Implementation of new tasks for old dams: a non ...

Regional Management of Water Resources (Proceedings of a symposium held during the Sixth I A H S Scientific Assembly at Maastricht, The Netherlands, July 2001). I A H S Publ. no. 268, 2 0 0 1 .

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Implementation of new tasks for old dams: a non-trivial problem

G E R T A. S C H U L T Z & A N D R E A S H. S C H U M A N N Institute for Hydrology, Water Resources and Environmental Ruhr University Bochum, D-44780 Bochum, Germany

Techniques,

e-mail: gert.a.schultz@ruhr-uni-bochum,de

Abstract In Central Europe many dams were built in the first years of the twentieth century. Now, 1 0 0 years later, many of the objectives valid in the design period are not relevant any more. Thus a decision has to be made, as to whether such a dam shall be removed or new functions be found. Such a decision requires a thorough analysis of potential new functions for the dam. In these cases the most important aspects are: dam safety, economy, scale of the water resources system, ecology and necessary technical modifications. Three examples are presented, in which such modifications had to be imple­ mented. The individual systems and problems are discussed and methods for their solution are presented. It is demonstrated, that certain common features are obvious, although the projects are significantly different from each other. In all three cases continuation of the use of the reservoir was suggested. This became possible, however, only if the ownership of the dam changed. K e y w o r d s b e n e f i t - c o s t r a t i o ; d a m safety; d e c i s i o n s u p p o r t ; n e w t a s k s ; old d a m s

E V A L U A T I O N O F P O T E N T I A L N E W TASKS FOR O L D D A M S Since in most countries of the developed world many water management systems were designed and operated on the basis of criteria established about a century ago, there is a pressing need to adapt such systems to the "changing water paradigm" (Gleick, 2000). This requires re-consideration of the system's performance in view of modern design criteria such as the Principle of Sustainable Development and the Principle of Integrated River Basin Management, while conventional principles like economic efficiency, technical safety and social demands remain valid. In this sense the first author was asked by various water authorities to re-evaluate existing water resources systems with respect to potential future changes in tasks and performance. Since all these consultancy jobs were done in the last four years, and they show certain common features, it seems worthwhile to draw conclusions from these experiences valid to a growing number of similar tasks expected in the future. Since the tasks relevant at the time of construction have become (partially) irrelevant, the question arises whether the cost of maintaining old dams are still justified. The reasons for the change of conditions are e.g. (a) reduction of water demand due to decreasing population, recycling, more efficient systems, reduced losses etc., (b) trend towards more efficient integrated larger systems, (c) changing perception of the population of acceptable risk, and (d) new water demands, e.g. for recreation, improvement and rehabilitation of ecosystems and replacement of dams by more natural flood protection systems. The results of re-evaluations of old dams usually lead to the question of whether the old dam

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Gert A. Schultz & Andreas H. Schumann

should be removed or new alternative uses for the dams be found. While in the United States in recent times more than 500 dams have been removed, this is in Europe, and particularly in Germany, almost impossible because during the decades of existence of the dams new ecosystems have developed with a high ecological value, and most of the old dams (see Fig. 1) have been classified as historical monuments. Therefore in most cases the need to find new tasks for old dams receives high priority and economic solutions need to be found which meet technical, social and ecological demands. In order to better understand this new and interdisciplinary task, three case studies are presented, in which the authors were involved, and potential problem solutions are high­ lighted. The three dams are located in three different river basins in northwest Germany.

E X A M P L E 1: T H E D A M SAFETY P R O B L E M — E N N E P E D A M Present situation The Ennepe dam ( 5 1 m high, built in 1913, reservoir capacity 12.6 x 1 0 m ) is owned by a small local water authority providing drinking water to the nearby cities and industry and low flow augmentation in the Ruhr River, to which the Ennepe River is a tributary. Strong competition between water supply companies resulted in the Ennepe water authority no longer being profitable. Thus it was looking for a new owner for the system who might be able to find an economical way to operate the system for new potential water uses. 6

3

Future tasks for the system A regional water authority (Ruhr-Verband), became interested in managing the Ennepe dam within its integrated, multipurpose reservoir system. The drinking water supply function should be kept but new functions, particularly low flow augmentation in the River Ruhr, were of interest. Main problems to be solved Ruhr-Verband was prepared to take over the reservoir only if it could be proved that the dam safety was guaranteed, even under the most extreme flood conditions. This was a sensitive issue, since it was known that dam safety could not be guaranteed if the reservoir was inundated up to the dam crest. Thus the first author was asked to make an analysis to determine the most extreme potential conditions. Methods applied The hydrological loading on the system under flood conditions was computed as the Probable Maximum Flood (PMF) derived from the Probable Maximum Precipitation (PMP) under extreme initial conditions. The response of the system was also assumed to be based on highly unfavourable initial conditions at the dam. The extreme initial flood conditions chosen were: (a) frozen soil, (b) deep snow cover, (c) rapid rise of temperature. P M F was computed with the aid of PMP as input to a distributed rainfall-runoff model. About 200 different scenarios had to be computed for a large range of potential PMP durations and time distributions within the duration. The system's response depends on the characteristics of the dam and its assumed (highly unfavourable) initial conditions. Figure 1 shows that there are 13 spillway elements in the centre and another 18 spillways at a higher elevation on both sides of the dam. There are two bottom outlets. Reservoir routing was computed for the relevant P M F scenarios under the following assumed initial conditions at the dam: both bottom outlets not functioning, the 13 spillways in the centre part of the dam

Implementation of new tasks for old dams: a non-trivial

problem

59

Fig. 1 Ennepe Dam with spillways.

are 100% clogged, out of the 18 spillways at the sides of the dam only 12 are functioning and the reservoir is full up to the spillway level (at the start of the flood). The central question to be answered was: Is it possible that under the most unfavourable conditions a P M F can fill up the reservoir to the dam crest? Figure 2

shows

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Time increments (hours) Fig. 2 Hydrographs of reservoir water level for different operation alternatives during extreme floods (PMF), Ennepe Dam, Germany. (Alternatives differ in status of bottom outlets, spillways in dam centre and dam sides and pre-flood releases.)

60

Gert A. Schultz & Andreas H. Schumann

hydrographs of water level in the Ennepe dam for seven different scenarios representing different P M F alternatives and different initial conditions at the dam site. It can be seen, that the maximum water level was—even under these most extreme conditions—6 m below the dam crest (315.80 m above sea level). Solution Due to the usefulness of the Ennepe dam for integrated river basin management, the moderate price and the non-existent danger of dam failure during extreme floods the Ruhr-Verband took over the dam from the previous owner.

E X A M P L E 2: T H E SCALE P R O B L E M — N E Y E D A M Present situation Until recently the city of Remscheid (Fig. 3) was supplied with drinking water from the nearby Neye dam built in 1908. After the new Dhûnn reservoir started operation in the mid-eighties, the city of Remscheid (among others) received its water partially from the Dhunn reservoir and partially from the Neye dam. Receiving its full water demand from the Dhunn reservoir would be economically more favourable, however, in the future. Thus the question arose, what to do with the now unnecessary Neye dam which was just costing money. The idea of selling or renting Neye dam to the Wupper River Authority, active in the larger catchment in which Neye dam is located, was discussed. It was interested, since the Neye dam could improve the performance of its water resources system (Fig. 3). Future tasks of the system If the Wupper Authority took over Neye dam it should mainly serve the purpose of improvement of low flow and thus water quality conditions. Before the dam could change ownership the city of Remscheid wanted to

Fig. 3 Wupper-DMnn multipurpose multi-unit water management system.

Implementation of new tasks for old dams: a non-trivial

61

problem

clarify its water supply situation under emergency conditions, i.e. if the Dhiinn reservoir were not functioning, e.g. due to technical problems, sabotage, etc. The city wanted to reserve the right to use water from the Neye dam in an emergency. Main problems to be solved There is obviously a conflict of interests between the city of Remscheid and the Wupper Authority as far as the conjunctive use of the Neye dam is concerned. Therefore the authors were asked to propose solutions which would be acceptable to both authorities. Methods applied The task was to specify for how many months the Neye dam, if initially full, could supply the city during a drought period. For the given water demand of the city (25 000 m day" ) the reservoir balance computations were carried out for historic extreme drought periods. Furthermore the potential maximum water supply, which could be maintained during critical drought periods of up to 24 months, was computed. Figure 4 shows the result of this computation for the (most severe) drought of 1959. The water demand could be satisfied for 11 months only. If the demand were reduced to 19 400 m day" the Neye dam had no difficulty in always delivering the desired water. Thus the results show that the city would need the full capacity of Neye dam to secure an emergency water supply, leaving no water to the Wupper authority for their intended uses. Alternative solutions were investigated, e.g. combined supply of Remscheid by Neye and Kerspe dam (Fig. 3) which produced results which were not much better. 3

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Solution Since the Neye dam was unable (even if combined with the Kerspe dam) to make available additional water for the purposes of the Wupper authority, a third alternative is now being considered. The city of Wuppertal receives water via a pipeline from the River Rhine (Fig. 3). Since the water supply network of the city of Remscheid is connected to that of the city of Wuppertal, it would theoretically also be possible under emergency conditions to supply the city of Remscheid with water from the River Rhine.

Reservoir content (10°nf)

Jan

Feb

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Fig. 4 Hypothetical hydrograph of the Neye reservoir content during the extreme drought of 1959, if no water were supplied to the city of Remscheid by the Dhiinn reservoir. Water demand 25 000 m day" and 19 400 m day" . 3

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Gert A. Schultz & Andreas H. Schumann

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E X A M P L E 3: T H E E C O N O M I C P R O B L E M — G L Ô R D A M 6

3

Present situation The Glôr dam (2 x 1 0 m capacity) was built in 1906 mainly for the purpose of supplying the nearby developing metal industry. Since many of the factories no longer exist, and others recycle water or use other sources of water supply, the dam is not needed anymore. Thus the owner, a small water supply company, wishes to avoid the costs of operating the dam in the future. Future tasks of the system At present the dam serves mainly recreation purposes, which were originally not intended by the owner. While the water supply company wished to remove the dam, strong public and political interest was raised to maintain the dam, mainly for recreation. Thus an interest group was formed b y various cities and a regional authority interested in operating and using the dam in the future. Main problems to be solved The problem was to find a solution between the two alternatives, removal or rehabilitation of the dam. Neither the present owner nor the newly formed interest group was prepared to pay for the high costs of the necessary rehabilitation of the dam. The state government offered to pay for most of the rehab­ ilitation costs, if proof were given that the benefit-cost ratio of further operation of the dam was greater than 1.0. Thus the authors were asked to perform such a benefit-cost analysis versus the alternative of dam removal. Methods applied The benefit-cost analysis was carried out for the purposes of low flow augmentation, flood control, fishery and hydropower. The main problem was, however, to quantify the monetary benefits of recreation, which dominated all other benefits. Glôr dam is located close to various larger cities. The so-called "gravitation model", which is built analogous to the gravitation theory known from astrophysics, was applied (Fig. 5) to compute the fluxes of excursionists based on the four parameters: number of inhabitants, their mobility, attractiveness of, and distance to the lake. A formula was developed computing the flux of excursionists from a settlement unit i to a lake at site j (trips/weekend), which was used to compute the number of excursionists coming to Lake Glôr every weekend, thus creating a time series. Depending on weather conditions and the various activities of people at the lake it is possible to compute the expenses of the persons for various purposes. Economically we distinguish in this case

Astrophysics

Water Management

Fig. 5 Analogy between astronomic gravitation and the attractiveness of reservoirs for recreation.

Implementation of new tasks for old dams: a non-trivial

63

problem

between three types of benefits, i.e. individual benefit (IB) (per excursionist), contribu­ tion to the Gross National Product (GNP) and regional income transfer (RIT). Since removal of the Glôr dam would not influence the contribution to GNP, this parameter is neglected here. IB and RIT are computed according to economic theory (Tiedt, 1992). For an assumed 80 years of reservoir operation and an interest rate of 3 % , these values amounted to 5.74 x 10 D M for IB and 2.26 x - 1 0 D M for RIT. Rehabilitation costs for the dam amounted to 4 x 10 D M and discounted O M R costs to 3.9 x 10 D M . 6

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Solution The sum of all the benefits of the Glôr dam amounted to 10.2 x 10 D M , while costs were 8.5 x 1 0 DM, from which a benefit-cost ratio of 1.2 resulted. This is higher than 1.0, thus justifying the financial aid from the state government. This means that the dam will not be removed, but operated in the future b y the new owner. 6

C O M M O N F E A T U R E S OF D A M T A S K C H A N G E P R O B L E M S A N D CONCLUSIONS Although the three case studies presented above are dealing with different systems, different problems and different administrative structures, they show common features, which are representative for this type of problem, i.e. finding new functions for old dams. These features are: Historic uses of dams have declined in importance or even became irrelevant. The option was to either remove the dam or define new objectives and determine the expected performance of the reservoir to meet them. It was shown that partial or complete change of a d a m ' s objectives in most cases is not only feasible, but also economically profitable (Neye, Glôr dams). Increasing public awareness of risk requires re-consideration of critical safety conditions of old dams to be improved by adequate measures, usually at the expense of performance (Ennepe dam). The scale problem of water resources systems became obvious: independent small systems are not profitable anymore and should be integrated into larger systems (Ennepe and Neye dams). Change of the objectives of the system as well as integration of smaller systems into larger ones always gave rise to change of ownership (Ennepe, Neye and Glôr dams). The implementation of new tasks for old dams often requires modification of the structure of the dam or water resources system (Ennepe and Neye dams). During a dam's life time its objectives and its role within a larger infrastructure system may change. Therefore designs of dams should be made more flexible such that structural modifications in the future are technically and financially feasible without too much effort.

REFERENCES Gleick, P. H. (2000) T h e c h a n g i n g water paradigm: a look at 21st century water resources development. Wat. 25(1), 1 2 7 - 1 3 8 .

International

Tiedt, M . & Schultz, G. A. (1991) Evaluation of recreational use of reservoirs b y the "gravitation m o d e l " . In: Hydrology of Natural and Manmade Lakes (ed. b y G. Schiller, R. L e m m e l a & M . Spreafico) (Proc. Vienna S y m p . , A u g u s t 1991), 1 5 9 - 1 6 6 . I A H S P u b l . n o . 206.