on which this river is divided just below the Slovak capital Bratislava: the Danube and the. Small Danube. The area of the Island is approximately 2000 km2 and ...
SEDIMENT IMPACT ON SURFACE WATER AND GROUNDWATER INTERACTION AT GABČÍKOVO-TOPOĽNÍKY CHANNEL (ŽITNÝ OSTROV) Renáta Dulovičová, Yvetta Velísková Institute of Hydrology, SAS, Račianska 75, 831 02 Bratislava 3, Slovak Republic
Abstract The aim of the paper is to study the interaction processes between groundwater and surface water along the Gabčíkovo-Topoľníky channel. This channel is one of the Ţitný Ostrov (Rye Island) channel network. Ţitný Ostrov is the area between two branches of the Danube River, on which this river is divided just below the Slovak capital Bratislava: the Danube and the Small Danube. The area of the Island is approximately 2000 km2 and represents about 4 % of the Slovak territory. It forms a flat plain with only small differences in altitude. Its average slope is about 2,5.10-4 and this was one of the reasons for building a channel network within this area. At first we investigated and analyzed the silting of a drainage channel and the influence of the silt on the groundwater recharge (hydraulic conductivity of silt). Determination of the total recharge amount was done by a numerical simulation (model SKOKY) and by the so-called method of interaction formulas. These two approaches were applied at the Ţitný Ostrov channel network, in the Gabčíkovo-Topoľníky channel. There were field measurements performed in this channel and adjacent to obtain correct input data. Finally, we calculated the total recharge amount for four alternatives of the surface water levels in the channel and the surroundings groundwater respectively. KEY WORDS: silting, groundwater recharge, interaction between surface water and groundwater Introduction The Danube River, the largest river flowing through Slovakia’s territory and its nearby area has brought in many benefits, but sometimes also problems. Therefore already long before it had been necessary to modify its stream and build various protective facilities. The nearby Ţitný Ostrov (Rye Island) has been important agricultural production area. Water resource management have to solve problems during dry periods or floods, which are connected with irrigation or drainage function of the Ţitný Ostrov channel network. To regulate water regime at the Ţitný Ostrov area needs information about mutual interaction between channel network of this area, and groundwater. The aim of the built channel network has been to drain wet area, to irrigate fields during dry period and to regulate the groundwater (as a source of drinking water). Groundwater at this area is strongly related to water regime of channel network. Various factors have changed their mutual interaction (e.g.designe and operation of Gabčíkovo water work). One of them is also bed aggradation of channels with suspended load silts, which affects the channel permeability. Channel network The channel network at Ţitný Ostrov region was built up for drainage and also to provide irrigation water. The water level in the whole channel network system affect groundwater level and vice versa. The Gabčíkovo-Topoľníky channel is the largest channel of this channel network at the Danube Lowland (Podunajská níţina) area. The Danube River flows through a relatively
narrow valley along the Austrian-Slovak border. The river crosses a strip of mountains connecting the Alps and the Karpatians, passes Bratislava, and divides into two branches just below Slovak capital, i.e., the Danube (Dunaj) and the Small Danube (Malý Dunaj) – see Figure 1.
channel Gabčíkovo-Topoľníky Figure 1. Location of channel Gabčíkovo-Topoľníky at Ţitný Ostrov
Two branches of the Danube flow separately approximately 100 km and then join each other again near the town Komárno. The area between the two branches of the Danube river is called Ţitný Ostrov. Ţitný Ostrov is part of the Danube Lowland and is one of the most productive agricultural areas of Slovakia. The average width of Ţitný Ostrov is 20 km; its area is approximately 2000 km2 , which represents about 4 % of the Slovak territory, but about 10% of the most productive arable land. Ţitný Ostrov area forms a flat plain with only small differences in altitude. Its surface decreases in the south-east direction. Its average slope is about 2,5.10-4 and this was one of the reasons for building channel network at this area (as a drainage system). Ţitný Ostrov is the result of sedimentation of the Danube River, with sediments from upstream mountains (Alps) being spread over its territory. The annual amount of sediments deposited along the Danube watercourse is about 700 to 800 thousand tons. These sediments are deposited in areas where the stream velocity decreases. Sedimentation of transported materials has created thick layers of gravel, with a tendency to elevate the river bed. The creation of river branches, which flow according to the Danube River water level, is another aspect of the sedimentation process. During previous centuries, the river branched into multiple streams and frequently changed course within its own alluvial sediments. At present, the process of river migration has stopped or is being controlled, because irregular sedimentation provides changes in the river bottom and changing conditions for shipping. Water in alluvial deposits of the Danube River serve as a source of water for such agglomeration as Vienna, Bratislava, Budapest, Belgrade, and others. The body of sediments between Bratislava and Gabčíkovo is of great importance in terms of supplying water to surrounding areas. The surface of this body creates a system of river branches inhabited by a variety of rare plants and animals. This area is protected as a water reserve, with many industrial and recreational activities prohibited.
The general direction of groundwater flow during periods of low discharges in the Danube River along the length of Ţitný Ostrov is in a south-eastern direction, more or less parallel to the Danube flow direction. When the water level in the Danube is relatively high, large amounts of water infiltrate into the Ţitný Ostrov aquifer, with velocity vectors of the seepage water being close to 90 degree to the Danube River flow direction (Kosorin, 1997). In order to secure recharge into the Ţitný Ostrov aquifer, it is necessary to keep water levels in the Danube River relatively high, and run seepage channels which allow one to control the water table. It is possible to use to regulate and control the groundwater table at Ţitný Ostrov area by existing channel network. As it was mentioned above, the slope of the Ţitný Ostrov area very low, so natural draining has not developed. For this reason the network of channels has been built at this area since the end of 19th century. Today the channel network length is nearly 1000 km. Its density is approximately 1 km/ 1,25 km2. System of channels can be utilized for drainage and irrigation. The problem of water level regulation at this area is complex. Therefore, many specialists were interested in solution of it (Kosorin, 1975; Kosorin, K. 1997; Novák a kol., 1998; Šútor, Štekauerová, 2000; Burger, 2003; Kališ, Bačík, 1983; Fratrič, 1963). Channels, manipulating objects and pumping stations as basic elements of this channel network allow to control water level in channels to achieve optimal position of groundwater table during vegetation period. Water vegetation affects flow conditions in channels. The thickness and structure of bed silts are factors influencing this interaction, therefore it has been necessary to assess also the impact of channel network silting up on it. Measurement of channel network silting up The measurements of silt thickness were performed in 1993 at following channels: Aszód, Gabčíkovo-Topoľníky, Aszód-Čergov, Čergov-Komárno, Čalovo-Holiare and HoliareKosihy. The scheme of the area is in Figure 2.
Figure 2. Scheme of Ţitný Ostrov channel network (1 – Danube; 2 – Small Danube; 3 – Gabčíkovo– Topoľníky; 4 – Aszód; 5 – Čalovo–Holiare–Kosihy; 6 – Aszód–Čergov; 7 – Čergov–Komárno; 8 – Dudváh; 9 – Komárňanský channel)
Measurements were done at the distance 1,0-2,0 m in cross-section direction and distances of evaluated cross-sections were in the range 2,0-5,0 km. The measurement data included distance of a measured point from the right channel bank, water depth, silt thickness and average silt thickness in the profile. The average silt thicknesses were determined as a ratio of a cross-section profile silt area and width of the channel. The average silt thickness distribution along Gabčíkovo-Topoľníky channel is shown in Figure 3.
Average silt thickness - channel Gabčíkovo-Topoľníky average silt thickness [m]
0,6 0,5 0,4 0,3 0,2 0,1 0 0
5
10
15
20
25
30
channel distance [km]
Figure 3. Average silt thickness of channel Gabčíkovo-Topoľníky in 1993 The volume of suspended load sediments was calculated from average silt thickness and from its distribution along a channel by the formula: p 1
V ( S Ni S Ni 1 )Di / 2
(1)
i 1
where SNi is a cross-sectional silt area in a profile [m2], Di is a distance between neighbouring profiles [m] and p is a number of all measured profiles. The total volumes of suspended load sediments are summarized in Table. 1. Table 1. Total volume of suspended load sediments in the Ţitný Ostrov channel network(1993) channel Aszód Gabčíkovo-Topoľníky channel Komárňanský channel channel Aszód – Čergov channel Čalovo – Holiare - Kosihy channel Čergov - Komárno
111 436 m3 93 793 m3 197 824 m3 38 419 m3 113 059 m3 58 350 m3
Control measurements were performed at selected profiles of the three largest channels Gabčíkovo-Topoľníky, Aszód and Komárňanský channel - for checking of the silting up changes. In 2004 the silt thickness changes along the channels were supposed to be linear, so these profiles were selected at the beginning, middle and ending part of channels and the measurements were carried out approximately at the same profiles along channels like in
1993. It is important to remind that all measurements were done from surface water level, so only the differences between silt top and channel bottom levels were estimated. Measurements of silt thickness at the Gabčíkovo-Topoľníky channel were done in 1993 more detailed and in 2004 for checking of the silting up changes. For determination of silt granularity and hydraulic conductivity we took the samples at the same time as we measured the silt thicknesses (in 2004). We measured the silt thickness in 18 to 22 points in each cross section of the irrigation channel, depending on the width and accessibility of the channel. In 2004 we performed measurements at three locations: station km 0.8 (Trhová HradskáTopoľníky, at the begging part of channel), at station km 10.0 (Kútniky-Dolný Bar, middle part) and at station km 26.0 (Baka, end of channel). The silt layers at begging part of channel were in 1993 on average only 0,14 m thick. It has to be noted that in 2004 we were not able to perform measurements at the same cross section as in 1993. Therefore the results measured in 1993 and 2004 do not represent a compatible set of data to analyze at this station. However, increasing tendencies of silting from 0,38 m in 1993 to 1,2 m in 2004, (215 % increase) were recorded at station km 10. The measurements at station km 10 were completed only partially because of dense vegetation, however they show significant increase as mentioned above. Additionally, the silt sampled in this channel contained a large amount of organic matter. At station km 27.0 the average silt thickness 0,09 m in 1993 increased to 0,22 m in 2004 (144 % increase).The result of the silt layer thicknesses comparison is summed up in Table 2. Table 2. Percentage of silt increases between 1993 and 2004
Channel
G-T
downstream middle upstream
Average silt thickness, 1993 [m] 0.14 0.38 0.09
Average silt thickness, 2004 [m] (0.04) 1.20 0.22
Percent increase of silt thickness ---215 % 144 %
Silt permeability The water level in the irrigation channels effects the groundwater level at Ţitný Ostrov area and vice versa. The mutual interaction of these two variables triggers not only a question about the quantity of silt building up in the channels, but also what the permeability and hydraulic conductivity of the silt is. In the process of this mutual infiltration, the silt in the irrigation channels functions as an impeding factor. The water infiltrates into and out of the irrigation channels through the sides and the bottom. However, most of the silt is deposited at the bottom of the channels, which are covered by the layers of silt acting as a sealant. We took the samples of the silts to determine the granularity and hydraulic conductivity. However, many locations along the channels were not accessible because of vegetation. Also some alluviums were of such a liquid consistence that the analysis of the permeability and determination of the granularity were impossible. We had to determine granularity of the silt as the first step. For illustration, the granularity curve for bottom and top layer of silt in km 26.0 (Baka) is shown in Figure 4.
Channel Gabčíkovo-Topoľníky
% of soil mass
100 80 60
Baka_t
40
Baka_b
20 0 0,001
0,01
0,1
1
10
particle size d [mm]
Figure 4. The granularity curve of top and bottom layer of silt
Then we computed the permeability of silts in the channels represented by hydraulic conductivity. We investigated numerous empirical relationships and excercised the highest level of caution as the formulas have a limited validity. Many formulas refered to evaluation of the undisturbed samples, which we were not able to take. The only two formulas, we were able to use, were the one by Beyer - Schweiger (cited in Mucha - Šestakov, 1987) and the one by Špaček (1987). Beyer-Schweiger formula for the hydraulic conductivity (m.s -1): 2 K 7 ,5 .10 6 C d10 (2) 3 d 60
0 ,20371
where C 1,5961.10 d 10 d10 – particle diameter in 10% of soil mass (m) d60 – particle diameter in 60% of soil mass (m) d 1 60 20 0 ,06 d 10 0 ,6 and conditions of validity ; . d 10
Špaček formulas for the hydraulic conductivity (m.d-1) are: 1,013
0 ,5 d d 10 60
K I . 20 ,577 .d10
K II . 108 ,4386 d10
0 ,8866
0 ,059
d60 0 ,7726
(3)
(4)
Conditions of validity for application of Equation (2) are:
or
1.
d10 < 0,01mm
(5)
2.
0.01 d10 0 ,13 d60 0 ,0576 0 ,5765d10
(6)
Conditions of validity for application of Equation (3) are:
or
1. d10 0,13 mm
(7)
2. 0.01 d10 0 ,13 d60 0 ,0576 0 ,5765d10
(8)
The values of hydraulic conductivity are summed in Table 3. Table 3. Values of grain and filtration characteristics of silts by Bayer – Schweiger and by Špaček I and Špaček II
Layer top bottom
Conductivity by Beyer Location Špaček I Špaček II Schweiger [mm] [mm] [m.s-1] [m.s-1] [m.s-1] -7 -6 Baka *0.008 0.088 11.05 *(4.70) (10 ) (1.99) (10 ) *(2.66) (10-6) Baka *0.0093 0.291 *31.29 *(5.13) (10-7) *(2.16) (10-6) (7.64) (10-6) The values with * are located outside of the validity interval. d10
d60
d60/d10
Using these infiltration values we can estimate the time it will take the water to infiltrate through the layer of silt. Determination of infiltration quantity Theoretical basis Interaction between groundwater and surface water is an integral part of the hydrological cycle. The interactions of surface-water bodies with groundwater systems are governed by the positions of the water bodies relative to the ground-water flow system, the characteristics of surface-water beds and underlying geologic materials and the climatic setting (Encyclopedia of Hydrological Sciences, 2005). When estimating the water transport and mass motion in the system of groundwater and surface water in their mutual interaction, we have to realise that there are very different processes respecting the time (large difference between the flow velocities of the groundwater and surface water) and there are different qualitative characteristics of the surface water and groundwater surroundings (large difference between the coefficients of hydraulic conductivity for groundwater surroundings and bed silts in the stream). The exchange of water quantity between open channel and groundwater reservoir is very important due to the mutual interaction between the surface flow and the groundwater. This exchange is realized through their contact areas (through the bottom and slopes of the channel or reservoir). Seepage from the groundwater into the stream (or out of the stream to the groundwater) is a key hydraulic characteristic in this process. This characteristic is given as an inflow/outflow in m3.s-1 over the channel bottom area which occurs on one meter of channel length. It is so called „specific discharge“ qbm in m2.s-1. This parameter is expressed by equation:
qbm
S Q t x
(9)
The intensity of this characteristic determines the stream impact onto the surrounding groundwater and vice versa. Discharge q constitutes the input characteristic in case of surface water or the boundary condition in case of groundwater. In areas where the groundwater table (GWT) is close to a soil surface (less than 3 m), water is transferred from the groundwater to the root zone and thus the soil water regime and the groundwater regime affect each other. The intensity of the interactions between the water table and the root zone of the soil profile depends especially on the groundwater table level, hydrophysical properties of the soil, the properties of the roots and on the meteorological properties. Calculation methods (a) Model SKOKY We have used three dimensional hydrodynamical model SKOKY for simulation of groundwater and surface water flow in their mutual interaction, which was developed at the Institute of Hydrology SAS (Bratislava) by K. Kosorin (Kosorin, 2001). It simulates threedimensional groundwater flow below and around rivers, reservoirs, dams, wells, tunnels, galleries, tubes and other structures including effects of singular domains of flow i.e. preferential ways, boils effects, bottom aggradations, gaps in the basin walls and leakage effects. The variables are geometry of the structures and hydraulic properties of the environment. The three-dimensional numerical simulation model SKOKY is based on a mathematical model, consisting of equations: grad P +v/k = O div v = 0
(10) (11)
where P is pressure function (potencial) P = y + p/gρ + cons t, v is a velocity vector, p is the hydrodynamic pressure, y is the vertical space variable (in direction of the gravitational acceleration g) and k is hydraulic conductivity. Model is completed with correctly stated boundary conditions. (b) Interaction formulas Kosorin (2001) derived a general relation for the inflow from the groundwater into a stream or outflow from a stream into the groundwater regarding their mutual interaction: qbm = a1j1 + a2j2 + a3j3 + …. +aiji
(12)
where j1, j2,…., ji are the interaction formulas and a1, a2,…, ai are the parameters of the interaction formulas, which we had to find. The interaction formulas include the influence of variable parameters for the evaluation of the inflow/outflow. It means that they will depend on the water level in the stream and at the same time on the groundwater level in the adjacent area. This relationship can be expressed as follows (Dulovičová, 2005): ji = f ( h, H0, H1, H2)
(13)
where H1 is groundwater level in input cross-section X1, H2 is groundwater level in output cross-section X2. Cross sections X1 and X2 are at symmetrical distances from the cross section centre line and H is the sum of the groundwater body thickness from impermeable bedrock to a channel bottom and water level in the stream, refer to Figure 5, 6.
The impact of geological characteristics of the surroundings at cross section and its geometrical parameters are expressed by the relationship: ai = fi ( H0, b, kfp , kn )
(14)
where i =1,2,3,…., kfp is a coefficient of hydraulic conductivity of the groundwater body, kn is a coefficient of hydraulic conductivity of the bed silts of a single cross section and b is the channel width in the bottom of this cross-section. We have to choose a number of interaction formulas to reflect the variability of the geology and stream geometry, which are subject to change in space and in time (we have to choose larger number of interaction formulas when geology and stream geometry are more variable). Y HPV
B
Hh
D0 Hb H0
H
H2
X
X1
X2
Figure 5. Schema of mutual interaction (gaining stream) between the surface flow and the groundwater for a symmetrical stream cross section with water depth h. The silt width D0 is symmetrical across whole bottom width. HPV is the groundwater level. Y B
HPV
Hh
D0 Hb H0
X1
H
H2
X X2
Figure 6. Schema of mutual interaction (losing stream) between the surface flow and the groundwater for a symmetrical stream cross section with water depth h. The silt width D0 is symmetrical across whole bottom width. HPV is the groundwater level.
The first alternative is a symmetrical double-sided groundwater inflow into a symmetrical stream cross-section with the water level h. The value of the total recharge for this alternative reflecting the surface and groundwater mutual interaction, is: Alternative I: qI = + q1 + q2 (m2.s-1) where q is total outflow/inflow from/into a stream, q1 is discharge through input cross section X1 and q2 is discharge through output cross section X2. The second alternative is a symmetrical double-sided outflow from a symmetrical stream cross-section (with water level h) to the groundwater. The value of the total recharge for this alternative is: Alternative II: qII = - q1 - q2 (m2.s-1) Both alternatives use the groundwater level in the input crosss section X1 identical with the groundwater level in the output cross section X2 , that means H1 = H2 and at the same time the water depth in stream is h. Alternative III is a double-sided groundwater inflow into a symmetrical stream crosssection with the same groundwater levels H1 = H2 and with water depth in the stream h/2. The relation for the value of the groundwater recharge for this alternative is the same as the first alternative, only water level in the cross section is h/2. Alternative IV is a symmetrical double-sided outflow from a symmetrical stream crosssection with water depth 2h and with the same groundwater levels H1 = H2. The relation for value of recharge for this alternative is the same as in second alternative, only the water level in the cross section is 2h. Discussion We computed the average silt thicknesses as a ratio between the area of the silt deposition and the width of the channel. The average silt thickness distribution in 1993 and 2004 in the Gabčíkovo-Topoľníky channel is shown in Tables 4 and 5. Table 4. Percentage of silt presence in the Gabčíkovo – Topoľníky channel in year 1993
Station [km] 3.0 10.0 27.0
Average silt thickness [m] 0.14 0.38 0.09
Silt area [m2] 1.53 6.16 1.18
Area of cross section [m2] 15.08 12.83 14.57
% silt area in cross section 10 % 48 % 8%
Table 5. Percentage of silt presence in the Gabčíkovo – Topoľníky channel in year 2004
Station [km] 0.8 10.0 26.0
Average silt thickness [m] 0.04 1.20 0.22
Silt area [m2] 0.26 2.90 2.69
Area of cross section [m2] 4.76 5.44 9.64
% silt area in cross section 5% 53 % 28 %
As logically expected, most of the silt is deposited at the bottom of the channel, and much less was observed at the sides. Otherwise the data does not seem to show clear and simple relationship between the silt thickness and any of variables which we believed would influence the silt depositions. As mentioned previously, the area of Ţitný Ostrov is very flat,
slope of 0,025 %, which is fairly uniform along the channels. Therefore the velocities of the flow in the channels are very slow. The deposition of the silts is certainly attributed the low velocities in the channels. At the same time, the velocities do not seem to control whether the heaviest deposition of the silts occurred in the downstream, middle, or upstream segments of the channels. In the Gabčíkovo-Topoľníky channel the thicknesses of the silt were the largest in the middle parts of the channel, 215 % increase respectively. We assumed the smaller amounts of the silt deposition in the upstream and downstream parts of the channel (by manipulating with pumping station) and larger amounts in the middle part and assumed this increase to be gradual and linear. These assumptions were confirmed in part. From view of mutual interaction between channel network and groundwater it has been necessary to know not only the silt thickness in the channels, but also permeability of silts. However, in some cases there was not possible to take the samples of silts (for determination of its granularity and hydraulic conductivity). For example, at the profile Topoľníky of the channel the alluvium had liquid consistence and thin thickness, so we did not manage to take samples of it; at profile Kútniky, there was flagrant silt, but as we already mentioned, the silt included large content of organic mass. Therefore it was not possible to establish its granularity. It is possible to use several empirical relationships for determination of hydraulic conductivity values, but we can apply them very carefully for their limited validity. In our case we could use only the relationships by Beyer-Schweiger (cited in Mucha - Šestakov, 1987) and by Špaček (1987) because we could not take of undisturbed samples. Each of them determines hydraulic conductivity as a function of d10 - particle diameter in 10% of soil mass and d60 - particle diameter in 60% of soil mass. Conditions of validity for application of these formulas also depend on value of d10 and d60. We determined these characteristics for top and bottom layer of silt samples. The values of d10 in channel Gabčíkovo-Topoľníky range 0,0080,0093 mm, d60 were from 0,088 mm to 0,291 mm. The valid values of hydraulic conductivity range (1,99 up to 7,64).10-6 m.s-1. Next we try to assess how impact a contents, texture and thickness of bed silt in drainage/irrigation channel on surroundings groundwater level through their mutual interaction. We applied two approaches for computation of recharge quantities: model SKOKY and method of interaction functions. Both of them are described above and were applied at Ţitný Ostrov region for channel Gabčíkovo-Topoľníky. We calculated recharge for four hydrological variants, in which we used gained results from field measurements described above. At the start we calculate it with model SKOKY and then we had to calculate unknown interaction formulas parameters a1 - a4 for each crosssection profiles. Calculation of parameters ai was made by matrix method solution of linear equations system. When the difference between q=(SKOKY) and q= f(φi), which was calculated by interaction formula, was negligible (less then 10%), we considered selected number of interaction formulas as sufficient. When the difference was not negligible (differences are higher then 20 %), we could not consider the calculation as sufficient, that means we would increase the number of interaction formulas and input another dependencies. However, the aim of this study is judgement of sediment influence on recharge, so in this phase of study we did not solve number of interaction formula and accuracy of this approach. The recharge calculated through interaction formulas, of which differences are higher then 20%, we did not accept for judgement. The values of total recharges from all four variants are in Table 6. Profiles, in which it was calculated, were distributed along the whole channel. The variant I. and III. represent inflow to channel from surroundings. They differ in heights of water level in surroundings and in the channel: in I. variant the water level difference was equal h and in III. variant it was h/2. So we assumed that values of total recharge in the variants would have
similar tendency. But results of simulation in conditions with bed sediments in drainage channel and their comparison shows that the change of total recharge value is approximately 6%. On the other hand, the variant II. and IV. represent outflow from channel to surroundings. They again differ in heights of water level in surroundings and in the channel: in II. variant the water level difference was equal (-h) and in IV. variant it was (-2h). In this case comparison of simulation results for these two variants shows 50% increase of total recharge in IV. variant. Table 6. Parameters and total recharges (4 variants)
Profile h [m] H0 [m] q f
i
1 2
3 2,1
4 2,2
5 2,3
6 2,4
7 2,3
10 2,5
15 2,5
17 2,4
86,3
88,0
67,8
30,7
6,5
6,4
6,1
4,8
4,3
(Δ= h) 5,89
1,67
1,11
5,90 0,01 0,10
1,67 0,00 0,00
1,11 0,00 0,09
I. variant 12,97 11,80 10,28
q = (SKOKY) 13,50 Difference 0,52 % 4,04
11,93 0,13 1,13
9,22 -1,07 -10,36
H 1 = H2 = H0 + 2h 10,65 7,63 6,94 9,85 -0,80 -7,47
5,75 -1,88 -24,63
5,90 -1,04 -14,92
II. variant -4,73 -6,04 -4,81 q f i q = (SKOKY) -4,21 -5,92 -2,91 Difference 0,53 0,12 1,90 % -11,12 -1,99 -39,46
H1 = H2 = H0 (Δ= -h) -2,29 -1,80 -1,51 -1,83 -3,62 -2,03 -3,54 -1,83 -1,33 -0,22 -2,03 0,00 58,13 12,42 134,44 0,00
-0,35 -0,34 0,00 -0,29
-0,53 -0,53 0,00 0,00
III. variant 13,72 11,44 4,67 q f i q = (SKOKY) 14,06 11,73 12,12 Difference 0,35 0,29 7,45 % 2,54 2,57 159,42
H1 = H2 = H0 + h (Δ= h / 2) 9,78 6,60 6,01 2,46 10,23 6,87 6,28 5,22 0,45 0,28 0,27 2,76 4,61 4,25 4,46 112,03
1,44 1,52 0,07 4,99
0,89 1,29 0,40 45,27
IV. variant -17,48 -7,13 -6,33 q f i q = (SKOKY) -6,74 -8,14 -5,51 Difference 10,74 -1,02 0,82 % -61,46 14,25 -12,97
H1 = H2 = H0 -6,02 -4,66 -6,74 -3,33 -0,72 1,33 11,99 -28,50
-3,06 -2,91 0,15 -4,87
(Δ= -2h ) -3,40 -0,88 -2,40 -0,65 0,99 0,23 -29,25 -26,02
-0,74 -0,76 -0,03 3,40
Summary and conclusions Aim of this paper was to analyse and evaluate the impact of bed silt quantity and quality on mutual interaction of water level of groundwater surrounding and drainage channel. Locality, at which the problem was solved, was the Gabčíkovo-Topoľníky channel. It is one of the drainage channels from Ţitný Ostrov channel network. We performed detailed measurements of the silt thicknesses in 1993. Then, in 2004 we performed the check measurements at the selected cross sections to verify the changes in silting.
The silt thicknesses changed from 1993 to 2004 according results in Tab. 2, 4 and 5. It seems that along the Gabčíkovo-Topoľníky channel the bed silts increased in generally, but at the same time in its beginning part the bed silt decreased. The silt depositions seemed to follow rather random patterns. We did not observe a clear relationship between silt depositions and any of variables we analyzed such as stream velocity, depth of the cross section or location of the cross sections with respect to the begin, middle or ending segment of the channel profiles, or presence of the vegetation. Some of the cross sections were not accessible because of dense flora, which developed in the channels over the years. The other objective of our field measurements was a determination of the silt permeability. As a first step we determined the granularity and then we computed hydraulic conductivity from the empirical formulas. The values, based on the material within the same silt sample, are in many cases rather different. In some cases we could not perform analyses because of large content of organic mass in the samples. The values of d10 and d60 seem to be the most critical entry for the empirical formulas so it will be necessary to determine them extremely carefully. These characteristics were used for simulation and computation of total recharge along the channel. We chose four simplified variants with the same geological conditions in surroundings area of channels, only water level of groundwater and in channels were modified. The results of the simulations seem to show greater impact of the silt in the case of outflow from the channels to the surroundings than the inflow into the channel from the surroundings. The results of our research are useful for channels’ maintenance program purposes. The approximate estimates of the silt depositions in 1993 and 2004 will be helpful to predict future depositions in the channels and serve a planning tool. The silting information in the channel supplemented by the hydraulic conductivity and values of total recharge through „specific discharge“ could be helpful for regulation of groundwater level in surroundings of the channel and from point of view that this territory is the most productive agriculture land of Slovakia, it seems to be very useful for plant water supplying.
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