Flash flood hazard maps are the result of two-dimensional hydraulic modelling and show the areas that are particularly endangered during heavy rainfall events. They illustrate where water accumulates and which areas may be especially affected by flooding. These maps are crucial tools for urban planning, disaster management, and risk assessment. Together with the analysis of the damage potential, they form the basis for analysing the flood risk.
Method and required data
In general, the hazard of flash floods can be analysed using different methods, which differ in terms of the required data basis, software used, processing effort and accuracy, and significance of the results. Two-dimensional hydraulic modelling can be used to calculate inundation extents with water levels and flow velocities on the surface. Required input data are at least: a digital elevation model (DEM, with high spatial resolution, 1 m to 2 m), building polygons and information on land use as well as selected rainfall scenarios. Based on the DEM and the buildings, a two-dimensional calculation grid is generated as basis for the model. The hydraulic simulations can be performed for various precipitation scenarios. The plausibility of the results can be checked, for example, based on previous experience with heavy rainfall, aerial photographs or site inspections. According to German DWA-M 119 standard, the classification of the water levels and flood hazard is as follows: 1 = low (< 10 cm), 2 = moderate (10 – 30 cm), 3 = high (30 – 50 cm) and 4 = very high (> 50 cm).
Challenges
Creating flash flood hazard maps can present various challenges. Data availability can be an issue, as not all required data is always available at the necessary accuracy and resolution. In particular, the resolution and the quality of the DEM is crucial and heavily effects the accuracy of hydrological models. A poorly processed DEM can e.g. lead to unrealistic depressions in which water accumulates. Without detailed information of the sewer system, the runoff on the surface can be simulated only and the capacity of the culverts is not considered. Furthermore, a high temporal (1 – 5 min) and spatial (radar data or ~ 1 rain gauge/ 20 km²) resolution of rainfall data is necessary when modelling heavy rainfall which typically occur very locally and in a short period of time. Additionally, modelling requires time, computational power and expert knowledge.
Flash flood hazard map
The modelling results are evaluated and visualized in map form, with flood-prone areas highlighted in different colors to clearly indicate the risk levels. Depending on the size of the study area, it may be advisable to produce detailed maps in addition to overview maps for the entire area. This will ensure that both the overall events and small-scale results are clearly visible in the finalised map set.
Fig.1: Flash flood hazard map for Downtown Amman
Flash flood hazard maps Amman
Hazard map Amman (complete) Baseline rainfall scenario
Hazard map Amman (complete) Moderate rainfall scenario
Hazard map Amman (complete) Intense rainfall scenario
Hazard map Amman (complete) Catastrophic rainfall scenario
Hazard map Amman (Downtown) Baseline rainfall scenario
Hazard map Amman (Downtown) Moderate rainfall scenario
Hazard map Amman (Downtown) Intense rainfall scenario
Hazard map Amman (Downtown) Catastrophic rainfall scenario
Flash flood hazard maps Wadi Musa
Hazard map Wadi Musa Baseline rainfall scenario
Hazard map Wadi Musa Moderate rainfall scenario
Hazard map Wadi Musa Intense rainfall scenario
Hazard map Wadi Musa Catastrophic rainfall scenario
References
- DWA, 2016: German Guideline, DWA-Merkblatt 119. Risikomanagement in der kommunalen Überflutungsvorsorge für Entwässerungssysteme bei Starkregen (Risk management in municipal flood prevention for drainage systems during heavy rain), DWA German Association for Water, Wastewater and Waste, e.V., Hennef, ISBN: 978-3-88721-393-0.