Hi there
I reran my model with some updated inputs. I noticed my overall exposure and exposure without habitats show the exact same colours for the shore points. I’m not sure why that is. Anyone know what could have caused this?
Kind Regards
N
Hi @n-marine -
When you say “the exact same colors” I assume that you’re talking about their symbology when you look at them in a GIS. Are the actual numeric exposure values (not the colors mapped to those values) the same between runs? Sometimes the colors can look the same, but the actual values are different. If the numeric values are the same, then what are the differences between your previous and updated inputs?
~ Stacie
Hi Stacie 
Yes i mean the symbology!
So i changed the habitats. I received an updated habitat map, and in this one there were categories like corals, seagrass, seagrass with macroalgae, seagrass with turf algae, seagrass with sand. So i grouped the lone seagrasses as 1 habitat, then created a habitat called seagrass_mix which contains the other seagrasses. Seagrass mix was set with a higher exposure rating of 5, while pure seagrass is 4, and corals as 1. Previously there were only 2 habitats, corals and seagrasses.
I replaced the bathymetry with a much larger one because my max fetch distance is quite large.
I changed the elevation radius down to 55 as the max elevation on my island is 43m. It was previously set as 250 i think.
My max fetch distance is now is the 100,000s. Previously it was like 200 (which was completely wrong)
I also added a geomorphology shapefile for the whole island.
I also decreased my resolution from 300 to 175m.
The values between the runs are different yes. but i think i didnt notice exposure and no_habitat having similar values because i had not done the quantile grouping previously. I think now that they are grouped it looks the same (the symbology). I have also changed the previous data into 3 quantile groups and its the same issue, the coastal exposure and exposure_no_habitats have the same symbology. Below is an image showing the output values from both (previous and current) runs:
Thanks
Nawal
I also have to say if these results are correct, that means my habitats dont contribute much to the protection of the coast.
Which is confusing because the areas where the coast is shown to have high vulnerability, is where the habitat role is also high.
I can only think of a few reasons why this might be:
1- My fetch distance quite large, in the 100,000s
2- My continental shelf is quite far from my island
I think these two alone exacerbate the wave size.
Heres how i fixed it
I combined the exposure and no habitat exposure excel coloumns. You will need to extend the ID coloumn as well, and also add an X and Y coloumn with the coordinates of each of your shorepoints. I used a formula on QGIS for this.
I then copy pasted the entire table into my notepad and saved as a TXT file and named it style. I went into QGIS and inserted it as a delimited text. Make sure your X and Y coloumns are correct. Then i inserted it and opened symbology. I selected the exposure value as that is the coloumn i combined the values into. Then i clicked classify and my legend pops up showing and it sets the min and max of my exposure and no habitat exposure. I then apply and right click on the layer and copy style, and go to my coastal exposure layer and paste style. I go into properties>symbology and in value i make sure its set as exposure. DO NOT CLICK CLASSIFY.
Because now the min and max values of the coastal exposure and no habitat exposure is being used. This is relevant if you want to compare between your maps, you need to have the same min and max values.
for habitat role, do not apply this as the values are meant to be very small. Just use what the output from the model is straight up
hope this can help someone else
Thanks
Hi there @n-marine I wanted to respond to a couple points from your thread. A couple small notes - the elevation radius is the sampling radius for the DEM over which the model is computing an average elevation value for a given shoreline point. It’s not related to the actual elevation of the terrain, but rather to resolution of your DEM. Having the elevation averaging radius set to 250m, for example is telling the model to look within 250m from a given shoreline point and compute the average elevation. Please revise your value based on this - 55 is likely too small a value, what is the resolution of your DEM?
The fetch value is a parameter that the model uses to help understand whether the coast in a given location is exposed to oceanic waves or wind waves. If it’s exposed coast then it’s primarily exposed to ocean waves, but it is in a sheltered bay, for example, it’s exposed to wind waves only. The fetch distance is set based on looking at your coastline and helping the model to understand how ‘far’ it should look from each shoreline point to understand if it’s being protected by land offshore. Please read the user’s guide section on setting fetch length and let us know if it’s still unclear.
The continental shelf distance being far offshore doesn’t pose problems for the model, it just means the relative risk of storm surge is higher. It’s important to keep in mind that this is a RELATIVE model, such that it’s capturing areas along that coast that might be more or less exposed to storm surge relative to other areas along the coast.
Regarding the habitat role, it makes sense to me that you might see areas of the coast that are highly vulnerable where habitats are not playing much of a role to reduce risk. Some stretches of the shoreline are places where exposure is high, there’s not much in the way of ecosystem-based coastal protection and so whether you maintain or lose ecosystems won’t really affect coastal exposure in that area.
Lastly, typically the ‘habitat role’ values are quite small even when they’re significant in terms of risk reduction. The index only goes from 1-5, and habitats are just one contributing factor in that index … so a habitat role value of 0.7 for example means that the loss of habitats from that stretch of shoreline increase coastal vulnerability by almost a whole indexed value - that’s quite a large change! If you were to symoblize the ‘habitat role’ column it looks like there are values >0 where there is an important role for habitats in reducing risk.
Hopefully this is helpful to you in refining your model run.
Jess
Hi Jess
My DEM is based off of LiDAR data, and so my DEM resolution is 1 metre. Is 55 metres okay for the max elevation radius?
For fetch value, im a bit confused. One side of my island is unexposed to the Red Sea and about 1000 m away from the mainland, while the other is exposed to the entire Red Sea with no landmass adjacent to the Red Sea facing coast. For the fetch distance, I created a point from the northern most point of my island and followed the wind direction and saw how much the distance for that would be. (i basically went in the opposite direction of the actual wind direction and stopped when i met the edge of any landmass, in my case it was a small island).
Now my island has a fringing reef attached to the landmass as well as a fringing reef surrounding the north and west area, seperating the landmass by a shallow lagoon. My island also has 2 bays. Im confused when bays are mentioned in replies because im not sure what to do in that case. Do you think my method of determining the fetch distance is correct?
For habitat role, i guess in my case where i am a marine environment consultant advising clients on if habitats do or dont protect a coastline, i cannot simply show them that the model says the leeward side of the island has very low protection contribution from habitats. If interpreted literally, to them this means, in order to develop the area they are free to destroy those reefs because 1- the exposure for that side is very low and 2- the habitats dont contribute much protection. We as scientists obviously understand the importance of these habitats, so i have had to find a way to explain why the model thinks this way for the leeward side of my island. Simply, because it is less exposed to local winds (its accomponied by cliffs which help block wind), and bathymetry on that side is small so waves cant get big. Because of these factors, the model rates the leeward side as very low in exposure, and thus it sees the habitats not having anything to protect (as exposure is already low).
Hi @n-marine , yes, if the DEM is based on LiDAR that elevation averaging radius should be fine. I just wanted to make sure that it was clear that it was an radius for getting an average elevation.
In terms of fetch length it sounds like you’re doing the right thing, but hard to really know without seeing a visual. As I said, you want to make sure the model is able to detect areas of the shoreline that are sheltered from ocean waves (it sounds like this would primarily be the back side of your island that is facing the mainland). Your fetch length should be long enough that when the model looks offshore of those sheltered areas it can extend far enough to ‘see’ the mainland and know it’s sheltered. For those areas offshore you, the user, knows that there’s no landmass offshore so you don’t need to set an exceedingly far fetch length to send the model looking for land - that will just increase the computation time. There are a couple of intermediate outputs produced by the model that can be useful in helping to both understand the wind/wave calculation and make sure you’re setting your fetch lenght correctly. Take a look at wind_wave/fetch_rays.gpkg and the wind_wave/wave_energies.gpkg. The fetch rays shapefile will help give you a visual depiction of the fetch length and you’ll be able to see right away if your fetch length it too short and your rays are not hitting the mainland, for example. The wave energies shapefile can help you understand how the model is classifying each shoreline point in terms of wave exposure (local waves vs. ocean waves).
Lastly, regarding habitat role, yes, I think in the case of a sheltered lagoon it’s likely that habitats have a smaller role in providing coastal protection from ocean waves, but sometimes storm surges can be high in lagoons so it’s not a foregone conclusion to me that you wouldn’t see a strong role for habitats in lagoons. But, in your case it seems like the lagoon side is relying less on the protection provided by coastal and marine habitats. As you said, you’ll want to explain why - and looking at the other ranked inputs in the model can be helpful, as you’ve already done. You should also make sure your habitat ranks and protective distances are correct for your area - for example if you had a barrier reef very far offshore that was breaking waves, you’d probably need to extend your protective distance for coral.
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