by F. Lempérière, Hydrocoop (France)
Africa has specific conditions, as well physical as economical. Criteria for dams designs used in other continents may need an in depth review for adapting them to these conditions and to the association of hydropower with other renewable energy sources.
The design of medium or low irrigation reservoirs and relevant criteria or regulations may be very specific to Africa.
Europe and North America have built most of their possible dams 50 years ago, Asia is doing the same now; Africa will probably do the same before 2050. It may thus appear easy to use for African dams designs similar to those used elsewhere recently. But it is advisable:
– To precise the specific African physical conditions which may favour more specific designs.
– To guess which may be along the century the Africa economic conditions and the needs for power and water.
– To adapt accordingly the designs prevailing elsewhere with care of their recent progresses and avoiding their remaining drawbacks.
– To design Africa dams for 100 years with due care of environment, climatic change and siltation.
– To avoid unnecessary extra costs.
– To avoid safety regulations unadapted to specific African data.
1. Specific African physical data
– The temperature is high and will increase. Air conditioning needs will be very high.
– Evaporation in reservoirs may be over 2m/year. It reaches 500 m3/second in Aswan Lake and may be a key point for the efficiency of low irrigation dams, especially in flat areas.
– Possibilities of hydropower production are high only in the central part; two thirds of the total potential is in the catchment of the 4 main rivers: Congo, Nile, Zambezi, Niger.
– Possibilities of solar energy at an attractive cost are high along the year in most Africa. Wind energy is significant in limited areas.
– The rains are high in the central part, low in North and South. These rains as well as the relevant river discharges are in many places limited to few months.
– Irrigation dams are also necessary in flat areas, requiring very long embankments.
– Earthfill materials may be very specific to some African areas.
2. Economic conditions and needs along the century
The total population is presently one Billion, one third in North and South with an average income per capita of 2 000 $/year and two thirds in the Central part with an income per capita of 500 $/year. The total African income is thus 1 000 Billion $/year. Within 50 years the population will probably reach 2 Billion; with a likely increase of the income per capita over 2% per year the total African income may then increase over tenfold if it is backed by the necessary energy at an acceptable cost.
The present electric power includes essentially 100 Twh/year of cost effective hydropower and 500 TWh/year from oil, coal or gas which is in fact expensive. The overall need will probably increase as the income and reach 5 000 Twh/years within 50 years. Possible hydropower will be implemented but is limited to 1 000 TWh/year. Electricity from fossil sources will be probably limited to 1 000 or maximum 2 000 TWh/year for lack of resources, or cost and/or environment reasons. The balance (half of needs) will be essentially from solar power in most Africa and from wind power in some places. Their cost will be acceptable, the potential is well over needs but a significant storage and/or optimized utilization with other sources will be necessary for such intermittent sources. Many hydropower plants will thus be in the future operated in association with intermittent energies and it seems thus advisable to increase initially or later the hydroplants capacity for operating mainly during lack of solar or wind energy.
Much long range electricity transport will be used; however many countries will need energy storage and the best solution will probably be by PSP (Pumping Storage Plants), i.e. hydropower plants operating between 2 reservoirs which may be along rivers or out of rivers. For an average intermittent electric production over 200 GW, the African need of PSP will probably be about 100 GW. A part of PSP, for instance in North Africa, may use the sea as low reservoir and a basin on a cliff as high reservoir.
From 30 GW now, the hydroplants capacity may thus increase to 300 GW for production and 100 GW for storage.
The increase of population, better living conditions and the climatic change will much increase the need of water storage in most Africa. Over years storage may be increased but the main need will be a seasonal storage from some months of the floods season to six months of the dry season. It may be by large reservoirs but also through a great number of medium or small reservoirs: their design should take great care of siltation and evaporation problems; the possibility of later increase of the dam height should be favoured.
There are in Africa many rather flat areas with storage needs where a large part of water storage by traditional low dams is lost by evaporation. Beyond specific dams designs which will be presented further in this paper a very cost effective solution may be to create in a flat area out of the river but close to it a basin closed by dykes about 10 m high; this basin will be filled by pumping from the river along few months of wet season.
– Floods mitigation may be one target of very large schemes: the High Aswan Dam has avoided Cairo inundations and in South Sudan it is possible to avoid inundation of dozens thousands km². It may also be by rather small dams upstream of cities only devoted to flood mitigation. And for irrigation dams storing most of the yearly flow, associating a free flow spillway and a bottom gate spilling the yearly flood may be efficient for mitigating floods downstream.
3. Progresses and drawbacks of worldwide dams designs
Many new efficient solutions appeared since 30 years but old design criteria and habits mail prevail a long time in many countries. Main progresses are underlined below:
– Using Roller Compacted Concrete (R.C.C.) has been a great progress; a relevant review and optimization of the gravity dams cross section according to foundation, materials, floods and earthquakes has been undertaken in many countries but is not finalized.
– Recent developments in CFRD (Concrete Faced Rockfill Dams) or rockfill dams with bituminous cores may be of interest in African areas where there is lack of clay for cores or much rain.
– Problems of siltation are very difficult for many rivers. They have been often overlooked or the chosen solutions have been poorly efficient with a reduced reservoir life or harmful damages downstream. The basic layout and data of reservoirs and dams basic design should sometimes be based mainly upon siltation problems. Useful relevant comments may be found in ICOLD Bulletins 115, 144 and 152.
– The main cause of fatalities from dams failures is linked to floods. The risk has been mainly for reservoirs over 10 hm3 and, surprisingly, from dams between 20 and 30 m height.
Failures were mainly caused by floods beyond the spillway capacity but also by gates jamming and especially by total gates jamming. Since 30 years there have been serious progresses in free flow spillways (new labyrinth shapes such as P.K. Weirs fig.2) and various fuse devices (earth or concrete fuse plugs fig.1, fusegates fig.3). Fully gated spillways which were usually the solution twenty years ago for large discharges may be in the future replaced by free flow spillways or associated with them or with fuse devices. Efficiency of earthfill fuseplugs may however be questionable after many years.
– The design criterion for spillways was usually 30 years ago the “Design Flood” associated with a freeboard. The criterion of a “Check Flood” of lower probability for a reservoir level close to the failure level is now preferred in many countries.
It is now acknowledged that the evaluation of extreme floods will always be difficult and uncertain and the problem will be increased by the climatic change. If the downstream risk is high, two solutions may be favoured:
– Concrete gravity dams with a cross section withstanding significant overtopping along most dam length.
– Embankment with a fuse part in a place chosen for reducing incremental damages downstream.
– Floods control during large dams construction has been usually by diversion tunnels and a rather high upstream cofferdam diverting along one or two years floods lower than 1/20 or 1/100 probability. This solution which was well adapted to Europe or North America may not be the best in Africa where floods are very important and happen during a short well known floods season. It may be less expensive and safer to divert only the floods of the dry season and to accept an overtopping of the dam under construction during a flood season. This applies to concrete dams but also may apply to rockfill dams if levelled horizontally some m above the river level during a flood season.
4. Avoiding unnecessary extra costs
For most dam sites it is usually rather easy to find a possible safe dam design. It is often more difficult to find the less expensive safe solution because there are usually many alternative for the dam place, foundation, cross section, best use of local materials, siltation and floods control, construction methods and schedule and unit costs vary with each solution.
It is often better to increase quantities and reduce unit costs.
References to recent ICOLD Bulletins about cost savings n° 144 and n° 152 and relevant advices may be useful. Cost optimization does not mean safety reduction.
5. Future dams in Africa
5.1. General design
It should be usually made for one century, not for 30 years.
– In rivers with significant siltation, the choice of the dam place and height and the design should take in account the relevant problem.
– The possibility of raising later the reservoir level should not be overlooked, especially for irrigation reservoirs.
– For hydropower schemes, the possibility of future operation associated with wind or solar energy may impact the future power plant capacity and the storage capacity.
5.2. Arch dams
Arch dams are now mainly used worldwide for very high dams, often over 100 m high. The number of such possible very high dams seems much lower in Africa than in Asia and the African arch designs may be rather similar to Asian or European designs.
Multiple arches or buttress dams seem to have little future, gravity dams being preferred for cost and safety reasons.
5.3. Gravity dams
Many gravity dams will be built as well for low or very high dams.
Masonry dams which were dangerous will preferably not be used and a large part of dams will use R.C.C. in the future especially for large quantities.
Thirty years ago quite all gravity dams had a vertical or quite vertical upstream face. Presently for half of R.C.C. designs, the upstream face has a significant slope, at least in the low part and some dams have a symmetrical cross section, favourable for earthquakes, huge overflooding and medium quality foundation: the cross section and concrete data may thus be adapted to all local conditions. It may be safer and often less expensive to increase quantities and reduce unit costs: the best quality of a gravity dam is the gravity.
Few failures of concrete gravity by floods have been reported, and were for rather low dams for which an unforeseen water level increase of some m has a rather high impact on stability. However the failure of a gravity dam may be sudden and then very dangerous downstream and the uncertainty on floods evaluation is high and will increase with the climate change. Where the downstream risk may be high, the dam should be designed for withstanding significant overtopping along most of the dam length.
5.4. Rockfill dams
The great majority of dams higher than 40 or 50 m will probably be gravity dams or rockfill dams, the choice between them being mainly linked with the floods and foundations conditions.
The traditional solution of rockfill dams with clay core will probably remain of interest.
Concrete faced rockfill dams (CFRD) have probably much future including for very high dams. Dams with bituminous core, presently used mainly under 100 m height, may also have much future.
Many rockfill dams, founded on rock, are in rather narrow valleys. The spillway may be expensive, sometimes in tunnel. Using a single gated spillway may be dangerous as the risk of total gates jamming cannot be overlooked. An emergency free flow spillway may possibly be placed upon the rockfill dam itself. And in case of tunnels spillways it may be possible to associate a traditional costly lined tunnel with gates for the 1/1000 flood and a less expensive unlined quite horizontal tunnel controlled by upstream fuse devices for extreme floods. Some downstream damage for such emergency spillway may be acceptable.
More high rockfill dams failed by overtopping during construction than in operation. For large catchment areas and significant floods it may be better to build the low part of the dam during a dry season and to accept during the next flood season overtopping of the dam levelled some m above the river level. This is less expensive than a costly diversion tunnels and high upstream cofferdam and the damages by an exceptional flood during construction are much smaller.
5.5. Earthfill dams
Worldwide rockfill dams are used mainly on rock foundation, often in rather narrow valleys for heights above 50 m. In Africa as in most other places, eartfill dams will be mostly used for height under 40 m, and often on soft foundations and in many rather flat sites. The reservoir closure will be made by a long eartfill dam and a short concrete spillway.
Some world data may be useful as reference, even if not applying directly to Africa (the number of dams in Africa may be 10 or 20% of the world number). There are worldwide over 30 000 “large” Earthfill dams, i.e. higher than 15 m, half being 15 to 20 m high, and few thousands higher than 30 m. About 10% store over 10 million m3 and half store 1 to 10 million m3.
2% of these dams failed but they were mainly failures and fatalities for dams built before 1930 in industrialized countries and before 1980 in developing countries. The safety has been much improved for large dams built since 1980; the yearly rate of failures for small Earthfill dams remains significant and may be close to 10-3 yearly but with limited damages and usually no fatalities. Great progress has been linked with better designs and by the fact that the cohesive material placed now by mechanical equipment with the right water content may withstand much better the internal or external erosion.
About half of failures were from internal erosion and half from floods with most fatalities from floods and for dams higher than 20 m with storage over 10 hm3. Some discharges from failures were above 50 000 m3/s for 25 m high old dams storing over 100 hm3 because the breaches did extend hundreds m.
Risks in the future will however be much reduced, not only by the better designs and material, careful first filling, and for floods risk by weather forecasting. Easy telecommunications and alarm systems will also reduce fatalities. And for Earthfill dams failing from floods, the full failure may take hours and incremental damages may be a small part of damages from the natural floods. Comparison between failures hydrograms is giving in the fig.4.
For most of these dams which are rather low and long, the part in river may be built within a dry season which is often long in Africa but the management of floods in operation will be a serious problem. Fully gated spillways should be usually avoided for avoiding the risks linked with gates operation or total gates jamming. Modern labyrinth spillways may have much future. Various fuse devices may be used for floods beyond 1/100 or 1/1000 yearly probability. Overtopping of the dyke may be foreseen with well adapted relevant lining.
As many dams are long and as it is difficult to assess the precise floods values of extreme floods it may be possible to choose the place of failure by an extra ordinary flood where the height is under 10 m. This may be obtained there by a lower crest level or by fuse devices.
5.6. Low Earthfill dams
If safety is a key design criterion for large dams, i.e. higher than 15 m or storing over 3 hm3 cost savings may be the key criterion for most lower dams and especially lower than 10 m which will be usually eartfill dams. Such dams may be very useful for irrigation or fresh water with storages often in the range of 0,1 or 1 hm3 and needed as well in very flat areas. There are worldwide over 10 000 small dams storing between 1 and 10 hm3, over 100 000 storing between 0,1 and 1 hm3 and millions storing 10 to 100 000 m3.
Most such dams will be 5 to 10 m high, i.e. with an average reservoir depth in the range of 2 or 3 m. In most Africa a large part of such storage may be lost by evaporation along over 6 months of dry season. Loosing 1 m of reservoir level may be a huge loss of useful storage; and in flat areas increasing by 1 m the dam crest level may increase very significantly the dyke volume and cost. The traditional design of a free flow concrete spillway with freeboard and questionable waves impact calculation may place the reservoir level 1,5 or 2 m under the dam crest.
As a flood of yearly probability 10-2or 10-3 is usually for such dams some m3/s or dozen of m3/s it may be much more cost effective to spill a low nappe depth along one or several hundred m.
The discharge may be along the dam crest itself designed or lined accordingly or, for flat areas, along the bank as per this figure where the weir level may be at the natural ground level.
The reservoir level may thus be about 0,5 m under the dam crest and the storage doubled.
A bottom pipe will be used for water with drawal. Where there is a significant risk of reservoir silting and for reservoirs storing a small part of the yearly flow it may be efficient to increase the diameter of this pipe for sluicing the river discharge at the beginning of the flood season when most sediments are brought, the reservoir being then quite empty. It is an usual solution in China.
The failure of such small dams will have usually little impact and a failure probability of 10-3 may be reasonable. However some reservoirs of about 1 hm3 may be a serious risk if very close to a village and their failure probability should be closer to 10-4.
5.7. Offrivers reservoirs
Offriver reservoirs in rather flat areas may be cost effective for storing water (for people and irrigation) or for storing energy as one or two reservoirs of a PSP (Pumping Station Plant).
The average depth of such reservoir may be 5 to 20 m and areas between 0,2 and 10 km². As the cost of dykes is proportional to the reservoir diameter D and the storage to D², such solution will be mainly cost effective for significant storages (over few hm3) and very attractive for storages of hundred hm3.
– When used for water storage, these reservoirs should be rather close to river and filled by pumping along some months of the rainy seasons, for instance along 2 000 hours. This solution is much better than traditional ones for environment and siltation. Pumping capacity and pipes diameter are not very important. The risks and costs linked with floods are avoided and the construction schedule is not linked with the river conditions. However the storage is well under the possible storage by a traditional dam because part of the floods is not stored. The dykes of the reservoir may have usual designs and the unit costs may be rather low; the bottom of the reservoir should be unmodified and does not request special lining.
– Offrivers reservoirs for PSP may be large ones, most often in the range of 1 or several km² and dyke height may reach over 20 m. The frequency of quick drawdowns will favour dykes with upstream facing.
Advices or regulations for dams safety have been published in many industrialized countries. They are essentially based upon local specific conditions such as climate or usual dams designs; they may be very different according to the country and poorly adapted to conditions of many African countries.
As example in Europe rain depth on reservoirs is about the same as yearly evaporation but in many African countries, the evaporation is 2 m over the rain depth. For dams lower than 10 m, i.e. with an average reservoir depth of 2 or 3 m using. European regulations on design flood, freeboard, waves impact, could totally prevent in African countries the utilization of cost effective solutions such as overtopping of low embankments by a nappe depth of some dozens cm and in fact prevent very useful reservoirs. Regulations for such low dams could be specific to each country, favouring low cost solutions, for instance limited to a check flood between 10-2 and 10-4. probability, adapted to the downstream risk.
F. Lempérière has been involved in construction and/or design of 15 large hydraulic structures in large rivers, including Zambezi and Nile. He has been Chairman of several ICOLD Committees on Dams costs and costs savings.