DAM OPERATION DURING EXTREME FLOODS

H. Haufe, H.-B. Horlacher, J. Stamm Institute of Hydraulic Engineering and Technical Hydromechanics

Department of Civil Engineering Technische Universitt Dresden

Germany

ABSTRACT In the 19th and 20th century safe drinking and industrial water supply and flood protection issues were the main reasons for the construction of dams in the Ore Mountains of Saxony (Germany). Due to these still existing multi-purpose requirements Saxon dams today are in the centre of various conflicts of interests. These conflicts are particularly visible during extreme floods as those which occurred in the past. In August 2002 strong precipitation led to the excess of the spillway design flood at numerous dams. The combination of extremely high reservoir inflows, limited outlet works capacities and short lead times inevitably led to the rapid filling of the flood control space. Spillways started operation and created flooding along the rivers downstream with partial catastrophic effects. Rising reservoir levels endangered the dam crests fortunately not affecting dam safety. Since 2002 flood storage capacities were increased to reduce critical situations in future flood events. According to new hydrological predictions rising reservoir inflows and the effect of the ongoing climate change must be considered. Efficient reservoir operation strategies require modern outlet works at the existing dams. Extensive reservoir routing calculations show that flood protection effects of dams can be optimised by using additional outlets which allow the release of significant discharges right at the beginning of the flood or even before. The paper will focus on these complex issues, considering both hydrological and structural aspects and will present first results and recommendations for adequate reservoir operation during extreme floods and dam modernisation in the 21st century.

INTRODUCTION AND PROBLEM Reservoirs in the Eastern Ore Mountains and historical floods In the 19th and 20th century safe drinking and industrial water supply and flood protection issues were the main reasons for the construction of dams in the Ore Mountains of Saxony (Germany). Due to these still existing multi-purpose requirements Saxon dams today are in the centre of various conflicts of interests. These conflicts are particularly visible during extreme floods as those which occurred in the past (1897, 1927, 1957, 2002). Flood of 2002 In August 2002 in the Eastern Ore Mountains a daily precipitation sum of 343 mm was registered (86% of the probable maximum precipitation (PMP)). The combination of extremely high reservoir inflows, limited outlet works capacities and short lead times inevitably led to the rapid filling of the flood control spaces. Spillways started to operate (Figure 1 & 2) and created flooding along the rivers downstream with partial catastrophic effects. In the Eastern Ore Mountains 12 people died and damage sum exceeded 1 billion Euros. Rising reservoir levels endangered the dam crests fortunately not affecting dam safety. The public discussion about appropriate reservoir operation started immediately after the flood. The relationship of cumulated inflow to the available flood storage capacity led to problems. Some reservoirs were not able to reduce the flood significantly.

Figure 1: Malter Dam Spillway operation 2002 Figure 2: Malter Dam Reservoir flood outflow 2002

Recent changes and further need for action Since 2002 flood storage capacities were increased to reduce critical situations in future flood events. According to new hydrological predictions rising reservoir inflows and the effect of the ongoing climate change must be considered. Efficient reservoir operation strategies require modern outlet works at the existing dams. The flood of 2002 had also dramatic effects on the water supply by the substantial entry of suspended sediments originating from fields, settlements and forests. At some drinking water reservoirs with typical summer thermal stratification the warm flood inflow with its polluting load was stored in an upper layer (approx. 10 m) under displacement of the existing water. Some days later the mixture of both layers started and caused substantial problems for the water treatment. Many of the existing dams have a lack of facilities for an effective release of wasted water layers in upper reservoir zones. Today the only solution for reservoir level draw down is the use of bottom outlets. In this case high quality water has to be released.

Table 1: Operating data of selected reservoirs in Saxony (2002-08-12/13) [1] Reservoir name

Spillway capacity

Design inflow BHQ1 HQ1,000

Design inflow BHQ2 HQ10,000

Peak inflow 2002-08-13

Peak spillway outflow 2002-08-13

Precipitation Watershed HQinflow 2002-08-13 vs BHQ1

HQinflow 2002-08-13 vs BHQ2

HQoutflow 2002-08-13 vs spillway capacity

Return period

[mm/48h][mm/24h]

295.3239.5313.6280.6250.9219

227.9201.4204.5178.8

[%] [%] [%][m/s] [m/s] [m/s] [m/s] [m/s] [a][km]

112

20

10,000

10,000

10,000

10,000

10,000

38.4

60.7

150 110

153

158 140

100

104 127

107

130.5

174

141

60.4 152

90.4 178

Lichtenberg

Saidenbach 98

156.3

43

39.8 45 63 20

39.2 60 60 48

147

Klingenberg 160

220200 220Malter

15086 150

12094.2

90

Lehnmhle 85.4 125 130

Considering the existing outlet works, it can be analysed whether these are still up-to-date and allow sufficient operational options for flood management or not. Deficits can be diagnosed by comparing the hydrological impacts to the existing discharge capacities with consideration of flood routing processes. Antiquated outlet works should be replaced if they are no longer able to meet operational requirements for flood releases. This rehabilitation often does not lead to a discharge increase due to the limited space available in narrow galleries or gate houses/chambers for the new installations. Then the implementation of additional outlets has to be checked. The investigation for an optimal solution must consider different aspects (hydrology, hydraulic, economy). The prioritisation of increased spillway capacity to guarantee dam safety against overtopping will not satisfy the expectations of people living downstream concerning risk reduction and damage minimisation. In this case a fast filling of the flood control zone up to the full reservoir level and the following spillway operation with fast discharge rise would only improve dam safety but creates new and higher threats to the downstream area causing damage to the public acceptance of dams. Therefore an optimisation should be achieved within the technical limitations. Minimisation of peak outflow and thus the highest possible flood protection are major objectives of reservoir flood operation. The peak outflows result however not only from the dam outflows but also from the discharges which are generated in the intermediate catchment (IC) downstream of reservoirs. Depending upon size of the IC it can have a significant influence on the discharge. If the IC is small the discharge from the dam dominates and in reverse the influence of the dam can become negligibly small since a large IC can generate large discharges. This factor can not be investigated separately, but have to be examined in relevant combinations.

INVESTIGATIONS Questions Against the background of the experiences from the flood of 2002 two key questions arise.

- How can the safety against dam overtopping be ensured? - To what extent is it possible to reduce the reservoir peak outflow and therefore flood damage?

Considering aspects of reservoir operation one solution might be an optimal utilisation of the existing flood control space. This depends on physical limitations of outlet works and quality of flood management. Of course an ideal operation is not realistic due to the mountainous character of the watershed with extreme short time of runoff formation finally reducing flood forecast options. Nonetheless efficient operating equipment could be installed for a more effective outflow (pre- and parallel-releases). Definitions for pre- and parallel-releases according to the German Standard DIN 19700-11 [2] are: Pre-release = water release by outlet works before spillway operation starts Parallel-release = water release by outlet works after spillway operation was started

Todays operation At the beginning of a flood the outflow usually will be increased by using bottom outlets or other outlets for limitation of reservoir water levels. The discharges of the installed outlets are limited by the hydraulic capacity. The outlet capacity depends on the hydraulic head and dimensions. Since the pre-discharge is not the primary function of bottom outlets, often the bankfull flow (allowed) of river downstream can not be used with this operating equipment. Bankfull flow is the discharge at which flow from the main channel begins to spill over into the floodplain. Valuable storage volume in the reservoir can not be cleared. In this case the reservoir water level rises rapidly. After the filling of flood control storage volume the spillway operation causes increased releases. At dams with ungated spillways the rate at which water is released to the river downstream can not be controlled. Flood routing Usually reservoirs with existing flood control storage have an absorbing effect on a flood, so the inflow hydrograph curve is transformed into an outflow hydrograph curve with a reduced and deferred peak value. Usually the inflow hydrograph can be derived from a design rainfall or snowmelt model. The outflow hydrograph is not known in advance. Both are linked by the reservoir routing equation which is based on the conservation of mass. The inflow (QIN), outflow (QOUT) and storage (S) are related by: Inflow (QIN) - Outflow (QOUT) = S/t (Figure 3). Where S is the change in storage during time increment t. Both QIN and QOUT vary with time and are defined by inflow and outflow hydrographs. This equation is the basis for all possible options for action. Finally the outflow hydrograph and the decrease of the peak discharge from HQIN to HQOUT is needed. To determine is the type and physical characteristics of the outlet structure, the storage volume vs. time relationship and the depth-discharge relationship. A stage-storage curve defines the relationship between the depth of water and the associated storage volume in a storage facility and can be derived by e.g. frustum of a pyramid formula, prismoidal formula for trapezoidal basins, circular conic section formula or analytically. A stage-discharge curve defines the relationship between the depth of water and the discharge or outflow from a storage facility and can be computed for various values of h once the physical characteristics of the weir or orifice are defined.

Figure 3: Reservoir routing Approaches If additional outlet works without limitations of discharge capacities would be available a significant pre-release would be possible. The potential of bankfull flow could be used or even be exceeded. The exceeding can be useful if peak outflow reduction could be achieved. In addition the question arises at what time pre-release should start. Furthermore the concept of outflow operation must be differentiated (e.g. constant outflow or adaptive outflow adjustment). Option 1 Pre-release starts before flood begins On the basis of reliable precipitation prognoses the time of the beginning of major reservoir releases could be shifted to the time before the flood begins. The flood begin in this investigation is at 0 h (Figure 4). At present large efforts towards an increased reliability of precipitation prognoses are undertaken so in the future better forecasts are to be expected. Figure 4 and Table 2 show results of such an operation of a given reservoir with constant outflow through fictitious outlets and following unregulated spillway operation for a 1,000 year flood. IC-generated flow is 28% of QIN and bankfull flow is 17% of HQIN. The gauge is downstream of IC. Table 2: Results of reservoir routing simulation - option 1

Operation Peak flow reduction at the gauge to Minimum time to start pre-release for lowest peak flow at the gauge

constant outflow 47% -10h adaptive outflow 53% -20h

0.0

0.2

0.4

0.6

0.8

1.0

-20 -10 0 10 20 30 40t [h]

Q/H

QIN

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

h [m

]

-20h

0h

-5h

-10h-15h

QOUT(t)+QIC(t)Present

QIN(t)

QOUT(t)+QIC(t)QPRE=const.

0h

-20h

Presenth(t)Normal operating water level

Full reservoir water level(Spillway operation)BHQ1 = HQ1,000QIC(t) = 0.28 QIN(t)

QPRE = 0.17 HQIN = QBankfullQPRE =const. vs. QPRE =adapt.

QOUT(t)+QIC(t)QPRE=adapt.

-20h

0h-5h

-10h-15h

Figure 4: Option 1 Results of reservoir routing calculations for QPRE = const = 0.17 HQIN

0-5-10

-15-20

0.6 0.7 0.80.9 1 1.1

1.2 1.3 1.40

1

(QO

UT +

QIC

)/HQ

IN

Start of Pre-

release [h]

QPRE/QBankfull

BHQ1 = HQ1,000

Figure 5: Option 1 Results of reservoir routing calculations for 0.6 < QPRE/QBankfull < 1.4 (QPRE = const.) Figure 5 shows possible peak-flow reduction at downstream gauge for different ratios QPRE/QBankfull. Major results of a reservoir simulation for option 1 considering an IC-influence are:

- significant reduction of peak discharge - early flood damages if QPRE > QBankfull (due to constant release and IC-influence)

New questions would arise about the appropriateness of such a flood operation. Option 2 - Pre-release starts after flood begins Comparing option 1 & 2 it can be stated that advantage of option 2 is the unnecessary head start of the flood-forecast since the water release begins with increasing reservoir inflows. Thus one of the main points of criticism that operators would not empty the reservoirs on basis of uncertain precipitation and discharge prognoses can be eliminated. Figure 6 left part shows the inflow hydrograph curve for HQ1,000 of a given reservoir (stage-storage-function and spillway capacity invariably with a single fictitious additional outlet (D=2.05m) situated approx. 14 m under operating water level and 18 m under full reservoir water level) and resulting outflow with variable flood control storage and adaptive outflow operation. Figure 6 right part shows for HQ100, HQ200, HQ1,000 and HQ10,000 the reservoir outflow for variable flood control storage. Left of the inflexion point of each graph spillway operation starts. Then outflow values increase rapidly. For low outflow rates spillway operation should be avoided. This requires greater available flood control storage capacities.

FCS vs. HQout Spillway + Add. Outlet 2.05 m QPRE adapt.

0

20

40

60

80

100

120

140

160

180

200

220

0 2 4 6 8 10

FCS [Mio. m]

HQ

out

[m/

s]

Present FCS Q BankfullHQ100 HQ200HQ1,000 HQ10,000

HQ1,000 Spillway + Add.Outlet 2.05m QPRE=>adapt.with variable Flood Control Storage (FCS)

0

20

40

60

80

100

120

140

160

180

200

220

0 10 20 30 40

t [h]

Q [

m/

s]

QinQout FCS=0.0 Mio.mQout FCS=2.0 Mio.mQout FCS=4.0 Mio.mQout FCS=6.0 Mio.mQout FCS=8.0 Mio.m Qout FCS=10.0 Mio.m

Figure 6: Results of reservoir routing calculations for variable flood control storage

FIRST RESULTS For the optimisation (minimisation) of reservoir peak outflow for any given:

- stage storage curve (reservoir) - inflow hydrograph curve - available flood control storage - bankfull discharge of downstream river (with restrictions)

an optimal pre-release value (option 1 & option 2) and an optimal starting time (option 1) can be derived! Considering this fictitious possibility of an exhaustion of the pre-release deficit by efficient operating equipment the following facts arise:

- more effective usage of flood storage capacities - reduction of reservoir peak outflow - reduction of dam overtopping risk - increased time delay of the maximum water level downstream (extended emergency evacuation time, later

damage occurrence) - reduction of necessary flood protection investments

Pre-releases as primarily water-quantity determined issue can positively affect aspects of water-quality if additional operating equipment also creates the technical possibilities for post-releases. Thus after a flood hit a reservoir water can be released from different horizons without bottom outlet operation.

CURRENT RESEARCH ACTIVITIES AND PERSPECTIVES Analytical optimisation Apart from the presented example an analytical optimisation must consider different variability:

- Reservoir o inflow hydrograph o stage-storage-function o existing flood control storage

- Dam o discharge functions of existing and additional operating equipment (spillways, outlets)

- Downstream river o bankfull discharge capacity

- Management objectives o peak outflow minimisation vs. damage occurrence time

These input-data can be partly adapted to the nature values with analytical approaches (Table 3). The variability regarding number, dimensions and altitude of additional operating equipment influences its discharge function.

Table 3: Input data for analytical approach

Inflow hydrograph

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5T

qzn = 2n = 5n = 10

)T1(nnZ eT)T(q

= [3]

Stage-storage-function

0

10

20

30

40

0 5 10 15 20

[Mio. m]

H [m

]

realanalytical

a = 0.0047b = 2.2492

bHa)H(V = [4]

Discharge functions

23

3Spillway hg2b32

Q =

++=

dL

1

hg2AQ 2Outlet.Add

++=

dL

1

hg2AQ 3Outlet.Bott

[5]

Perspectives The analytical optimisation with consideration of risk aspects forms the emphasis of further research activities. Subsequently technical-structural aspects with special consideration of the existing dam-type are regarded (concrete/masonry gravity dam or embankment dam). Fundamental construction principles for outlet works are to be considered. That includes intakes (incl. screens), outlets, the number of vales/gates, the operating reliability (vibrations/cavitation) as well as energy dissipation. The integration into the existing dams is a technological challenge. On the basis of the described need for action at some Saxon dams, this investigation creates a tool which can be used as basis for the modernisation of operating equipment. The improvement of the flood protection effect of dams represents an important component for an up-to-date flood management. There is good evidence of the benefits of additional mid level outlets in being able to optimise reservoir operation at the beginning of the flood and thus reducing risks, protecting people and averting severe damage downstream of dams in the 21st century.

REFERENCES [1] Sieber, H.-U.: Hochwasser 2002 Kurzbericht, http://www.smul.sachsen.de, read on 2008-19-05 [2] DIN 19700 Stauanlagen - Teil 11: Talsperren, Beuth-Verlag Berlin, 2004 [3] Sinniger, R.; Hager, W. H.: Retentionsvorgnge in Speicherseen, Schweizer Ingenieur u. Architekt, 26/1984 [4] Khne, A.: Charakteristische Kenngren schweizerischer Speicherseen, Geographica Helvetica, 4/1978 [5] Bollrich, G.: Technische Hydromechanik Band 1 Grundlagen, 4. Auflage, Verlag fr Bauwesen Berlin, 1996