“Daugava HPP Cascade”, SJSC UP “Latvenergo”: hydrogeology(0796/1-b)

Background

The Plavinu hydropower plant (HPP), built 1968, is the largest in the chain of HPPs on the main Latvian stream, the River Daugava. The raise of the waterlevel is the 40 m, and its reservoir volume is approx. 0.5 cubic kilometers. The HPP is built on the right floodplain of the River Daugava, its main earth dam is built on the riverbed but wing dams extended along the bedrock hills. HPP is located on the intersection of the contemporary valley of the River Daugava with the ancient buried valley filled with moraine. This valley intersects the sandwitch – type structure of the Devonian aquifers (dolomites, sandstones and sand) and aquitards (clay). The engineering solution to place the main building on the soft morainic loans and sandy – loams reduced the filtration directly beneath the HPP; however it caused leakage through the heterogeneous deluvial material on the slopes of pre – quaternary valley. Cascade of Daugava HPPs, the operator of Pļaviņas HPP requested the modelling tools to investigate the geological structure and filtration processes beneath the HPP.

Topographic surface elevations near the Plavinas HPP

Problem

There exist more than thousand of survey boreholes drilled in the vicinity of Plavinas HPP. The series of the observations of the piezometric heads are available at about 350 wells for all (6 to 7) nontemporary and Devonian aquifers in the range of absolute height from -60 m to +75 m. Additionally the recording of the data of discharges through the drainage system and relief wells are available since 1968. Almost all of this data was available only in paper form in early 1996. The aim of the hydroengineering measures (as, for instance, changes in the drainage and relief regime) is the maintenance of the dam and buildings safety. The operational model of groundwater filtration, equiped with up-to-date visualisation functionality would serve as a powerful tool for consulting hydrologist and safely engineer. The digitizing of all raw data, building the software for visualisation of the geological structure, development of the model for the groundwater filtration, its calibration were among the basic goals of the long – term project aiming evaluation of the impacts of the different engineering solutions on the dam safety.

The geological structure and the distribution of calculated piezometric head in the vertical cross-section along the axis of Plavinas HPP building

Scope of work and results

The scope of work and basic results are listed according to the stages (1st to 3rd during Years 1996, 97, 98, respectively).

The distribution of calculated piezometric head and filtration velocity in the horizontal cross section at level 0 m.

1. The data regarding the geological structure were digitized from the passports of operational (stage 1) and surveys’ (stage 2) boreholes.
2. The database of the observations of the filtration regime (stages 1, 2), and mechanical building parameters (stage 3) with respective software FiData was developed, exceeding 50 Mb of row data in late 1998.
3. The three – dimensional geological structure of the region was developed, filling layered structure of 2D interfaces between aquifers/aquitards by 3D finite elements. The available geological surveys’ data was the major source for interpretation during stages 1 and 2, whilst interpretations arising from filtration modeling dominated during stage 3.
4. The steady – state three – dimensional mathematical model of the groundwater flow, based on 3D finite elements (> 300000 elements by the end of 1998) was developed. The spatial resolution of the model increased during stages; it varies from few cm (thickness of thin lenses) to hundreds of m (horizontal distance in periphery of 4 to 4 km modeling region).
5. The calibration of the model were performed (a) showing principal possibility to modelize the groundwater flows during stage 1; (b) reaching 10% discrepancy with observation values by means of calibrating model coefficients during stage 2; (c) reaching maximum discrepancy of predicted piezometric heads below 1 m by means of structural improvements during stage 3.
6. The evaluation of the impact of simple engineering measures as changing relief height of several wells or drainage system was performed during stages 2 to 3. This evaluation was extended by the development of possible scenario of structural changes that hypothetically were responsible for emergency raise of the piezometric heads in the observations system in spring 1979.
7. The software for the visualisation of the 3D geological structure and various fields related to groundwater filtration HIFiGeo was developed for Linux (stages 1, 2) and Windows 95/98/NT operational systems. The software serves as a tool at working place for hydrological engineer.