I have a question regarding an appropriate continental shelf contour for my project. I am looking at coastal vulnerability in the northern coast of Cambodia, and I have been using the raster imaging from GEBCO for the bathymetry requirement. Based on that raster, I made a continental shelf contour line (required for the tool) at 50m deep, because the Gulf of Thailand’s depths are between 45-80m deep.
The tool’s user guide states that the continental shelf contour must be within 1500 km of the area of interest, and the continental shelf contour that inVEST provides is technically within that distance, but I don’t know if it is appropriate to use the inVEST provided contour because it doesn’t extend into the gulf. I have attached an image below, with the orange line being the continental shelf contour provided by the tool and the distance between the farthest point and the shelf contour is 762 km away. The 50m deep contour line I made based on the GEBCO raster for Cambodia is the reddish line close to the area of interest (with the green points).
It would be super helpful to get some insight on which contour is more appropriate for my project! Or if there is another route I should be taking, that would be great to know as well. Thank you in advance for your help!
Hi @meghanm.7 , good question. The model uses the shelf contour to approximate the storm surge exposure of the coastline. In theory, coastline near long, wide areas of shallow water is more exposed to storm surge than coastline near deep water. To approximate this exposure, the model uses a simple metric: “distance from shore point to nearest point on the shelf contour”.
In this case, I think it makes a lot of sense to use a local contour that extends into the Gulf so that the distance being measured actually represents a realistic path that the storm surge would travel. The distance from the Cambodia shore to that global contour line east of Vietnam is not very relevant because storm surge cannot travel over land.
This limit only exists in the software so that we can optimize the distance measurements and raise an error if no shelf contour is found within 1500 km. We have this comment in the source code regarding the 1500km:
# By examing the global continental shelf layer in our sample data,
# all global coast is far less than this distance from the nearest shelf edge:
Hi @dave! This is very helpful. Thank you for taking the time to explain the reasoning behind the contour.
Currently, there seems to be no previously created contour line for the Gulf of Thailand, so I have substituted my own. I have very little knowledge of bathymetry, so I am hoping you can point me in the right direction as far as creating a good contour.
The contour line I made before was at 50m deep based on the GEBCO bathymetry raster tile for the ocean surrounding Cambodia. Should I extend this contour to the entire Gulf? And considering the depths of the Gulf are between 45-80m, would 50m be an appropriate depth to create contour at?
I don’t think it will matter too much which depth you choose. The model compares the relative distances from each shore point to the contour, so the absolute distance is not important. And it’s probably only important to create the segment that is close to your study area. The model will find the nearest spot on the contour for each point, so there’s no real benefit to extending the contour far from your study area.
