6. Relationship between surface water and ground water

Between falling as precipitation and passing out of an area as stream-flow, water moves over (surface water) or through the ground (ground water). This is the typical relation between precipitation and river flow. The peculiarity of the Danube section of Danubian Lowland in the Gabčíkovo project section between Bratislava and Sap is that the Danube flows above the local terrain, on top of its own alluvial fan. This fan is made up of layered gravel and sand typical of piedmont situation, where a mountain stream enters the plain. The Devín gate at Bratislava, through its bedrock river bottom, acts as the mountain front. Immediately downstream the river gradient is relatively steep while downstream near Sap it is gentler and thereby thick layers of gravel and sand are deposited on the Neogene bedrock. This provide a highly permeable aquifer as much as 450 m thick near Gabčíkovo. As the river splits over its banks and shifts its position a low-angle alluvial cone is built on whose upper surface the Danube flows (as far as the Malý Danube to the north and the Mosoni Danube to the south) (Fig. 1.4 and 1.6, Chapter 1).

At all water stages the elevated river bed supplies the ground water, which flows partly towards the Malý Danube and Mosoni Danube and partly towards the system of drainage canals. The significance of this arrangement is also vital during the flood events. The effect of the bursting of the Danube banks and protective dikes is extreme inasmuch as there is no natural raised terrain or higher terraces to contain the escaping water flow, as it is the case in normal alluvial conditions. Because of the thick and highly permeable aquifer, there is also a secondary form of flooding. During high water levels in the Danube the ground water recharge is high and the ground water level is pushed upward to the ground surface. This leads to a flooding of the lower lying areas even at some distance from the river, a situation called inland water flooding.

6.1. Infiltration - recharge of ground water

Infiltration or exfiltration of the river water into the aquifer or from the aquifer into the river is conditioned by the hydraulic gradient between the river water and ground water. When the water level in the river is higher than the ground water level, water from river infiltrates into the aquifer. When the water level in the river is lower than the ground water level, water flows from aquifer (exfiltrate) into the river. If the water level in the river and ground water level equals, there is no water flow via the river bed. The relationship between the river water level and the ground water level is decisive for the water flow direction from the river or into the river, see Fig. 6.1.

The amount of water flow from or into a river (measured in m³/s per selected area or per unit area of river bed) depends on the following physical phenomenon:

• distribution of difference between river water level and ground water level,
• distribution of permeability of the river bed and thickness of the river bed sediments,
• permeability of aquifer,
• geometric characteristics of aquifer,
• geometric characteristics of the river bed.

The distribution of permeability of the river bed sediments and the thickness of the river bed sediments having this permeability is called clogging or colmatation of the river bed. If the river bed is not permeable, there is no water flow across the river bed. Colmatation means reduction of infiltration to some degree.

6.2. Gravel pits at Vojka

Gravel pits at Vojka illustrate typical examples of ground water relationship to the surface water in the river branches (Fig. 6.2). Water in the gravel pits - lakes - originates from ground water. At the upper part of the lakes ground water flows into the lake, then from the lake flows downstream, and at the lowest part of the lake it flows out and infiltrates again into the gravel aquifer. Ground water, and thus water in the lakes, originates from the water of river branches, reservoir and in some places also from the seepage canals (Fig. 6.3). Water in the lakes is therefore of ground water quality, and is continuously exchanged by fresh ground water. This water and area are suitable for recreational purposes.

6.3. Colmatation - clogging of the riverbed or reservoir bottom

Colmatation is an important phenomenon, which creates a more or less impermeable barrier between the river water and the water in an aquifer. The aquifer downstream from the granite threshold at Bratislava is recharged mainly by water from the Danube. If the Danube riverbed, river branch bed, or the bed of the reservoir or lake were to become clogged with fine sediments, for example, recharge would be impeded to some degree.

Colmatation is a process, and the most normally measured characteristic of colmatation are permeability and thickness of the river bed (river bottom) sediments. The permeability of the riverbed varies from the permeability values of the underlying aquifer, down to some low values depending on the permeability of the sediment just on the river bottom. The finer the river bottom sediments, the smaller the permeability. In the Danubian Lowland conditions no absolute colmatation exists, neither in the old dead river branches. The permeability of river bottoms vary in time, and is influenced mainly by the sedimentation - erosion processes and the morphological changes in the river bed. The protection of a sufficiently high permeability means keeping the river bed free of finer particles, such as mud. The sedimentation of fine particles - mud - is a function of water flow velocity. If flow velocity is at least higher than some 0.2 m/s for part of the year, the river bottom is usually free of mud and the permeability of the river bed is usually fairly high.

An example of such a situation is mentioned in the EC Working Group report [1]: "However, after discharging water into the side channels in the Slovakian flood plain from May 1993 onwards, the ground water levels have increased above those corresponding to pre-dam conditions. This demonstrates that a considerable recharge now takes place from the side channels. This has become possible because the running water has removed the fine material, previously clogging the bed of these river arms". And the report goes on to say: "?it is evident that a good hydraulic connection between the side river channels and the ground water system has been established. Thus, a substantial ground water recharge takes place from the river side channels resulting in up to 1.5 m increased ground water levels".

Colmatation is manifested by the difference between water levels in the reservoir and aquifer just under the reservoir bottom, or between water levels in river or river branches and aquifer. The difference between water levels in reservoir or, for example, the river arm and piesometer (observation well with short filter - screen) consists of one part characterising the flow line between the top and bottom of the colmatation zone, and of another part between the bottom of the colmatation zone and the entrance into the observation well. Thus, by recharging an aquifer from river or reservoir there exists always a water level difference (if there is no water level difference, there is no flow, no recharge). During aquifer recharge the water level in an observation well is below the water level in the river, river arm, or reservoir. Therefore, the main criterion of colmatation is not the water level difference, but the water recharge of the aquifer or the water infiltration rate via the river bottom. If the ground water level increases too much, because of high infiltration rate (low colmatation), the drainage canals carrying away the surplus infiltrated water should be constructed. Typical examples are the seepage canals bordering the reservoir and bypass canal (Fig. 6.4). A flow of water in the seepage canal is a sign of small or no colmatation.

In the case that the reservoir or river is draining an aquifer, clogging of the reservoir or river bottom does not really exist, because the small particles are, at least at some places, lifted upwards as point sources lifting or rising of sand.