Westergouwe

The Zuidplaspolder (ZZP) is situated in the Western part of the Netherlands in the triangle Rotterdam, Zoetermeer and Gouda (RZG), see figure below:

 

zuidplaspolder

The physical characteristics can be found in table 1 below:

Indicators Characteristics
Size 4902ha[1]
Inhabitants 41,000 inhabitants (CBS xx, municipality Zuidplas)
Elevation -4.5m NAP (Moerkapelle) to -7m NAP (Nieuwerkerk aan de IJssel)
Soil subsidence 7 till 10mm per year
River Hollandse IJssel
Land use

Land reclamation processes

Until 1960s: pastureland and horticulture (Gemeente Zuidplas, 2010: 24).

In 1970s Urban expansion (Gemeente Zuidplas, 2010: 127).

 

Flood risk and strategies

The Zuidplaspolder is potentially prone to flooding from the river (Hollandse IJssel), the sea and to a minor degree pluvial flooding. However, it is protected by a primary flood defence (dike ring 14) and the storm surge barrier Krimpenerwaard, which reduce the probability of flooding greatly (safety standard of 1: 10, 000). If the Hollandse IJssel dike breaches in the South of the polder, the water level will be 0.5m to 1.3m within 6 hours[2], because the Southern area is situated lowest, has instable soil and is prone to soil subsidence. The area in the North of the polder is less flood-prone, because it is higher, with better soil conditions and highway A20 will function as a compartmentation (Ministerie van Verkeer en Waterstaat 2006, (interview 26). Therefore, the floor level of new development should be heightened till at least NAP -4.70 m for Westergouwe (HHSK & Grontmij, 2007: 29). It could also be that the Waal-dike breaches. In that case it would take 10 days before the Zuidplaspolder would be flooded, enough time for evacuation.

Pluvial flood risk is increased due to increase in paved area, and the fragmentation of the groundwater levels, which is on the one hand caused by the different soil types, but also because every new development area got their own groundwater level (HHSK & Grontmij, 2007: 5). The storage capacity of two water level areas (peilgebieden) need to be increased by broadening the channels.

Table 2 below illustrates the measures per strategy present in the Zuidplaspolder.

  Flood prevention Flood defence Flood mitigation Flood preparation
Pluvial   Increased water storage capacity of water system (broaden channels) Heightened floor level 1.20m above groundwater level
fluvial Prohibition of construction in certain areas, e.g. in parts of the South, or close to the dike

Hollandse IJssel dike

Storm surge barrier

Heightened floor level to -4.70 NAP

Compartmentation of the polder via the A20

Evacuation exercise

Flood risk management plans

 

 

[1] http://ontwikkelingzuidplaspolder.nl/project/feiten-en-cijfers/

[2] The River Hollandse IJssel is very regulated, it can be closed off from the Delta by Capelle and by Gouda. Threfore it is assumed that only the water which is in the in-between part of the river can enter the polder (interview 26, 28)