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= Introduction =
In thisFMA challengeChallenge 1, 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 and of the FMA challengeschallenge are provided [httphttps://www.tuflow.com/Download/TUFLOW/Demo_Models/2012_FMA_Challenge/All/FMA%20Challenge%201%20Documention.TUFLOW.pdf here]<br><br>▼
The FMA Challenge Wiki's provide example TUFLOW models submitted by BMT WBM for the Floodplain Management Association 2-Dimensional Model Challenges, 2012.
This Wiki assumes an intermediate to advanced user level so if you're just starting out or haven't already completed the tutorial models, please see this <u>[[Tutorial_Model | tutorial model]]</u> page. <br>
 
A fullyThe functional exampleFMA modelChallenge 1 (1D-2D linked) has been developed by BMT WBM, allowing you to review the model setup, run the model and review results developingimproving your skills 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 and of the FMA challenges are 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 by BMT WBM, allowing you to review the model setup, run the model and review results developing your skills in:
*Nested 1D/2D models;
*UrbanUnderstanding 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;
*The impact of data quality on DEM development;
*TheUnderstanding the influence of hydraulic structures on open channel hydraulics;
*Using scenariosbc andlayers variableswith toHX, determine a suitable cell sizeXP, timestepCD and log outputCN scenariostypes; and
*UsingUnderstanding bcthe layersimpact withof HX,data XP,quality CDon andDEM CN typesdevelopment.<br><br>
 
Data for this model is provided viain ZIPa compressedvariety fileof posteddifferent onGIS compatible formats. Download the <fontdataset color="red">that internet/FTPmatches WHEREthe ISGIS THIS???</font>software foryou download.are using:
*[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>
*<font color="blue"><tt>1D-2D Linking - </ttu></font>- [[Tutorial Module02Tutorial_M03|Tutorial Module 23]] </u>
*Running Scenarios - <u>[[Tutorial_M08|Tutorial Module 8]]</u>
*<font color="blue"><tt>Running Events and Scenarios</tt></font>- http://www.tuflow.com/forum/index.php?showtopic=1149&hl=scenario
*Running Events - <u>[[Tutorial_M09|Tutorial Module 09]]</u>
 
=Model Setup=
This section detailsprovides thean setupoverview and discussion of the fully functional model developeddomain by BMT WBMsetup.<br> All files required to re-run the model can be found at '''FMA_Challenge_Models/FMA_Scenario1/'''. It is at your discretion which GIS package, text editor and method of model simulation to use (batch mode or within the text editor).
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==▼
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 digitizeddigitised in .shp format.</li> ▼
<li>GIS layer of 1D/2D interface lines along the left and right banks of the channels.</brli> ▼
</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. <br>▼
 
==Cross Section Spacing, Grid Size and Mesh Element Size==
1D sections were used for the in-bank topography. The overbank (2D) areas were modelled using 10ft (~3m) and 15ft (~4.5m) resolutions. The 2D grid dimension (rotated) was 17,500ft by 8,000ft. A total of 87 cross-sections were used for the 1D in-bank domain.
 
Structures were modelled as a combination of bridges and culverts. For bridges, height varying energy loss tables were used based on the Hydraulics of Bridge Waterways (Bradley, 1978). For culverts, TUFLOW simulates all possible inlet and outlet controlled flow regimes with automatic switching between regimes. The calculations of culvert flow and losses are carried out using techniques from “Hydraulic Charts for the Selection of Highway Culverts” and “Capacity Charts for the Hydraulic Design of Highway Culverts”, together with additional information provided in the literature such as Henderson 1966.
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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 digitized in .shp format.
▲<li>GIS layer of 1D/2D interface lines along the left and right banks of the channels.</br>
 
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.
▲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.
 
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>
Peak flood depths and water levels were exported to ESRI 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.
 
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.
▲==2D Grid Resolution==
Each variable is then placed in the TCF and TGC using the << >> syntax. For example for the 10ft scenario:
A 15ft (~4.5m) 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).▼
 
*Within the TCF, <<2D_Timestep>> is replaced with 1 and <<logInt>> is replaced with 30; and
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!▼
*Within the TGC, <<2D_CELL_SIZE>> is replaced with 10.
 
This example shows the powerful features of scenario modelling in TUFLOW.
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.▼
 
===Review of Varying DEM Results===
▲A 15ft (~4.5m) 2D grid resolution is extensively used for urban modelingmodelling, 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 thetopographic roadsfeatures such as discussedroads, levees aboveetc).
▲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 modelingmodelling. 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==
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=Constraints=
=Challenges=
The typical challenges experienced in situations similar to this usually relate to:<br>
*The use of contour data to create the DEM; and
*The model boundary/terrain data does not extend beyond the flooded area.
 
The presented solutions to these challenges are asprovided follows:in the following sections.
 
==Using Contour Data to Create the DEM==
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[[File:Contour Data.jpg|600px]]
 
For 2D modelingmodelling, 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.
 
=Conclusion=
We have explored flood behaviour on an urbanised floodplain. Through the provided example model, we now we have a better understanding of:
We have explored an Urban Riverine conditions with an urbanized over-bank floodplain. Now we have a better grasp of nested 1D/2D models, influence of hydraulic structures, use of scenarios and variables to determine cell sizes, timesteps and log outputs, and using bc layers with HX, XP, CD and CN types in synergy.<br>
* Nested 1D/2D models;
 
* The influence and implementation of hydraulic structures;
 
* Use of scenarios and variables to determine cell sizes, timesteps and log outputs;
* Using bc layers with HX, XP, CD and CN types; and
* The limitations of contour data that should be considered when developing new hydraulic models.<br>
Congratulations on finishing Challenge 1!
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