Hydraulic conductivity of saturated bed silts in Chotárny channel, ŽO area, Slovakia

This paper deals with the comparison of two different ways used for determination of saturated hydraulic conductivity values of bed silts located along the Chotárny channel, obtained from field measurements in 2018. The Chotárny channel is one of three selected monitored channels at the ŽO area – the flat lowland with channel network. The permeability of bed silts impacts water flow between surface water in the channel and surrounding groundwater in the scope of their interaction at this area. It is expressed by its value of saturated hydraulic conductivity. The bed silts along Chotárny channel were extracted and obtained by two ways, as disturbed samples and as undisturbed samples. The selection of sampling place was made by thickness of bed silt in the measured profiles. The samples were extracted in three different vertical parts of bed silt – from top, middle and bottom part of bed silt layer. In case of the disturbed samples the empirical formulas based on the grain size analysis were used for determination of saturated hydraulic conductivity value. In case of undisturbed samples was used the falling head method for determination of saturated hydraulic conductivity value. The values of saturated hydraulic conductivity obtained from disturbed samples of bed silts K d were calculated by using of several empirical formulas: 1. Bayer – Schweiger; 2. Špaček I.; 3. Špaček II.; 4. Hazen I.; 5. Bayer; 6. USBR and 7. Orechova. The results in Table 1 show that the valid values K d for Chotárny channel in 2018 were from 1.00x10 -10 to 1.19x10 -04 m s -1 . The recommended formula for calculation of K d of bed silts by empirical formulas in Chotárny channel, based on criterium of the largest number of valid obtained results, is Hazen I. formula. The values of saturated hydraulic conductivity obtained from undisturbed samples of bed silts K u were determined according the relation for calculation of average value of saturated hydraulic conductivity (by falling head method in laboratory). These values are illustrated in Table 2. The values K u for Chotárny channel in 2018 reached values from 7.68x10 -08 – 4.53x10 -05 m s -1 . All the results from both methods of determination of saturated hydraulic conductivity were compared.


Introduction
Žitný Ostrov (ŽO), Rye Island -Fig.1a on the leftwas created by sediments transport from upper part of the Danube River.This area formed as a flat plain with only small differences in altitude.Its average slope is about 0.25 ‰ and it was one of the reasons for building channel network here.The channel network at 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.The silting up of channel bottom by bed sediments has impact to bottom permeability of channels in time and by this way to interact with surrounding groundwater.This permeability is expressed by the value of saturated hydraulic conductivity of bed silts.Engineering practice often requires the investigation of ground water movement, volumes in storage and computation of the amount of infiltrated water into or from the aquifer.Hydraulic engineers, hydrologists and hydrogeologists have been studying this topic with a variety of conclusions.In the current literature research papers usually focus on a wide variety of saturated hydraulic conductivity related topics.Habtamu et al. (2019) evaluate saturated hydraulic conductivity with different land uses of disturbed and undisturbed soil, Duong et al. (2019) clarify the effects of soil hydraulic conductivity and rainfall intensity on riverbank stability, Říha et al. (2018) present the verification of validity of various published porosity functions and empirical formulae with the use of the experimental data, Wang et al. (2018) present an alternative model to predict soil hydraulic conductivities, Hwang et al. (2017) compare saturated hydraulic conductivities of sandy soils to characterize properties of water retention, Gadi et al. (2017) studied spatial and temporal variation of hydraulic conductivity and vegetation growth in green infrastructures, Hussain and Nabi (2016) used seven empirical formulas to calculate hydraulic conductivity, based on grain size distribution of unconsolidated aquifer materials, Kutílek (1978) or Kasenow (2010) Fig. 1a.ŽO within Slovakia and three monitored channels at ŽO area.2018) used simplified equipment for measuring of saturated hydraulic conductivity from undisturbed samples by falling head method, etc.This paper relates to results of field measurements along Chotárny channelone of three main monitored channels at the ŽO area (Fig. 1a, Fig. 1b).Saturated hydraulic conductivity values of bed silts were determined on the base of two ways of bed silt samples extraction (disturbed and undisturbed samples).The channel network aggradation at ŽO has been monitored and studied by many our regional specialists -Kosorin (1997), Mucha et al. (2006), Štekauerová et al. (2009), Baroková and Šoltész (2014), Kováčová (2017), etc.This paper shows some results of field measurements along the Chotárny channel in 2018 -Fig.1b.

Material and methods
Chotárny channel is one from three channels of the ŽO channel network monitored by Institute of Hydrology SAS since 1993.Geometrical parameters of this channel observed during the last measurements were: the channel length was approximately 27 km, the channel width was in range 11-17.5 m, the channel depth run into maximal values up to 3.15 m (according to cross-section location).The registered values of hydraulic conductivity in aquifers nearby this channel Kfs are 0.40-3.4x 10 -3 m s -1 (Mišigová, 1988).Chotárny channel receives water from drain channel of Gabčíkovo waterworks.The tributaries of Chotárny channel are channel Gabčíkovo -Ňárad, Čilížsky stream, channel Jurová -Veľký Meder, channel Kračany -Boheľov, Belský channel and Býčí channel.The water from Chotárny channel is used for irrigation by another channels which are connected to this channel network by sluices (Dušek and Velísková, 2014).The collection facility for the Chotárny channel is situated in left-hand embankment of the drain (outflow) channel (in km 1.25) and is permanently closed.Water supply to Chotárny channel is provided by underground seepage pipe.In km 10.15 of Chotárny channel occures the water measuring station Jánošíkovo na Ostrove.The Chotárny channel connects to Little Danube through pumping station eastward of village Topoľníky.The measurements of bed silt thicknesses along the Chotárny channel in 2018 were performed from the displaceable inflatable boat by simple drill hole.The distance of cross-sections along the channel varied between 1.0-1.5 km.In all channel cross-sections there was measured the water depth and bed silt thicknesses with step 1.0-2.0m along the channel width.The samples of channel bed silt were extracted in these selected cross-sections where the largest channel bed silt thickness was noticedin 8 cross-sections of Chotárny channel.Sediment sampling was conducted using the 04.23 Sediment Core Sampler, a rod operated type Beeker.This instrument collected the samples of silts in 1 m long acrylic tube as shown in Fig. 2a.The Fig. 2b illustrates the sediment sampling from field measurement in one of cross-sections of Chotárny channel.There was possible to take undisturbed samples of bed silt with this device and from each whole sample there was extracted a part from top, middle and bottom layer of silt.After experimental determination of saturated hydraulic conductivity for each layer of sample from one cross-section, the sample was broken and changed to disturbed sample.Next, for each disturbed sample it was done the granularity analysis and determined the value of saturated hydraulic conductivity.

The disturbed samplesdetermination of saturated hydraulic conductivity
Determination of saturated hydraulic conductivity from disturbed samples of bed silts can be calculated by empirical formulas coming out from grain size analysis (Kutílek, 1978;Kasenow, 2010).Several empirical formulas for determination of hydraulic conductivity from granularity exist, but it is possible to apply only a few of them because their limited validity.Therefore we used at first the relationships by Beyer-Schweiger and Špaček (Špaček, 1987) and later we added yet using of another four empirical formulas according to authors Hazen I., Bayer, USBR and Orechova (User's manual of software GeoFil).In all these mentioned relationships the value of saturated hydraulic conductivity Kd is function of d10, d17, d20 or d60 (particle diameters in 10%, 17%, 20% and 60% of soil mass).These parameters were determined from granularity curves of all extracted disturbed samples at Chotárny channel.Beyer-Schweiger formula, used for determination of saturated hydraulic conductivity from disturbed samples KdBS [m s -1 ], is: where C=1.5961x10 -3 (d60/d10) -0.20371 , d10particle diameter in 10% of soil mass [m], d60particle diameter in 60% of soil mass [m] and conditions of validity are:  where d10 is particle diameter in 10% of soil mass [cm]; d60 is particle diameter in 60% of soil mass (cm); a, b are constant factorsfor consolidated soils a=0.01; b=-0.23;U is coefficient of uniformity; the conditions of validity are: 0.06 (cm)  d10  0.6 (cm)  U  20.

USBR formula Kd
where d20 is particle diameter in 20% of soil mass [mm]; the condition of validity is: 0 where d17 is particle diameter in 17% of soil mass [mm]; the condition of validity is: g0,063 [mm]  35%, it means that is valid for soils with fraction less than 0.063 mm content  35 %.
The valid values of saturated hydraulic conductivity from disturbed samples of silts along the Chotárny channel Kd according to these 7 formulas were calculated and summarized in Table 1.The samples of silts were extracted in those selected cross-sections along the Chotárny channel where the larger thicknesses of silt were measured within that cross-sectionin 8 locations (as the 1 st column of Table 1 and Table 2 cites)in km 1.2, 6.0, 8.6, 16.3, 18.0, 20.0, 24.6 and 25.5.

The undisturbed samplesdetermination of saturated hydraulic conductivity
In case of undisturbed samples of bed silts the values of saturated hydraulic conductivity were obtained by using of falling head methoddirect measurement in laboratory.There was used the simplified equipment for measuring of saturated hydraulic conductivity from undisturbed samples -Fig.3a (Šurda et al., 2013).The formula for calculation of average value of saturated hydraulic conductivity Ku according to scheme on Fig. 3a is: where Ku is the saturated hydraulic conductivity of undisturbed samples [cm.s -1 ], l is a sample height [cm], h1, h2variable static head [cm]see Fig. 3a.
The values of saturated hydraulic conductivity from undisturbed samples, extracted from 8 selected cross-sections at Chotárny channel (as was described above also in case of disturbed samples), were determined by the relationship (8) according to scheme on Fig. 3a.
In case of comparison of the values of Ku in its single layers -between top, middle and bottom layer within one cross-sectionthere was no significant impact of the level and the results were scattered throughout the cross section, except in two casesthe cross-section in km 6.0 and in km 18.0.
Fig. 2b.Sediment sampling from boat at Chotárny channel in 2018.
Špaček formulas I., II.[m d -1 ] for saturated hydraulic conductivity from disturbed samples Kd are: d10 is particle diameter in 10% of soil mass [cm], the condition of validity is:d15  0.6 [cm].Bayer formula Kd B [m s -1 ] is:

Fig. 3b .
Fig. 3b.The demonstration of application of falling head method to 24 undisturbed samples from Chotárny channel.

Fig. 5a .
Fig. 5a.The illustration of saturated hydraulic conductivity values from disturbed samples at Chotárny channel in 2018.

Fig. 5b .
Fig. 5b.The illustration of saturated hydraulic conductivity values from undisturbed samples at Chotárny channel in 2018.