ABSTRACTS 'Extreme Discharges' - CHR-KHR

Hydra-Models, a way to assess the influence of climate
change and river programs on future dike heights and the
probability of dike failure.
Robert Slomp
Institute for Inland Water Management and Waste Water Treatment (RIZA)
P.O. Box 17, NL 8200 AA Lelystad, the Netherlands
r.slomp@riza.rws.minvenw.nl
The use of Hydra-models is mandatory in the Netherlands for verifying the minimum safety levels of primary
flood defences in the deltas of the Rhine and Meuse and in the IJssel lake area. Hydra-models can also be used
in policy studies, the outcome of large river projects can be evaluated in combination with climate change. This
paper will cover the possibility to assess the combined influence of climate change and river programs on future
dike heights and the chance of dike failure. In this way a robust strategy can be determined. Realistic examples
of climate change and project proposals can be combined.
Flood defences have been built to accommodate for the two major threats, high discharges and storm. Storms
generate storm surges and waves but also impede drainage, so combinations of high water levels and waves
threaten dikes. The largest estuaries have been closed off with storm surge barriers. However the locally built up
storm surges to the land side of the storm surge barriers in combination with the impeded drainage remain a
threat. In the IJssel lake area, at the mouth of the IJssel these storm surges amount up to 2 m and in the lower
reaches of the Rhine and Meuse up to 50 cm. Impeded drainage to the sea, because of storm surges, adds between
1 m to 2,5 m to the water levels at design conditions, especially in the closed off estuaries Haringvliet and
Hollandsch Diep. At the moment two large river projects are being prepared Room for the River (Ruimte voor
de Rivier) and the Meuse works, (de Maaswerken), these projects will be finished in 2015. A number of studies
on the impact of climate change on design water levels have been published. But up till now no studies combine
both issues, the influence these large projects and climate change, on necessary dike heights in a detailed and
precise manner for all stretches of the Rhine and Meuse rivers.
The main purpose of the Room for the River project is to accommodate for the difference in official design water
levels caused by the high river discharges of 1993 and 1995. The Room for the River project mainly lowers
design water levels on the Rhine Branches and lower Meuse (from Meuse km 201 to km 250) where the design
water levels are primarily caused by high discharges. The project consists of increasing the discharge capacity
of the river by enlarging the cross section. In the short term dikes will be laid back, side channels dug, river
groins lowered. In some areas dike reinforcement will be inevitable, especially where the river is no longer the
predominant threat but combinations of storm and river discharges are. On account of climate change large scale
measures are being considered as measures for the long term e.g. flood retention reservoirs and bypasses
through existing polders.
The Meuse works mainly improve the safety for people living in the Meuse flood plain, by lowering the design
water levels, by raising the existing levees and by the construction of a number of flood retention reservoirs.
The return period for the levees, built in 1995-2000, will be raised from 50 years to 250 years. In fact the river
cross section has been diminished for most discharges, except for the design discharge of the Meuse, which has
a return period of 1250 years. The Meuse Works have to compensate for these negative effects, no rise in design
water levels downstream of the project area is permitted, from km 150 to 250.
The Hydra-models calculate
- The exceedence frequency of a fixed level of the hydraulic load. This fixed level can be the design water
level or the dike height of a dike section or enclosed combination of dike sections, called a dike ring.
- The design water level or design height of a dike can also be determined for an exceedence frequency
or return period.
- The wave overtopping rate for a given dike height and return period.
The Hydra-models make it possible to evaluate a certain project proposal in detail in a situation with and without
climate change. Up till now the possibility to assess the combined influence of climate change and river
programs on future dike heights and the chance of dike failure is only possible on the Rhine branches and the
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