Institute of Biology, UL and State Forest Agency:Rivers hydraulics and hydrodynamics; water quality
(1098/1-b)

Background

The pulp mill near Livani is the largest proposed investment project in Latvia. The tender for the assessment of the impact of the pulp mill effluents on the ecosystem of the River Daugava was announced by Latvian Forest Agency in 1998. The Institute of Biology, UL won this tender, assuming subcontracting Laboratory for modeling hydrology, hydrodynamics, and water quality of the River Daugava.

The River Daugava is the largest Latvian stream. It is of national significance, among other being also a major source of drinking water and energetic resource. Thus, any anthropogenic impact on the River Daugava has to be considered with special attention.

Problem

The River Daugava flows in its natural, non – regulated riverbed in the stretch Daugavpils – Jekabpils. The mean annual discharge is ca. 600 cubic meters/s, whilst the minimum summer low – flow water discharge can be below 100 cubic meters/s, reaching water temperature above 220C. The River Daugava enters the reservoir of Plavinas HPP 40 km downstream Livani, just below town Jekabpils. The self – purification potential of the reservoir is low. Therefore, to assess the impact of the pulp mill emissions on the River Daugava ecosystem we have

• to assess present state of the water quality of the River Daugava;
• to evaluate the changes of the water quality parameters for most dangerous, i.e. summer low – flow, situation.

Calculated velocity modulus distribution in the stretch of the river Daugava below Livani

Scope of work

• The dataset of the cross – river profiles and longitudinal profile of the River Daugava was extended to cover the stretch Daugavpils – Aizkraukle (data from Institute of Melioration).
• The one – dimensional hydrological model for the stretch Daugavpils - Aizkraukle was developed and verified for different discharges.
• Several input data sets (characteristic for 80-ies, 90-ies, and for typical and critical summer low – flow) were generated from the long – term (1977 to 1997) data series of the hydrochemical and hydrobiological observations.
• The one – dimensional water quality model HeliFlow was calibrated and verified reaching good agreement with measured data.
• The prognostic calculation series were performed with the calibrated model to forecast the typical and critical distributions of the water quality parameters along the River Daugava after the beginning of pollutant emission.
• The digital terrain distribution was developed for the topographic surface of the approx. 13 km long stretch below Livani.
• The calculations of the hydrodynamics and water quality were performed by two – dimensional software SwEvolver. This yields analysis of the current patterns showing the zones of pollution spreading and full mixing for different scenarios.

Results

The project implementation indicated that

• the water quality in the River Daugava during 90-ies has essentially improved in comparison with 80-ies;
• the self – purification capacity of the River Daugava stretch Livani – Jekabpils is high enough to expect the nutrients concentration raise due pulp mill emissions below 10% even during critical summer low – flow.

We should mention the following essential results of the project:

• The development and calibration of the hydrological and water quality model for the stretch Daugavpils – Aizkraukle.
• The two – dimensional digital terrain model for the riverbed, foodplains and bedrock hills along 13 km stretch below Līvāni.
• The development of the method for interpolation of two – dimensional elevation distribution from the cross – profilings of [meandering] river.
• The development of interface for transition from one – to two - dimensional models for a particular river stretch.

The prognostic calculations for the typical and critical summer low – flow allowed:

• estimation of the expected sedimentation sites of suspended particles and proposal of sediment monitoring stations;
• evaluation of two alternative effluent discharge sites and recommendation of more suitable of them;
• estimations of the full mixing zones (taking into account hydrobiochemical transformations) and recommendations for the water quality monitoring site downstream the proposed pulp mill.

Calculated distribution of water stage and COD concetration along the stretch of the river Daugava between Daugavpils and Plavinas HPP reservoir