Flood risk in the Netherlands

Flood risk in the Netherlands

Flood risk in the Netherlands

Willem Schellinks The breach of the St. Antony Dike near Houtewael, 1651

About the lecturer

Mark Voorendt

Lecturer and researcher Hydraulic Structures, Department of Hydraulic Engineering

Main tasks:

• PhD-research on multifunctional flood defences

• Teaching

− Engineering projects, BSc-thesis − Hydraulic structures

− Field work Hydraulic Engineering − MSc-thesis

Organizational tasks Contact details:

- coordination BSc-thesis M.Z.Voorendt@tudelft.nl

- misc. others Delft University of Technology, faculty CiTG office 3.76

Contents of this presentation

• Present situation in the Netherlands

• History of flood protection in the Netherlands • Early history • Zuiderzeewerken • Deltaplan • (Newest developments) • Flood defences • Definitions • Types

flood prone areas elevation map

Possible causes of floods

Basically:

• from sea

- spring tides - storm set-up - sea level rise

- tidal channels (scour)

• from rivers

- rain fall - melting water

- changing river course

• from precipitation • from groundwater

secondary causes:

• failing flood defences • failing drainage system

Flood threat from sea: tides

Flood threat from sea: storm set-up

Tjerk Zitman

wikimedia commons

Storm at the North Sea, Scheveningen 2012 picture: Gerard Stolk

Dune erosion

Flood threat from rivers

RIZA

Flooding of the Vltava near Prague, 2012 picture: AP Photo/ CTK, Vit Simanek

Flood threat by precipitation

KNMI

The Netherlands is a dry country, especially in winter Precipitation is only 75 cm/year

Flood caused by precipitation

Flood threat from ground water

(deep) seepage

The Netherlands during the Roman empire

Many low-lying areas with frequent floodings

Artificial mounds ('terpen') in Oostfriesland

De Bosatlas van Nederland waterland

First Dutch dikes

XI century

in Friesland, later also in Zeeland

Zeedijken in het noorden

Dijken van Nederland

Polders, reclamation, entrepeneurship

1626 Wijde Wormer (1,620 ha) 1635 Heerhugowaard (3,500) 1635 Schermer (4,770 ha)

Polders - definition

A polder is a low-lying tract of land enclosed by embankments known as dikes, that forms an artificial hydrological entity. The groundwater level is lower than the surrounding outer water levels.

It has no connection with outside water other than through operated devices.

Water enters the low-lying polder through: • water pressure of groundwater

• rainfall

Wind mills

Steam pumping stations

Modern pumping stations (diesel / electrical)

Plans for the

Zuiderzee

30 30

The result

31 31

Construction of the Afsluitdijk 1932

The Afsluitdijk in 2015

1836 people died 1800 km2 _flooded

Extreme waterlevels in the Netherlands

1953

Technical aspects

• The storm surge was higher than anticipated • The waves were not very high

• Mainly an overflow problem, not overtopping • Most problems occurred at leeward dikes

Social aspects

1835 people died, a large part because of freezing

Inhabitants in deep polders could not escape, because of the surge flowing into the polder

There was no communication

Warning by telegraph, but to office addresses (weekend) Radio did not broadcast during the night

Only after 24 hours the magnitude of the flood became clear

Everyone in South-West Netherlands was affected Large impact on national economy

The response

Detailed (technical) report on what happened, but no investigation to administrative and political failure Fast action to close all gaps, with lot of creativity and

The Delta Plan

Delta committee _{was formed to advise on further action:}

Plans did exist, but were not considered before 1953 High quality scientific basis of the advise

Focus on safety and economic development (including freshwater management)

A new start with the Deltaplan:

• Safety standards, based on economics • National dike improvement plan

• Closing of estuaries with dams and storm surge barriers (shortening coastline 700 km)

Shortening the coast line

The Deltaplan

(this may never happen again !)

The change in thinking

Around 1970 in society much more emphasis was given to the environment

Strong lobby for not closing the Oosterschelde with a dam Compromise was found in the construction of the

Ammerstol, 2012

After bringing the sea defence to the standard level, also the river dikes needed to be improved

Possible dike improvements: inner berm

reduces piping and instability inner slope

Sheet piling

The rivers

Raising the dikes in the centre of Rotterdam and Dordrecht was in fact not possible:

• too expensive

• socially not acceptable • culturally not acceptable

The "living with water" campaign

detain store

discharge

This is applicable both on large scale as well as on small scale

Definition of a flood defence:

A flood defence is a hydraulic structure intended to protect land from irregular covering by water

"anything that can be used to divert, restrict, stop or otherwise manage the natural flow of water" - USACE

Note: the flood defence has to be part of a system:

• abstract system: organisation of the operation and

maintenance of the concrete part. Also the settling of safety levels and assessment methods

• concrete system: a continuous protective line around the

area that has to be protected (dike ring)

Dike ring areas

per area equal risk, but varying flood probability

Natural soil structures

Natural soil structures

dunes

Artificial soil structures

_dikes

Boven Merwededijk (Gorinchem, 2012)

Specific water-retaining structures

sheet pile wall

Hydraulic structures in flood defences

storm surge barrier

(Hartelkering)

Objects

non-water retaining object

Flood defences in urban areas

Flood risk reduction

Dutch policy:

Multi-layered safety (Meerlaagsveiligheid)

Flood risk reduction in three layers: 1. Prevention

2. Spatial planning

Layer 1: prevention: flood defences Room for the River project

Layer 2: spatial planning: floating houses

Layer 2: spatial planning: houses on piles

Layer 2: spatial planning: houses on mounds

Layer 3: disaster management: precaution, creating awareness

Layer 3: disaster management (precaution / evacuation)

Layer 3: disaster management: evacuation, shelters

Multi-layered flood safety

Results of recent research

• "A flood defence system heavily based on dike rings does not lend itself to implement the multi-layered approach." (Hoss, 2011)

• Measures to reduce the probability of flooding (first layer) and non-expensive measures of risk

management (third layer) can be cost-effective. Physical measures in spatial planning (second layer) are less or not at all cost-effective (Kolen, 2011, 2013)

Conclusion: prevention of floods still is the best method

Multi-layered flood safety

Measures in the second layer, however, can be justified if other values than flood protection become prevalent

(Kolen 2011).

Drawbacks:

• Only process requirements for 2nd and 3rd layer • Each layer has a different responsible authority • Different response of people to flood risk