Some questions about Area Reduction Factors and Width Reduction Factors

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    I have a few questions about Area Reduction Factors and Width Reduction Factors (ARFs/WRFs) and how to best incorporate them into a flood model that includes a few residential structures:

    – Do you need both ARF and WRF in order to accurately model an obstruction such as a building? Based on my model runs, it seems that adding WRFs simulates the obstruction in flow and has a significant impact on depth and velocity output in the vicinity of the structure. However, only adding ARFs does not seem to have a drastic impact on hydraulic output. Do the ARFs only effect overall storage volume in the model?

    – Is the best way to incorporate ARF/WRFs by using the “compute area and width reduction factors” tool in the grid menu? When I use this tool it seems to vary the ARF and WRF based on the location of the grid cell within the structure: grid cells that only contain a portion of the structure outline have low ARF/WRF values whereas cell inside of the structure are shown as fully blocked, is this correct?

    – The “Guidelies for Applying the FLO-2D model to the Central Valley Floodplain Evaluation Delineation Program” contains a table (Table 8) that gives various ARF/WRF values for different types of structures. How can I incorporate these values into the compute area and width reduction factors tool in the GDS?

    Any additional information or advice is greatly appreciated. I can also send along a couple screen shots of my model showing the ARF/WRF factors that were generate in the GDS using the tool.




    Hi Joey,

    I think you should start by asking yourself whether the structures impact the flood wave progression or area of inundation. If he answer is no, you can ignore them.

    If they do have an impact on the movement of water, you can import the ARF’s and WRF’s with a shapefile that has a polygon for the outline of the building. The GDS will then calculate the building for you and that will comply with the table in the guidelines.

    Even if you don’t have a shapefile, it is usually easier to build a shapefile than to code the buildings individually in the GDS.



    Yes, using a shapefile is simply.

    But when I load the shapefile (ARF.shp) with value 1 (polygons=1), so SGD doesn´t perform complete blockage of polygon/area. (see URL)

    It is true?



    Hi Robert,

    This is true because the GDS doesn’t use any Values from the shapefile. It only calculates the ARF and WRF from the geometry of the polygon. When I use this system, I add a little extra modification to the ARF and WRF values. For example, if the ARF blockage is 0.9 or higher, I just make that a totally blocked cell by moving it to the top of the file and putting a T in the line. I do the same thing with the WRF values. Any WRF that is 0.9 or greater, I reset to 1. That is just my style of modeling. I do it so there aren’t any cells that are causing stability because they are almost fully blocked.


    I have imported my building shapefile (as I imported other shapefiles), but I am struggling to find the process to use the shapefile to calculare ARF and WRF values. What is the workflow for this?



    mschutte: See Lesson 11

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