Dam Safety andLow Cost Spillway Designs

By F. Lempérière & J.P. Vigny (Hydrocoop France)

THE CHALLENGE

Increasing dams safety by taking into account more importantfloods passing through spillways, is a major concern all around theworld.But corresponding costs may become very high and sometimesunbearable for developping countries:Is it possible to find safe and economical solutions well adapted tothe 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 yearlyprobability between 1/100 and 1/1,000) and the “Check Flood”.The true safety of dams refer to the Check Flood (sometimeschosen as the Probable Maximum Flood) to which the dam mustresist without failure, and not to the Design Flood, during which thedam must be normally operated without damages and the water levelof the reservoir is kept well under the dam crest (freeboard).

The P.M.F. discharge may be 3 or 4 times the discharge of theflood of probability 1/100. It may be in the range of 500 m3/s for 10km² catchment area, 10,000 m3/s for 1,000 km²: The challengeremains to withstand such high floods at low cost.

5 th International Conference on Dam EngineeringLisbon – 14-16 February 2007

Accepting a reservoir level close to embankments crest (nofreeboard) and possibly over concrete dams crest and somedamages (such as loss of fuse devices) during very exceptionalfloods, allows for optimizing various economical solutions asdescribed 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 shouldbe 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 existingdams 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 CRESTRaising the reservoir level close to the dam crest

automatically increases the hydraulic head and thus thedischarge. Optimizing may include: - Steepening the slopes of upper part of earthfill dams.

- 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 isLa/3e√H.

For usual values of L/e (5 to 10) and H (2 to 4 m), this cost isin the range of 1 or 2a, i.e. few hundreds U.S. $ per additionalm3/s.

2 - EMBANKMENT OVERTOPPING

If an embankment dam includes a long part 5 to 10 m high, it may bepossible, as additional spillway, to line there the downstream slopewith Roller Compacted Concrete and to spill for exceptional floodsthe water depth corresponding to the freeboard.For a freeboard of 3 m and 20 or 30 m3/m of R.C.C. the cost perextra m3/s is 2 or 3 m3 of R.C.C., i.e. some hundreds U.S.$. Suchsolution has been used for 100 low dams in U.S.

Placing spillways over a Concrete Faced Rockfill Dam will probablybe more accepted in the future.

3 – INCREASING THE FREE FLOW DISCHARGE

3 – 1 TRADITIONAL LABYRINTH WEIRS

R.C.C. lining

Worldwide, dozens of spillways are labyrinth weirs and are operatedsuccessfully since many years.They are usually made of thin verticalreinforced concrete walls with a trapezoidal and symmetrical lay out.Inlets and outlets designs are identical.

With overall length of walls “L” often 4 times the spillway length“W” (conventional ratio N = L/W =4), the discharge is about thedouble of a traditional weir. Most have walls 3 or 4 m highincreasing 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 forrather low structures.Where cost of labour is low, the cost perm3/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 concernshydraulic efficiency, easiness of construction and cost (parallelismof walls, unequal widths of inlets and outlets, partial filling ofcells..).

The main drawback of this structure is the need of largeplace: it cannot be built upon a gravity dam section, i.e. on most

spillways structures. Its cost may also quickly increase forlarge discharge because of the need of high walls withcorresponding high water pressure.

3 – 2 PIANO KEYS WEIRS (P.K.Weirs)

Flow

Inlet

OutletA

A

Plan viewSection AA

Flow

To overcome these drawback, a new solution of Labyrinth Weirs hasbeen 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 materialsThe 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 theaspect of “piano keys”.

Nota : Height « H ».is measured at crossing of inclined walls(maximum free height of the vertical walls) and is a maincharacteristic of a P.K.Weir.

inlet Outlet

A

A

A

A

B

B

B

B

H H H H

Section AA Section BB Section BBSection AA

P.K.W. type A P.K.W. type B

Model test of a P.K.Weir type A

Model tests leads to “standard” designs which seems close to thebest compromise between hydraulic efficiency, easiness ofconstruction 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)

Model testing showed that, if “h” is the head of water, the specific flowfor 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 byalmost 4 when h = 0.25 H, by 3 when h = 0.4H and by 2 when h isaround 0.8H. The increase in specific flow (in m3/s/m) is nearly 1.8H1.5. A saving in head of water (i.e storage) of nearly 0.5 H isobtained.Type B layout can generate an outflow of nearly 4.5 h H.

0 1 2 3 4 5Diagrams comparison for Creager andP.K.W type A weirs in case of H = 4 m

• P.K.Weirs may be made of reinforced concrete or of steel (thislast solution being limited to “H” below 2 or 3 m for practical andeconomical reasons).• Construction may be made in situ or using total or partialprefabrication.• For a type A concrete solution, requested quantity of reinforcedconcrete varies from about 0.6 to 0.4 H2 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 m3 of reinforced concrete or 250 kg of steelplates allow to increase the discharge of a classic weir by about1 m3 /sec.

Q (m3/sec/m)

P.K.Weir Q = 4h H = 8h

Creager weir Q = 2.17 h1.5

Saving in head of water

Increase in specific flow

h (m3/sec)

20

30

40

10

P.K.Weirs may be used either on new dams or on existing ones:

For the cost of about 1 m3 of reinforced concrete, it is possibleto modify an existing weir so as to increase

- either the discharge by 1 m3 /sec- or the storage by about S/3L m3 (S reservoir area in m2, 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)

Area to be removed

PKW

0.5 H 1.5 H

2HH

0.75 H

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 thenusually lost. Ordinary floods may overtop them or be discharged byan other spillway.• Such devices may be very cost effective: the drawback is theexceptional loss of elements and the corresponding cost andtemporary loss of storage: first opening for floods of yearlyprobability between 1/100 and 1/1,000 appears thus a reasonablechoice.• Very economical fuse devices were used since a long time insimplified manners such as the “flash boards” in the U.S.A. and theearthfill fuse plugs in China.• New solutions, more reliable and precise, have been recentlystudied, tested on models and, for some of them, implemented andsuccessfully 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

pipes fit in the sill concrete. They are dismantled by hand beforethe flood season or the steel pipes bend for a given nappe depth overthem.This solution, quite inexpensive, may be used for reduced heightbut 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 theusers. - height limited to about 1/3 of the difference between weir leveland dam crest level.Flash boards are thus well adapted to small irrigation dams withelements about 1 m high. They may also be temporarily used beforea more precise and efficient solution.

4-2 EARTHFILL FUSE PLUGS

Used in China mainly as emergency spillways, their cost per m3/sis rather low, but they require much place and may thus be adaptedto few sites.This device, particularly when the maintenance is poor, may lack ofprecision and reliability: Along years, cohesion is progressivelyincreasing in the materials so that the head of water necessary forthe 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 justlaid side by side on the sill of a spillway, free standing and safestable until the water level in the reservoir reaches a certainelevation and tilting when this elevation is reached.

The elements (blocks) laid on a same sill are of the sameheight but may have different thickness, and therefore differentdeadweight, so that they will tilt for different water level.

For each block, this level may be foreseen rather accuratelyprovide that the value of the uplift under the block is well known.

For this reason, it is convenient to design the block so that the upliftis either total or nil: this is easy to realize with a void under theblock and a seal either downstream (full uplift) or upstream (no uplift).

• Blocks may be designed to tilt before overtopping. They maybe rather high when compared to their length (height “H” up to 2times 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), thusallowing an important depth “h” of the water nappe above them (h/Hup to 2). Attention must be paid to the problem of the aeration of thenappe, which can be obtained easily.

Tilting before overtopping Overtopping before tilting(tests of separating walls)

Model tests

seal

E

H

E

abutmentBlock without uplift and tiltingbefore overtopping

Block with full uplift andovertopped before tilting

abutment

seal

Blocks tilting after overtopping are particularly interesting toimprove 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.

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 arevery long.

Uplift chamber

ProfilingChamfered edge

• Otherwise, separating walls, fixed in the spillway sill, increasethe accuracy on the water level corresponding to tilting by limiting theincidence, on the stability of the remaining block, of the deformationof the nappe after tilting of an adjacent block.• Theoretical calculations are easy only before overtopping.• Model tests have shown the reliability of the ratio betweenthe thickness “E” of a block and the depth “h” of the waternappe above it at tilting time.• The very rough following formula may be used for a preliminarydesign using the shape described here above and a concrete of usualdensity:

h = E – 0.4 H• It means that for a block with E = 1.5 H, h is about equalto H.• Such blocks are easy to build (for instance pouring theconcrete upon a layer of random materials covered by a plasticmembrane 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 morethan 1 m3 of concrete so that the cost is rather low. For an existing free flow spillway, the cost may be few hundredU.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

obtained by a sudden total uplift created in the chamber througha connected well when the required water level is reached.• Uplift in the chamber is nil for ordinary floods so that theelements are very stable until the tilting level is reached and thesystem is very precise. Ordinary floods are discharged over thefuse gate crest which can be straight or labyrinth, depending ofthe shape of its vertical facing.• Such fuse gates have been now used since over ten yearsin many countries around the world and some of them alreadytilted for the foreseen flow. They may be used up to 100 m3 /sec/m, are much less expensivethan gates and seem very attractive for large spillways. Inparticular, they may be used as emergency spillway when verylarge spilling capacity is required and are a safe answer to theproblem of gates jamming.

5 - COMBINING GATES AND OTHER SOLUTIONS

• For most dams in the past, the extreme flood was totallydischarged through gates where it was over some thousands m3/secand totally discharged by free flow spillways where it was under 1000m3/sec. In the future, combining gates with one of the solutionsabove may be most often safer (avoiding total gates jamming)and/or less expensive.• For check floods over few thousands m3/sec, best solution maybe to discharge 30 or 50% by gates (to reduce loss of storage andmitigate floods) and the extra discharge by P.K.Weirs or fuse gates orconcrete fuse plugs. For check flood in the range of 1000 m3/sec or less, combiningabove solution with gate discharging 10 to 30% of the check floodmaybe attractive if it requires no permanent operator ( low gateopened during the flood season for sluicing sediment or upper gate,possibly automatic, used only during the flood season).

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 notto the “design flood”.

• Accepting an higher reservoir level and some damages forexceptional floods favour many new solutions; Spillage of extremefloods may thus be obtained at low cost for most dams.

• Relevant costs may be particularly low where costs of labour andconsultancy are low, for instance in Asia where are most existingand future dams and where floods discharges are high.