This paper explores the vulnerability of two estuaries, the Clyde in Glasgow, Scotland, in the developed world with an established infrastructure, and where finance can be made available for adaption and mitigation. The other is in Gambia, West Africa, where poverty is endemic, the infrastructure undeveloped and where the finances for adaption and mitigation are severely limited. The paper employs a numerical modeling approach, first for the Clyde Estuary in Glasgow with a rich data-set modeled with commercial 1-d and 2-d models, and then the Gambia estuary in West Africa with a very sparse data-set, modeled with a 1-d model and an idealized 3-d CFD model. The paper demonstrates different modeling techniques appropriate to each case and the very different scale of threats, vulnerability and different responses required to ensure greater sustainability in each estuary.