Observed variations in stratification and currents in the Zuari estuary, west coast of India

Abstract

25-hour time series measurements of vertical salinity profiles and currents in Zuari estuary were done during both spring and neap tidal cycles of pre-monsoon (April–May), south-west monsoon (June–September) and post-monsoon (October–December) seasons during 2011 at four different stations along the estuary. Measurements were done at locations, near the mouth (called the bay station), lower middle estuary, upper middle estuary and upstream. Based on these observations, variations in stratification at different time scales (daily, spring–neap cycle and seasonal) are described. In the mixed tidal regime with semi-diurnal dominance, stratification at higher low water succeeding lower high water is more intense than that at lower low water succeeding higher high water over a tidal cycle. During pre-monsoon, when river discharge is negligible, the spring–neap cycle hardly causes any change in peak stratification factor (stratification factor, ΔS, is difference between the bottom and surface salinity), attained within a tidal cycle at the bay and upstream stations. However, during spring, the peak stratification factor (ΔS) of lower and upper mid-estuarine stations decreases by about two and one, respectively. During monsoon at the lower middle estuary station, salinity value peaks only at high water which is similar to the pre-monsoonal conditions at the upstream station. High stratification observed during high waters at the lower middle estuary during monsoon weakens marginally at the peak of the high waters, during both spring and neap phases. Due to the incursion of high-saline downstream water along the bottom, the bottom water gets saturated slightly before the peak of high water. Because of this, the salinity of bottom layer remains constant for about 2–3 h, while the surface salinity still continues to increase up to the peak of high water, resulting in the weakening of stratification. Moreover, during monsoon, the magnitude of the flood flow progressively reduced from mouth to upstream and no flood flow was observed at the upstream station. Variation of computed potential energy anomaly is found to match with that of stratification factor (ΔS) closely.