ACTUAL VALUES OF SATURATED HYDRAULIC CONDUCTIVITY OF CHANNEL BED SILT AND ITS DISTRIBUTION ALONG KOMÁRŇANSKÝ CHANNEL

This contribution deals with the evaluation of channel bed silt permeability along the Komárňanský channel based on the measurements during the year 2019. Komárňanský channel is the largest one from three great channels of channel network at Žitný ostrov area. The channel bed silt permeability is expressed by parameter of its saturated hydraulic conductivity. This paper describes the current state of channel bed silt distribution along this channel during the year 2019 and simultaneously brings the values of saturated hydraulic conductivity of channel bed silt. The channel bed silts were extracted and obtained by two ways, as a disturbed samples and as an undisturbed samples. The samples were taken in each selected profiles of channel in three layers – top, middle and bottom layer of channel bed silt. The selection of sampling was made in dependency on channel bed silt thickness in the measured profile. The values of channel bed silt saturated hydraulic conductivity from disturbed samples Kd were calculated according to empirical formulas of BayerSchweiger and Špaček. The valid values Kd reached from 1.09 x 10 – 1.98 x 10 m s. The values of channel bed silt saturated hydraulic conductivity from undisturbed samples Ku were determined by the laboratory falling head method. The acquired values Ku for Komárňanský channel reached from 3.73 x 10 – 2.01 x 10 m s. The current state of longitudinal distribution of channel bed silt along this channel was demonstrated in the paper graphically and values of saturated hydraulic conductivity were demonstrated numerically.


Introduction
The channel network at Žitný ostrov (ŽO) area was built already in the late 19th century with primary aim to drain the wet places of this area. Thereafter its utilization began also to surface irrigation during dry periods and also to groundwater resources regulation in some ŽO localities. ŽO area is a flat plain with only small differences in altitude and that's why the flowing velocities in channels are very low. Just these slow velocities cause the silting up on the bottom of channels. It is very important to observe the development of this channel bed silt aggradation continuously, forasmuch as the channel network is in close interaction with groundwater. The silting up of channel bed by sediments has impact to bed permeability in time and by this way to interact with surrounding groundwater. The permeability is expressed by value of saturated hydraulic conductivity. This parameter can be determined from granularity analysis of extracted bed silt samples. This paper is adressed to results of field measurements along the Komárňanský channel in 2019 with aim to find out the current state of silting up at this channel and the influence of bottom aggradation degree to interaction between channel and its surrounding groundwater. The values of saturated hydraulic conductivity were determined on the base of two ways of channel bed silt extraction (disturbed and undisturbed channel bed silt samples).

Material and methods
Channel network at the ŽO area is created by several main channels -e.g. channel Gabčíkovo-Topoľníky, Chotárny channel (Aszód), Komárňanský channel, channel Čalovo-Holiare-Kosihy, channel Aszód-Čergov, channel Čergov-Komárno, channel Dudváh and by thus network of smaller channelssee Fig. 1. Our research of channels silting up has been concentrated since 1993 to three main channels of this network: channel Gabčíkovo-Topoľníky, Chotárny channel and Komárňanský channel (KCH). The localization of KCH is shown in more details see Fig. 2. Mutual interaction between channel network and groundwater of ŽO is influenced not only by general conditions of groundwater flow at ŽO area, but also by water level regime of its channel network. For this reason, many Slovak researchers were interested in solution of groundwater regime at this specific area (Kosorin, K. 1997;Burger and Čelková, 2004;Čelková, 2014;Mucha et al., 2006;Štekauerová et al., 2009;Baroková and Šoltész, 2014;etc.). The regulation of water level in channel network should be able to affect the groundwater level in its surrounding. Channels, manipulating objects and pumping stations as basic elements of channel network enable to control water level in channels and by this way to achieve optimal position of groundwater table mainly during growing season. But on the other hand, during this season the aquatic vegetation affects flow conditions in channels and also the thickness and structure of channel bed silts significantly influence the interaction between surface water in channel network and groundwater in surrounding area of the ŽO. Komárňanský channel (KCH) is the largest one of three main channels of ŽO channel network. This channel, primary built for drainage, is now used also for irrigation  function. KCH is supplemented from the Váh river over pumping station Komárno -Nová Osada and it connects with Chotárny channel (in rkm 9.1 of Chotárny channel) through a manipulating objects northwest of the Okoč village. The last measured length of the KCH is about 28 km. The channel width during the measurements was in range 10-29 m, the measurements of channel depth registered maximal values up to 2.7 m (according to located cross-section profiles). The values of saturated hydraulic conductivity in aquifers nearby this channel Kfs were 0.40-3.4 x 10 -3 m s -1 (Mišigová, 1988). The measurements of channel bed silt thickness along the KCH were performed in 2019 from the displaceable inflatable dinghy by simple drill holesee Fig. 3. The measurements were realized along the whole length of the KCH. The distance of cross-section profiles along the channel varied between 1.0-1.5 km. In all channel cross-section profiles there was measured the water depth and channel bed silt thickness with step 1.0-2.0 m along the channel width. Besides of channels silting up also the velocity profiles and discharges were measured during field measurements in 2019. The RiverSurveyor S5/M9 from SONTEK was usedsee Fig. 4. The logged files from these measurements confirmed that the velocities in every channel cross-section profile are very low. In some profiles, the channel was overgrown by very dense aquatic vegetation inside the channel (under water level) and also by bank vegetation, so there was impossible to measure velocity profiles and discharges. In 2019 the samples of channel bed silt were taken in these selected KCH cross-section profiles where the largest channel bed silt thickness was noticed. The extraction of samples was done by equipment (see Fig. 5) which is so-called sediment beeker sampler. There was possible to take undisturbed samples with this equipment and from each whole sample there was extracted a part from top, middle and bottom layer of channel bed silt. After experimental determination of saturated hydraulic conductivity for each layer of sample from one cross-section profile, the sample was broken and changed to disturbed sample. Next, for each disturbed sample it was done the granularity analysis and determined the saturated hydrau-lic conductivity value.

Saturated hydraulic conductivity of channel bed silt
The values of saturated hydraulic conductivity of channel bed silt was determined from undisturbed samples and also from disturbed samples for the same localities. The values of saturated hydraulic conductivity from disturbed samples were calculated by empirical formulas coming out from granularity curves. The several empirical relationships for determination of SHC from granularity exist, but their validity is limited and for that reason we should apply them very carefully. In case of disturbed samples sampling we used the relationships by Beyer-Schweiger and Špaček (Špaček, 1987). These relationships are functions of d10particle diameter in 10% of soil mass [m] and d60particle diameter in 60% of soil mass [m]. Both of them were determined from   granularity curves of extracted samples of channel bed silt. The formulas of Beyer-Schweiger and Špaček (Špaček, 1987) were used for determination of saturated hydraulic conductivity from disturbed samples from KCH -Kd. The equations for Bayer-Schweiger, Špaček I., Špaček II. formulas and for calculation of saturated hydraulic conductivity values have been already cited in previous published papers (e.g. Dulovičová et al., 2016, Dulovičová et al., 2018. The values of saturated hydraulic conductivity from disturbed samples Kd along the KCH are summarized in Table 1.
In case of undisturbed samples the values of saturated hydraulic conductivity were assessed by falling head methoddirect measurement in laboratory. There was used simplified equipment for measuring of saturated hydraulic conductivity from undisturbed samples (Kutílek, 1978;Rožnovský et al., 2013)see Fig. 6 (methodology of measurement and calculation described e.g. Šurda et al., 2013;Dulovičová, 2014;Dulovičová et al., 2016;2018;etc. Fig. 6. The values of saturated hydraulic conductivity from undisturbed samples Ku, extracted from selected crosssection profiles of the KCH, were calculated by the relationship (1) according to scheme on Fig. 6. The values of Ku along the KCH are summarized in Table 2.

Results and discussion
As was mentioned before, flow velocity in the channels is very slow by low slope condition of whole area of ŽO, which is very flat. The low flow velocity caused the deposition of silt in the channel bottom. The distribution of channel bed silt along the channel depends also on flow conditions in channels junction. We supposed the smaller amounts of the silt deposition in the upstream and downstream parts of KCH (caused manipulating with pumping station) and larger amounts in the middle part. Also we supposed that this increase to be gradual and linear. Ours expectations were confirmed partially. The silt thicknesses along KCH during monitored period 2019 showed moderately increasing trend with rising stream log, but in middle part was noticed the decrease of silt thicknessessee Fig. 7. Globally, the KCH aggradation gradually enlarged with rising stream log (excepting local parts with small amount of deposits) and the channel bed silt thicknesses increased.

Values of saturated hydraulic conductivity from disturbed samples
The value of saturated hydraulic conductivity, as the indicator of channel bed silt permeability, was calculated for disturbed samples by Beyer-Schweiger and Špaček  Simplified equipment for measuring saturated hydraulic conductivity of undisturbed sample: 1sampling tube height, 2 -Kopecky´s roller, 3 rubber ring, 4filter paper and wire strainer, 5 -Petri dish. relationships, which are the function of d10 (particle diameter in 10% of soil mass) and d60 (particle diameter in 60% of soil mass). The characteristics d10 and d60 were determined separately for top, middle and bottom layer of extracted samples.
The valid values of channel bed silt saturated hydraulic conductivity in KCH reached to 1.09 x 10 -08 -1.98 x 10 -04 m s -1 . In comparison with values of saturated hydraulic conductivity in aquifers nearby this channel Kfs (Mišigová, 1988), the values Kd are severalfold lower. Detailed distribution of Kd values along the KCH shows Table 1.

Values of saturated hydraulic conductivity from undisturbed samples
The values of saturated hydraulic conductivity from undisturbed samples Ku extracted from selected crosssection profiles along the KCH during 2019 were determined by equation (1) in correspondence to scheme on Fig. 6. The valid values of Ku reached to 3.73 x 10 -08 -2.01 x 10 -05 m s -1 . In comparison with values of saturated hydraulic conductivity in aquifers nearby this channel Kfs (Mišigová, 1988), the values of Ku are also severalfold lower. Detailed distribution of Kn values along the KCH shows Table 2.
We also compared the values of saturated hydraulic conductivity obtained from disturbed and undisturbed samples of channel bed silt. As it is obvious from Table 1, the values of Kd from disturbed samples run into 10 -08 to 10 -04 , whereby the values 10 -07 predominated. Likewise the values of Ku from undisturbed samples run into 10 -08 to 10 -05 (Table 2), predominant values were 10 -07 -10 -06 . This fact only partly confirmed our assumption that the values from undisturbed samples Ku will be lower then the values from disturbed samples Kd. The differences betwen them were practically irrelevant. At comparison of single layers of channel bed silt, extracted as disturbed samples, we detected that between top, middle and bottom layers were little differences, maximally tenfold.
In case of comparison of undisturbed samples in its single layers the differences between top, middle and bottom layer were tenfold up-to hundredfold. The interesting results were obtained from mutual comparison of single layers of silt for disturbed and undisturbed samples. As it is obvious from comparison Table 1 and Table 2

Conclusion
This paper deals with the evaluation of bed silt permeability along the Komárňanský channel (KCH) on base of field measurements performed during the year 2019. The channel bed silt permeability and the thickness of bed silt determine the rate of mutual interaction between surface water of KCH and nearby groundwater in its surroundings. For this reason it is important to know actual state of channel bed aggradation. The permeability of channel bed silt is expressed by its saturated hydraulic conductivity.
According to results of distribution of average bed silt thickness along the KCH from 2019 it is visible that silting up of this channel has been changed, it gently increased with ascendant stream log, but in middle part there was noticed the local and also global decrease of silt thicknessessee Fig. 7. The values of saturated hydraulic conductivity of channel bed silt were determined by two ways: as channel bed silt extracted and obtained as disturbed samples and as undisturbed samples from top, middle and bottom layer of silt. From disturbed samples the values of saturated hydraulic conductivity of channel bed silt were calculated according to Bayer-Schweiger and Špaček formulas and they are presented in Table 1, the valid values Kd reach from 1.09 x 10 -08 -1.98 x 10 -04 m s -1 . From undisturbed samples of channel bed silt there were determined values Ku by falling head method and they are summarized in Table 2. The values Ku reached values from 3.73 x 10 -08 -2.01 x 10 -05 m s -1 . Some discrepancies in distribution and values of saturated hydraulic conductivity obtained in 2019 and from previous study period exist. For this reason, it is necessary to put these results under the more detailed analysis in the next step in the future.
As it was mentioned, the aggradation along KCH enlarged in 2019, the channel bed silt thicknesses increased. Because the mutual interaction between surface water in channel and groundwater in its surroundings depends on channel bed permeability, which is mainly determined by the channel bed silt thickness and permeability, it is necessary to know these parameters for correct evaluation of the mutual exchange of water amounts between surface water in channel and groundwater in its surroundings. All obtained information about actual state of silting up of KCH, supplemented by values of saturated hydraulic conductivity channel bed silt, will be helpful for numerical simulation models, also for any way of regulation of groundwater level in surroundings of this channel and water source management at ŽO area.