Dealing with flood debris:reviewing the performance of

hydraulicsteel structuresR. Kälin, D. Ballini and J. Meier, ETtI, Switzerland

When flooding occurs, enormous quantities of floating debris can accumulate in front of inletscreens. The removal of flotsam carpet by screen clearing machinery can sometimes not bepossible as a result of the immense quantity of materiaL and the method can therefore be

inefficient. A practical technical solution for this overload situation is to wash away the flotsamover a weir system.

Considering the large arnount of flotsam whichcan accumulate during a flood, and the size andweight of the individual pieces of floating

debris (trunks, roots and massive branches), the feasi-bility of washing it all away can be questionable. Dothe loads that will result and the possible risks liewithin an acceptable range?

On the basis of these concerns, the safe load-bearingcapability and the performance capability of the fol-lowing elements has to be checked:

. hydraulic steel structures (segments, overflow gates);

. inlet screens; and,

. screen-clearing machinery.

Research into standards, and in particular DIN19704 (hydraulic steel structures), reveals that theeffects of floating debris are not defined. Comparisonswith ice loadings and ice pressure can only be relevantup to a point, as the types of hazard are different.

A possibility to evaluate this loading capability can

An overloaded trashrack rake.

Hydropower & Dams /ssue Six, 2004

be developed, starting with a definition of a standardtree trunk. The flotsam will be assigned a specificcharacter, which will serve as an indicator for the eval-uation of dimensioning and the approval.

Tree: 0 =0.5 mL =5.8 mPWood = 900 kgm-3

From this, we obtain a mass of mTree= 1025 kg or aweight of around FTree=1OkN.

As a simplification, the calculation can proceed froman impact which is normal to the sheet metal field. Itwill be assumed that the floating debris has the samespeed as the water.

Influencing factors will be determined for the speed,direction and impact surface. With these influencingfactors, it is possible to determine the effective possi-ble loading for each case individually.

Structural safetyThe following formulae can be used to check theinfluences on the protective structures. They will beused here for the actual case of the Ruppoldingenpower station.

formulae for the elastic case

For simplification, the deformation of the plate will beconsidered as being round, while the sheet metal fieldis considered to be approximately circular and f1Il1llyclarnped.

2

[

2

]

-1(j 2m1t s a r

Fmax= s (m+l)log-+(m+l)~3 ~ 4a

maximum permanent force (N)

Flotsam moves in

the direction of thebottom outlet.

77

4n E m2 S3k=

3(m2 -1)a2

Spring constant (Nimm)p2

EeZ =--1lli!2l.2k

Energy in elastic case (Nm)

M = 2EeZV2

Mass of tree trunk (kg)

where:crs = yie1dpointro = radius of the load distributionE = modulus of elasticitym = transversal contraction figure-1s = thickness of the sheet metala = radius of the sheet metal fieldk = spring constantv = flow speed

formulae for the plastic deformation

Ezug-1-

5 - 1t0"s !:>.pm

S!:>Pm2

36

1

Validity: !:>Pm< 1.3 -( E;:g r

1 2E = -mvzug 2

21t0",!:>Pm[36i +5!:>p~]-.c;M=36v2

1

Validity:!:>Pm < 1.3 -( ~:g r

L1Pm=maximum plastic deformationEzug = added energy (mgh)M = mass

MaterialThe steel qualityused by themanufacturerfor the seg-ments is S355NLthroughout.S355NL: crs= 350 Nmm-2

crslYPevalue= 380 Nmm-2

flow speed over the overpour gate

v =3..11~2 g hü3

78

11= (0.65...0.73)

hü = overfall height

flow speed under the segment

v = ~2 gHA more accurate ca1culation can be obtained using theformulae and data originally used by the manufactur-er in the design of weIT.In the formula below, the dis-charge quantity is described in terms of the opening ofthe segment. In this way, the flow speed at the loweredge of the segment can be determined.

v = ~ * 11* ~2 gH

where: ~=A + B(a - D) + C(a - D)411:(0.75)~: A=0.97, B=0.025, C=0.0035, D=2.75a: 1.80 m and 3.20 m (lifting height of the segment)HZOOm3/s398.4- (391.2 + 3.20) = 4.0 m

flotsam speed based on the surfacing effectThe flotsam is washed over the overflow gate, fallsinto the stilling basin and surfaces from the wateronce. There is now the possibility that the surfacingwood could be thrown against the segment frombehind.

a = Pwater - Pwood * gPwood

v = ~2ah

h = water deptha = acceleration of the trunk

With 900 kgm-3, the pWoodcorresponds consistentlywith earlier assumptions.

Results for the Ruppoldingenhydraulic steel structureThe values given in Table 1 have been ca1culated forthe 'worst-case' scenario, that is, the radius of the loaddistribution at impact has been selected to be verysmall. The maximum possible flow speed for the weirinstallation has also been used for the ca1culation. At900 kg/m3, the density of the wood has also beendeliberately selected to be high. Factors such as lowerflow speed, more acute impact angles and largerimpact surface have not yet been taken into account.

The results are briefly summarized below.

VerificationThe results can be verified using the formulae givenabove. It is now necessary to estimate how stronglythe influencing factors should be weighted.

If we consider the influencing factors selectedabove, it can be assumed that no plastic deformationwill take place at the overflow gate. Any impacts willbe absorbed elastically. The possibility of damage tothe anti-corrosion coating cannot be excluded, how-ever.

Furthermore, the seal between the overflow gate andthe segment could be damaged by the impact of thefloating debris. Above all, in cases where the overflowstructure has been flattened, the seal will be exposed.

Small deformations could occur during the washingaway of flotsam at the segment lock which is abovethe water, although in practice this has not been thecase so far.

Hydropower & Dams Issue Six, 2004

Table 1: Results for the Ruppoldingen steel structure

Max. mass of tree trunk Max. plastic deform-without plastic deformation ation of the element(kg) with def. mass of

1025 kg (mm)

Overflow gate 497 4

Segment lock (above 13.3 44water side)Segment box 29.3 28(underwater side)Side plate 76 19

Cabling for the side plate heating and the piping for the flapgate cylinder.

The segment locks can therefore apparently with-stand the additional influence of flotsam.

ServiceabiIityProtective installationsThe layout of the hydraulic wiring, with hoses and theelectrical cabling of the segments, has been arrangedconscientiously on the installation at Ruppoldingen.Wherever possible, the wiring has been installed fromabove, so that it cannot come into contact with theflowing water. Only the cable guides for the cylinderof the overflow gate and the heating of the side platescan come into contact with water with high under-water levels. These hoses/cabling are led to the seg-ment in the area of the pivotal bearing of the support-ing arms (Illustration 4).

Stainless covers protect all the hoses and cables inthe area which could be vulnerable. A plate has alsobeen mounted on the supporting arm from below toprotect the piping from floating debris when the seg-ment is raised.

Laying the cable directly within the supporting armwould provide an even higher level of protection.

The hydraulics for the overpour gate are weIl pro-tected.

Until now, when the waterways have becomeblocked with by flotsam, it has always been possibleto solve the problem with the methods available. Thisrequires, however, the availability of sufficient waterin the upper reaches of the Aare. If the water pressureshould be insufficient to release the blockage, othersolutions would have to be found. In this respect, it isalso advisable not to keep the segment fully open, sothat it could be opened further if necessary to clear any

blockage. No additionalloading should arise for theweir installation with regard to clearance of block-ages.

Experience has shown that washing away the float-ing debris could endanger operating personnel andthird parties. Both the area on the bridge, that is, abovethe segments, and the area beneath the opened seg-ments is endangered by the floating debris.

In the area of the segment, a piece of floating debriscould be tumed upright by the water flowing downand this could endanger anyone in the area of the rail-ings (see photo). After crossing under the segmentlock, the water and the floating debris flow into theunderflow at higher speed. Because of the strong CUf-rent, the floating debris could be catapulted out of thewater into the air. The photograph below shows afac;ade which has been damaged by a piece of floatingdebris during the process of washing flotsam away. Atree trunk was hurled through the railing, and into thefac;ade.

A wide area around the weir must be fenced offthrough which the floating debris can be washed away.The risk of accidents among the operating personnelcan thus be reduced. In particular, it must be ensuredthat no passers-by can enter the power station site, toensure accident-free operation.

Inlet screensIn the case of an emergency shutdown of the turbines,the control apparatus will be closed, and apressureimpulse will build up as a result. This happens if themoving water column (mass x speed = impulse) isslowed down. The faster the closing takes place, thelarger the pressure impulse will be. In the case of

Ruppoldingen, the control ~aratus is closed within20 seconds.

Hydropower & Dams Issue Six, 2004

Fa,ade that hasbeen damaged byfloating debris.

79

Table 2: The infIuencing factors

Speed Direction Impact Max. mass Max. mass Max. plasticof Iree trunk of Iree trunk deformationwithout without of elementplastic def. plastic def. withdef.without (kg) mass ofinfluencing 1025 kgfactors (kg) (mm)

Overflow gate 1.2 1.5 2 497 1790 1

Segment lock 1.2 1.5 2 13.3 48 12

(above water side)

Segment box 1.8 1.5 2 29.3 159 5

(underwater side)

Side plate 1.8 2 2 76 548 3

It should be ensured that the screens and their fixingmechanisms can withstand the resulting pressureimpacts against the normal direction of flow in thecase of a blockage. The screens will withstand load-ings caused by the emergency shutdown. This alsoapplies if the screens are partially blocked with flot-sam.

J.MeierD.BalliniR.Kälin

Screen clearing installationThe enormous amount of floating debris which accu-mulated at Ruppoldingen during the summer of 2002seriously exposed the limits of the screen-dearingmachinery installed. To some extent, the problems arecaused by the operating concept, but they are alsopartly caused by the equipment design. The gripperonly plunges into the water under the influence of itsown weight. If, however, a large number and, aboveall, large volume of floating debris have accumulatedin front of the screens, the gripper simply stops on thesurface of the flotsam carpet. Also, it cannot beopened wide enough to pick up large pieces of float-ing debris. The crane arm of the screen-dearingmachinery is too shürt, and, together with the grippercurrently installed, is not suitable for grippiITg[thicktree trunks.

By using a dear concept and well-thought-out logis-tics für the rem oval of tree trunks, large accumulationsof floating debris could be managed better.

The corresponding basic data für long-term planningand a cost-effective solution are the subjects of a fur-ther investigation currently being carried out atSITEC. 0

Roland Kälin obtained his degree in MechanicalEngineering from the HSR, University of Technology inRapperswil, Switzerland. He currently works as a SafetyEngineer at SITEC.

David Ballini, obtained his degree in MechanicalEngineering from the HSR, University of Technology inRapperswil, Switzerland. He currently works as a SafetyEngineer at the SITEC.

Prof. Jürg Meier, obtained his degree in MechanicalEngeering from the federal Institute of Technology inZurich, He is Manager at the Institute of Plant and SafetyTechnology, SITEC, at the HSR, University of Technologyin Rapperswil, Switzerland.

ETH HSR Hochschule für Technik Rapperswil Institut fürAnlagen- und Sicherheitstechnik Oberseestrasse 10 CH-8640 Rapperswil, Switzerland.

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