Riga port authority: rivers hydraulics and hydrodynamics (0198/2-b)

Background

Riga harbour is the 2nd largest Latvian seaport by cargo turnover. It is situated in the estuary of the largest Latvian stream, the River Daugava. The terminals of the harbour are located along the riverbanks, channels and artificial aquatories. The intensive reconstruction of the Riga harbour continuously expands during 90-ies, including widening and deepening of the navigation channel, building new terminals, dredging aquatories of existing ones. The Riga Port Authority requested the development of the physical model of the lower River Daugava from the Institute of water and earth sciences, Latvia University of Agriculture in early 1998, before the reconstruction of Rinuzi oil terminal. The aim of the physical modelling was to investigate the changes in the flow and wave regime after the reconstruction and dredging works. Riga Port Authority requested “PAIC”, Ltd. to perform mathematical modeling of the hydrology of the lower River Daugava parallely to the physical modeling.

The water depth distribution of the lower stretch of the river Daugava

Problem

The hydrology of the lower River Daugava is determined by the interaction of the Riga HPP work regime with the water level fluctuations in the southern part of the Gulf of Riga and the wind action over the estuary. The discharge maximum can exceed 10000 cubic meters/s during spring snow – melt flood when HPP lowers the waterlevel of this reservoir. The elevation fluctuations in the Gulf of Riga are caused mainly by the wind forcing, the minimum/maximum levels may reach –1.5/+2.0 m. The modeling of the lower River Daugava may become quite complex due to (a) multiple connections of the main stream with inland water basins as Vecdaugava, Kisezers; (b) confluence with Buļļupe, connecting the River Daugava with another major river Lielupe just few km before entering the Gulf of Riga; (c) generation of the wind waves over the prolonged straight stretches of the lower River Daugava; (d) saltwater edges in the deeper layers of the river. These aspects cannot be accounted in the physical model and are omitted also in the mathematical model to ensure comparability.

Scope of work

• The digital surface map was constructed from the depth survey data supplied by the Riga Port Authority. The model region covers ~ 13 km long stretch from the Vansu bridge to the wave – breakers’ heads, and 2.5 to 3 km of the coastal zone of the Gulf of Riga.
• The depth distribution of the physical model was constructed covering 6 km lower stretch of the River Daugava.
• The mathematical model of the river hydrodynamics was developed in the shallow water approach extending software package SwEvolver for the vertical resolution of the logarithmic velocity profiles.
• The calibration of the 2D model (i.e. determination of friction coefficient) was performed for the average discharges, involving measured waterlevel differences from the physical model.
• The series of flow pattern calculations were performed for four different discharges (2400 to 13800 cubic meters/s), three different Gulf of Riga elevations (-1.65 to 2.00 m) in two modeling regions (i.e. for the estuary of the River Daugava, and for its physical model).
• The wave field calculations were performed for the propagation of the monochromatic NW wind waves in the River Daugava estuary.
• The additional calculation series were done to investigate influence of (a) hypothetical discharges in the River Bullupe and Milgravis channel; (b) wind fetch and Coriolis forces.
• The wide analysis of the hydrology of the lower River Daugava for the different discharge/waterlevel combinations paid special attention to the region of the Rinuzi oil terminal.
• The comparison of the results of physical and mathematical modeling included almost all measurements on the hydraulic model, i.e. longitudinal waterlevel distribution, surface velocities, and velocity distribution in the vertical cross – sections.

Calculated velocity distribution in the two selected transversal vertical cross-sections of the physical model of the river Daugava

Results

The calculations indicated that

• The main flow of the River Daugava follows the deepened navigation channel; the flow velocities change mainly due varying cross – section area of the stream.
• The core of the stream flows closely along the right bank near Rinuzi oil terminal; a transit flow through its aquatory cannot be neglected.
• The wave height up to 20% of the respective values in the Gulf of Riga can be reached in the terminal area during the most unfavourable conditions.

Evaluation of the results of physical modeling yields conclusion that the assumptions of the physical model allow analysis of the flow fields near the Rinuzi terminal. These flow patterns have only minor influence from (a) the extending of the model area from 6 to 13 km; (b) the account for wind drag and Coriolis force; (c) discharges through the River Bullupe and the Milgravis channel and (d) the vertical/horizontal scale distortion in the physical model.

The comparison of the results of the physical and mathematical modeling shows that

• mathematical model reasonably reflects the measured elevation differences along the longitudinal profile of the River Daugava; it allows to detect uneven distribution of the bottom roughness and its changes between experimental campaigns in the physical model;
• the quantitative values of the surface velocities as well as the velocity values in the vertical cross – sections are well predicted by mathematical model being in the range of measurement accuracy.

Comparison of measured and calculated depth distribution.