RESEARCH OF SEDIMENT TRANSPORT AND DEPOSITION IN THE WATERSHED –RIVER SYSTEM ON THE BASE OF TRACER TECHNIQUES

Valentin N. GOLOSOV

The Laboratory of Soil Erosion and Fluvial Processes, Faculty of Geography,
Moscow State University, GSP-2, Moscow, 119992, Russia,
e-mail golosov@river.geogr.msu.su

INTRODUCTION

Quantitative assessment of sediment redistribution within the river system is the base for elaboration of soil and water conservation programs. Famous Russian scientist V.V. Dokuchaev was the first who formulated given statement at the beginning of XX century. The most complicated task is to evaluate the sediment input from the different parts of the basin for different time intervals as well as for extreme events, which are the most essentially influenced on the sediment transfer within the basins of different scales, and to define the sediment sources for the large river basin. It is already well-known that increasing of basin area promote to decreasing the influence of sediment from the slope on sediment transport within the main river channel. The reason of that is redeposition of sediment along the pathway from the slopes to the small rivers and further to the large rivers. Quantitative assessment of soil/sediment loss and sediment deposition within different locations along pathway from cultivated slope to the large river channel or reservoir allows evaluating the sediment budget for study basin. Application of radionuclides tracers for assessment of soil loss/gain is the one of the best way for solution given task. In addition some other tracers are used for evaluation of sediment loss/gain from the different parts of the fluvial system.

¹³⁷Cs, ⁷Be and ²¹⁰Pb_ex are the most widely used radionuclides in erosion study. Caesium-137 is a man-made radionuclide with a half-life of 30.2 years, which was release into stratosphere by the atmospheric testing of thermonuclear weapons during the period 1954-1963. Additional release of ¹³⁷Cs into atmosphere after Chernobyl disaster in 1986 promote to inputs of ¹³⁷Cs fallout over large areas of Europe and adjacent regions. Beryllium-7 has a very short half-life (53 days). ⁷Be is produced by the bombardment of the Earth’s atmosphere by cosmic rays and is afterwards deposited as fallout. Lead-210 is a naturally occurring product of the ²³⁸U decay series (half-life 22.2 years), that is derived from the decay of gaseous ²²²Rn, the daughter of ²²⁶Ra. Radium-226 exists naturally in soils and parent rocks. Diffusion of a small proportion of the ²²²Rn from the soil introduces lead-210 into atmosphere and its subsequent fallout provides an input of these radionuclides to surface soils and sediments which is not in equilibrium with its parent ²²⁶Ra. The amount unsupported ²¹⁰Pb in sediment sample can be calculated by measuring both lead-210 and ²²⁶Ra and subtracting the supported or in situ component.

Several directions of application of radionuclide tracers in the sediment study can be distinguished:

· Evaluation of soil loss/gain from cultivated and uncultivated slopes and slope catchments;

· Determination of sedimentation rate within dry valley bottoms, floodplains and river deltas;

· Study of sedimentation rates in ponds, lakes, reservoirs and other water bodies;

· Assessment of sediment budget for small catchment;

· Identification of sediment sources within the basin

Evaluation of soil loss/gain from cultivated and uncultivated slopes

Numerous investigations on application of different radionuclides for assessment of erosion and deposition rates were done during last decade in different part of the world (Wallbrink, & Murray, 1999; Montgomery et al., 1997; Turnage et al, 1997; Bajracharya et al., 1998; Bernard et al., 1998; Blake et al., 1999; Walling et al., 1999; Basher, 2000; Fulajtar, 2003; Belyaev et al., 2004 etc.). They demonstrate that ¹³⁷Cs can be successfully used for evaluation of soil loss/gain in case of application technique according guidelines (Handbook.., 2002). Detail maps of soil redistribution on cultivated slope or slope catchment can be composed on the base one single field visit. Given maps can be used for calculation of net losses from cultivated area. Results of evaluation of sediment redistribution within slope catchment with known history of cultivation are presented in tab. 1. It was found the proportion of eroded area and deposition area within the catchment. Finally total denudation rates were calculated. The results of such studies also can be used for elaboration of conservation practices for the slope of different configuration. ²¹⁰Pb_ex and ⁷Be are also used for evaluation mean soil loss/gain for about 100 years and one event correspondingly. However they are not used so widely because of the techniques should be verified for different landscape zones.

It was found that high variability of initial fallout of bomb-derived ¹³⁷Cs in areas with high spatial variability of dominant type of precipitation is the most essential limitation in application of given radionuclide in this particular areas (Bajracharya et al., 1998). Area receiving additional small input of Chernobyl-derived ¹³⁷Cs also has some problems with interpretation of results because of considerable lag between last essential input of bomb-derived fallout and Chernobyl input (Higgitt et al., 1993). Areas with high input of Chernobyl-derived ¹³⁷Cs are usually characterized by relatively low random variability, but spatial ¹³⁷Cs variability is corresponded with trend of initial ¹³⁷Cs fallout (Golosov et. al., 1999). Also it is still need to evaluate random and spatial variability of initial fallout of ²¹⁰Pb_ex in different environment. Available information about variability of initial fallout of ²¹⁰Pb_exdemonstrates that it is higher than variability ¹³⁷Cs.

Table 1 Average soil redistribution rates for the case study arable slope hollow catchment estimated by different methods (Belyev et al., 2005a)

Method	¹³⁷Cs		Soil-morphological	USLE
Method	proportional model	mass-balance model	Soil-morphological	USLE
Period	1954-2002	1954-2002	1864-2002	1864-2002
Eroded area (ha)	9.0	9.7	10.5	16.9
Average erosion rate (t ha^-1 year^-1)	13.7	24.1	8.7	11.6
Volume eroded (t year^-1)	123	233	91	196
Volume eroded over the period (t)	5918	11222	12613	27048
Total eroded layer (mm)	44	77	80	106
Depositional area (ha)	0.9	1.0	7.7	-
Average deposition rate (t ha-1 year-1)	6.4	11.8	12.6	-
Volume deposited (t year-1)	6	11	97	-
Volume deposited over the period (t)	278	566	13386	-
Total deposited layer (mm)	20	38	115	-
Total slope catchment area (ha)	18.7
Average net erosion rate (t ha^-1 year^-1)	6.5	12.2	2.2	-
Average annual sediment export (t year^-1)	117	222	40	-
Sediment export over the period (t)	5640	10655	5615	-
Total denudation layer (mm)	21	39	20	-

Figure 1. Change of erosion rates along the slope, established by different methods for different time intervals. Legend: 1 – soil morphological method (entire period of cultivation); 2 – erosion models; 3 – direct measurement of rills after the rain-storm; 4 – ¹³⁷Cs technique (for one event);
5 – ¹³⁷Cs technique (for last 17 years) (Kuznetsova et al., 2007)

Detail study of sediment redistribution on the cultivated slope located in the Zusha river basin (Central Russia) demonstrates that application ¹³⁷Cs techniques simultaneously with other approaches allows to avoid possible errors in assessment of soil losses (Kuznetsova et al.., 2007). For example, in given case it is more likely, that overestimation of soil losses on the base of ¹³⁷Cs technique directly connects with losses of part Chernobyl-derived ¹³⁷Cs during late spring and summer 1986 before the first cultivation, because of erosion event occurred on given slope. According of meteorological data several rain-storms with layer more 10 mm were observed in area around study slope.

Determination of sedimentation rate within dry valley bottoms, floodplains and river deltas

Radionuclide markers are very useful for determination of deposition rates in different localities within the fluvial system (Golosov, 1998; Walling, & He, 1999; Saxena et al., 2002; Owens et al., 2003; Ritchie et al., 2004; Linnik et al., 2005 etc.). Detail profile of vertical distribution of ¹³⁷Cs in areas with bomb-derived and Chernobyl-derived ¹³⁷Cs fallout can be used for assessment dynamic of sediment storage for the last 50 years on the territory of Europe, contaminated in April-May 1986. In combination with application ²¹⁰Pb_ex as a marker for sediment dating it is possible to evaluate the influences of changes in agriculture for the last 100 years on the basin sedimentation. Changes of concentration of radionuclides along the vertical profiles in combination with description of sediment allow to suggest the main sources of sediments. It is possible to evaluate the deposition rates spatial distribution if to take bulk samples from different localities within floodplain. However in given it is necessary to assess the initial fallout value in the reference location not so far from study area.

Detail study of sediment deposition in dry valley located in the Vorozhba River basin, tributary of the Seim River basin (Kursk region) was undertaken. One of the tasks of study was to evaluate the differences of deposition rate along the dry valley during different time intervals. Dry valley has mostly cultivated watershed with convex slopes 100-800 m length. Several pits with depth 150-200 cm were dug on the distance 50-200 m from each other from the top of valley bottom (after junction of two large slope hollows) and incremental samples for radionuclide analysis were taken with interval 3 cm for the top 30 cm and 5 cm for the more deep layers. Detail morphological survey of the valley bottom was done using DGPS in differential regime. In addition two boreholes were made in each bottom cross-section where sampling pit was located. This was done for determination of total storage of sediment deposited during period of intensive cultivation. It was found that the highest deposition rate is observed on the dry valley bottom during 1954-1986. It is connect with beginning of application of heavy tractors and combine harvesters and increasing proportion of sugar beats in crop rotation.

Study of sedimentation rates in ponds, lakes, reservoirs and other water bodies

Radionuclide markers are widely used for evaluation sedimentation rates in different water bodies (Hasholt et al., 2000 ;Appleby.2002; Abril, 2003; Cundy et al., 2003 ; Corbett et al., 2004 etc). Because of high variation of sediment grain size in column of bottom sediment is it necessary to take this into consideration for correct calculation of vertical distribution of radionuclide. Sediment mass movements are very typical for reservoir bottoms. It can lead to serious disturbance of vertical radionuclide profile and to it’s wrong interpretation. So it is better to have at least two independent profile for one morphological unit within the bottom to be sure, that you got correct results. Interpretation of ¹³⁷Cs vertical distribution profile for the northern hemisphere allows receiving minimum two peaks connected with 1963 and 1986. However in the southern hemisphere it is possible to use 1958 as the first detectable appearance of ¹³⁷Cs (Hancock et al., 2002). 1963 peak do not exist in vertical distribution curves of ¹³⁷Cs. ²¹⁰Pb_ex vertical distribution profile should be validated using one independent tracer, because different factors can influenced very seriously of depth distribution of ²¹⁰Pb_ex . Bioturbation and mass-movements on the bottom are the main factors between others. Variability of initial fallout of radionuclide is increased with increasing of water surface of reservoir. From the other hand bank erosion processes is influenced very seriously on the depth distribution of radionulides if sampling location is situated very close to the reservoir banks.

Detail study of sediment deposition in the ponds using radionuclide markers was undertaken. Two typical ponds were selected for detail study. One of them was located in Timiryazevsky park on the north of Moscow, and the other was located in agricultural area on the south of Moscow region. Comparison of ¹³⁷Cs and ²¹⁰Pb_ex vertical profiles for one pond located within Moscow park area and the other located in agricultural area of northern part of Srednerusskaya upland allows evaluating the differences in sedimentation rates during different time intervals. It was found that sediment deposition in the urban (Moscow) pond is very low and mostly associated with storage of organic matter. Deposition rate in agricultural pond is higher, but it is decrease during last decade, because of reduction of cultivated land.

Assessment of sediment budget for small catchment

Radionuclide techniques can be used for evaluation of sediment budget for relatively small catchments (Owens et al., 1997; Golosov et al., 1999; Bernard & Laverdière, 2000; Panin et al., 2001; Walling, et al., 2002, 2003; Belyaev et al., 2004a, 2005a,b etc). Study catchment should be divided on morphological elements (units) with relatively uniform relationship between erosion and deposition within each of them. This may be done on the base of detail large scale geomorphological mapping. Intensity of sediment loss/gain within each of morphological unit can be defined using one or two radionuclide tracers. In addition some independent methods should be applied for evaluation of erosion or deposition rates. Precision of sediment budget depends from accuracy of determination of area of each morphological unit and mean annual erosion or deposition rates within it. In addition detail information about changes of land use and crop rotation for study catchment should be collected. Given data should be taken into consideration for assessment of sediment input/output for each morphological unit.

Sediment budget was evaluated for different time intervals using different methods for small cultivated catchment in Tver’ region, the Osuga river basin (tab. 2).

Application of different methods, including ¹³⁷Cs and ²¹⁰Pb_ex techniques allowed to define the differences in sediment transfer during entire period of cultivation (300 years), last century and the second half of last century. Evaluation of soil losses was done for slope with different configuration. Highest erosion rate was established on the base of using ¹³⁷Cs technique. It is likely that it is connect with high intensity of tillage erosion in the second half of XX century because of wide use of tractors for cultivation. Increasing of surface runoff after beginning of use heavy machinery in the same time is the other reason of increasing of soil loss during second half of XX century. At least it is possible that part of ¹³⁷Cs was washed out immediately after fallout before the first cultivation because of erosion event. Hence application of ¹³⁷Cs technique can lead to overestimation of soil losses.

Table 2. Summary of extrapolations of soil and sediment redistribution characteristics obtained from different methods for the entire studied subcatchment area (Belyaev et al., 2004a)

Method	Soil-morphological	USLE-based model		¹³⁷Cs	²¹⁰Pb_ex
Period (year)	300	300	48	48	100
Erosion (t ha^-1 year^-1)	6.4	14.5	12.0	24.2	10.0
Area of erosion zones (ha/%³)	35.2/65.3	-	-	42.2/78.3	31.9/59.1
Erosion over the period (t)	67350.0	220770.3	29323.4	49019.5	31914.0
Within-slope redeposition (t ha^-1 year^-1)	6.7	-	-	6.4	5.1
Area of within-slope redeposition zones (ha/%)	13.0/24.1	-	-	8.6/16.0	16.1/29.8
Within-slope redeposition over the period (t)	26190.0	-	-	2640.0	8211.0
Sediment yield from slopes over the period (t)	41160.0	220770.3	29323.4	46379.5	23703.0
Sediment delivery ratio (t)	61	-	-	95	74.3
Deposition in the valley bottom (t ha^-1 year^-1)	21.2	21.2	15.7		22.5
Area of the valley bottom (ha/%)	3.1/5.8
Deposition in the valley bottom over the period (t)	19690.0	19690.0	2332.5		6975

Identification of sediment sources within the basin

Fingerprinting technique is widely used for identification of sediment sources within the basin (Carter et al., 2003; Motha et al., 2003; Belyaev et al., 2004b; Collins, Walling. 2004; Wallbrink, Olley. 2004 etc). Given approach is based on the suggestion, that different sediment sources are characterized by different physical and chemical properties, including concentration of radionuclides. It is assumed relatively uniform fallout of radionuclides across the study catchment or it is necessary to define the trend of initial fallout that should be taken into consideration for elaboration of sampling program. Statistically sufficient number of samples should be taken from each possible sources area. In addition sufficient volume of suspended sediment should be collected for analysis. Sediment mixing model should be applied for providing quantitative sediment sources ascription (Walling & Collins, 2000).

This approach is very useful for area where several possible sediment sources are observed. Investigation in the upper Kaleya River basin (Zambia) is an excellent example of application of fingerprinting approach. Four potential sediment sources types was identified within the Kaleya basin: surface soils beneath each of communal and commercial cultivation and bush grazing and eroded banks of channel and gullies (Walling et al., 2001). 20 representative samples were taken from each of potential source types. Bulk suspended sediment samples were collected from the outlet of the study basin. All samples were analyzed for a very wide range of fingerprint properties, including between others radionuclides (²¹⁰Pb_ex, ¹³⁷Cs, ²²⁶Ra). In the result of application of multivariate mixing model load-weighted mean source type contribution was determined (fig.2).

Figure 2. Local-weighted mean relative contribution from main sediment source type (the upper Kaleya River, Zambia, Walling et al., 2001),. Legend: 1 – Surface soil under communal cultivation; 2 – channel banks and gullies; 3 – surface soils under bush grazing; 4 – surface soil under commercial cultivation.

ACKNOWLEDGEMENTS

This research was undertaken under the financial support of the Russian Foundation for Basic Research (RFBR grant no. 07-05-00193) and President of Russian Federation program for support of leading scientific schools (grant no. NI-4884.2006.5)

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