Small dams are playing vital role for the life of local communities in sub-Saharan Africa providing water for irrigation livestock development, and building. These small reservoirs are overexploited due to the population growth and increasing economic activities. Due to sedimentation due to lack of soils and water conservancy measures in the watershed the loss of storage capacity for many of these reservoir are important. Low cost technology like fuse plugs and PK weirs provide solutions to restore and increase water storage for local communities. In Burkina Faso, Hydrocoop France, “Eau sans Frontières” a French NGO and a local NGO ODE have developed an example project for the Wedbila reservoir using the new technology of fuse plugs. The project increases water availability by one third of the reservoir capacity and allow the development of more irrigated area for vegetable and cereals production. The second ongoing project is the Bambakary Dam in the north of Burkina Faso, where a PK weirs will be use to ensure the dam safety against floods and improve the reservoir storage capacity for one of the driest area of Burkina with important economic potential. The paper will present these cases and the impacts on local communities’ livelihood.
1. Presentation of Burkina Faso and Projects location
Burkina Faso is a landlocked country located in West Africa within the Niger River bend. It is comprises within the latitude 09°20’ South and 15° North and longitude 05°3’ West and 02°30’ East. The neighbouring countries are Côte d’Ivoire, Ghana, Togo and Benin in the south, Niger in the east and Mali in the North and North-West. The total Area of the country is 274,000 Km2. The total inhabitants are estimated to 13.5 Million people (2006 Census) and a mean density of 49 inhabitants per Km2.
Fig 1: Bambakary and Wedbila dams projects location
2. Role and importance of small dams for local communities
Due to the fact that Burkina Faso is a semi-arid country located in the west African Sahel water stress is important. Associate to the low rainfall, more than 80% of the country is covered by crystalline rocks with low ground water resource.
2.1 Water resources potentialities and constraints
There are three main river basins in Burkina Faso which are all International River shared by more than 15 countries: the Niger River basin, the Volta river basin and the Comoé River basin. The map on figure n°2 presents these basins.
The water resources potential is summarized in the table 1 below. The total annual renewable water is estimated to 41 billion m3 representing a per capita 2800 m3 per year. The trend in rainfall as shown by the figure n° below is to decrease since the 1970’s.
Table 1: renewable water resources in Burkina Faso
2.2 Role of small dams
In Burkina Faso, around one thousand and half of small dams have been built since the 1920’s and more than 5 000 villages out of 8 000 and around 8 million inhabitants out of 14.5 millions depend on these small reservoirs for water supply, food production and livestock breading during the long dry season. The flowing map shows the distribution of dam in Burkina Faso.
Fig.2: Map showing the main river basins and the existing dams
2.3 Main issues in small reservoirs management
The main important issue for these small reservoirs are their sustainability for these reasons:
– Huge losses of water due to evaporation, as the dams are located in arid and semiarid zones with long insulation and high temperature and generally these reservoirs have a low mean depth of 2 m and rarely up to 5 or 6 m.
– Loss of storage capacity linked to sedimentation by fines transported with the early seasonal flows on a denuded watershed due to desertification and human activities such as bad cropping techniques and clearing of wood.
– Overexploitation due to population growth and increasing water demand.
3.Fuseplugs for heightening the WEDBILA Dam
The Wedbila dam heightening project is located in Burkina Faso. The purpose of the Project is to raise the spillway of the dam with Fuse plugs which are low cost innovative solution to improve reservoir water storage capacity when keeping or improving the safety of the dam against floods.
3.1 Local environment
Wedbila is situated to 30 Km south from Ouagadougou the capital city of Burkina Faso. The dam has been constructed in 1980 by the World Federation of the Evangelical Churches (FEME). The geographical coordinates taken in the GPS are:
– Longitude: 01°25’00 ” W
– Latitude: 12°09’09 ” N
It is a small earth fill dam with a storage capacity of 2 million m3 intended for the irrigation of 45 Ha of land t downstream of the dam. Water stored serves also to supply livestock in dry season as well as fore domestic and construction needs of the riparian populations. The perimeter employs hundreds of peasants during rainy and dry seasons. The dam contributes to generate important incomes for the riparian populations and constitutes one of the rare sources of water in dry season in the zone.
With the growth of the activities around the dam, the volume of water stored in the reservoir becomes more and more insufficient to fulfil the needs because of the increase of population (growth rate of population: 2,5%) and the progressive reduction of the reservoir capacity due to sedimentation in the reservoir. This project is an answer to this situation and a response to the demand of the farmers who s for a long time are seeking support for the increase of the capacity of the dam to allow them to secure and develop their activities.
3.2 Technical aspects of the project
The features of the dam are given below:
- Capacity of reservoir: 2 080 000 m3
- Length of the spillway: 70 m
- Length of the dam: 600 m
- Useful maximal height: 4 m
- Surface of irrigated perimeter: 45 ha
The technical features of fuseplugs have been obtained by design model developed by Hydrocoop France team with the collaboration of IFEC Consulting Engineers.The nature, principles and purpose of this technology are:
- The fuseplugs are simply massive concrete blocks placed side by side on a spillway sill. They are free standing and self stable until the water level in the reservoir reaches a certain elevation, but start tilting when this elevation is exceeded;
- The fuseplugs are designed to be overtopped by a water nappe before tilting. The blocks placed on the same sill may have the same height but different thickness, so that they tilt progressively for different water levels;
- The water elevations for tilting can be predicted quite accurately since the blocks are designed with a total uplift at their base.
- Possibility of increasing the storage capacity or/and improving spillway performance of small dams without changing the MWL and consequently the dam height.The existing free overflow spillway has a length of 70 m. the project consisted of installation of seven (07) concrete fuseplugs, 0.50 m high and with US/DS width varying between 0.78 m to 1.03m and unit length of 9.70 m as presented in table 2 below.The first block tilt for the 10-year flood and all the blocks tilt in order to keep the existing MWL for the Design Flood. The tilted blocks can be easily and quickly replaced by provisional bags before replacing the block.The implementation method of blocks can be summed up as follow:
- Preparation of the existing spillway: the goal of this operation is to widen the crest of the spillway in place which was narrow to receive the fuseplugs. (see photo 1)
- Construction of fuse plugs and separation walls. (see photo 2)
- Installation of joints between the fuse plugs and the walls
3.3 The social and economic impacts of the project
This project answers a strong demand of the operators that for years solicits the increase of the capacity of the dam to allow them to secure their activities above all and to spread them possibly.
The project with an increase of 50 cm of the reservoir normal level will increase the reservoir storage capacity to 25% and the irrigated area to 47% with an increase in rice and vegetables production in the range of 85.6 tons per year and 428 tons per year which can feed 450 additional persons and increase yearly income to 45 000 Euros for an investment of 15 000 Euros.
4. Using the PK weirs to improve storage and safety of the Bambakari dam
4.1 Local environment
Bambakari project is located in northern region of Burkina Faso, close to the border with Mali, on the Beli River, a tributary of the Niger River. The geographical coordinates taken in the GPS are:
– Longitude: 00° 10′ 50 ” W
– Latitude: 14° 58′ 10 ” N
Bambakari is located in the driest zone of the country. The region receives only two months of rain and has a mean annual rainful of approximately 350-500 mm. During the 1970s, this area was subject to severe drought, resulting in the loss of livestock and distress for local population. The trend of rainfall decreased continuously until the 1970s, when a gradual improvement occurred in the rainfall pattern since the 1990’s. Fig.2 below shows the rainfall trend in the northern region of Burkina Faso.
The main activities of this region are livestock development and rainfed agriculture of cereals and beams. The population of this region is well known for its handicraft leaver products.
The region is rich in mineral like limestone and manganese with some planed development of clinker factory and mining industries.
Ecologically this region constitutes the zone of destination of many birds migrating from northern Europe in Africa during winter. The Bambakari dam project is a multipurpose project with the aim to store the water resources of the Béli River for the development of:
- irrigated agriculture for around 1000 ha;
- livestock development for more than existing in the riparian villages and remote one fromMali and Niger;
- fisheries and fish products development;
- hydropower production with the installation a micro power station;
- regulation of the Béli river;
- Touristic and recreational activities.
4.2 Technical aspects
The catchments area of the dam is evaluated to 7860 km2 and is dominated by a sand hill landscape with gentle slope. The annual mean flow is estimated to around 115 hm3 per year. The reservoir capacity is 95 x 106m3 with a storage area of 43.3 km, the mean depth of the reservoir is estimated to 2.20 m which is just above dry period (September to July) evaporation rate. The live reservoir capacity is estimated to 90 x 106m3.
Bambakari is an earth dam with a maximum height of 10 m above the river bed and a length of 2 400 m, the total volume of earthworks is estimated at 500 000 m3.The dam foundation consists of fractured limestone, apparent or covered by a thin layer of laterite soils to sandy, silty soils on the right bank. in the centre of the valley, the earth cover of the limestones around 5 m, consisting of fine alluvial deposits; on the left bank, the limestone is located at a depth of 15 m cover by thick, multi-layered wind deposit sands, laterite and residual sands. These one are composed by alteration of limestone. Despite the fact that the dam is small, the foundations are complex as a result of the fractured behaviour of limestone which contains a karstic area.
As the river is a large one in the northern zone of Burkina Faso and the main tributary of the Niger River in this region and despite the low rainfall, the river flows are very large. In order to reflect current trends in the profession whose purpose is to properly protect the earthfill dams while limiting the cost overrun, it is proposed that the following criteria: design flood with a return period of one thousand years and the check flood with a return period of ten thousand years.The one thousand years return period discharge has been estimated to 1200 m3/s and for the 10 000 years return period the value is 2 000 m3/s.
Considering the low head of the dam and the necessity to store enough water for the project purpose it was important to design a high performing spillway. For the choice of spillway a comparative analysis has been done between four types of spillway: free Ogee overflow spillway, straight drop spillway, improved labyrinth spillway and a PKWeir. A multicriteria comparison has been carried.
After examination of these diverse solutions the PK Weir spillway has been retained as the best solution. Indeed PK weir is a low cost technology and is a high performance spillway with a discharge flood 3 to 4 time higher than and free Creager spillway.
The management of flows will be insured by two spillways. A main spillway with a PK weirs profile to spill important floods with a minimum head and an emergency spillway located on the right bank at the end the dam to spill the check flood.
H is the height of vertical walls.
The hydraulic design is to determine the load (h) of water over the PK Weir to flow 1000 m3 / s for the length chosen. The spillway has a length of 200 m and is calculated to evacuate 1000 m3/s. The by following formula system allows to compute H and h values.
As the maximum level of the reservoir is 4.06 m above the foundation of the PK Weir, a first relation is h + 1.5H = 4.06.
As the flow discharged by such a PK Weir is in the range of 4h√H (in m3/sec/m), a second relation is 4h√H = 1 000/200 .
The resolution of this equation with two unknown variables has been done numerically and graphically. Hence H = 2.13 m and h = 0.86 m. The features of the spillway are given below:
Fig.5: Cross section of PK weir designed for the Bambakary dam
Fig.6: Plan view of PK weir designed for the Bambakary dam
A checking of operating conditions of the PK Weir has been done by evaluating the flow in the river downstream of the dam. It may be noted that according to the results of these calculations that the spillway will not be submerged by downstream level for the ten thousand years return floods.
The spillway is being located on the left bank in the area where the outcrops of limestone are found. The rock is very heterogeneous in terms of quality. RQD tests indicate a rock with good quality even fractured and cracked to a rock with medium quality highly fractured and cracked. Consequently it is planned in the one hand to strip the weathered rock surface to the healthy rock to found the spillway and in addition to proceed with grouting of cement and bentonite on two grouting lines with 10 m deep on average. The stability computation of the PK Weirs, founded on limestone, is ensured for different load cases.
4.3 The social and economic impacts of the project
The project will improve water availability for livestock and agricultural activities development. It is planned to develop 1000 ha of irrigated land which will provide cereals and vegetable to improve the nutrition conditions and increase incomes for the population which are one of the poorest in the country and in the world. The reservoir will create a permanent wetland in this arid zone and attract all living animal and improve the biodiversity of the zone. It will help to reduce internal and external migration.
An environment and social management plan has been developed to mitigate the adverse effects of the dam. The contractor for the work has been procured and work will start by October 2009.
These two projects are example of how water mobilisation by dam can improve the life conditions of local communities in a semi arid country and how new cost effective solutions are contributing for these achievements. The cost of additional m3 stored in the Wedbila dam is around 3 US cent and the cost of the PK weir per m3 spilled is around 500 US$.
1. HYDROCOOP FRANCE &IFEC, « Projet de renforcement de capacités et de la sécurité des petits barrages réservoirs pour les communautés villageoises au Burkina Faso- Décembre 2007 »;
2. GERTEC « Mémoire descriptif du Projet de barrage de Bambakary – Juillet 2008 » The Authors
Adama Nombre is graduated in electrical and civil engineering at the University of Besançon and the International Institute for water resources and environment engineering school. He has worked for 20 years as hydraulic engineer in the government water resources and dam’s offices. Since 2001, he is the manager of IFEC consulting firm. He is also the chairman of Burkina Faso committee on dams and past vice president of ICOLD.
François Lemperiere is graduated from Ecole Polytechnique, Paris. He has an experience of 40 years in engineering and construction of large dams and was involved in construction of some twenty large dams, canals and river embankments projects on the Rhone, Rhine, Nile, Zambesi and other rivers and in the design of ten of these projects. He is chairman of the committee of cost savings of ICOLD and chairman of Hydrocoop.
Jean Pierre Vigny is graduated from Ecole Polytechnique, Paris. He has an experience of 35 years in worldwide construction and was involved as civil engineer or manager in construction or design of several large dams. He his member of the French committee for dams and reservoirs and general manager of Hydrocoop.”