An important problem in terrain analysis is modeling how water flows across a terrain and
creates floods by filling up depressions. The accuracy of such modeling depends critically
on the precision of the terrain data, and available high-resolution terrain models of even
fairly small geographic regions often exceed the size of a computer's main memory. In such
cases movement of data between main memory and external memory (such as disk) is often
the bottleneck in the computation. Thus it is important to develop I/O-efficient modeling …

Our algorithm is I/O-efficient, meaning it is able to handle terrain models that are too large to
fit in the RAM, which is the case for the 1.6 m resolution model of Denmark. Our algorithm
can process the 1.6 m model in 2 hours on a standard workstation, which shows that our
algorithm can be useful in near-term flood risk assessment.