FMA Challenge 1 (1D-2D linked): Difference between revisions
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Peak flood depths and water levels were exported to ESRII ASCII grids, and the flood extent was created by contouring the grid into a single region. Flows are outputted in .csv format and directly loaded into Excel. Profiles were created using the post processing utility TUFLOW_to_GIS and outputted into a .csv file.
==2D Grid Resolution==
A 15ft 2D grid resolution is extensively used for urban modeling, and in this case provides a good trade-off between resolution and run time. 15ft cells are small enough that flow paths down roads are adequately represented (provided the DEM accurately represents the roads as discussed above).
To test the effect of different resolutions, simulations were made using grid resolutions of 5, 10, 15, 20 and 40 ft. Upon examination of the results, the flood extents varied by unexpected amounts between different resolutions. For example, the 10ft grid scenario (provided as part of the ftp download) produces a more extensive flood extent, even though the profile down the 1D channel is almost identical to the 15ft case. The extended flooding is the result of very shallow flow (less than 0.01ft deep) over large flat (horizontal) areas caused by the use of contours to create the DEM as discussed above. Due to the slightly coarser resolution the 15ft grid does not let water on to some of these flats, and they remain dry. Should an accurate DEM be made available for this Challenge, the flood extents are likely to be very different and much more consistent between different grid resolutions!
This does raise one issue indirectly of flood mapping in urban areas where the flood depths are very shallow, or if using direct rainfall modeling. In these instances, mapping of urban areas may specify that flooding must be of a minimum depth to be mapped. For example, where the flooding is less than say, 0.05m, it is not mapped.
==Manning's n Values==
The Manning’s n values were based on the aerial photo and structure photos as tabulated below. A sensitivity simulation was carried out increasing the Manning’s n value along the main channel from 0.03 to 0.04. The results for the sensitivity simulation are provided in the long-profile of maximum water surfaces along the channel (depth grids and other data can be provided upon request). The maximum increase in peak water level along the profile is 1.94 ft.<br>
{| align="center" class="wikitable" width="50%"
! style="background-color:#005581; font-weight:bold; color:white;"| Land Use
! style="background-color:#005581; font-weight:bold; color:white;" width=50%| Manning's n
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| Main Channel || 0.03
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| Roads || 0.02
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| Properties (Buildings, Gardens and Fences) || 0.1
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| Crop || 0.05
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| Parkland || 0.035
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| Vegetated area adjacent creek || 0.06
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| Pasture || 0.045
|}
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For 2D modeling, especially in flat urban areas, contours should not be used to create the DEM. If the contours were generated from a DEM, then the original DEM should be used or the original terrain data should be provided and the DEM recreated from this data. If contours are used, additional point data and/or 3D breaklines along the low and high points need to be provided to prevent the terraced effect from occurring.
==The Model Boundary and Terrain Extent==
The flooding in the overbank 2D domain extends to the edge of the model boundary and terrain data. The terrain data needs to be extended further afield to high ground.
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