Dam Safety and Low Cost Spillway Designs

Transcription

1 Dam Safety and Low Cost Spillway Designs By F. Lempérière & J.P. Vigny (Hydrocoop France) THE CHALLENGE 5 th International Conference on Dam Engineering Lisbon February 2007 Increasing dams safety by taking into account more important floods passing through spillways, is a major concern all around the world. But corresponding costs may become very high and sometimes unbearable for developping countries: Is it possible to find safe and economical solutions well adapted to the existing local situations? This needs a review of the usual floods and design criteria. FLOODS AND DESIGN CRITERIA The criteria for spillway design are based upon two floods: the traditional Design Flood (chosen most often with a yearly probability between 1/100 and 1/1,000) and the Check Flood. The true safety of dams refer to the Check Flood (sometimes chosen as the Probable Maximum Flood) to which the dam must resist without failure, and not to the Design Flood, during which the dam must be normally operated without damages and the water level of the reservoir is kept well under the dam crest (freeboard). The P.M.F. discharge may be 3 or 4 times the discharge of the flood of probability 1/100. It may be in the range of 500 m3/s for 10 km² catchment area, 10,000 m3/s for 1,000 km²: The challenge remains to withstand such high floods at low cost.

2 Accepting a reservoir level close to embankments crest (no freeboard) and possibly over concrete dams crest and some damages (such as loss of fuse devices) during very exceptional floods, allows for optimizing various economical solutions as described here after. COSTS ANALYSIS If - q1 is the discharge of the design flood, - q2 is the gap between q1 and the discharge of the check flood, the discharge of the check flood is q1 + q2 and should be as high as possible for best safety. If - c1 is the cost per m3/s for discharge of q1, - c2 is the cost per m3/s for discharge of q2, the total cost for the check flood is c1q1 + c2q2 and should be as low as possible for cost saving. It is difficult to reduce c1(cost of gates, loss of storage..) which is often over 5,000 U.S. $, and it is much easier to reduce c2. It may then be also attractive to reduce q1 and increase q2. c2 may be only hundreds U.S. $ with low cost solutions presented here after. LOW COST SOLUTIONS FOR CHECK FLOOD These solutions apply for increasing safety and/or storage of existing dams as well as for optimizing design of new dams. These solutions include: 1 - Optimising the embankment crest. 2 - Embankments overtopping. 3 - Increasing the free flow discharges (labyrinth weirs). 4 - Various fuse devices. 5 - Combining such solutions with gates. 1 - OPTIMIZING THE EMBANKMENTS CREST Raising the reservoir level close to the dam crest automatically increases the hydraulic head and thus the discharge. Optimizing may include: - Steepening the slopes of upper part of earthfill dams.

3 - Adding a crest parapet. - Improving of crest imperviousness and waves protection (stones or concrete lining against erosion or scouring). If we call L the dam length in m, e the spillway length in m, H the freeboard, and a the cost per meter to raise the crest by 1 m, the discharge of a free flow spillway is increased by about 3e H (in m3/s), and the cost per m3/s of discharge increase is La/3e H. For usual values of L/e (5 to 10) and H (2 to 4 m), this cost is in the range of 1 or 2a, i.e. few hundreds U.S. $ per additional m3/s. 2 - EMBANKMENT OVERTOPPING If an embankment dam includes a long part 5 to 10 m high, it may be possible, as additional spillway, to line there the downstream slope with Roller Compacted Concrete and to spill for exceptional floods the water depth corresponding to the freeboard. For a freeboard of 3 m and 20 or 30 m3/m of R.C.C. the cost per extra m3/s is 2 or 3 m3 of R.C.C., i.e. some hundreds U.S.$. Such solution has been used for 100 low dams in U.S. R.C.C. lining Placing spillways over a Concrete Faced Rockfill Dam will probably be more accepted in the future. 3 INCREASING THE FREE FLOW DISCHARGE 3 1 TRADITIONAL LABYRINTH WEIRS

4 Worldwide, dozens of spillways are labyrinth weirs and are operated successfully since many years.they are usually made of thin vertical reinforced concrete walls with a trapezoidal and symmetrical lay out. Inlets and outlets designs are identical. Inlet Flow A Flow Outlet A Plan view Section AA With overall length of walls L often 4 times the spillway length W (conventional ratio N = L/W =4), the discharge is about the double of a traditional weir. Most have walls 3 or 4 m high increasing the discharge by about 5 m3/s/m. This solution is easy to build. The quantity of reinforced concrete for increasing the discharge by 1 m3/s is about 1 m3 for rather low structures.where cost of labour is low, the cost per m3/s is few hundreds U.S. $, possibly 500 for higher walls. Studies and model tests are presently under way at Biskra University, Algeria, to improve such kind of weir in what concerns hydraulic efficiency, easiness of construction and cost (parallelism of walls, unequal widths of inlets and outlets, partial filling of cells..). The main drawback of this structure is the need of large place: it cannot be built upon a gravity dam section, i.e. on most spillways structures. Its cost may also quickly increase for large discharge because of the need of high walls with corresponding high water pressure. 3 2 PIANO KEYS WEIRS (P.K.Weirs)

5 To overcome these drawback, a new solution of Labyrinth Weirs has been studied, tested and optimized since 5 years, in five countries, which allows to - reduce the required space, - decrease the height of the walls, and - improve the hydraulic efficiency. This new design leads also to substantial savings in materials The solution include - either two hangovers, one upstream and one downstream, usually symmetrical (P.K.Weir type A), - or only one upstream but longer hangover (P.K.Weir type B). Part of walls are inclined and the lay out is rectangular, giving the aspect of piano keys. A B A B inlet Outlet A B A B H H H H Section AA Section BB Section BB Section AA P.K.W. type A P.K.W. type B Nota : Height «H».is measured at crossing of inclined walls (maximum free height of the vertical walls) and is a main characteristic of a P.K.Weir.

6 Model test of a P.K.Weir type A Model tests leads to standard designs which seems close to the best compromise between hydraulic efficiency, easiness of construction and cost. Such standard designs include - hangover slopes between 2/1 and 3/2 - width of the inlets 1.2 times that of outlets e (for type A) For H between 3 to 5 m, the ratio N=L/W should be about 4 to 6. For smaller H, the ratio N could be more important. P.K.Weir type A Standard (for e = 8/11 H, ratio N becomes 6)

7 Model testing showed that, if h is the head of water, the specific flow for type A is close to 4h H for the heights most often used (0.25H<h<1.5H), instead of 2.2h h for a traditional Creager spillway. The outflow of a traditional Creager crest is hence multiplied by almost 4 when h = 0.25 H, by 3 when h = 0.4H and by 2 when h is around 0.8H. The increase in specific flow (in m3/s/m) is nearly 1.8 H 1.5. A saving in head of water (i.e storage) of nearly 0.5 H is obtained. Type B layout can generate an outflow of nearly 4.5 h H. 40 Q (m3/sec/m) P.K.Weir Q = 4h H = 8h 30 Creager weir Q = 2.17 h Saving in head of water Increase in specific flow h (m 3 /sec) Diagrams comparison for Creager and P.K.W type A weirs in case of H = 4 m P.K.Weirs may be made of reinforced concrete or of steel (this last solution being limited to H below 2 or 3 m for practical and economical reasons). Construction may be made in situ or using total or partial prefabrication. For a type A concrete solution, requested quantity of reinforced concrete varies from about 0.6 to 0.4 H 2 for H between 2 and 6 m. A type A steel solution requires about 700 x H kg of steel plates. It means that 0.5 m 3 of reinforced concrete or 250 kg of steel plates allow to increase the discharge of a classic weir by about 1 m 3 /sec.

8 P.K.Weirs may be used either on new dams or on existing ones: Area to be removed PKW 0.75 H 2H 0.5 H 1.5 H H For the cost of about 1 m 3 of reinforced concrete, it is possible to modify an existing weir so as to increase - either the discharge by 1 m 3 /sec - or the storage by about S/3L m 3 (S reservoir area in m 2, L reservoir length in m). P.K.Weirs may be used in many occasions: - Increasing storage of reservoirs with free-flow spillway - Increasing safety of dams - On rivers or canals - Improving morning glory spillways Creating an additional spillway on Goulours Dam (EDF- Electricité de France. October 2006)

9 Creating an additional spillway on Goulours Dam (EDF- Electricité de France. October 2006) 4 - VARIOUS FUSE DEVICES They open only for floods of rather low probability and are then usually lost. Ordinary floods may overtop them or be discharged by an other spillway. Such devices may be very cost effective: the drawback is the exceptional loss of elements and the corresponding cost and temporary loss of storage: first opening for floods of yearly probability between 1/100 and 1/1,000 appears thus a reasonable choice. Very economical fuse devices were used since a long time in simplified manners such as the flash boards in the U.S.A. and the earthfill fuse plugs in China. New solutions, more reliable and precise, have been recently studied, tested on models and, for some of them, implemented and successfully operated since ten years. 4-1 FLASHBOARDS Thousands of small dams in U.S. have used them since 100 years. They are usually vertical wood boards standing against steel

10 pipes fit in the sill concrete. They are dismantled by hand before the flood season or the steel pipes bend for a given nappe depth over them. This solution, quite inexpensive, may be used for reduced height but is not precise and requires some precautions: - boards placed only at the end of the flood season. - checking that steel pipes have not been strengthened by the users. - height limited to about 1/3 of the difference between weir level and dam crest level. Flash boards are thus well adapted to small irrigation dams with elements about 1 m high. They may also be temporarily used before a more precise and efficient solution. 4-2 EARTHFILL FUSE PLUGS Used in China mainly as emergency spillways, their cost per m3/s is rather low, but they require much place and may thus be adapted to few sites. This device, particularly when the maintenance is poor, may lack of precision and reliability: Along years, cohesion is progressively increasing in the materials so that the head of water necessary for the plug to fuse increases accordingly, making the plug hardly fusible. There are also questions about the relevant downstream hydrogram. 4-3 CONCRETE FUSE PLUGS Concrete fuse plugs are massive elements in concrete just laid side by side on the sill of a spillway, free standing and safe stable until the water level in the reservoir reaches a certain elevation and tilting when this elevation is reached. The elements (blocks) laid on a same sill are of the same height but may have different thickness, and therefore different deadweight, so that they will tilt for different water level. For each block, this level may be foreseen rather accurately provide that the value of the uplift under the block is well known.

11 For this reason, it is convenient to design the block so that the uplift is either total or nil: this is easy to realize with a void under the block and a seal either downstream (full uplift) or upstream (no uplift). Blocks may be designed to tilt before overtopping. They may be rather high when compared to their length (height H up to 2 times length L ) and narrow (thickness E about 1/2 H). Blocks may be designed to allow overtopping before tilting. They may be rather long and thick (L/H up to 10; E/H up to 3), thus allowing an important depth h of the water nappe above them (h/h up to 2). Attention must be paid to the problem of the aeration of the nappe, which can be obtained easily. E E seal H abutment abutment Block without uplift and tilting before overtopping seal Block with full uplift and overtopped before tilting Tilting before overtopping Overtopping before tilting (tests of separating walls) Model tests

12 Blocks tilting after overtopping are particularly interesting to improve free flow spillways: - increasing discharge capacity by lowering the sill level. - increasing storage by elevation of the water level in the reservoir. - combining both increases. For such blocks, the total uplift option appears more precise as for the water level leading to tilting. Profiling Chamfered edge Uplift chamber Cross section View from upstream View from beneath Typical block allowing overtopping before tilting Blocks general arrangement Blocks may be laid directly side by side provided that they are very long.

13 Otherwise, separating walls, fixed in the spillway sill, increase the accuracy on the water level corresponding to tilting by limiting the incidence, on the stability of the remaining block, of the deformation of the nappe after tilting of an adjacent block. Theoretical calculations are easy only before overtopping. Model tests have shown the reliability of the ratio between the thickness E of a block and the depth h of the water nappe above it at tilting time. The very rough following formula may be used for a preliminary design using the shape described here above and a concrete of usual density: h = E 0.4 H It means that for a block with E = 1.5 H, h is about equal to H. Such blocks are easy to build (for instance pouring the concrete upon a layer of random materials covered by a plastic membrane to form the chamber, then taking out these materials). They do not need too much concrete: in usual cases, increasing the discharge by 1 m3/sec does not require more than 1 m3 of concrete so that the cost is rather low. For an existing free flow spillway, the cost may be few hundred U.S.$. 4-4 FUSE GATES They are also gravity units in reinforced concrete or steel, laid side to side on a spillway sill and free standing, but tilting is

14 obtained by a sudden total uplift created in the chamber through a connected well when the required water level is reached. Uplift in the chamber is nil for ordinary floods so that the elements are very stable until the tilting level is reached and the system is very precise. Ordinary floods are discharged over the fuse gate crest which can be straight or labyrinth, depending of the shape of its vertical facing. Such fuse gates have been now used since over ten years in many countries around the world and some of them already tilted for the foreseen flow. They may be used up to 100 m3 /sec/m, are much less expensive than gates and seem very attractive for large spillways. In particular, they may be used as emergency spillway when very large spilling capacity is required and are a safe answer to the problem of gates jamming. 5 - COMBINING GATES AND OTHER SOLUTIONS For most dams in the past, the extreme flood was totally discharged through gates where it was over some thousands m 3 /sec and totally discharged by free flow spillways where it was under 1000 m 3 /sec. In the future, combining gates with one of the solutions above may be most often safer (avoiding total gates jamming) and/or less expensive. For check floods over few thousands m 3 /sec, best solution may be to discharge 30 or 50% by gates (to reduce loss of storage and mitigate floods) and the extra discharge by P.K.Weirs or fuse gates or concrete fuse plugs. For check flood in the range of 1000 m 3 /sec or less, combining above solution with gate discharging 10 to 30% of the check flood maybe attractive if it requires no permanent operator ( low gate opened during the flood season for sluicing sediment or upper gate, possibly automatic, used only during the flood season).

15 CONCLUSION The traditional design methods based upon the design flood and traditional structures deserve an in depth review. The true safety of dams refers to the check flood and not to the design flood. Accepting an higher reservoir level and some damages for exceptional floods favour many new solutions; Spillage of extreme floods may thus be obtained at low cost for most dams. Relevant costs may be particularly low where costs of labour and consultancy are low, for instance in Asia where are most existing and future dams and where floods discharges are high.