Effects of sediment and contaminated sediment on structural and functional components of experimental stream ecosystems

Three experimental stream ecosystems were used to determine the effects of sediment and contaminated sediment: one stream received 1.7 g L^?1 uncontaminated sediment for 2 hr each week for 6 wk; one stream received 1.7 g L^?1 contaminated sediment (50 to 1600 ppm triphenyl phosphate applied in increasing doses each week) for 2 hr each week for 6 wk; and the third stream was maintained as a control. Each stream was monitored for changes in nutrient dynamics, leaf decomposition, primary production, and invertebrate dynamics. Both sediment and sediment/triphenyl phosphate altered the drift dynamics of benthic invertebrates. Invertebrates in the sediment treatment exhibited delayed nocturnal drift, while those in the sediment/triphenyl phosphate treatment drifted immediately once a threshold of toxicity was reached. Both sediment and sediment/triphenyl phosphate decreased the percent similarity of benthic invertebrates, reduced the drift of filamentous algae, increased the production of rooted flora, and increased net nutrient retention. However, neither treatment altered leaf decomposition rates, nor affected benthic invertebrate dynamics (total number, number of species, or diversity) or insect emergence.     

草本植被覆盖对坡面降雨径流侵蚀影响的试验研究

通过野外人工模拟降雨试验,研究了草本植被覆盖对坡面降雨径流侵蚀的影响,并从径流侵蚀功率和降雨侵蚀力两个方面对比分析了草本植被对坡面侵蚀动力的调控效果,结果表明草本植被覆盖深刻影响降雨侵蚀动力,并最终对坡面径流侵蚀量产生较大的影响。植被覆盖度为0%～60%时,产流产沙量随植被覆盖度的增加迅速降低,植被覆盖度>80%时,覆盖度的增加不能引起产流、产沙量的大幅度下降,植被水沙调控作用趋于稳定,确定本研究的临界植被覆盖度为60%～80%;以径流深和洪峰流量模数表示的坡面径流侵蚀功率以及降雨侵蚀力等侵蚀动力指标均与侵蚀产沙量呈正相关关系,但径流侵蚀功率与产沙量具有更强的相关性,说明径流侵蚀功率能更好地模拟侵蚀动力;以径流侵蚀功率/侵蚀量表示植被覆盖度对侵蚀结果的影响,反映了临界植被覆盖度的存在,可以作为评价植被侵蚀动力调控效应的一个指标。     

Toward generalizable sediment fingerprinting with tracers that are conservative and nonconservative over sediment routing timescales

Purpose

The science of sediment fingerprinting has been evolving rapidly over the past decade and is well poised to improve our understanding, not only of sediment sources, but also the routing of sediment through watersheds. Here, we discuss channel–floodplain processes that may convolute or modify the sediment fingerprinting signature of alluvial bank/floodplain sources and explore the use of nonconservative tracers for differentiating sediment derived from surface soil erosion from that of near-channel fluvial erosion.

Materials and methods

We use a mathematical model to demonstrate the theoretical effects of channel–floodplain exchange on conservative and nonconservative tracers. Then, we present flow, sediment gauging data, and geochemical measurements of long- (meteoric beryllium-10, ¹⁰Be) and short-lived (excess lead-210 and cesium-137, ²¹⁰Pb_ex and ¹³⁷Cs, respectively) radionuclide tracers from two study locations: one above, and the other below, a rapidly incising knick zone within the Maple River watershed, southern Minnesota.

Results and discussion

We demonstrate that measurements of ¹⁰Be, ²¹⁰Pb_ex, and ¹³⁷Cs associated with suspended sediment can be used to distinguish between the three primary sediment sources (agricultural uplands, bluffs, and banks) and estimate channel–floodplain exchange. We observe how the sediment sources systematically vary by location and change over the course of a single storm hydrograph. While sediment dynamics for any given event are not necessarily indicative of longer-term trends, the results are consistent with our geomorphic understanding of the system and longer-term observations of sediment dynamics. We advocate for future sediment fingerprinting studies to develop a geomorphic rationale to explain the distribution of the fingerprinting properties for any given study area, with the intent of developing a more generalizable, process-based fingerprinting approach.

Conclusions

We show that measurements of conservative and nonconservative tracers (e.g., long- and short-lived radionuclides) can provide spatially integrated, yet temporally discrete, insights to constrain sediment sources and channel–floodplain exchange at the river network-scale. Fingerprinting that utilizes nonconservative tracers requires that the nonconservative behavior is predictable and verifiable.     

Anthropogenic activities drive the microbial community and its function in urban river sediment

Journal of Soils and Sediments - Understanding the dynamics of the structure and function of the microbial community in sediment across freshwater environments will help to predict how these...     

Fine-sediment dynamics: towards an improved understanding of sediment erosion and transport

Purpose

Using Ells River, Alberta, Canada bed sediments, this study aims to determine (1) the erosion, transport, and deposition characteristics of cohesive bottom sediments, and (2) the influence of the microbial community in this regard.

Materials and methods

A 2-m annular flume was used to generate bed shear to assess cohesive sediment dynamics for eroded beds with consolidation/biostabilization periods of 1, 3, and 7 days. Additional optical particle sizing, image analysis, densitometry, and microbial analysis were employed to further the analysis with respect to bed erosion and eroded floc characteristics.

Results and discussion

Sediment dynamics can influence the benthic and planktonic community health within aquatic systems. The critical bed shear stress for erosion increased from 0.05 to 0.19 Pa (for 1- to 7-day runs). Consolidation (dry density) increased with time and depth and eroded biofilm biomass was observed to increase with time. The community structure of the eroded sediment did not change with time suggesting a stable well-established and highly selected community. Hydrocarbon-degrading bacteria were present within the microbial consortium. The sediment was highly hydrophobic (96 %) due to a high natural oil content which likely had a profound effect on sediment dynamics, flocculation, and sediment cohesion. Eroded sediment settled poorly, which will result in the long-range transport of associated contaminants.

Conclusions

The Ells River possesses some unique properties which should be considered when assessing contaminant source, fate, and effect. The most significant of these are small floc size, the hydrophobicity of the sediment, and the biological community as these were found to be influential in both the erosion and flocculation processes. It is important that any management strategies and operational assessments of reclamation strategies that may have implication on river health incorporate the sediment compartments (SS and bed sediment), biology, and the energy dynamics within the system in order to better predict the downstream flux of sediments.

     

Modeling of hydrophobic cohesive sediment transport in the Ells River Alberta,Canada

Purpose

In this paper, a novel modeling approach is applied to assess the unique transport characteristics of hydrophobic (bitumen containing) cohesive sediment for the Ells River, AB, Canada. The modeling offers a new way of treating the transport and fate of fine sediment in rivers and points to the importance of including a sediment entrapment process in the modeling of the Ells River sediment dynamics.

Materials and methods

The modeling approach involves combining two existing models (RIVFLOC and MOBED). Using fine sediment transport parameters derived from laboratory flume experiments (e.g., settling velocity of sediment as a function of floc size and the critical shear stresses for deposition) and the calculated flow field from the MOBED model (using field survey data such as, cross-sectional geometry, river slope, grain size of bed material, and discharge), the RIVFLOC model is used to predict the transport characteristics (including entrapment) of the hydrophobic Ells River sediment.

Results and discussion

The application of the connected RIVFLOC and MOBED models, demonstrated the unique hydrophobic sediment dynamics of the Ells River. The model showed no deposition (in the classical sense) of the hydrophobic sediment as the bed shear stresses, even at base flow, are well above the critical bed shear for deposition (flocculation is shown to occur, but its impact on settling is negligible given the high shear stresses). However, the model showed the possibility of fine sediment ingression into the river bed (interstitial voids) due to the entrapment process which is known to occur at bed shear stresses well above the critical shear stress for deposition.

Conclusions

The salient features of RIVFLOC and MOBED models and their applications for understanding the transport and fate of unique hydrophobic fine sediments are presented. The models are shown to be useful for the understanding and projection of flow characteristics and sediment dynamics (including entrapment), and will be of benefit for the adaptive management of riverine monitoring programs given various flow scenarios including extreme events and climate change.

     

裸露耕地土壤风蚀跃移颗粒分布特征的试验研究

以内蒙古阴山北部的典型农牧交错带耕地为试验区，利用DEM6型轻便三杯风向风速表和LV50便携式风速仪(可测瞬时风速和任一时段的平均风速)，采用SC—I型沙尘采集器收集跃移沙粒，并对收集到的沙粒进行筛分、称重，详细地分析了不同高度风蚀物百分含量、输沙量等风沙运动主要影响因素的变化特征，得出了耕地土壤在风沙跃移运动中的粒径分布特征，初步了解了该地区土壤风蚀的特点，为今后风沙运动力学理论的深入研究，以及实施保护性耕作和其他土地荒漠化的防治提供依据。     

Comparison of sampling designs for sediment source fingerprinting in an agricultural watershed in Atlantic Canada

Purpose

Sediment fingerprinting is increasingly being used to improve the understanding of sediment dynamics within the critical zone and provide information that can help guide management decisions at the watershed scale. The objectives of this study were to investigate both the implications of different sediment fingerprinting sampling designs and spatial scales on the characterization of sediment dynamics in a predominantly agricultural watershed in northwestern New Brunswick, Canada.

Materials and methods

Color and radionuclide fingerprints were used to discriminate between three potential sediment sources: agricultural topsoil, agricultural streambanks, and forested areas (topsoil and streambanks). Suspended sediment was collected seasonally, between 2008 and 2014, at five sites with drainage areas ranging from 3.0 to 13.4 km². Using the same source and sediment data set, multiple-, nested-, and local-location fingerprinting sampling designs were employed to investigate the influence of scale of observation, geomorphic connectivity, land use, and the heterogeneity of source fingerprints on apportionment results.

Results and discussion

Sediment collected in the headwaters was primarily derived from forested areas while the sediment collected at the outlet of the watershed was primarily from agricultural topsoil. When comparing the multiple- and nested-location designs, it was found that accounting for the spatial variability in the fingerprint properties of each source had a small difference in the sediment apportionment results. Furthermore, the local-location design demonstrated that the sediment collected at each location was composed of predominately local sources as opposed to upstream sediment entering the local catchment.

Conclusions

Assessment of the sources of sediment at a range of spatial scales better accounts for both geomorphic connectivity and differences in land use throughout the watershed. Overall, each of the three fingerprinting sampling designs provided different information that can be used to guide soil and water conservation management.

     

Tracing sediment dynamics and sources in eroding rills with rare earth elements

Eroding rills evolve morphologically in time and space. Most current studies on rill erosion use spatially averaged soil erosion data, providing little information on soil erosion dynamics. A method is proposed to use rare earth elements (REEs) to trace sediment distribution in eroding rills. Laboratory flume simulation experiments were conducted at three flow rates (2, 4 and 8 litres minute^?1) and five slope gradients (5, 10, 15, 20 and 25°) with three replicates of each treatment. The rills, of 8 m length, were subdivided into 10 equal segments of 0.8 m length and 0.1 m width, with a different REE applied to each segment. We derived computational formulae for estimating the distribution of eroded amounts along the rills. The actual erosion distribution along rills was then estimated with the data from the experiments. The precision of the REEs for tracing rill erosion was analysed. The results showed that sediment concentration increased with rill length, but the increased rate (the slope of the curve) flattened gradually. Sediment yields increased with slope gradients and flow rates, but the slope gradients had a greater effect on sediment concentration than flow rates, and greater flow rates caused more rill erosion and soil loss under the same slope gradient. The results also demonstrated the feasibility of using REEs to trace the dynamic processes of rill erosion.     

Process-based modelling of erosion,sediment transport and reservoir siltation in mesoscale semi-arid catchments

Purpose

To support scientifically sound water management in dryland environments a modelling system has been developed for the quantitative assessment of water and sediment fluxes in catchments, transport in the river system, and retention in reservoirs. The spatial scale of interest is the mesoscale because this is the scale most relevant for management of water and land resources.

Materials and methods

This modelling system comprises process-oriented hydrological components tailored for dryland characteristics coupled with components comprising hillslope erosion, sediment transport and reservoir deposition processes. The spatial discretization is hierarchically designed according to a multi-scale concept to account for particular relevant process scales. The non-linear and partly intermittent run-off generation and sediment dynamics are dealt with by accounting for connectivity phenomena at the intersections of landscape compartments. The modelling system has been developed by means of data from nested research catchments in NE-Spain and in NE-Brazil.

Results and discussion

In the semi-arid NE of Brazil sediment retention along the topography is the main process for sediment retention at all scales, i.e. the sediment delivery is transport limited. This kind of deposition retains roughly 50 to 60 % of eroded sediment, maintaining a similar deposition proportion in all spatial scales investigated. On the other hand, the sediment retained in reservoirs is clearly related to the scale, increasing with catchment area. With increasing area, there are more reservoirs, increasing the possibility of deposition. Furthermore, the area increase also promotes an increase in flow volume, favouring the construction of larger reservoirs, which generally overflow less frequently and retain higher sediment fractions. The second example comprises a highly dynamic Mediterranean catchment in NE-Spain with nested sub-catchments and reveals the full dynamics of hydrological, erosion and deposition features. The run-off modelling performed well with only some overestimation during low-flow periods due to the neglect of water losses along the river. The simulated peaks in sediment flux are reproduced well, while low-flow sediment transport is less well captured, due to the disregard of sediment remobilization in the riverbed during low flow.

Conclusions

This combined observation and modelling study deepened the understanding of hydro-sedimentological systems characterized by flashy run-off generation and by erosion and sediment transport pulses through the different landscape compartments. The connectivity between the different landscape compartments plays a very relevant role, regarding both the total mass of water and sediment transport and the transport time through the catchment.     

Modelling sediment transport from bare rilled hillslopes by areally averaged transport equations

Treating the dynamics of sediment transport as two-dimensional on interrill-areas and as one-dimensional in rill sections, areally averaged sheet sediment transport equations are developed. The two-dimensional sheet sediment transport equation is averaged over an individual interrill-area width and then along the interrill-area length to obtain local-scale areally averaged interrill-area sheet sediment transport equation (local-scale areal averaging). Similarly, the cross-sectionally-averaged rill sediment transport equation is averaged along an individual rill length to obtain local-scale areally averaged rill sediment transport equation (local-scale areal averaging). In order to minimize computational effort and economize on the number of model parameters, the local-scale areally averaged equations are then averaged over a whole hillslope section (large-scale areal averaging). These equations constitute the areally averaged model. The expectations of the terms containing more than one variable are obtained by the method of regular perturbation. In the large-scale areal averaging it is assumed that all the randomness in the state variable is due to the randomness in the parameters of the process. Comparison of the results obtained from the areally averaged model with those of the point-scale model indicates that the areally averaged model uses far less data and yet it performs as well as the point-scale model. The results of the developed model indicate that on a rilled-surface most of the sediment loads comes from rill sections. The developed model is successfully tested against experimental data obtained from a bare rilled hillslope. It predicted measured runoff and sediment rates with mean absolute errors of 11.07 l/min and 0.382 kg/s, respectively.     

Coupling hysteresis analysis with sediment and hydrological connectivity in three agricultural catchments in Navarre,Spain

Purpose

Rain storm events mobilise large proportions of fine sediments in catchment systems. Sediments from agricultural catchments are often adsorbed by nutrients, heavy metals and other (in)organic pollutants that may impact downstream environments. To mitigate erosion, sediment transport and associated pollutant transport, it is crucial to know the origin of the sediment that is found in the drainage system, and therefore, it is important to understand catchment sediment dynamics throughout the continuity of runoff events.

Materials and methods

To assess the impact of the state of a catchment on the transport of fine suspended sediment to catchment outlets, an algorithm has been developed which classifies rain storm events into simple (clockwise, counter-clockwise) and compound (figure-of-eight; complex) events. This algorithm is the first tool that uses all available discharge and suspended sediment data and analyses these data automatically. A total of 797 runoff events from three experimental watersheds in Navarre (Spain) were analysed with the help of long-term, high-resolution discharge and sediment data that was collected between 2000 and 2014.

Results and discussion

Morphological complexity and in-stream vegetation structures acted as disconnecting landscape features which caused storage of sediment along the transport cascade. The occurrence of sediment storage along transport paths was therefore responsible for clockwise hysteresis due to the availability of in-stream sediment which could cause the “first flush” affect. Conversely, the catchment with steeper channel gradients and a lower stream density showed much more counter-clockwise hysteresis due to better downstream and lateral surface hydrological connectivity. In this research, hydrological connectivity is defined as the actual and potential transfer paths in a catchment. The classification of event SSC-Q hysteresis provided a seasonal benchmark value to which catchment managers can compare runoff events in order to understand the origin and locations of suspended sediment in the catchment.

Conclusions

A new algorithm uses all available discharge and suspended sediment data to assess catchment sediment dynamics. From these analyses, the catchment connectivity can be assessed which is useful to develop catchment land management.

     

Evidence for transport of bedload in waves: Analysis of fluvial sediment samples in a small upland stream channel

Data pertaining to the transport of fluvial sediment sampled in detail during a five hour long event in a small upland stream channel in the extremely arid Southern Negev are presented. Considerations based on comparative evaluations of water discharge, stream power, total and fractional sediment concentration, and total and fractional sediment discharge suggest that bedload transport in channels of even very small upland catchments practically devoid of continuous alluvial cover takes place in waves, which are not a priori related to any water discharge characteristic, but rather reflect some intrinsic feature of watershed dynamics, such as slope-channel topographic contrasts or fluvial bifurcation. The regular spacing of pools and riffles and of gravel bars in the more alluvial reaches downstream may therefore be initiated from upstream.     

The impact of environmental changes on the sediment loads of Norwegian rivers

The impact of unusual events on the sediment dynamics in rivers is discussed in this paper. The increase in the number of extreme precipitation events and other unusual weather events in Norway strongly suggests that weather conditions are changing. It is difficult to monitor sediment transport during large magnitude floods. It is however well known that they may deliver substantial amounts of sediment. The suspended sediment load in the upper Atna river during the springflood in 1995 was 44 times the mean load measured during the preceding years. The recurrence interval of this flood was estimated to be 100–200 years. The conditions during the occurrence of a particular flood event may also be of considerable importance and measurements during rainfall on frozen ground have indicated unusually high runoff and erosion rates.The seasonal distribution of runoff in several parts of Norway has changed in recent years and higher winter temperatures have produced an increase in the frequency of flood events. A very long lasting flood caused by a prolonged period of rain occurred in the Vikka stream during September to December 2000. The availability of sediment increased with the increasing soil moisture content in the area. As the ground became saturated, more active slope processes caused erosion rates to increase markedly. The combined effects of climate change and human impact on sediment transport in rivers appear to enhance downstream sediment delivery. Flood protection works along river channels prevent sediment from being deposited on the floodplain. River channels that have been lowered in order to reduce groundwater levels in agricultural floodplain areas will experience the same effect. During the last decade extensive ecological changes have taken place in the coastal areas of southern Norway as 90% of the sugar kelp forest has been lost. This change has been attributed to an apparent increase in sediment delivery to the sea along with a rise in sea temperatures.     

Establishing a Holocene sediment budget for the river Dijle

A Holocene sediment budget was constructed for the 758 km² Dijle catchment in the Belgian loess belt, in order to understand long-term sediment dynamics. Hillslope sediment redistribution was calculated using soil profile information from 809 soil augerings, which was extrapolated to the entire catchment using morphometric classes. As large parts of the forests within the catchment prove to have undergone little or no erosion since medieval times, a correction was applied for the presence of forests. Total Holocene erosion amounts 817 ± 66 Mt for the catchment, of which 327 ± 34 Mt was deposited as colluvium. This corresponds with a net Holocene soil erosion rate of 10.8 ± 0.8 × 10³ Mg ha^− 1 for the entire Dijle catchment. Alluvial deposits were studied through 187 augerings spread over 17 cross-valley transects. The total alluvial sediment deposition equals 352 ± 11 Mt or 42% of total eroded sediment mass. Results indicate that at the scale of a medium-sized catchment the colluvial sediment sink is as important as the alluvial sediment sink and should not be neglected. As a result the estimation of erosion through alluvial storage and sediment export would yield large errors. Dating of sediment units show an important increase in alluvial deposition from medieval times onwards, indicating the important influence of agricultural activities that developed from that period. Mean sediment export rates from the catchment for the last 1000–1200 years range between 0.8 and 1.3 Mg ha^− 1 a^− 1 and are consistent with present suspended sediment measurements in the Dijle. Erosion for agricultural land for this period is 9.2 ± 2.2 Mg ha^− 1 a^− 1. Sediment budgets for the various tributary catchments provide an insight in the sources and sinks of sediment at different scales within the catchment.     

Estimate of sediment yield in a basin without sediment data

In this study, three mathematical models for the estimate of sediment yield, due to soil and stream erosion, at the outlet of a basin are presented. Each model consists of three submodels: a rainfall-runoff submodel, a soil erosion submodel and a sediment transport submodel for streams. The rainfall-runoff and the stream sediment transport submodels are identical in the three mathematical models. The rainfall-runoff submodel that is used for the computation of the runoff in a sub-basin is a simplified water balance model for the soil root zone. For the estimate of soil erosion in a sub-basin, three different submodels are used alternatively, owing to the fact that erosion or sediment yield data are not available. The soil erosion submodels are (a) a modified form of the classical Universal Soil Loss Equation (USLE, [Foster, G.R., Meyer, L.D., Onstad, C.A., 1977. A runoff erosivity factor and variable slope length exponents for soil loss estimates. Transactions of the ASAE, 20 (4), 683–687]) taking into account both the rainfall erosion and the runoff erosion, (b) the relationships of Poesen [Poesen, J., 1985. An improved splash transport model. Zeitschrift für Geomorphologie, 29, 193–211] quantifying the splash detachment, as well as the upslope and downslope splash transport, (c) the relationships of Schmidt [Schmidt, J., 1992. Predicting the sediment yield from agricultural land using a new soil erosion model. Proceedings of the 5th International Symposium on River Sedimentation. Karlsruhe, Germany, pp. 1045–1051] including the momentum flux exerted by the droplets and the momentum flux exerted by the runoff. The sediment transport submodel for streams aims to estimate the sediment yield at the outlet of a sub-basin. This quantity results by comparing the available sediment amount in the main stream of a sub-basin with the sediment transport capacity by stream flow, which is computed by the relationships of Yang and Stall [Yang, C.T., Stall, J.B., 1976. Applicability of unit stream power equation. Journal of the Hydraulics Division, ASCE, 102, 559–568]. The mathematical models were applied to the basin of Kompsatos River, in northeastern Greece, with an area of about 565 km². The whole basin was divided into 18 natural sub-basins for more precise calculations. Monthly rainfall data were available for 27 years (1966–1992); therefore, the calculations were performed on a monthly basis. The deviation between the three mean annual values of sediment yield at the basin outlet, for 27 years, resulting from the three mathematical models is relatively small.     

Estimate of sediment yield in a basin without sediment data

In this study, three mathematical models for the estimate of sediment yield, due to soil and stream erosion, at the outlet of a basin are presented. Each model consists of three submodels: a rainfall-runoff submodel, a soil erosion submodel and a sediment transport submodel for streams. The rainfall-runoff and the stream sediment transport submodels are identical in the three mathematical models. The rainfall-runoff submodel that is used for the computation of the runoff in a sub-basin is a simplified water balance model for the soil root zone. For the estimate of soil erosion in a sub-basin, three different submodels are used alternatively, owing to the fact that erosion or sediment yield data are not available. The soil erosion submodels are (a) a modified form of the classical Universal Soil Loss Equation (USLE, [Foster, G.R., Meyer, L.D., Onstad, C.A., 1977. A runoff erosivity factor and variable slope length exponents for soil loss estimates. Transactions of the ASAE, 20 (4), 683–687]) taking into account both the rainfall erosion and the runoff erosion, (b) the relationships of Poesen [Poesen, J., 1985. An improved splash transport model. Zeitschrift für Geomorphologie, 29, 193–211] quantifying the splash detachment, as well as the upslope and downslope splash transport, (c) the relationships of Schmidt [Schmidt, J., 1992. Predicting the sediment yield from agricultural land using a new soil erosion model. Proceedings of the 5th International Symposium on River Sedimentation. Karlsruhe, Germany, pp. 1045–1051] including the momentum flux exerted by the droplets and the momentum flux exerted by the runoff. The sediment transport submodel for streams aims to estimate the sediment yield at the outlet of a sub-basin. This quantity results by comparing the available sediment amount in the main stream of a sub-basin with the sediment transport capacity by stream flow, which is computed by the relationships of Yang and Stall [Yang, C.T., Stall, J.B., 1976. Applicability of unit stream power equation. Journal of the Hydraulics Division, ASCE, 102, 559–568]. The mathematical models were applied to the basin of Kompsatos River, in northeastern Greece, with an area of about 565 km². The whole basin was divided into 18 natural sub-basins for more precise calculations. Monthly rainfall data were available for 27 years (1966–1992); therefore, the calculations were performed on a monthly basis. The deviation between the three mean annual values of sediment yield at the basin outlet, for 27 years, resulting from the three mathematical models is relatively small.     

The impact of environmental changes on the sediment loads of Norwegian rivers

The impact of unusual events on the sediment dynamics in rivers is discussed in this paper. The increase in the number of extreme precipitation events and other unusual weather events in Norway strongly suggests that weather conditions are changing. It is difficult to monitor sediment transport during large magnitude floods. It is however well known that they may deliver substantial amounts of sediment. The suspended sediment load in the upper Atna river during the springflood in 1995 was 44 times the mean load measured during the preceding years. The recurrence interval of this flood was estimated to be 100–200 years. The conditions during the occurrence of a particular flood event may also be of considerable importance and measurements during rainfall on frozen ground have indicated unusually high runoff and erosion rates.The seasonal distribution of runoff in several parts of Norway has changed in recent years and higher winter temperatures have produced an increase in the frequency of flood events. A very long lasting flood caused by a prolonged period of rain occurred in the Vikka stream during September to December 2000. The availability of sediment increased with the increasing soil moisture content in the area. As the ground became saturated, more active slope processes caused erosion rates to increase markedly. The combined effects of climate change and human impact on sediment transport in rivers appear to enhance downstream sediment delivery. Flood protection works along river channels prevent sediment from being deposited on the floodplain. River channels that have been lowered in order to reduce groundwater levels in agricultural floodplain areas will experience the same effect. During the last decade extensive ecological changes have taken place in the coastal areas of southern Norway as 90% of the sugar kelp forest has been lost. This change has been attributed to an apparent increase in sediment delivery to the sea along with a rise in sea temperatures.     

Dominant discharges for suspended sediment transport in a highly active Pyrenean river

Purpose

Dominant discharges and associated sediment dynamics of the River Isábena, a 445-km² catchment in the central Pyrenees of Spain that is punctuated by badlands, are analysed.

Materials and methods

Calculations of suspended sediment loads are based on continuous records of discharge and turbidity obtained at the basin outlet for the period 2005–2010.

Results and discussion

Dominant discharges for sediment load (i.e. effective discharge) present a bimodal distribution, with one peak falling in the range of low flows and the other associated to less frequent but higher magnitude floods (i.e. bankfull). The highly suspended sediment availability in the badlands, together with the high connectivity between the badlands and the stream network and the important in-channel fine sediment storage, causes both large and small events to remobilize fines. Baseflows, despite their low competence, generate resuspension and massive sediment loads. Thus, effective discharge (i.e. the discharge which transports most of the sediment) is not solely associated with bankfull (i.e. the discharge that dominates channel form), but to a wider range of discharges. Consequently, this river channel is not specifically adjusted to convey most of the sediment load during high floods, as in many other rivers, but instead large volumes of sediment are transferred downstream at an almost constant rate.

Conclusions

Results suggest that dominant discharge may play a lesser role in terms of (suspended) sediment load in non-supply-limited fluvial systems and/or in rivers that permanently work close to, or at, full transport capacity, as is the case of the Isábena.