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Coastal Vulnerability Parameter Help


I am trying to run the coastal vulnerability model.  I have read the user guide, but as I am not a marine scientist by background, I was wondering if anyone could point me in the direction of some guidance on how to select the appropriate values for the following input parameters:

  1. Rays per sector (required) – 1 default- max 9

  2. Fetch Distance Threshold

  3. Depth Threshold (meters, required)

  4. Exposure proportion (meters, required)

  5. Oceanic effect cutoff (meters, required)

The site I am looking into is for Semarang, Indonesia.




  • Hi, 

    We probably need to have better documentation on this.

    Read the section on 'wave exposure' in the user's guide again, and you'll see that the model computes two types of waves: oceanic waves and wind generated waves.  Oceanic waves are computed on exposed coastline and wind generated waves on sheltered coastline (coastline enclosed by land).  You, as the user, have to help the model distinguish sheltered coastline from exposed coastline and the parameters that you've listed above are all involved in that process. 

    The key ones are fetch distance and exposure proportion.  Fetch refers to the distance that wind can blow across water unobstructed.  Once you're out on the open ocean, fetch distances are very long - but in bays and enclosed areas, your fetch distance will be much shorter because you'll bump into land.  With setting the fetch length, essentially what you want to tell the model is 'If I have a fetch length that is X km long, I'm likely to be in a bay or other sheltered area', and if I have one that's longer than that, I'm probably out on the open ocean.  And, since the model sends out 16 fetch rays in different directions from each shoreline segment the sheltered/exposed categorization applies of the proportion of rays that are less than your fetch length cutoff (that's where the exposure proportion comes in).  See the little text of the user's guide below.

    Here's what I do in practice:
    1.  Look at my coastline and try and get a sense of what is sheltered and exposed.  Usually this is pretty intuitive even if you're not familiar with the region.  Semarang looks a bit more complicated, considering all the diked areas nearshore.  The whole coast looks fairly exposed to me, but I would be interested to know what you're considering the shoreline in this area.

    2.  Using the measure tool in your GIS measure some distances from coast to coast within sheltered areas (again, you're trying to get a sense of the typical distances that wind can blow unobstructed in these sheltered areas before it hits land).  
    3.  Use this value as your fetch cutoff and leave the exposure cutoff as default (and the other values as default).

    4.  Run the model using just the fetch info and the land polygon to minimize run times.

    5.  Look at the 1_a_shore_exposure.tif output where 1=exposed and 0=sheltered.  If this looks good, keep these values and move forward to running the model.  If not, tweak the exposure proportion and the fetch length and try again.

    6.  Often this requires a little trial and error.

    Take a look at the screenshot I've attached.  This is for a couple of islands in the Bahamas.  You can see the oceanic exposed coastline in red, and all the sheltered coastline in blue.  Not every pixel will be perfectly categorized, which is okay.

    I know this is complicated to explain - hopefully this hasn't confused you more!  I would be happy to troubleshoot your coastline specifically if you would like.  You can email me directly at

    I will leave it there and you can either email me directly or continue to post clarifying questions on the forum,


    1. Exposure proportion (meters, required). The model uses this input (between 0.0 and 1.0) to determine if shore segments are exposed or sheltered. This is done in four steps:

      1. Compute the number of fetch rays (N) that correspond to the proportion N: :math:’segments over water * exposure proportion’
      2. Determine if the current segment is in deep water (at least N sectors project over water that is at least “depth threshold” meters)
      3. Determine if the current segment is enclosed by land (at least N fetch rays have to be blocked by land, i.e. fetch distance is less than “ocean effect cutoff” meters).
      4. Determine segment exposure: a shore segment is exposed if it is both in deep waters, and not enclosed by land (facing open water), otherwise, it is sheltered.

    In other words, if the fetch threshold is 12 km and the depth threshold is 5 m, and the exposure proportion is 0.8, the model will classify a segment as sheltered if more than 80% of the segments have a fetch distance lower than 12 km or the average depth along each fetch segment is less than 5 m.

    1200 x 675 - 264K
  • Hi Jess,

    Thanks for this - it is really helpful!

  • Good, this part is rather complicated to explain - but fairly intuitive to do.  So, as you proceed please let me know if you have more questions.

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