Author(s): Tim Kerlin; Peter Oberle; Andreas Kron; Mario J. Franca

Linked Author(s): Tim Kerlin

Keywords: Urban flash flood; Sheetflow; Surface runoff; Overland flow; Roughness parameterization

Abstract: Heavy precipitation and resulting flash floods regularly cause serious property damage and endanger human life. In Germany the damage is in the order of 50 % of the total damage caused annually by floods. In particular, the State of Baden-Württemberg is promoting the creation of municipal hazard maps for heavy rain risk management; these should detail flow pathways with information on flood depths and flow velocities. Broad and positive experience exist with the use of multi-dimensional CFD models for rivers and streams in engineering practice for around 20 years. In contrast, methods to simulate heavy rain surface runoff presents significantly greater uncertainties. A direct transfer from river hydraulics of empirical values to parameterize the models is not possible due to the different scales of the flow depths, terrain gradients and morphodynamic processes. Besides difficulties on defining flow geometry, appropriate spatial resolutions and the adequate quantification of the flow resistances due to surface roughness and shape irregularity or vegetation is the main challenge. A comparative study shows a considerable impact of the uncertainty in the parameterization of flow resistance in the discharge hydrographs in the investigated area, and this will ultimately increase the uncertainty associated to risk assessment and emergency actions. This article presents the results of a comprehensive literature study) on the roughness parameterization of sheetflow. More than 250 international literature sources were studied and partially transferred to a meta database. Concentrating on the determination of roughness parameters, one can differentiate between field tests on natural surfaces such as meadows and arable land, and laboratory tests with synthetic material. It is noticeable that the roughness values of the field tests also include unexpectedly smooth roughness values which are within the scope of the usual roughness values for river modeling. It is presumed that this is due to runoff concentration based on small-scale geometric irregularities of the natural surfaces. Further considerations on the applicability of common flow formulas are discussed. In particular the application limits when using the Darcy-Weisbach formula are shown. This study was made on behalf and in collaboration with the LUBW (Institute for Environmental Protection from the State of Baden-Württemberg, Germany).

DOI: https://doi.org/10.3850/IAHR-39WC252171192022882

Year: 2022