Purpose of Modelling
Predicting inundated areas and response time is critical for planning emergency response in the case of flash flooding. To this end, the Urban Drainage and Flood Control District, in conjunction with the City of Boulder and DHI Water & Environment prepared a 2-dimensional (2D) model of overland flooding in the Boulder Creek drainage basin using the MIKE 21 modelling software developed by DHI Water & Environment. A 2D model was chosen for the ease of development and results presentation capabilities. “A 2D model can be developed rapidly and with much less effort than a 1D model, given that appropriate topographic data is available.” said Chad Kudym from UDFCD.
The objective of the modelling was to provide Urban Drainage and Flood Control District (UDFCD) with static inundation maps, travel times for the flood wave propagation and flood animations for Boulder Creek, based on MIKE 21 model results. The inundation maps and flood animations were used for illustrative purposes during an emergency preparedness exercise in Boulder, CO.
The project consisted of two main tasks, hydraulic modelling and result presentation.
The model bathymetry was developed from existing USGS 10m DEM data while the model extents were developed based on delineation of the area of interest provided by UDFCD. The model domain extends from approximately half a mile upstream of Orodell to Canyon Park. This represents a stream channel length of approximately 10 miles. The model extents are shown in Figure 1 below. Six inflow locations and corresponding dynamic flood hydrographs were used. The flood flow hydrographs, and inflow locations were specified and provided by UDFCD.
Figure 1: Flood Model Extents
Based on the hydraulic modelling result files, flood hazard maps were prepared delineating the maximum flood depths based on the simulation results, and 2D animations of the flood inundation were prepared. These maps and animations were used to aid in developing and testing an emergency response plan.
Figure 2: Sample Flood Hazard Map
“The animations and prepared mapping were extremely useful in understanding the areas at risk to flood and the travel time of the front of the flood” said Chad Kudym.
Mr. Kudym continues “Flood modelling for emergency response planning is quite different than flood mapping for insurance purposes. The main modelling goals for emergency response planning are a reasonable estimate of inundation areas and response times, while keeping the model development costs to a minimum. Using a 2D model precluded the need to spend a lot of man-hours developing model geometry, and defining flow paths and flow splits. All of these were taken directly from the specified topography. By using a tool like MIKE 21 we were able to quickly develop representative flood models for emergency response planning at low cost.”