The influence of spring and neap tide on salt intrusion and stratification in Sebou estuary (Morocco)

Estuaries, which are coastal bodies of water connecting the riverine and marine environment, are among the most important ecosystems in the world. Saltwater intrusion is the movement of coastal saline water into an estuary, which makes up estuary water, becomes salty due to the mixing of freshwater with saltwater. It has become a serious environmental problem in the Sebou estuary (Morocco) during wet and dry seasons, which has a considerable impact on residential water supply, agricultural water supply as well as urban industrial production. The aim of this paper is to study longitudinal salinity and the vertical salinity stratification under different hydraulic conditions in the Sebou estuary. Field observations were done during May 2014 and February 2016 and were interpolated using a Geographic Information System (GIS). Measurements results show that the vertically averaged salinity during spring tide is higher than that during neap tide. Also, the stratification increases a larger during the low-flow periods than during the high-flow periods in the Sebou estuary. Based on results of the longitudinal and vertical salinity observations and historical data, empirical equations were obtained for estimation of the intrusion length as a function (1) of upstream freshwater discharge and (2) tidal range in the Sebou estuary. Additionally, a correlation equation between the surface salinity and the vertical average salinity in estuary (at Kenitra station) was proposed. Finally, this paper provides simple equations that are useful to provide first estimates of intrusion length and vertical average salinity in the Sebou estuary, which can be obtained by a simple desk study without the use of every measurements day.