I'm a GIS specialist with a good background in watershed hydrology and geomorphology theories. In a project, I should analyse coastal and inland catchments separately. I have some neighbourhood coastal catchments and want to use archydro tool in ArcGIS to create flow accumulation.

In the archydro manual, esri suggested the users append coastal catchments (Overview of archydro terrain preprocessing workflows Page 8 of 13):

Append Coastal Catchments. This is an optional step that should be done for larger DEM with a large portion of coastal areas – areas defined as not draining into the “streams”. This function defines coastal catchments and appends them to the catchment feature class.

Why a user should append coastal catchments while the catchments logically have separate drainage area.

Thanks for advanced

  • $\begingroup$ Just an idea without knowing archydro: might it be that diffusive water flow from these coastal catchments into the ocean are not support? Hence, if you define a coastal catchment without appending it to river-bound catchments, there would be not outlet for the water in that catchment? Thus, one has to connect each coastal catchment to another catchment, which drains into a river? But that is just a guess ... . $\endgroup$ – daniel.neumann Feb 5 '18 at 12:30
  • $\begingroup$ Or might is just be that they just provide to different approaches to add river-draining catchments and directly-into-ocean-draining catchments? $\endgroup$ – daniel.neumann Feb 5 '18 at 12:32
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    $\begingroup$ @daniel.neumann Thank you for your logical idea but I need to know the real reason. $\endgroup$ – BBG_GIS Feb 5 '18 at 12:51
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    $\begingroup$ I think that the 'coastal catchments' need to use a different model process because they form an area that slopes towards the coastline without forming discrete streams (at the scale where you define the streams). It would be analogous to overland flow before it reaches a stream. So those grid cells won't ever flow to your drainage network. I don't know why you couldn't just define the coastline as a stream, though. $\endgroup$ – haresfur Feb 5 '18 at 21:07
  • $\begingroup$ @haresfur There is no different model process for inland and coastal watersheds in archydro. But your idea seems to influence the result of flow accumulation and as you said discrete streams are very important.Maybe the issue is discrete streams. Coastlines are not defined as a stream. could you please recommend to me some references about coastal watersheds hydrology-geomorphology? Thanks to advances $\endgroup$ – BBG_GIS Feb 6 '18 at 7:45

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