Development 1D model

B.2 Development 1D model

For the downstream parts of the rivers Rhine and Meuse a 1D model was already developed. This model is called “Noordelijk Delta Bekken model” and includes the water bodies near Dordrecht. The model was used for the 1D simulations. The boundary conditions were adjusted to correspond with the reference scenario’s (design discharge conditions).

B.2.1 Boundary conditions

Through statistical analysis, combinations of boundary conditions were determined which all result in the design water level (See paragraph A.2.2). The design water level for Dordrecht is NAP +3.01m. For Stadswerven it is expected that no flooding will occur for this water level. Therefore 1D simulations were done with a water level of NAP +3.10m, which has a return period of 4000 years.

To gain a better insight into the flooding behaviour of the area under more extreme conditions, flooding simulations were also carried out for a water level of NAP +3.4m. This is the predicted design water level in the year 2100. By simulating this situation, the Stadswerven area is evaluated for the future design water level taking rising water levels due to climate change into account. There are no statistical results available for such an extreme situation and therefore no set of boundary conditions. The boundary conditions for the extreme situation have therefore been obtained by upgrading the boundary conditions for a water level of NAP +3.1m (return period of 4000 years). This is further described in paragraph B.3.2.

B.2.2 Converting statistics design points to boundary conditions for 1D model Two design points for a water level of NAP +3.1m were selected; scenario A and B. Scenario A considers a situation in which the water level at sea is the dominant factor, while scenario B describes a situation where high river discharges are the dominant factor. Both scenarios assume an open storm surge barrier (Maeslantkering). The probability of a coinciding of extreme river discharges and extreme sea levels is extremely low. This means that the probability of a closed storm surge barrier during high river discharges is extremely low as well. Therefore no simulations were carried out for this situation. The variable values for the two scenarios are listed in Table B.1.

Table B.1

Variable values for the evaluated scenarios

Scenario Wind

wind

sea level (m Discharge

Discharge Water level

direction

speed m/s NAP)*

Rhine

Meuse (m NAP)****

(m3/s)**

(m3/s)***

3.10 * sea level at the Maasmond

** Discharge Rhine at Lobith *** Discharge Meuse at Lith **** Water level at Dordrecht

Downstream of Lobith, the river Rhine bifurcates into three branches. For the distribution of the discharge over the three branches, the following percentages have been used.

The conversions of the variable values to boundary conditions for the 1D model are as follows:

Scenario A •

The discharges for the Rhine and Meuse are applied as a constant value in stead of a time series.

• The time serie for the sea water levels are derived according to the method described in ‘onderbouwing HR 2001 voor het benedenrivierengebied’. This method adds a storm surge wave to the astronomic tide serie for the Maasmond. This is illustrated in Figure B.1.

Figure B.1

Derivation of the time series for the sea levels (Slomp, 2005).

A storm surge is trapezium shaped and is defined by three parameters; the maximum height (Hs), duration (Ts) and phase shift between the astronomic high water level and the maximum storm surge Height (Fs) (see Figure B.2).

Figure B.2

Storm surge parameters (Slomp, 2005).

For the derivation of the sea level time series, the values as listed in ‘onderbouwing HR 2001 voor het benedenrivierengebied’ were used (Ts = 29 hours and Fs = 4.5 hours).

The choice of Hs results in a maximum sea level equal to the sea levels from the two scenarios.

Scenario B •

The discharge series were deducted by downscaling the design discharge time series for the Rhine (16.000 m3/s) and Meuse (3800 m3/s). It was assumed that the peak of the discharges for the Rhine and Meuse coincide at Dordrecht.

• The Rhine boundaries are located downstream of the bifurcation point where the Rhine bifurcates into the Waal and the Nederrijn. No information was available on the simultaneous development of the peak discharge for the Waal and Nederrijn. The discharge over the Waal is much larger than the discharge over the Nederrijn and it was chosen to apply a constant discharge value for the Nederrijn corresponding to the maximum scenario discharge for this part of the river.

• The sea level at Maasmond were derived according to the method as described for scenario A.

B.2.3 Calibration of the 1D model

The water levels at Dordrecht are also influenced by the water levels on the Haringvliet lake. The scenarios do not provide water level values for the Haringvliet. Through an iterative process, the water level for the Haringvliet was varied until the water levels at Dordrecht reached a value of NAP +3.10m +/- 2cm.