FMA Challenge 1 (1D-2D linked): Difference between revisions
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= Introduction =
In
▲In this challenge, a fully two-dimensional model with a nested one dimensional model has been developed to explore in- and over-bank floodplain conditions. The model includes several hydraulic structures/bridges within the main stream system which impact flood elevations. Flooding of the urbanised over-bank floodplain is experinced. For interest, the requirements of the FMA challenge is provided [http://www.tuflow.com/Download/TUFLOW/Demo_Models/2012_FMA_Challenge/All/FMA%20Challenge%201%20Documention.TUFLOW.pdf here]<br>
▲A fully functional example model has been developed allowing you to review the model setup, run the model and review results developing your skills in:
*Nested 1D/2D models;
*Understanding urban riverine conditions and over-bank floodplains;
*Using the powerful functionality of scenarios and variables to determine a suitable cell size, timestep and log output scenarios;
*Understanding the influence of structures on open channel hydraulics;
*Using
*
Data for this model is provided
*[https://www.tuflow.com/Download/TUFLOW/Demo_Models/FMA_Challenge_Model_1_QGIS.zip QGIS Data Download]
*[https://www.tuflow.com/Download/TUFLOW/Demo_Models/FMA_Challenge_Model_1_MapInfo.zip MapInfo Data Download]
*[https://www.tuflow.com/Download/TUFLOW/Demo_Models/FMA_Challenge_Model_1_ArcGIS.zip ArcGIS Data Download]
=Relevant Tutorials=
Although all Tutorials are of relevance for the FMA Challanges, For FMA Challange 1, it may be useful to revisit the following:<br>
*
*Running Scenarios - <u>[[Tutorial_M08|Tutorial Module 8]]</u>
*Running Events - <u>[[Tutorial_M09|Tutorial Module 09]]</u>
=Model Setup=
This section provides an overview and discussion of the model domain setup.<br>
All files required to setup and run the models are available within the download package. You have the choice of running with shape file or mif for usage in ArcGIS/QGIS or Mapinfo respectively.
==Computational Domain Assembly==▼
<ol>▼
<li>A DEM TIN created from the provided terrain data (2ft contours) and exported to ESRI ASCII format as a 2ft DEM grid. When TUFLOW reads this DEM it interpolates the elevations onto the 2D computation grid.</li>▼
<li>GIS layers of cross-section locations and 1D network including structure details.</li> ▼
<li>GIS land-use layer digitised in .shp format.</li> ▼
<li>GIS layer of 1D/2D interface lines along the left and right banks of the channels.</li> ▼
</ol>▼
All model inputs are independent of the 2D grid cell size, orientation and extent, allowing for different 2D resolutions, dimensions and orientation to be easily simulated.
==Cross Section Spacing, Grid Size and Mesh Element Size==
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==Use of Scenarios to Modify 2D Grid Resolution==▼
▲==Computational Domain Assembly==
To test the effect of different resolutions, simulations were made using grid resolutions of 5, 10, 15, 20 and 40 ft (Noted that only the 10 and 15ft are presented in this example).
▲TUFLOW directly reads GIS data layers to construct models. The layers used/created for Challenge 1 are:
▲<ol>
▲<li>A DEM TIN created from the provided terrain data (2ft contours) and exported to ESRI ASCII format as a 2ft DEM grid. When TUFLOW reads this DEM it interpolates the elevations onto the 2D computation grid.</li>
▲<li>GIS layers of cross-section locations and 1D network including structure details.</li>
▲<li>GIS land-use layer digitised in .shp format.</li>
▲<li>GIS layer of 1D/2D interface lines along the left and right banks of the channels.</li>
▲</ol>
▲All model inputs are independent of the 2D grid cell size, orientation and extent, allowing for different 2D resolutions, dimensions and orientation to be easily simulated.
To run the model with varying cell sizes from a single TCF, scenarios were implemented with either '10ft' or '15ft' entered in the batch file using the -s switch. Importantly, if the -s switch is not used, the model will assume a default model scenario of 10ft as specified by the line <font color="blue"><tt>Model Scenario </tt></font> <font color="red"><tt>==</tt></font> <tt> 10ft </tt> within the TCF.
Within this example, the 2D timestep and log output frequency are variables dependent on the 2D Cell Size scenario. The variables are specified through an external read file (.trd). The .trd file is called from the TCF: <font color="blue"><tt>Read File </tt></font> <font color="red"><tt>==</tt></font> <tt> FMA_T1_CellSize_Commands_001.trd </tt><br>
▲==2D Grid Resolution==
A 15ft (~4.5m) 2D grid resolution is extensively used for urban modelling, 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).▼
When the model is initialised, the scenario determines the 2D_CELL_SIZE, 2D_Timestep and LogInt variables. The variables are set using the 'Set Variable' command in the trd.
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!▼
Each variable is then placed in the TCF and TGC using the << >> syntax. For example for the 10ft scenario:
*Within the TCF, <<2D_Timestep>> is replaced with 1 and <<logInt>> is replaced with 30; and
*Within the TGC, <<2D_CELL_SIZE>> is replaced with 10.
This example shows the powerful features of scenario modelling in TUFLOW.
===Review of Varying DEM Results===
▲A 15ft (~4.5m) 2D grid resolution is extensively used for urban modelling, 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
▲
This does raise one issue indirectly of flood mapping in urban areas where the flood depths are very shallow, or if using direct rainfall modelling. 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.
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For 2D modelling, 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.
=Conclusion=
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