Improving flood hazard prediction models

ABSTRACT

Inaccurate components of hydrodynamic models can lead to inaccurate flood hazard simulations, particularly when models are applied to floods larger than the model calibration conditions, yet hazard information is usually sought for events which are more extreme than those documented by accurate historical measurements. Model components subject to error include the input hydrographs (and their exceedance probabilities), input roughness maps, representations of topography, bathymetry, the numerical solver and the model flow resistance equation. There is particular uncertainty surrounding the treatment of roughness and flow resistance in 2D flood models. Such models typically have low-resolution mapping of roughness compared to the mapping of topography. Significant areas with high roughness and low flow depth can occur with flood rise and fall, particularly with ‘direct rainfall’ models. Conventional flow resistance equations break down under these high relative roughness conditions. The formulation of friction within a hydrodynamic model code and derivation of depth-averaged flow resistance equations for 2D models are investigated. This study gives recommendations for improved mapping of roughness, new equations for better representing flow resistance in the modelling code, a nomograph for converting the more common ‘n’ roughness values to ‘Z_o’ roughness values and makes suggestions for better communication of model results.

KEYWORDS:

• Annual Exceedance Probability
• digital elevation model
• roughness
• flow resistance
• Manning
• log law
• Z_o

Acknowledgement

This paper is based on a presentation given by the author at the RiverFlow 2016 Conference.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This research is funded by the New Zealand National Institute of Water & Atmospheric Research (NIWA), the NZ Resilience Challenge fund and the NZ Natural Hazards Research Platform.