EGL Ref: 9216 22 June 2022 Page 9 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. finite difference routing scheme to predict the flood wave over a computational domain. In the runout analysis, the slurry from TDBA Process I was analysed as water, i.e., Newtonian flow. 11.1.2. Terrain Model The terrain model used for flood routing was developed using the 2007/2008 0.5m contours (Ref. 30) developed from LiDAR survey. 11.1.3. Downstream Flood Extent The CDA Technical Bulletin on TDBAs recommends that the downstream flood routing should extend to the point where the incremental effects of a failure would no longer represent a threat to life, properties and the environment, or where sufficient warning time would exist. The Federal Emergency Management Agency (FEMA, Ref. 18) recommends that the dam breach flood routing for the Rainy Day breach scenario needs to be modelled as far downstream as required for the incremental increase in depth to be within 0.3 m to 0.6 m, or the distance travelled by the peak of the flood wave in 24 hours. Similarly, for fair weather scenarios, FEMA (Ref. 18) recommends the study extends until the flows decrease to be within the river channel (i.e., a bank full flood), or until a large water body such as a lake, reservoir, or an ocean is intercepted. For Storage 3 the flood routing model was terminated immediately upstream of the confluence of the Ohinemuri River and Waihou River. The Sunny Day breach flow is fully attenuated before reaching the boundary of the model. The Rainy Day breach flow incremental depth is less than 0.6 m at the confluence of the Ohinemuri River and the Waihou River. The incremental increase in depth is considered sufficiently low to set this point as the model boundary based on the recommendations in FEMA (Ref. 18). 11.1.4. Roughness Coefficients The roughness coefficient of the flow path was assumed to be a Manning’s n value of 0.04. This was selected according to the recommendations in the HEC-RAS manuals (Refs. 11 & 12). The Manning’s n value considers base surface roughness, obstructions, irregularities, channel alignment, vegetation, and distance to the breach location. 11.1.5. Natural Flows No baseflow was modelled in the Sunny Day breach scenario. This is because the dam breach flow is more than two times greater than the mean annual flow in the downstream watercourse, and thus the baseflows in the downstream watercourse can be ignored according to FEMA (Ref. 18). The Rainy Day breach scenario assumes a breach of the embankment occurs during a flood event. The natural flows from watersheds for downstream watercourses that are tributaries of the Ohinemuri River were modelled as multiple sources of inflows, i.e.,
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