1.1 General
This Guide is the description and supplement to relevant provisions of "Safety Requirements for Nuclear Power Plant Siting". This is a guidance document. Methods and proposals different from those specified in this Guide may be adopted in actual practices, but it must be proven to the National Nuclear Safety Administration that they are equivalent in the aspect of safety level and will not cause any unacceptable risk to the nuclear power plant staff and the public.
This Guide provides methods for determining design basis flood of coastal nuclear power plant siting and technical guidance for flood control of nuclear power plants.
The design basis flood for coastal sites is the flood which a nuclear power plant is designed to withstand. IT is the most severe among the following types of floods:
(1) The flood resulting from the probable maximum storm surge.
(2) The flood resulting from the probable maximum tsunami (if any).
(3) The flood resulting from the probable maximum seiche (if any).
(4) The flood resulting from a reasonable combination of severe events of the types listed in items (1)~(3) above.
The wind wave effects must be considered independently or in combination with the above floods.
A conservatively high reference water level is considered for each of these cases and allows, where applicable, for tides, sea level anomalies, and changes in lake level and flood level on rivers.
1.2 Scope
This Guide discusses various phenomena (storm surge, seiche, tsunami and wind wave) that may cause coastal floods, and gives an outline of the methods that can be used for and the critical factors involved in the evaluation of such events and of their associated effects. In addition, some treatment is presented of the possible combinations of two or more of these phenomena to produce a design basis flood. Methods are also provided for evaluating the reference water levels, taking into account the effects of tides, sea level anomalies and changes in lake level and river flow.
Sites vulnerable to coastal flooding are located on open coastal regions, semi-enclosed bodies of water and enclosed bodies of water. Open coastal regions are those portions of land directly exposed to and having a shore on a major body of water. Semi-enclosed water bodies are lagoons, river estuaries, gulfs, fjords and rias. Enclosed water bodies are lakes and reservoirs.
The phenomena of the lowering of the water level at coastal sites caused by offshore winds, low tides, wave effects or of drawdown caused by tsunamis are discussed. The static and dynamic effects of floods resulting from the various combinations (independent and interdependent) of surface waves of varying frequency are also discussed. Consideration is also given to coastline instabilities and to the effects of erosion.
The estimated flood levels and related effects on the nuclear power plants, which will vary according to the method of analysis and the type of flooding considered, shall be compared with available historical data where this is relevant, to check the conservativeness of the evaluated results.
1.3 Extreme Phenomena
For estimating the floods, a reference water level shall be established, which in this Guide allows, as appropriate, for tides, sea level anomalies and changes in lake level and river flow. For the extreme flood events, the reference water level to be considered and the combination of these events shall be selected and properly taken into account for evaluating the flood and the associated effects against which the plant has to be protected. Then the floods due to extreme events (probable maximum storm surge, probable maximum seiche or probable maximum tsunami) are evaluated together with wind-wave effects. In some coastal sites, floods due to wind-waves are the dominant flooding consideration; in these cases, care must be taken to determine the maximum wind-wave flooding.
The water-level increments resulting from the main events (storm surge, seiches and tsunamis) and wind waves shall be discussed separately, as is the reference water level. However, the principle for superposition does not apply here, so the various contributions to the total level cannot be calculated separately and summed. The calculation of the total level treats the effects jointly in order to take this non-linearity into account.
1.3.1 Method to determine probable maximum event
Two basic methods are available for determining the probable maximum flood-causing event (probable maximum storm surge, probable maximum seiche and probable maximum tsunami and wind wave): one is to make use of the knowledge of the physical model of the phenomena (deterministic methods); the other is to rely on the analysis of the historical data set of actual measured water levels in the region (stochastic method). The choice of which methodology to use depends on the availability of a large, complete and reliable set of historical data appropriate to the method and on being able to model adequately the relevant event. If sufficient information exists to permit the use of both methods, the predictions should be cross-checked one against the other.
Deterministic methods make use of models which may be empirical or may be based on physical relationships which describes the system. For a given input or a set of initial and boundary conditions, the model will predict a single value or a set of values to describe a state of the system. In order to obtain "conservative" estimates, appropriate extreme or conservative values of the input parameters are used.
Stochastic methods are based on the statistic analysis of sequences of data on historical events (i.e. time series of events) and on the separation of random component from time-dependent component. Deviation, abrupt change and tendency shall also be considered in the analysis. As for stochastic methods, the longer the historical record, the less the uncertainty in the derived values of the parameters which defined the distribution.
1 Introduction 1
1.1 General 1
1.2 Scope 1
1.3 Extreme Phenomena 2
2 Preliminary Investigation 2
2.1 Methods for the Systematic Survey of the Region 3
2.2 Site-specific Methods 4
3 Detailed Data 6
3.1 Data Collection 6
3.2 Description of the Coast 8
3.3 Data Related to Seiches 9
4 Flooding by Storm Surges and Seiches 9
4.1 Deterministic Evaluation of Probable Maximum Storm Surge 10
4.2 Probable Maximum Drawdown 14
4.3 Stochastic Evaluation of Probable Maximum Storm Surge 14
4.4 Seiche 15
4.5 Severe Surges and Seiches 16
5 Tsunami Flooding 16
5.1 Tsunami Source 17
5.2 Seabed Movement 18
5.3 Initial Form of Water Surface Displacement 18
5.4 Distant Tsunamis 19
5.5 Local Tsunami 19
5.6 Nearshore Tsunami Modifications 20
5.7 Wave Runup 20
5.8 Drawdown 21
5.9 Severe Tsunamis 21
6 Wave Effects 22
6.1 Introduction 22
6.2 General Points Concerning Methodology 22
6.3 Wind Field 23
6.4 Generation of Offshore Waves 24
6.5 Transformation of Offshore Waves 24
6.6 Near Shore Waves 25
6.7 Local Modifications of Waves 26
7 Reference Water Level 26
7.1 Astronomical Tides 27
7.2 Sea Level Abnormalities 27
7.3 Level in Enclosed Water Bodies 28
7.4 Level in River Flow 28
8 Ice Effects 28
9 Combined Flood Events 29
9.1 Introduction 29
9.2 Criteria for Selecting Combinations of Events 29
9.3 Application of the Criteria 30
10 Design Basis for Flood 31
10.1 Flood Level 31
10.2 Wave Force 31
