The combined effects of mountainous terrain, sea and urban settlements on local circulation have been rarely investigated, albeit 50% of the world population lives in coastal areas and many of them are replete with varying terrain. Based on the results of field measurements performed in a narrow semi-urbanized valley along the eastern Adriatic coast of central Italy (Biferno Valley), we have recognized certain mechanisms that may change the previously identified flow structures and morning and evening transition processes in mountainous terrain due to complexity introduced by coastal and urban features, including the urban heat island (UHI). Such changes also affect air pollution dispersion in such valleys. For example, the morning transition becomes sharp and abrupt in the presence of combined effects, while the evening transition is delayed and prolonged, thus concentrating pollutants near the ground. Observations included the collapse of turbulence and simultaneous flow reversal from upvalley to downvalley at different locations along the valley axis during the evening transition, the initiation of upslope flow in the morning and development of convective turbulence. A general conceptual framework for flow evolution in complex terrain in the presence of coastal and urban influence is proposed, together with some new parameterizations to quantify the relevant phenomena.