Riga port authority: rivers hydraulics and hydrodynamics (1298/1-b)

Background

The Riga harbour, situated in the mouth of the River Daugava, is the 2nd largest in Latvia by annual cargo turnover. The extensive reconstruction of the harbour continuously grows in 90-ies. The essential shaping of coastline, building of terminals, deepening of aquatoria and ship access channel were planned in the Mangalsala region during early 1999. The Riga Port Authority being convinced in the efficiency of the previous modelling efforts requested “PAIC”, Ltd. for an assessment of the reconstruction impact on the flow and wave regime near Mangalsala.

Problem

The Mangalsala terminal is situated on the right bank of the River Daugava just 2.5 km upstream the entrance of the harbour. The navigation channel in this region shifts from the right toward left bank under small angle. The significant deepening up to 14 m depth of extended area may cause reasonable changes in the character of the velocity and wave fields. The analysis of these changes was the major aim of this project. The secondary aim was an interest in evaluation of the adequacy of the shallow water approach by means of an investigation of the vertical density stratification.

Depth distribution near the Mangalsala before and after planned reconstruction

Scope of work

• The hydrological cruise on the research vessel “Antonija” was organized in the lower Daugava jointly with Latvian Fisheries Research Institute. The measurement campaign involved vertical profiling of temperature, salinity and oxygen content along 13 km long lower stretch of the river.
• The depth distribution of the Riga port was updated by the data from the surveys performed in the late 1998.
• The calculation series for the hydrodynamics of the River Daugava estuary were performed by program SwEvolver for 4 different discharges (2400 to 13800 cubic meters/s), 3 different waterlevels of the Gulf of Rīga (-1.65 to 2.00 m), situations before and after reconstruction.
• The propagation of the monochromatic NW waves in the lower River Daugava was calculated.
• The flow and wave field character was analysed for the region of the Mangalsala terminal. The changes of the hydrology in this region as well as their influence on the overall situation were evaluated.

Calculated velocity distribution near the Mangalsala before and after planned reconstruction

Results

The hydrological measurements indicated the intrusion of saline water 5 to 6 km upstream the estuary. The riverine and coastal water masses are divided by the distinct pycnocline, their mixing is negligible.

The analysis of the calculated hydrodynamic fields allows to draw several concrete conclusions regarding the impact of the reconstruction of Mangalsala terminal:

• the decrease of the velocity of the flow core by 10 to 20% is expected due the increase of the channel cross – section area;
• the shift of the current for ~ 50 m towards the center of the River Daugava will occur downstream the terminal;
• the intensification of the recirculation gyre in the terminal itself is expected;
• the wave regime in the terminal area will become more uniform, the wave heights may reach 20 to 35% of the far sea wave height;
• the deepening of the river will stimulate further penetration of the wind waves into the estuary; thus, increase of wave height by ~ 20% is expected in the vicinity of Rinuzi terminal.

Mesured salinity distribution in the longitudinal cross-section of the lower stretch of the river Daugava