Combined storm surge – riverine 

flood events

Joost Beckers

NRC Workshop on PFHA, Rockville MD, January 2013
Outline

• Tidal rivers – combined influence from surge and river

• Challenge for PFHA: need to consider many combinations of surge 

and river discharge 

• Account for correlations

• Example of how this is done for Rotterdam 
Rhine-Meuse estuary

Rotterdam

Rhine

Meuse

North Sea

Barrier

Wind
Probabilistic model 

Wind River

discharge 

Water level 

at sea

Water level 

at the levee

hydraulic 

model

To get the exceedance probability of a water level at the levee

- Consider all relevant combinations of forcing variables

- Calculate the water levels at the levee

- Determine their probabilities

- Integrate probability over all combinations

Barrier

Failure 

model

Failure 

probability

statistics

Resistance
Sobek 1D hydraulic model
Calculate water levels and draw contours

Sea

River

Water levels at location 

of interest computed 

by hydraulic model

2.75

3

3

3.25

3.25

3.5

3.5

3.5

3.75

3.75

3.75

3.75

4

4

4

4

4.25

4.25

4.25

4.5

4.5

4.5

4.75

4.75

5

discharge [1000 m3/s]

sea water level [m+NAP]

local water level [m+NAP]

1 2 3 4 5 6 7 8 9 10

1

1.5

2

2.5

3

3.5

4
Upstream location

Sea

River

Water levels mainly

determined by river

discharge

3.5

3.5

3.75

3.75

3.75

4

4

4

4.25

4.25

4.25

4.5

4.5

4.5

4.75

4.75

4.75

5

5

5

5.25

discharge [1000 m3/s]

sea water level [m+NAP]

local water level [m+NAP]

1 2 3 4 5 6 7 8 9 10

1

1.5

2

2.5

3

3.5

4
Downstream location

Sea

River

Water level mainly 

determined by sea

water level

2 2 2

2.25 2.25 2.25

2.5 2.5 2.5

2.75 2.75 2.75

3 3 3 3

3.25 3.25 3.25

3.5 3.5 3.5

3.75 3.75 3.75

4 4 4 4

4.25 4.25 4.25

discharge [1000 m3/s]

sea water level [m+NAP]

local water level [m+NAP]

1 2 3 4 5 6 7 8 9 10

1

1.5

2

2.5

3

3.5

4
Compute exceedance probability

Sea

River

2.75

3

3

3.25

3.25

3.5

3.5

3.5

3.75

3.75

3.75

3.75

4

4

4

4

4.25

4.25

4.25

4.5

4.5

4.5

4.75

4.75

5

discharge [1000 m3

/s]

sea water level [m+NAP]

local water level [m+NAP]

1 2 3 4 5 6 7 8 9 10

1

1.5

2

2.5

3

3.5

4

Example: flooding occurs 

if h>4.5 m+NAP
Compute exceedance probability

Sea

River

Failure domain

Integrate probability

over failure domain
Results Rotterdam – contour plot
Forcing variables for Rotterdam

Variables:

 North Sea water level (1,2,3,4,5,6m) 6 values

 Rhine and Meuse discharge (2 times 9) 18 values

 Wind direction 16 wind sectors

 Wind speed 5 values

 Open/closed storm surge barrier 2 possibilities

Total number of combinations: 7000 

Several weeks on a single PC, some days on a 10 PC cluster

Store in a database. Next, determine their probabilities
Statistics and dependencies

Consider 16 wind sectors:

 Separate sea water level statistics for each wind sector

Probabilities of all wind sectors should add up to onmidirectional

 Separate wind speed statistics for each wind sector 

Correlated with sea water level:
Statistics and dependencies (2)

 Open/closed storm surge barrier:

- Open due to erroneous forecast 

- Additional 1% probability of failure

 Rhine and Meuse discharge

- mutually dependent

- independent of other variables
Numerical integration of probability

Probability of exceedance of critical water level h*

X1

X

2

h*

∆x

1

∆x

2

exceedance

no exceedance

Failure domain

Vary h* to get the water

level for a given probability
Results: 1/10,000 year water levels

3.62

2.77 3.25

3.03

3.42

6.70

5.53

Large contribution from combinations of less than extreme surges and discharges 

and of ‘open barrier’ situations to exceedance probability
A few other examples: Hong Kong

Correlation between

rainfall and surge

during typhoons
Ho Chi Min City (Saigon)

Strong seasonal effects

in rainfall and tide
Bangladesh
Jakarta – Indonesia 

Land subsidence of several cm per year
Conclusions

• Number of combinations of forcing variables in tidal river 

systems poses a computational challenge to PFHA

• Specific model is required for each situation 

(forcing variables and correlations)

• In the transition zone, often a large contribution from 

combinations of less than extreme values of the individual 

variables
Thanks for your attention 

Deltares, Delft, The Netherlands

Joost Beckers 

+31-88 335 8336

Joost.beckers@deltares.nl