https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN36290-FM_3-0-000-WEB-2.pdfthere is no mention of a 10km required brigade frontage for an amphibious landing that I could find. Not only that, even the shorelines shown in that Bloomberg map you linked are mostly >10km. I can count maybe 3 out of a dozen or so that may not be 10km long. Regardless you have not established that it even needs to be "10 km" in the first place.