Compound flooding from storm surges, rivers, and groundwater -
Hydrodynamic modelling in a coastal catchment
Abstract
Coastal zones are particularly vulnerable to flooding. Several climatic
and state variables may drive the occurrence of such events, e.g., storm
surges, sea level rise, heavy rainfall, and high river and groundwater
levels. The co-occurrence of such events, i.e. compound or cascading
effects, has been shown to escalate flooding impacts and extent, but the
contribution of groundwater is routinely overlooked. Here, we apply an
integrated hydrological/hydrodynamic/groundwater model to investigate
underlying causes and compound effects in a Danish Wadden sea catchment.
Two models were developed: a long-term model and an overbank-spilling
model. The long-term model was calibrated and used to simulate 30-year
periods. Extreme value analyses were carried out for sea levels,
precipitation, simulated river water stages, and groundwater levels. The
co-occurrence of extremes was used to identify compound effects on high
river-stage incidents (as a flood proxy). The overbank-spilling model
was then used for simulating flooding for a subset of the largest river
stage events identified from the long-term model. The analysis showed
that the river-stage events were closely correlated to the sea level
extremes, but that the largest river-stage events were almost
exclusively compounded by precipitation or groundwater, or both. High
groundwater tables seem to correlate to the flooding events with the
largest spatial extent, as well as prolonged extreme events where either
precipitation or sea level were elevated during long periods. Thus, this
study shows that there is a general need to acknowledge the potential
effect of groundwater levels on the resulting flooding on terrain in
coastal zones.