Simulation of flood scenarios on the Lower Pembina River flood plains with the Telemac2D Hydrodynamic Model Phase 3

In phase one of the Pembina modelling with the Telemac 2D software, a first 2 dimensional model was prepared to simulate the flooding along the lower Pembina River downstream of Walhalla, ND. This model did not extend far enough North and South of the Pembina River to be able to properly simulate scenarios where the elevated roads and the infrastructure could be adjusted. [Ref 1] In phase two of the project, the model was extended to cover the Aux Marais River and a large portion of Buffalo Creek, and Louden and Rosebud Coulees. Several scenarios were simulated where the road network was altered, and where proposed diversions and a floodway had been simulated to redirect some flood waters from the Pembina River to the adjacent coulees and rivers. [Ref 2]. The size of the second phase model was much larger than that of the first phase and represented accurately the lower Pembina River and its banks. It was successful in simulating flood propagation over the road network, but it described the other flow passages (coulees) with only a coarser cross section. This was designed to minimize the size of the model and the time it took to simulate a full month flood hydrograph. During some scenarios, extra water arriving from artificial diversions or from the removal of roads was added to the existing rivers and coulees, and the model was not able to reproduce with accuracy the flooding along their banks because of their coarse channel description. Also, all scenario simulations were performed with the same 2006 spring flood on Red River and Pembina River, which was a relatively large event that flooded the Pembina flood plain in many of the cases, providing only small differences between scenarios. In this phase 3 of the Pembina flood project, many of the small rivers and coulees have been described with a better description of the cross sections geometry, therefore providing a better representation of the conveyance of each channel. As an example, much of the Aux Marais River was described with a 3 to 6 m grid instead of the original 30 m. This fine description of the rivers has increased significantly the size of the model, and a new and faster cluster of processors were required to run with a reasonable speed. At the same time, other improvements were carried out such as extending the model from Letellier to Morris so that changes in the flow hydrodynamics on Aux Marais River or the Buffalo Creek would develop without being constrained by the downstream boundary. (The main features of importance are indicated in Figure 1. More bridges were added to the model, in particular on Buffalo Creek, the Aux Marais River, and under railway tracks that were affecting the propagation of floods. Another important modification to the modelling of the scenarios was that they were simulated with four different flood hydrographs (instead of the same 2006 hydrograph): 1:10, 1:50 and 1:100 year return period annual floods and the 1:20 year summer flood. This allowed a wide range of flood severity and consequently a progression in the protection required in each case. Spring flood hydrographs at Walhalla were developed by the USACE, summer flood hydrographs at Walhalla were developed by Manitoba Water Stewardship (MWS) and the remaining hydrographs were developed by NRC.

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