I am a new GIS student currently working on a coastal flood model in arc map, where I intend to create flood polygons at various heights above 0m contour (MSL)to represent the extent of area flooded on the coast. However, the DEM that was created using elevation spot height and contour data shows areas in the ocean >0m. Is this a correct representation? I would think the areas covered in ocean should be 0m or less. Therefore, when I create the flood polygons portions of ocean are shown to be flooded as well. Is there anyway to rectify this?
The ocean is a dynamic environment, depending on conditions (thermal expansion, swell heights, bathymetric profile, location, etc.) this can be normal. Additionally, MSL is really in the area of the intertidal zone (somewhere between MHHW and MLLW), so depending on the DEM data, tidal conditions during the survey could be showing. Coastal flood modelling commonly uses MHW or MHHW reference contours. Also, a vertical projection in NAVD88 is not the same as local MSL (see fig). Depending on where you are in the world, you should find your local tidal datum and use that value to draw a reference contour.