Deposition of riverine suspended solids in a shallow bay (Bay of Koper, Gulf of Trieste, northern Adriatic Sea)

Abstract

Deposition of riverine suspended solids affects operations in ports located in the proximity of river mouths. The Rižana River is the main source of riverine solids in the shallow Bay of Koper (Gulf of Trieste, northern Adriatic Sea). The mean discharge increases rapidly in relation to high precipitation, typical of spring and autumn. During such events, the runoff leads to torrential river flow with high concentration of suspended solids which are consequently deposited in the port zone. Turbidity (NTU) measurements were performed during 2011–2013 in the Rižana and Badasevica Rivers and in the inner part of the bay influenced by fresh water inflow. In parallel, the concentrations of total suspended solids (TSS) and suspended organic matter (SOM) were measured gravimetrically, the latter after combustion, and their grain size distributions were determined using a laser granulometer. The maximum turbidity values in the bay were found in the sea surface layer during the river flood. The linear regression model between TSS and turbidity (NTU) TSS = 1.01 × NTU + 0.91 showed strong positive correlation. The regression analysis between turbidity and SOM also showed strong correlation with a regression equation SOM = 0.12 × NTU + 0.85. The impact of SOM on measurements was hypothesized by the rather poor model fit of the regression analysis between TSS and river discharge, which is mainly related to high SOM variations. The grain size spectra of riverine and nearshore surface TSS in the proximity of the river mouth were skewed and sorted with the mode size mostly around 10 μm. Considering the TSS mass in the sea surface layer, the yearly TSS input into the bay is estimated to be ~18 × 103 t. Using the average TSS concentration in the Rižana River and the average Rižana River discharge, the yearly input would be ~2 × 103 t. The discrepancy between both calculated inputs is probably related to the TSS input during high river discharge which occurs in short periods. A maximum deposition rate of 80 g m2 day−1 is mostly restricted to the second port basin. Less than 50 % of the introduced TSS settles within the second port basin during mean river flow conditions. The regression model was found to be a useful tool to estimate TSS and SOM concentrations from turbidity data in rivers and the coastal sea, thus providing an evaluation of riverine TSS and SOM input and deposition into the small bay, which in turn affects local port activities.