Determination of River Water Level Exceedance Frequency Curves

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1 Swiss Federal Nuclear Safety Inspectorate ENSI Determination of River Water Level Exceedance Frequency Curves G. M. Schoen, R. C. Hausherr, A. Ramezanian PSA 2017 International Topical Meeting on Probabilistic Safety Assessment and Analysis Pittsburgh, PA, September 2017

2 Content 1. Introduction 2. Proposed Method 3. Example 4. Conclusions 2

3 Introduction Water level exceedance frequency curves are used as an input for PSA Design Basis or demonstration of acceptable safety Exceedance Frequency [1/yr] 1E-3 1E-4 Hazard Curve 1E-5 1E-6 1E Design Basis Water Level [m.a.s.l.] 3

4 Introduction A comprehensive assessment of the hazard curve requires the consideration of many attributes, like: measured data analysis of preinstrumental flood events weather scenarios extreme value statistics flood-induce failures of hydraulic facilities landslides log jam erosion, bed load deterministic hydraulic simulations 4

5 Introduction How to integrate into the hazard assessment the various analyses (including their uncertainty) with available tools e.g.: Assessment of measured data and historical flood events Extreme value statistics Evaluation of (interacting) flood phenomena like partial or complete log jam of water control systems, landslide, flood-induced dam failures, sediment transport, Deterministic hydraulic simulations of specific flood scenarios This paper presents an approach to integrate probabilistic and deterministic calculations and the relevant phenomena into the external flood hazard assessment. 5

6 Method analysis of preinstrumental flood events measured data weather scenarios extreme value statistics discharge flood-induce failures of hydraulic facilities partial log jam landslides erosion, bed load deterministic hydraulic simulations water level calculation flow 6

7 Method Assumption: The phenomena can be delineated between system-relevant phenomena affecting to the discharge over a large part of the river, and local phenomena impact the water level at the considered site but have a negligible impact on the discharge of the system. Failure of a large dam Partial blockage of the bridge Site Site system-relevant phenomena local phenomena 7

8 Method Consideration of system-relevant phenomena: Given a site-specific discharge exceedance frequency curve P D valid for the discharge range [r 1, r 2 ): P D (X x) for x > 0 A numerical approach is used to include system-relevant phenomena into P D. Based on P D n initiating event frequency denoted as H D (y i ) are derived: H D (y i ) = P D (X x i ) P D (X x i + 1 ) Let q S (x) [0,, 1] be the discharge-dependent probability that a system-relevant phenomenon increases the discharge by Δx. 8

9 Method Example of the reshuffling of the initiating event frequencies H D (y i ) given a hazard increasing phenomenon such that y i + 3 y i = Δx H [Frequency] q S (y 1 ). H D (y 1 ) = q S (y 4 - Δx). H D (y 4 - Δx) q S (y 4 ). H D (y 4 ) y 1 y 2 y 3 y 4 y 5 y 6 y 7 x [Discharge] x 9

10 Method For a hazard increasing phenomenon the reshuffled frequency H DR (y i ) is: For a hazard decreasing phenomenon the reshuffled frequency H DR (y i ) is: Given H DR (y i ), the reshuffled discharge exceedance frequency curve is 10

11 Method Illustration by an artificial example: Phenomena absent in data will not be captured solely by extrapolation. 11

12 Method Consideration of local phenomena: The discharge at the site is given by P DR Event trees are used to consider local phenomena. The initiating events of the event trees are derived by P DR Site-specific flood level exceedance frequency curve: For each sequence of an event tree the frequency, discharge and considered local phenomena are known. Flood levels are determined for the sequences based on deterministic hydraulic calculations. Given a flood level and the frequency for each sequence the desired flood level exceedance frequency curve can be computed. 12

13 Method Overview on the suggested concept Discharge Exceedance Freq. [considering systemrelevant phenomena] Initiating Event Freq. Event Trees [considering local phenomena] Scenarios Water Level Freq. [hydraulic calculations for given scenarios] Water Level Exceedance Freq. Exceedance Freq. P DR H DR (y 1 ) H L (w 1 ) Exceedance Freq. P L H L (w 2 ) H DR (y 2 ) Discharge Water level w 13

14 Example Illustration of the concept by an artificial example: Site Bridge Potential landslide 14

15 Example Site-specific discharge exceedance frequency curve (including a system-relevant phenomenon): 15

16 Example Event tree structure: Initiating Event [ H DR (y i ) ] No landslide No total blockage of the bridge No partial blockage of the bridge Scenario S i, j 16

17 Example Assumed river profile: a 1 r 1 w r 2 a 2 h b m.a.s.l 17

18 Example Water level exceedance frequency curve: 18

19 Conclusions The presented approach allows to systematically consider the various data on flood events, the relevant phenomena, and to integrate the results of deterministic estimation of water level with probabilistic assessments. A key element of the approach is to distinguish between phenomena, relevant for a large part of the river concerning discharge the so-called system-relevant phenomena and local phenomena which are less relevant for the overall discharge but important for the flood level at a specific site. A way to incorporate system-relevant phenomena into a given discharge exceedance frequency curve was presented. The described approach can be extended to include the impact of uncertainties in the expert judgement in a systematic and integrated manner. 19

Monte Carlo Simulations for Probabilistic Flood Hazard Assessment

Monte Carlo Simulations for Probabilistic Flood Hazard Assessment Jemie Dababneh, Ph.D., P.E., 1 and Mark Schwartz, P.E. 1 1 RIZZO Associates, Monroeville, Pennsylvania Presentation to PSA 2017 International