Looking at inexpensive methods for water supply schemes in rural areas of developing countries, this book examines the suitable conditions and economic factors for different types of groundwater dam and describes planning and investigation methods. Åke Nilsson holds a Master's Degree in natural resources management and environmental engineering, and a Licentiate Degree in hydrogeology. His main working areas have been assessment and management of natural resources, especially water resources, and small-scale technical solutions for water supply. Groundwater Dams for Small-scale Water Supply By Åke Nilsson Practical Action Publishing Ltd Copyright © 1988 Intermediate Technology Publications All rights reserved. ISBN: 978-1-85339-050-0 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, 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