Variability of In-Channel Sediment Storage in a River Draining Highly Erodible Areas (The Isábena, Ebro Basin)
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J Soils Sediments (2014) 14:2031–2044 DOI 10.1007/s11368-014-0957-6 ANALYSIS AND MODELLING OF SEDIMENT TRANSFER IN MEDITERRANEAN RIVER BASINS Variability of in-channel sediment storage in a river draining highly erodible areas (the Isábena, Ebro Basin) Gemma Piqué & José A. López-Tarazón & Ramon J. Batalla Received: 27 September 2013 /Accepted: 30 July 2014 /Published online: 30 August 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract winter, transferred to the main channel during spring, stored in Purpose In-channel sediment storage is a fundamental com- the river during summer and, finally, exported out of the basin ponent of a river basin’s sediment budget. Sediment remains by the autumn floods. Marked spatial variability was ob- stored until a competent flow re-suspends and transfers it served; sections located immediately downstream from the downstream. The objectives of this paper are: (1) to quantify main tributaries (i.e. mainly Villacarli) generally held larger in-channel sediment storage and its spatial and temporal dy- amounts of sediment in the bed. Runoff and sediment inputs namics in the River Isábena, a mesoscale mountainous catch- from the tributaries were the most important factors determin- ment draining highly erodible areas (badlands) in the south ing sediment storage and its spatial and temporal dynamics. central Pyrenees (Ebro basin) and (2) to analyse changes in The overall sediment yield of the Isábena was much higher storage in the mainstem channel in relation to sediment yield than the in-channel sediment storage, despite the large from the main tributaries. amounts stored in the channel. Materials and methods In-channel sediment storage was mea- Conclusions This finding corroborates a previous published sured seasonally (from winter 2011 to winter 2012) at 14 hypothesis that fine sediment in the drainage network has a mainstem cross-sections using a re-suspension cylinder. A mean residence time of the order of 1 year and that the basin’s minimum of three locations were sampled at each section, delivery ratio exceeds 90 %; both of these characteristics can and two levels of agitation were applied. Samples allowed be related to the high connectivity between production areas determination of the amount of sediment accumulated per unit (badlands) and the river network, and to the role of baseflows surface area at a given point in the river; estimates of the total allowing continuous export of sediment from the catchment. storage in the bed of the mainstem Isábena were derived from these data. In addition, main five tributaries were monitored Keywords Badlands . In-channel sediment storage . River for discharge and suspended sediment transport. Results and discussion Results show an annual sedimentary Isábena Storage variability Suspended sediment cycle, with the sediment being produced in badlands during Responsible editor: Axel Bronstert 1 Introduction G. Piqué (*) : R. J. Batalla Catalan Institute for Water Research, 17003 Girona, Catalonia, Spain In-channel sediment storage is an essential component of the e-mail: [email protected] sediment budget of basins (Dietrich and Dunne 1978; Trimble : : 1983). The amount of sediment stored in the drainage network G. Piqué J. A. López-Tarazón R. J. Batalla varies greatly, from rivers in which it constitutes a relatively Fluvial Dynamics Research Group, Department of Environment and Soil Sciences, University of Lleida, 25198 Lleida, Catalonia, Spain small fraction of the total basin sediment yield to others where it can be of similar or higher magnitude to the sediment export J. A. López-Tarazón (e.g. Walling 1983; Meade et al. 1990). Whatever the case, its Liverpool John Moores University, Liverpool L3 3AF, UK storage and episodic transfer downstream exert an important ’ R. J. Batalla control on both the temporal variability of the river sloadand Forest Science Centre of Catalonia, 25280 Solsona, Catalonia, Spain the instantaneous magnitude of sediment being transported. 2032 J Soils Sediments (2014) 14:2031–2044 Bed sediment is mobilised during floods and transported authors have paid attention to in-channel sediment storage, downstream; in periods between floods, water is generally not its magnitude and dynamics, using different methods to sufficiently competent to transfer sediment so the material is quantify the amounts of material involved. Lambert and deposited on the riverbed surface (e.g. Wilson et al. 2004)and Walling (1988) developed a technique consisting of a metal within the gravel matrix (mainly silt and clay) (Walling et al. cylinder that allowed quantification of the amount of sediment 1998). Thus, not all the available sediment (i.e. material stored in the bed at a given point and time. Recently, Gartner produced by erosion and transferred to the drainage network) et al. (2012) used radionuclides to measure the residence time reaches the outlet immediately (Reid and Dunne 1996; of stored fine sediment. Seasonal patterns of in-channel bed Walling et al. 1998). Once supplied from the catchment, the accumulation have been also described by different workers. transport capacity of the flow and channel morphology control For instance, Duijsings (1986) described a seasonal trend in the distribution of sediment storage along the channel (Miller the River Schrondweilerbaach (Luxembourg), with dominant and Shoemaker 1986; Pryor et al. 2011). Stored sediment can storage in winter and re-mobilisation in summer, whereas remain from days to millennia, depending on the magnitude higher accumulations were found during spring months. and frequency of competent floods (i.e. as a function of the Furthermore, Collins and Walling (2007) described temporal relations between flow intensity to particle calibre and bed storage variations in two rivers of the UK (the Frome and the sedimentary structure). Suspended sediment concentrations Piddle). However, the dearth of studies characterising fine increase on the rising limb of flood hydrographs with flood sediment storage is still evident from the literature. Within intensity also entraining larger sediment and, if available, this context, results in the River Isábena—a highly dynamic releasing large amounts of sediments from the bed subsurface. mesoscale catchment representative of the southern Pyrenean In particular, gravel-bedded rivers show marked spatial and region—offers an opportunity to highlight the importance of temporal variability of sediment storage, both in longitudinal channel storage, illustrating the fundamental link between and transversal directions (Adams and Beschta 1980). The sources, transfers and yields of sediment in river basins. riffle-pool structure found in such rivers implies continuous Basic water and sediment budgets for the River Isábena were variation in flow depth and water velocity (depending on local established by López-Tarazón et al. (2012)fromsummer2007 morphology and slope) that ultimately controls fine sediment to spring 2009. Later, a quantitative assessment of in-channel accumulation. When flood discharges are competent enough sediment storage and its relations with suspended sediment to mobilise both surface and subsurface material, the riverbed was presented by López-Tarazón et al. (2011). This study was acts as a sediment source (Diplas and Parker 1992). carried out at four locations close to the basin’soutlet(ina4- The sediment budgets of river basins are conceptualised as km river stretch) for the period 2007–2008. black-boxes in which inputs from sediment sources are related Building on these studies, and by following a similar (both in absolute terms, i.e. tonnes eroded vs. tonnes exported, approach to that of López-Tarazón et al. (2011), the and relative terms, i.e. delivery ratio) to the sediment yield at current paper aims to further develop knowledge of in- the outlet. However, generally, little or no attention has been channel fine sediment dynamics in the River Isábena. devoted to the transfer sequence through the drainage net- Sampling was thus extended to a larger number of work. Results of fine sediment stored in the channels are, sampling sites, whereas sub-basins were simultaneously however, particularly important since storage may play a monitored for runoff and suspended sediment transport. critical role in controlling key aspects of the physical and even The novelty of this work in relation to previous studies biological functioning of a river. It may, for instance, control in the Isábena and beyond relies on: (1) the analysis of the temporal variability and magnitude of the suspended sed- the temporal and spatial variability of sediment stocks iment concentrations and load at a given river reach and for in the riverbed; and (2) the interpretation of these re- particular flow conditions. It may also affect channel form and sults in light of the water and sediment contributions roughness, bank stability, bedload entrainment and porosity of from the main sub-catchments. Specific objectives of the the riverbed, amongst others. The amount of sediment stored paper are thus to: (1) determine the spatial and temporal in a given channel is a direct reflection of the degree of sediment storage along the whole mainstem of the connectivity between sources of particulate material and the Isábena River and (2) assess the influence of the main fluvial network. Furthermore, in-channel storage has impor- tributaries on the magnitude and variability of the in- tant effects on the composition, richness, distribution and channel sediment storage. The first objective