundwater level in the aquifer actually dammed, or in surrounding or underlying aquifers.
All dry, monsoon and tropical wet-and-dry climate areas of the world have been marked on the map in Figure 1.3. These are the parts of the world where the rainfall conditions are most suited for damming groundwater and there is a good correspondence with the actual groundwater dam sites.
Arid areas of the world are by Köppen's definition those where the potential evaporation is larger than rainfall. The relative advantage of damming groundwater as compared to common surface storage depends to a large extent on the losses that would result from open-surface evaporation. Some values of potential evaporation from relevant areas where groundwater dams have been constructed or proposed are presented in Table 2.1.
It is evident that evaporation losses from open water surfaces in these areas are considerable. The loss of say 2 metres from a reservoir may represent quite a large portion of its total capacity.
Depending upon the level of the groundwater table and the capillarity of the aquifer material, there is some limited evaporation also from groundwater. The evaporation of water from sand has been studied quantitatively by Hellwig (1973) at experiments at Swakop River in Namibia. The evaporation from a saturated sand surface was found to be approximately 8 per cent less than from an open water surface. The lowering of the water table by 0.3m below the sand surface reduced the evaporation from a fine sand to 50 per cent of that from an open water surface. The corresponding figure when keeping the water level at 0.6m depth in medium sand was 10 per cent. The relation between evaporation and depth of water table is shown in Figure 2.1. The sorting of the material has an influence on the extent of evaporation losses. It was found that a reduction of particles of less than 0.1mm diameter from 9 per cent to 0. 7 per cent in a layer of medium sand, reduced evaporation by 25 per cent at 0.30 metres depth to the water table. It is important therefore that the accumulation of fine particles at shallow depths in sand-storage dams is avoided. Evaporation as a function of particle size is shown in Figure 2.2.
2. Topography
The topographical conditions govern to a large extent the technical possibilities of constructing the dams as well as achieving sufficiently large storage reservoirs with suitable recharge conditions and low seepage losses.
The basin in which water is to be stored may be underlain by bedrock or unconsolidated formations of low permeability. It is generally preferable to site groundwater dams in well-defined and narrow valleys or river beds. This reduces costs and makes it possible to assess storage volumes and to control possible seepage losses. On the other hand storage volumes have to be maximized keeping at the same time dam heights as ?mall as possible. In mountainous areas with very high gradients, it might be difficult to find an acceptable relation between storage volumes and dam height.
The depletion of groundwater storage through natural groundwater flow is the most basic reason for building a subsurface dam. The gradient of the groundwater table and thus the extent of flow is generally a function of the topographic gradient. This fact indicates that the construction of subsurface dams is feasible only at a certain minimum topographical gradient, which will vary according to local hydrogeological conditions.
Examples of topographical gradients at some construction sites are presented in Table 2.2. Generally the gradient is between 0.2–4 per cent but in extreme cases construction has been made on slopes of 10-16 per cent.
The particle size of sediments accumulated along streams and in river beds is generally proportional to the topographical gradient whereas on the other hand, the depth and lateral extent of such deposits is inversely proportional to the gradient. The optimum relation between these two factors, and thus the most favourable sites for sub-surface dams, is generally found on the gentle slopes in the transition zone between hills and plains.
The topography of the impermeable beds or bedrock underlying the storage reservoir determines storage efficiency and methods of dam construction. Figure 2.3 shows how natural underground dams in the form of rock bars improve the natural groundwater conditions (Skibitzke et al., 1961). They may also constitute promising locations for the construction of groundwater dams that would further increase the amount of exploitable water (Sörlie, 1978). Also natural dikes may have a damming effect that could be enhanced by the construction of groundwater dams (Newcomb, 1961). The presence of surface rock bars is generally necessary for the construction of sand-storage dams as is described in the case history from Machakos, Kenya in Chapter 6.
An example where optimum topographical conditions prevail is shown in Figure 2.4. An aquifer of even thickness in a wide valley with gentle gradient is drained through a narrow passage between outcropping rock. A dam at this site will create a large amount of storage at comparatively low cost.
3. Hydrogeology
The most favourable aquifers for construction of sub-surface dams are river beds made up of sand or gravel. In-situ-weathered layers and deeper alluvial aquifers have also been dammed with success, even if such aquifers generally have less favourable storage and flow characteristics.
The specific yield of such water-bearing strata may vary from 5 to 50 per cent depending on grain-size distribution, particle shape and compaction (Davis and de Wiest, 1966). Wipplinger (1958) reports a specific yield of approximately 25 per cent from a typical river bed in Namibia, whereas the specific yield at the site of a sub-surface dam constructed in a residual-soil aquifer in south India was 7.5 per cent (Ahnfors, 1980). These figures represent fairly well what can be expected in the two types of aquifers.
Hydraulic conductivity values are more sensitive to the type of material constituting the aquifer. The hydraulic conductivity of coarse sand for instance, may be a hundred times higher than that of a very fine sand (Bedinger, 1961) and the presence of clay in a sand aquifer may reduce its hydraulic conductivity thousandfold. The problems that will be encountered when a sub-surface dam is constructed in an aquifer with fine-grained material is thus less related to available storage volumes than to extraction possibilities. The techniques that may be used to solve such problems will be treated briefly in Chapter 5.
A typical profile through a weathered layer is shown schematically in Figure 2.5. According to Taylor (1984) four zones can generally be identified within the profile. The uppermost zones (a) and (b) both have high porosity values but low permeability, whereas zone (c) has low porosity but high permeability. If such an aquifer were dammed, it would provide storage in zones (a) and (b), and zone (c) would act as a natural, and, due to its lateral extent, very effective drain transmitting the water for extraction at the dam wall.
(Continues…)
Excerpted from "Groundwater Dams for Small-scale Water Supply"
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Copyright © 1988 Intermediate Technology Publications.
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Table of Contents
Preface, ix,
Chapter 1 Introduction, 1,
Chapter 2 Physical Conditions, 7,
Chapter 3 User Aspects, 17,
Chapter 4 Planning and Investigation Methods, 23,
Chapter 5 Design and Construction, 27,
Chapter 6 Case Histories, 45,
List of References, 63,