ICOLD Question 84,
By J.-P. Vigny
FLOODS AND DESIGN CRITERIA
•The modern criteria for spillway design, as advocated by ICOLD, are based upon two floods:
– The traditional “Design Flood” chosen most often with a yearly probability between 1/100 and 1/1,000. The reservoir level is kept well under the dam crest (freeboard).
– The “Check Flood” of much lower probability (or chosen as the Probable Maximum Flood) for which the reservoir level is close to embankments crest and possibly over concrete dams crest. Damages are acceptable but not the dam failure..
•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.
•Is it possible to withstand such high floods at low cost?
•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.
TEN LOW COST SOLUTIONS FOR CHECK FLOOD
These solutions include:
– Optimising the embankment crest.
– Increasing the free flow discharges (labyrinth weirs).
– Various fuse devices.
– Embankments overtopping.
– Combining such solutions with gates.
1 – OPTIMIZING THE EMBANKMENTS CREST
•Raising the reservoir level close to the dam crest increases the discharge at low cost. Optimizing may include:
– Steepening the slopes of upper part of new earthfill dams.
– Crest parapet.
– Improvement of crest imperviousness and waves protection.
•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,
the cost c2 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. $.
2 – TRADITIONAL LABYRINTH WEIRS
•Worldwide, dozens of spillways are labyrinth weirs made by thin vertical reinforced concrete walls with a trapezoidal lay out.
•The overall length of walls is often 4 times the spillway length and the discharge 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.
•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.
3 – PIANO KEYS WEIRS (P.K.Weirs)
•Since 5 years, a new solution of Labyrinth Weirs which may be placed on most existing or new free flow spillways has been studied and optimized in five countries. It includes one or two hangovers. Part of walls are inclined and the lay out is rectangular; construction may be made with prefab elements; the discharge may be 3 times the discharge of a Creager Weir. It is about 4h√H (m3/s/m), h being the nappe depth and H the maximum walls height in m.
•It requires about 0,5 m3 of reinforced concrete per m3/s of extra discharge, i.e. a low cost in most developing countries.
•It may be adapted to most existing free flow spillways for increasing the discharge or the storage.
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.
•Many solutions are possible, 5 are presented here after:
3 open by tilting, 1 by bending, 1 by erosion.
4/5 – CONCRETE FUSE PLUGS
•Concrete blocks simply laid side by side on a spillway sill may be washed away by water pressure. If sliding is avoided by small abutments they will tilt for a chosen upstream level if the uplift is well known. There are thus two simple solutions with a void under the block : no uplift and full uplift.
Solution with uplift and overtopping
Solution with no uplift and tilting before overtopping
•The solution with no uplift seems better if the blocks are designed for tilting before overtopping, i.e. for auxiliary spillways.
•The solution with full uplift appears more precise in case of huge overtopping before tilting, i.e. for improving new or existing free flow spillway.
•For new free flow spillways the extra cost of this solution compared with a traditional weir is very low and c2 is quite nil. It is thus possible to get maximum safety at a very low cost.
•For improving the safety of most existing free flow spillways, c2 may be in the range of few hundred U.S.$.
6 – FUSE GATES
•They have now been used in 10 countries. They are also gravity blocks; the uplift is nil for ordinary floods and they tilt in sequence when high uplift is created in a chamber through a well for a precise uplift level.
•If designed for being overtopped before tilting they may be labyrinth shaped thus adding advantages of labyrinths and of fuse devices.
•They seem mainly attractive for rather large spillways. They may be used up to 100 m3/s/m and are much less expensive than gates.
7/8 – OTHER FUSE DEVICES
There are other fuse devices, for instance:
– Earthfill fuse plugs. Their cost per m3/s is rather low but they require much place and may thus be adapted to few sites. There are questions about their long term reliability and about the relevant downstream hydrogram.
– Flashboards. Thousands of small dams in U.S. have used them since 100 years. They are usually vertical wood boards standing against steel 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. They are inexpensive but not precise.
9 – 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.
•Placing spillways over a Concrete Faced Rockfill Dam will probably be more accepted in the future.
10 – COMBINING GATES AND OTHER SOLUTIONS
• For check floods over 5,000 m3/s, using traditional gates as normal operation for about half, and, for exceptional floods, P.K. Weirs spilling 50 m3/s/m under the freeboard head(or Fuse gates discharging up to 100 m3/s/m) is less expensive than a fully gated spillway.
It avoids dam failure in case of total gates jamming.
• For smaller discharges it is possible to use free flow spillways with fuse devices or P.K. Weirs as normal operation and gates opened only for exceptional floods.
•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.