The conjunction of storm
surges and high discharges
Gouwe aquaduct, A12 (RWS beeldbank)
Contact
Deltares | R&D Highlights 2015 Flood Risk
The need to better understand the interactions of the various processes that contribute to high levels in the IJsselmeer lake is becoming increasingly pressing. Typical issues requiring answers are the pump capacity needed at the Afsluitdijk barrier dam in 2020 to compensate for sea-level rise, the feasibility of raising the lake level in early spring within the current safety standards in order to generate a better environment for birds, and the impact of a structural failure of pumping and discharge resources on the flood frequency curve for the lake level.
Information about the correlations between, and time shifts in, the most important stochastic variables contributing to high lake levels is needed to set up a probabilistic model. This research is investigating these correlations and time shifts using measured time series. As this information will be used for situations that occur less frequently, an understanding of the underlying meteorological processes is required to make it clearer whether these processes also hold for more unusual situations that have never occurred before.
The IJsselmeer lake is fed by the river IJssel, a branch of the river Rhine, and by “local” runoff from a number of relatively small drains, rivers and surrounding polders. The water is discharged from the lake through the locks at Den Oever and Kornwerderzand into the Wadden Sea under gravity during low tide.
These processes and mutual interactions have been explored on the basis of five historical events and statistical analysis using a thirty-year time series. The analyses show there is a
certain regularity in the occurrence of the processes causing high lake levels. First, the water level in the lake rises as increased seawater levels
reduce discharge capacity and heavy precipitation causes a large ‘local’ supply. The small local drains respond quickly to the precipitation brought by low-pressure areas (in other words, meteorological depressions) and add to the increased level. The discharge from the IJssel then increases, prevent the lake dropping back to normal levels. This sequence is explained by the arrival of low-pressure areas from the north-west moving in the upstream direction in the catchment of the Rhine. The response from the Rhine/IJssel river system arrives after a delay of about 5-7 days.
Some of the historical events analysed involved multiple successive depressions with concurrent rainfall. In these events the process repeats itself, resulting in higher and higher lake levels. The main contribution of the current study is the identification of the importance of local runoff for lake levels, which has been underestimated to some extent in the past. The study therefore provides crucial information for probabilistic analyses of flood risk and flood risk management for the IJsselmeer lake.
nienke.kramer@deltares.nl T +31(0)6 4643 2053
Discharge into and out of the IJsselmeer lake in the early months of 2002
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Stevin locks in the Afsluitdijk barrier
dam at Kornwerderzand discharge IJsel
at Olst discharge into the Wadden sea discharge local rivers and region level IJsselmeer lake
Rainfall events
