Advances in Data and Decision Making: The Impact on Design Flood Maps

Abstract:

Prediction of design floods can be considered on a range of scales, from modeling a design hydrograph along an individual river reach, to generating national-scale flood maps. Decisions around data selection and how those data are applied within methods, can lead to increasingly divergent results.

We discuss the development of national-scale flood hazard maps for a range of design return periods to highlight how decisions regarding fundamental aspects of flood map development can lead to significant differences in the flood maps generated. Examples include: how to define a river; what type of hydraulic model to use; and how to calculate design flows and rainfall totals.

We then explore how more complex considerations can further add to model disagreement. These considerations include: hydraulic model boundary conditions the extent to which runoff processes and hydrological losses are modeled; and post-processing techniques applied to hydraulic model outputs.

We highlight the significance of these decisions by using a case study to compare two different design flood maps for the US state of Texas. We compare the Federal Emergency Management Agency (FEMA) flood map against our commercially-developed US 5m Flood Map, analyzing flood affected areas from the Hurricane Harvey event (Aug-Sep 2017) to illustrate how differences in the flood map development process have led to two very different views of flood risk.