Sea outfall discharge is a practical way to dispose of brine waste stream from a coastal desalination plant. However, sandy beaches are changing as a result of coastal erosion. Coastline urban developments are the man-made permanent changing of beaches, where the coastlines are being dredged and reclaimed as artificial land. Therefore, if a plant is built and operated with an outfall to satisfy the imposed site's environmental regulation compliance but the beach is subsequently being eroded, what action needs to be done to make sure the imposed criteria that minimize the impact on the marine environment can still be met? A mathematical model is presented that accounts the effect of beach erosion for estimating the brine's outfall adverse impact on the environment.