RISK ASSESSMENT OF COASTAL DEFENCES

Transcription

1 RISK ASSESSMENT OF COASTAL DEFENCES TERRY HEDGES (University of Liverpool) MARIA TERESA REIS (Posford Duvivier) T.S.Hedges & M.T.Reis, June 1999

2 PARASODE (Probabilistic Assessment of Risks Associated with Seawall Overtopping and Dune Erosion) OVERTOPPING Owen s formula H&R formula DUNE EROSION Vellinga s model

3 PARASODE program listing is freely available written in FORTRAN 77 majority of code is generic and can be applied to different failure mechanisms

4 PARASODE employs First Order Reliability Method (FORM) ANALYSIS MODE: failure probability is calculated for a given value of the design parameter, e.g. the crest level of a seawall DESIGN MODE: value of a specific design parameter is calculated for a target probability of failure

5 SINGLE FAILURE MODE PROBABILITY ANALYSIS failure function or limit state function, Z, is defined Z = resistance load Z 0 denotes failure

6 SINGLE FAILURE MODE PROBABILITY ANALYSIS The probability of failure, during a specified reference period, can be expressed as: P P( Z 0)... f dx... dx f = = Z 0 X,..., X 1 N 1 N

7 SINGLE FAILURE MODE PROBABILITY ANALYSIS The above equation forms the basis of different methods of risk analysis. Level III: full distribution approach Level II: semi - probabilistic approach Level I: limit state approach PARASODE uses FORM which is a Level II procedure

8 PARASODE incorporates routines for transforming correlated variables to a set of non-correlated variables incorporates ten continuous pre-defined statistical distributions; distributions may be truncated either on the left or on the right side user may also add own distributions

9 APPLICATION OF PARASODE TO WAVE OVERTOPPING OF SEAWALLS

10 Owen s Model crest level of seawall, CL seawall slope roughness, r still-water-level, SWL significant wave height, H S mean wave period, T m Owen parameter, A Owen parameter, B model parameter, e B

11 Owen s Model Z Q A gh 3. S CL SWL s =..exp eb. B. ( ). sm / 2π rh. S. 2π m Q is the allowable overtopping discharge; s m is the deep-water wave steepness; (CL-SWL)/r is the effective freeboard when the front slope is rough rather than smooth.

12 H&R Model crest level of seawall, CL seawall slope roughness, r tangent of the seawall slope, tan α s still-water-level, SWL significant wave height, H S peak wave period, T p H&R parameter, A H&R parameter, B model parameter, e B

13 H&R Model B 3 CL SWL CL SWL Z= Q A. g. ( C. HS ). 1 for rch 0.. S rch.. S < 1 e. B Z= Q for CL SWL rch.. S 1 Q is the allowable overtopping discharge; CH S is the maximum run-up on a smooth slope; (CL-SWL)/r is the effective freeboard when the front slope is rough rather than smooth.

14 Allowable Overtopping Rates, Q m 3 /s/m PARASODE

15 H&R and Owen Models Rc (m) Slope 1:2 Hs = 2m H&R (LAD): 37% H&R (LAD): 99% Owen (LAD) Owen (Rec.) Q (m3/s/m)

16 CASE STUDY: Liverpool Bay

17 CASE STUDY: simple embankment seawall impermeable front slope of 1:2 with a relatively smooth surface toe level of 0m OD crest level of between 8m OD and 16m OD

18 CASE STUDY: distributions of the sea state parameters (H S and T m or T p correlated in Liverpool Bay) distributions of water levels (user-defined for either total water level or tide level) interrelationship between sea state and water level (assumed uncorrelated in Liverpool Bay) distribution of the tangent of the seawall slope distribution of the roughness of the seawall slope distributions of the parameters of the models

19 CASE STUDY: Example of user-defined distribution PDF Liverpool Tide Level (m LBD)

20 Results P f (%/Year) H&R Model - (R max ) 37% Slope 1:2 Q (m 3 /s/m) LEVEL II (FORM) CL (m OD)

21 Results P f (%/Year) H&R Model - (R max ) 99% Slope 1:2 Q (m 3 /s/m) LEVEL II (FORM) CL (m OD)

22 Results P f (%/Year) Owen's Model Slope 1:2 Q (m 3 /s/m) LEVEL II (FORM) CL (m OD)

23 Results

24 Observations acceptable probabilities of failure for coastal structures are generally between 10-2 and 10-5 (i.e. 1 to 0.001%/year) to ensure structural safety, Q must be less than about 10-2 m 3 /s/m, whilst for functional safety Q must be less than about 10-5 m 3 /s/m

25 Observations Owen's model predicts higher probabilities of failure than the H&R model for normal conditions, a crest level greater than about 10m would be required to satisfy structural safety in this example a level of at least 12m would be required to satisfy functional safety according to the H&R model; Owen's model would demand a crest level greater than 16m

26 GENERAL REMARKS probabilistic methods provide a powerful framework for the design of coastal structures process involves detailed knowledge about the individual structure; therefore, confidence in the calculated value of the probability of failure must change with the amount and quality of the information used for its calculation

27 GENERAL REMARKS probabilistic methods may be seen simply as a design tool based on scientific methods which can facilitate good engineering decisions, but not a process which will necessarily provide a precise assessment of safety comparison of design alternatives using these methods is a promising way in which to apply them

28 GENERAL REMARKS but it is important to be aware of the characteristics of the various probabilistic methods, their applicability and their limitations, otherwise wrong conclusions can be drawn, incorrect decisions can be made and unsound action may be taken

29 ACKNOWLEDGEMENTS William Allsop Allsop and and Ian Ian Meadowcroft (Hydraulics Research Ltd) Ltd) Jan Jan Van Van de de Graaff Graaff (Delft (Delft University of of Technology) David David Downham and and Geoff Geoff Tickell Tickell (University of of Liverpool) Marteen Koster Koster (Koster Engineering, The The Netherlands) Mario Mario Castanheta (National Laboratory of of Civil Civil Engineering, Portugal) David David Blackman (Proudman Oceanographic Laboratory)