Proposals of Measures for Enhancement of the Safety of … · Proposals of Measures for Enhancement ... overtopping danger was expressed in the design of safety spillway with one

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Paper N0: II.12

Proposals of Measures for Enhancement

of the Safety of Dams

Vlastimil Stara Miroslav Špano

Abstract: Extreme floods which occurred in the Czech Republic in 1997 and 2002 evoked questions relating to the safety of dams built during the last century. Most of the dams have to be assessed mainly with respect to their safety and the capacity of their existing operational structures. The design of new facilities of dams, designs for reconstruction of the existing structures, and/or both are necessary to enable safe management of extreme water passage. This paper presents specific proposals of measures to enhance the safety of some Czech dams. Keywords: dam, safety of dams, Znojmo Dam, hydraulic research

1. Introduction

Extreme floods had been affecting most of the territory of the Czech Republic from 1997 to 2002. Most Czech dams were exposed to the highest load so far. As a response to these events and according to a worldwide trend, a Guideline for Examination of the Safety of Dams during Floods [Metodický pokyn (1999)] was created by the Water Protection Section of the Ministry of the Environment of the Czech Republic. The guideline should amend the valid standard specification ČSN 73 6814 “Designing of dams.” The purpose of the guideline is to ensure the required safety of Czech dams according to ICOLD recommendations [Metodický pokyn (1999)]. There are 86 dams registered in The World Register of dams and more than 23 thousand small earth dams (ponds) in the Czech Rep. The process of examining the existing dams according to the new guidelines has now been in progress.

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2. Comparison of approaches to examination of the safety of dams

The present legislative ([Act No. 254/2001 of Coll. of Laws], also called “The Water Act”) divides hydraulic structures (HS) into four categories (I to IV) from the point of view of Technical and Safety Supervision. Probable life and property losses downstream of the dam play a decisive role for the division. The standard specification ČSN 73 6814 set the N-year interval of the so-called design flood and control flood, and some operational conditions. The maximum N-year interval does not exceed the value N = 1 000.

The Metodický pokyn (1999) guideline divides HS into three categories (A, B, C) by specific survey of losses in case of dam breach. The aspects examined are:

• hazard to life, • economic losses (on HS and on downstream area), • environment damage, • social and economic consequences for the owner, the region, and the country.

Assignment of the required safeguard for each category expressed in N-year control flood is summarized in Table 1. The law category of HS is also included.

Table 1 Required safeguard of hydraulic structures (HS)

Required safeguard of HS HS category (hazard indication)

The law category Survey of losses

N [years] P = 1/N I - II Considerable life losses are expected 10 000 0.0001 A

(VERY HIGH) II Life losses are not probable 2 000 0.0005 Some life losses are expected 1 000 0.001 B

(HIGH) III – IV Life losses are not probable 200 0.005 Losses prevail with third parties 100 0.01 C

(LOW) IV Owner’s losses prevail, other losses are negligible 50 - 20 0.02 - 0.05

As shown in the table, the present safeguard requirements are much higher compared with ČSN standards (max N = 1 000) especially for higher categories. A new special standard specification TNV 75 2935 “Assessment of dams during floods”, which follows the Metodický pokyn (1999) guideline, has been in force since August 2003.

2.1 Examination of the safety of dams The following terms are being introduced to evaluate the safety of a dam:

• maximum safety storage elevation (MSE): determined for a specific dam as the highest water elevation in the reservoir which does not endanger the safety of the dam,

• maximum water-surface elevation (MWE): determined by routing the inflow hydrograph of an N-year flood through the reservoir surcharge volume.

A positive result has to satisfy the following formula:

MWEMSE ≤ 2.1

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Many dams in the Czech Rep. do not satisfy this condition. Therefore, searching for ways of correction is required. Redistribution of the reservoir volumes can sometimes suffice. But sometimes reconstruction of the existing and/or construction of new safety structures are necessary. Special roles are played by small earth dams (ponds). Their safety during floods was underestimated. A warning example is the rupture of several ponds on the river Lomnice in southern Bohemia. Flood from breached ponds caused significant losses in downstream towns (e.g. the Metelský Pond) [Jandora, J. (2005)].

3. Examples of dam safety enhancement on some specific dams 3.1 The Skalka Dam – implemented modifications This 17 m high earth dam is located on the river Ohře close to the town of Cheb in Western Bohemia. It had been built between the years 1962 and 1964. Underestimation of the overtopping danger was expressed in the design of safety spillway with one section gated by a tainter gate. Within the framework of the prepared reconstruction of functional structures it was decided to build a new 17 m wide functional block containing a 7 m wide spillway section gated by a flap gate. The new spillway was proposed to safely handle the flow rate of 277 m3.s-1 (corresponding to Q100) itself and 700 m3.s-1 (corresponding to Q10 000) combined with the old one [Broža, V., Pondělíček, V., Satrapa, L. (1999)].

3.2 The Morávka Dam – implemented modifications The Morávka Dam is situated on the northern side of the Moravskoslezské Beskydy mountains on the river Morávka (catchment area of the river Odra). It had been built from 1961 to 1967. The earth dam is 39 m high. The upstream bituminous concrete face was damaged during floods occurred in September 1996 and July 1997. The analyses showed the necessity of the dam reconstruction. One of the aims of reconstruction was increasing the bottom outlets capacity. This was achieved by constructing two new bottom outlets situated in the right bank of the reservoir. For a full supply elevation in the reservoir the capacity of all bottom outlets has grown up from 24 m3.s-1 to 60 m3.s-1. The reconstruction had been realized between the years 1997 and 2000 [Morávka dam (2000)].

3.3 The Šance Dam – proposed modifications An earth dam located on the river Ostravice (catchment area of the river Odra), serving as flood protection and as a drinking and industrial water supply, had been built from 1964 to 1969. The dam height is 56.8 m. The functional structures are composed of a safety spillway and a concurring chute with a design capacity of 70 m3.s-1, water intake tower with two bottom outlets DN 3000 and DN 2200 with a capacity of 113 m3.s-1 with full supply level [Švancara, J. (2002)], and a common stilling basin. The capacity of safety instrumentations

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was exceeded during the flood in 1997. The spillway, stilling basin and escapade channel were significantly damaged. Therefore, the owner, Povodí Odry, s. p., ordered a study [Švancara, J. (2002)] which deals with proposals of measures for increasing the safety of the dam against overtopping. Safe handling of the control flood corresponding with Q10 000 = 730 m3.s-1 (a flow rate of approx. 110 m3.s-1 was estimated during the 1997 flood) was required. Seven variants of proposed measures are discussed in detail in the study. Construction of a new spillway, reconstruction, and modification of the old one with respect to current structures are proposed. The choice of the specific proposal will be based on a technical and economic evaluation of the variants presented.

3.4 The Bystřička Dam – proposed modifications The Bystřička Dam is the oldest dam in the catchment area of the river Morava. This 27.4 m high sandstone masonry dam, located on the river Bystřička near the town of Vsetín, had been built between the years 1908 and 1912. At present it serves mainly as a flood protection. Three pipes of bottom outlets (one inside the masonry and two in the diversion tunnel) have a capacity of 10.85 m3.s-1 for the full supply elevation. The dam almost overtopped during the 1997 flood. The cables anchoring the masonry into the ground were also damaged. A low capacity of the bottom outlets and the escapade channel (Q = 30 m3.s-1) disabled operative management during the flood. The dam is classed in category A by the Metodický pokyn (1999) guideline. The whole dam reconstruction was chosen as the solution [Bilík, J., Rotschein, P. (2004)]. The masonry reconstruction and enlargement of the bottom outlet inside the dam (from DN 700/500 to DN 1200/1100) are already done. An outlet capacity of 11.7 m3.s-1 is available now (for the full supply elevation). The reconstruction of the safety spillway is planned for the years 2006-2007. It should ensure safety handling of 10 000-year flood with a peak flow rate of 310 m3.s-1 [Švancara, J., Torner, V. (2004)].

4. The Znojmo Dam – proposed modifications

This dam is located on the river Dyje close to the town of Znojmo in southern Moravia. It had been built from 1962 to 1966. This rock-filled dam with a watertight core is combined with a spillway block which comprises two sections of safety spillway gated by flap gates, two bottom outlets, two small hydropower stations (HPS), and water intakes.

The capacity of the safety spillway was exhausted during the 2002 flood. A flood peak of Q = 379 m3.s-1 was estimated [Kadeřábková, J., Krejčí, V. (2004)]. Therefore, a study [Glac, F., Janků, O., Bubeník, M. (2003)] with variants of technical measures for safe management of extreme water passage was made. The study counts with a design flow rate of Q = 610 m3.s-1 and with a 10 000-year control flood with a peak of Q = 732 m3.s-1. To increase the spillway capacity of the Znojmo Dam the variant 2b was chosen as the best. The following modifications were proposed: to sink the existing spillway crest by 1.2 m and to replace the

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existing flap gates with tainter gates with flaps. The necessary modifications of the stilling basin were designed, including an increased height of basin walls. A breakwater was designed on the dam crest as well as a debris retaining system in front of the spillway ,see Figure 1.

With regard to the importance of the dam and the volume of the planned reconstruction, assessment on a hydraulic model was necessary. The client required:

• verification of the modified spillway capacity and the discharge coefficient evaluation,

• assessment of the spillway surface mainly with respect to the negative pressure occurring during extreme flow rates,

• proposal and design of modifications of the stilling basin at a design flow rate of Q = 610 m3.s-1,

• specification of the designed modifications if necessary.

4.2 Hydraulic research The hydraulic research was realised in the Laboratory of Hydraulic Research of the Faculty of Civil Engineering, Brno University of Technology, Veveří 95, Brno, Czech Republic.

A physical model of the Znojmo Dam spillway block was built respecting the rules of Froude’s similarity at a length scale of Ml = 35 (Figure 1).

Figure 1 Downstream view of a model of the

Znojmo dam spillway block

Figure 2 The stream leading plate set on top

of the lower part of side pier

4.2.1 Tests of modified spillway capacity There are 3.3 m wide separating piers with vertical grooves of cofferdam upstream of each section of the spillway (see Figure 1), whose modification considerably affects the spillway capacity. The results of the research led to the decision to sink the piers top elevation from 223.00 m a.s.l. to 221.60 m a.s.l. The influence of the tested piers shape modifications was negligible. The values of the discharge coefficient of the modified spillway surface are summarized in Table 2.

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Table 2 Discharge coefficient m values as a function of discharge, valid for final modification

Q [m3.s-1] 51 143 282 606 725 m 0.392 0.372 0.374 0.398 0.401

4.2.2 Regulation of flow conditions A model test proved that the contractions at the side piers have no effect on the spillway capacity because the side piers are ahead of the spillway surface. Wakes cause a water level increase of approximately (1.0 ÷ 1.1) m at side piers when the discharge is Q = 610 m3.s-1. An improvement of wake effects, i.e. decreasing the wave crest, was necessary due to the maximum gate opening during floods. Therefore several tests of flow improvement and wake elimination were performed by using wing-like steel elements attached to the front part of pier walls. The minimisation of plate dimensions was achieved by taking step-by-step measurements while using different plate shapes and sizes [Stara, V., Šulc, J. (2004)], see Figure 2.

The circular trailing end of the central pier was replaced by the rear part of the NACA profile [Stara, V., Šulc, J., Špano, M. (2005)] with the aim to eliminate the wave formed downstream of the pier, see Figure 3.

No values of pressure sufficiently negative to cause a cavitation effect on the spillway surface had been registered during the model measurements - even when the rate of flow reached Q = 732 m3.s-1.

Figure 3 Deformation of level arising behind the end of middle pier and its elimination by the NACA profile when Q = 610 m3.s-1

4.2.3 The stilling basin modifications The first model experiments have shown that the existent stilling basin of Znojmo Dam (length L = 33.0 m, depth h = 3.5 m) can no longer comply with the requirement of sufficient energy dissipation at a design rate of flow Q = 610 m3.s-1. It was necessary to propose its modification. The client required to keep the elevation of the stilling basin bottom at 207.65 m a.s.l., because the bottom is formed by natural rock. The baffle blocks at the bottom of the basin and the chute blocks at the end of the spillway surface with a combination of basin length were proposed and tested basing on preliminary computations

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[Stara, V., Šulc, J. (2004)]. The criterion for finding the optimum stilling basin modification was the extent of deformations of the bottom of the channel downstream of the basin sill.

The use of chute blocks had a negligible effect on the total energy dissipation in the basin. The use of chute blocks considerably increased the aeration of the stream entering the space above the basin and spread a more uniform load on its bottom. The use of the chute blocks by alt. 3 was proved as more suitable (Figure 4). A 48.5 m long basin combined with five baffle blocks ensures low deformations of the channel bottom. Baffle blocks may be laid out in one or two rows depending on the chute blocks used, see Figure 4.

Due to the spray formed by water impinging on the baffle blocks in the basin a full concrete barrier about 1.2 m high was added on top of the basin walls. A heavy riprap 15 - 20 m long with concrete filling in an approximately 5 - 7 m long section was recommended as channel bottom protection.

Figure 4 Vertical plane section of operational structure and the possible modifications and ground plan of optimal layout of baffle blocks: a) with chute blocks used; b) with no chute blocks used

4.2.4 Conclusion The modified and tested safety spillway enables safe handling of the required rate of flow Q = 610 m3.s-1. If the extreme water elevation corresponds with the breakwater crest level, the capacity of the spillway reaches up to Q = 730 m3.s-1. Prolongation of the existing stilling basin combined with baffle blocks leads to sufficient energy dissipation and enables safe

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connection of the river Dyje downstream. Physical modelling was entire part of presented assessments and proposals of suitable modifications of the Znojmo Dam spillway block.

5. Summary

This paper shortly describes proposals for reconstructions of the functional structures of some hydraulic structures (HS) which led to increased safety of dams during floods with special regard to an overflow danger. Special attention was given to Znojmo Dam because the authors participated in the hydraulic research and evaluation of designed modifications. The capacity of the outlet and safety instrumentations of the dams is necessary for safe management of extreme water passage over the HS. The functionality and reliability of valves and gates are also very important. HS as a whole represent a complex organism which needs adequate servicing. The time of construction of large dams is over in the Czech Rep. However, it is still necessary to solve the questions of servicing, operating, modernizing, and enlarging the lifetime of dams. We will probably need to solve the question of safe demolition of some of the dams that are unsatisfactory from both operational and safety points of view.

References Act No. 254/2001 of Coll. of Laws “The Water Act” Bilík, J., Rotschein, P. (2004). Flood protection measures on water reservoir Bystřička on Bystřička river, Water

Management 6/2004 p. 167-168. Praha 2004. Broža, V., Pondělíček, V., Satrapa, L. (1999). Problems of Waterworks Reconstruction in Accordance with Water

Management Requirements: the Examples WW Skalka and Nechranice, Water Management 3/1999 p. 47-52. Praha 1999.

Glac, F., Janků, O., Bubeník, M. (2003). VH soustava Vranov – Znojmo, převedení N-letých průtoků, studie. VODNÍ DÍLA - TBD a. s., Brno, 2003

Jandora, J. (2005). Small Earth Dam Failures in the Lomnice River Basin, Wasserbaukolloquium 2005 “Stauanlagen am Beginn des 21. Jahrhunderts.” Dresden 2005

Kadeřábková, J., Krejčí, V. (2004). Floods on the Dyje river and reconstruction of the dam Znojmo. Water management, 12/2004 p. 380-386, Praha 2004

Metodický pokyn (1999). Metodický pokyn odboru ochrany vod Ministerstva životního prostředí k posuzování bezpečnosti přehrad za povodní, Věstník MŽP Částka 4, Duben 1999 Ročník IX.

Morávka dam (2000). Reparation and Reconstruction of the Morávka Dam, Povodí Odry, a. s., Ostrava 2000 Reidinger, J. (1997). Posuzování bezpečnosti přehrad za povodní, Věstník MŽP. Prosinec 1997 Ročník VII. Sedláček, M. (2004). Bezpečnost vodního díla Římov při povodních, XXIX. Přehradní dny 2004 Sborník - svazek

1. České Budějovice 2004 Stara, V., Šulc, J. (2004). Hydrotechnický modelový výzkum VD Znojmo. Závěrečná zpráva VUT v Brně FAST

ÚVST-LVV, Brno, 2004 Stara, V., Šulc, J., Špano, M. (2005). Hydraulic Research of the Spillway Block of the Znojmo Dam, Water

Management 4/2005 p. 104-107. Praha 2004. Švancara, J. (2002). VD Šance-zvýšení odolnosti díla proti přelití, studie proveditelnosti. Aquatis, a. s. Brno

prosinec 2002 Švancara, J., Torner, V. (2004). Reconstruction of the Bystřička Dam, Water Management 6/2004 p. 155-158.

Praha 2004. Authors Doc. Ing. Vlastimil Stara, CSc.: Brno University of Technology, Faculty of Civil Engineering, [email protected]. Miroslav Špano: Brno University of Technology, Faculty of Civil Engineering, [email protected]