Modelling the impact of land-use change and farm dam construction on hillslope sediment delivery to rivers at the regional scale

Landscapes in southeastern Australia have changed dramatically since the spread of European colonisation in the 19th century. Due to widespread forest clearance for cultivation and grazing, erosion and sediment yields have increased by a factor of more than 150. In the 20th century, erosion and sediment yield were reduced again due to an increasing vegetative cover. Furthermore, during the last decades, thousands of small farm dams were constructed to provide drinking water for cattle. These dams trap a lot of sediment, thereby further reducing sediment delivery from hillslopes to river channels. Changes in sediment delivery since European colonisation are documented in sediment archives. Within this study, these changing rates in hillslope erosion and sediment delivery were modelled using a spatially distributed erosion and sediment delivery model (WATEM/SEDEM) that was calibrated for Australian ecosystems using sediment yield data derived from sedimentation rates in 26 small farm dams. The model was applied to the Murrumbidgee river basin (30,000 km²) under different land-use scenarios. First, the erosion and sediment yield under pre-European land-use was modelled. Secondly, recent land-use patterns were used in the model. Finally, recent land-use including the impact of farm dams and large reservoirs was simulated. The results show that the WATEM/SEDEM model is capable of predicting the intensity of the geomorphic response to changes in land-use through time. Changes in hillslope erosion and hillslope sediment delivery rates are not equal, illustrating the non-linear response of the catchment. Current hillslope sediment supply to the river channel network is predicted to be 370% higher compared to the pre-European settlement period, yet farm dams have reduced this back to 2.5 times the pre-19th century values. The role of larger reservoirs is even more important as they have reduced the current sediment supply downstream to their pre-European values, thus completely masking the increased hillslope erosion rates from land-use change. However, the model does so far not include valley widening and sediment storage in river systems. Therefore, modelled rates of sediment delivery are lower than observed values.     

Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m³/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m³/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection.     

多因素对三峡水库下游弯曲河型演变的迭加效应（英文）

Elucidating the influence of dams on fluvial processes can inform river protection and basin management.However,relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches.Using hydrological and topographical data,we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam(TGD).Our conclusions are as follows.(1)The meandering reaches can be classified into two types based on their evolution during the pre-dam period:G1 reaches,characterized by convex point bar erosion and concave channel deposition(CECD),and G2 reaches,characterized by convex point bar deposition and concave channel erosion(CDCE).Both reach types exhibited CECD features during the post-dam period.(2)Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars.However,changes in the river regime,river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches.(3)Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars.The point bars become scoured if the durations of these flows,which are close to bankfull discharge,exceed 20 days.In addition,the reduction in bedload causes the decreasing of point bar siltation in the water-falling period.(4)During the post-dam period,flood abatement,the increased duration of discharges ranging from 20,000 to 25,000 m3/s,and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features.Our results are relevant to other meandering reaches,where they can inform estimates of riverbed change,river management strategies and river protection.     

Sediment-related impacts due to upstream reservoir trapping, the Lower Mekong River

A sharp decrease in total suspended solids (TSS) concentration has occurred in the Mekong River after the closure of the Manwan Dam in China in 1993, the first of a planned cascade of eight dams. This paper describes the upstream developments on the Mekong River, concentrating on the effects of hydropower dams and reservoirs. The reservoir-related changes in total suspended solids, suspended sediment concentration (SSC), and hydrology have been analyzed, and the impacts of such possible changes on the Lower Mekong Basin discussed. The theoretical trapping efficiency of the proposed dams has been computed and the amount of sediment to be trapped in the reservoirs estimated. The reservoir trapping of sediments and the changing of natural flow patterns will impact the countries downstream in this international river basin. Both positive and negative possible effects of such impacts have been reviewed, based on the available data from the Mekong and studies on other basins.     

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.     

蚌埠闸及上游闸坝对淮河自然水文情势的影响

淮河流域建设了许多闸坝,为揭示如此密集的闸坝对河流自然水文情势的影响,选取比较典型的淮河干流上的蚌埠闸作为控制节点,用成熟的IHA法和RVA法,研究蚌埠闸及其上游闸坝对水文情势的影响程度,同时通过蚌埠水文站水文情势变化的估算来分析闸坝对淮河河流生态水文条件的影响。结果显示,这些闸坝对河流水文情势的影响强烈,特别是在枯水季节。由此得出的淮河蚌埠段生态水文目标可为蚌埠闸开展生态系统管理、生态修复以及进行生态调控提供理论支持。     

Downstream effects of reservoirs in areas suffering from water deficiency-the case of Hutuo River, China

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Sediment storage and release from Himalayan piggyback basins and implications for downstream river morphology and evolution

Piggyback basins developed at the mountain fronts of collisional orogens can act as important, and transient, sediment stores along major river systems. It is not clear, however, how the storage and release of sediment in piggyback basins affects the sediment flux and evolution of downstream river reaches. Here, we investigate the timing and volumes of sediment storage and release in the Dehra Dun, a piggyback basin developed along the Himalayan mountain front in northwestern India. Based on OSL dating, we show evidence for three major phases of aggradation in the dun, bracketed at ca. 41–33 ka, 34–21 ka and 23–10 ka, each accompanied by progradation of sediment fans into the dun. Each of these phases was followed by backfilling and (apparently) rapid fan‐head incision, leading to abandonment of the depositional unit and a basinward shift of the active depocentre. Excavation of dun sediment after the second and third phases of aggradation produced time‐averaged sediment discharges that were ca. 1–2% of the modern suspended‐sediment discharges of the Ganga and Yamuna rivers that traverse the margins of the dun; this sediment was derived from catchment areas that together comprise 1.5% of the drainage area of these rivers. Comparison of the timing of dun storage and release with upstream and downstream records of incision and aggradation in the Ganga show that sediment storage in the dun generally coincides with periods of widespread hinterland aggradation but that late stages of dun aggradation, and especially times of dun sediment excavation, coincide with major periods of sediment export to the Ganga Basin. The dun thus acts to amplify temporal variations in hinterland sediment supply or transport capacity. This conceptual model appears to explain morphological features of other major river systems along the Himalayan front, including the Gandak and Kosi Rivers, and may be important for understanding sediment flux variations in other collisional mountain belts.     

Cellular modelling as a tool for interpreting historic braided river evolution

Results are presented from a new cellular model of braided river dynamics that simulates flow, sediment transport, morphological change and the effects of braidplain vegetation. This model is used to investigate the effect of changes in upstream sediment supply on braided river systems over simulation periods of 200 years. Modelled changes in channel morphology, associated with both aggradation and degradation, were seen to be consistent with those reported in the literature. In addition, simulation results allowed the identification of diagnostic characteristics of aggrading and degrading reaches, in the form of relationships between the age, extent and relative elevation of fluvial surfaces. Interpretation of spatial patterns of valley floor surface characteristics in the Avoca River, New Zealand, on the basis of these relationships, allowed the identification of channel reaches that appear to be experiencing either aggradation or degradation. These inferences are shown to be consistent with independent evidence of spatial patterns of sediment supply to the main valley floor, derived from aerial photographs and an existing sediment source inventory. These results illustrate the potential for using cellular models to develop an improved understanding of natural river behaviour.     

Hydrological impacts of flow regulation associated with the Upper Nepean water supply scheme,NSW

The upper Nepean River has been progressively regulated for water supply to Sydney and Wollongong since 1886 by the Upper Nepean Water Supply Scheme which consists of four large dams, two small dams and two diversion weirs. Secular rainfall changes produced periods of high rainfall and large floods (flood‐dominated regimes) between 1857 and 1900 and 1947 and the present, and an intervening period (1901–46) of low rainfall and small floods (drought‐dominated regime). Upstream impoundment and flow regulation significantly reduced flood magnitudes for most return periods during both types of flood regimes. The probability distribution of mean daily flows was also changed significantly by flow regulation such that during the drought‐dominated regime, the high and low frequency flows were reduced substantially but the moderate frequency flows were increased due to dam releases; the change from a regulated drought‐dominated regime to a regulated flood‐ dominated regime resulted in a substantial increase in discharge for most durations; and increased water diversions to Wollongong during the current flood‐dominated regime produced a marked downward shift in the whole flow duration curve. Nepean Dam reduced downstream suspended sediment yields by two orders of magnitude because it traps in excess of 99 per cent of the inflowing suspended sediment load. Streamflow releases are urgently required from the two diversion weirs to improve downstream water quality and to ensure the viability of the resident ‘potentially threatened’ eastern Macquarie perch (Macquaria nov. sp.).     

Channel Bed Mobility Downstream from the Elwha Dams,Washington*

Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross‐sections. The relationship of average boundary shear stress (τ_o) to critical shear stress (τ_cr) served as the basis for determining channel bed material mobility under the two‐year and ten‐year flood events. The channel had the greatest channel bed mobility at the natural cross‐section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run‐of‐the‐river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River.     

河流阶地形成过程及其驱动机制再研究

河流阶地的形成是在内因(河流内部动力变化)和外因(低频和高频气候变化、构造运动、基准面变化)共同作用下的结果。受单一气候变化制约的河流阶地发育模式可以解释由于沉积物通量和径流量变化引起的河流堆积-侵蚀过程,但它难以解释形成多级阶地的逐步(或间歇性)下切过程。多级阶地的形成可能同时受到构造抬升和周期性气候变化的制约。由于下切过程的滞后效应,侵蚀和冰川均衡抬升、河谷的侧向侵蚀过程等影响,山地的构造抬升与河谷的下切之间并非一种简单的线性关系,应当慎用河谷的下切速率来代表山地的抬升速率。     

黄河内蒙古头道拐断面形态变化及其对水沙的响应

根据黄河上游位于弯曲一顺直过渡段的头道拐断面的多年实测水文、泥沙数据和断面资料,分析了断面对上游一系列水利工程在时间尺度上的调整响应过程.结果发现,头道拐断面的形态变化强烈受到上游水库运行的影响,在一系列水库投入运行后,河床粗化现象一直持续到三湖河口断面以下.头道拐断面悬移质泥沙中值粒径在水库运行后表现为先增大,然后减小,最后再增大的三阶段性特征,并且自1986年龙羊峡与青铜峡、刘家峡等水库联合运用以后,该三阶段性特征的变化幅度明显较只有青铜峡、刘家峡水库单独运行时段(1969-1986年)小.另外,自1987年以来,头道拐断面河道的横向摆动速率大幅减小,断面形态及位置趋于稳定,有利于当地百姓正常的生活和生产.     

无定河流域产沙量变化的淤地坝效应分析

无定河流域1971~1989年的年均流量、悬移质含沙量及输沙率比1954~1970年的明显变小,而月均水沙过程曲线也发生了明显变化。上述水沙过程的变化受到1970年以来人类活动的强烈影响,而淤地坝建设是主因。为探讨淤地坝的减沙效应,提出淤地坝有效减沙面积这一概念,并拟合了动态变化的淤地坝有效减沙面积与年份之间的关系,发现该关系曲线与无定河流域各年代产沙量的变化情况相符。自1990年以来无定河流域淤地坝有效减沙面积呈明显递减趋势,导致了自上世纪90年代以来该流域的产沙量出现增大现象。为了抑制该流域的产沙量,势必需要加大淤地坝建设的力度。如果想使该流域的产沙量逐渐减少,则至少要使流域内年淤地坝有效减沙面积逐年增加。     

Reconceptualising coarse sediment delivery problems in rivers as catchment-scale and diffuse

This paper assesses river channel management activities in the context of the interaction between coarse sediment delivery, climate change, river channel response and flood risk. It uses two main sources of evidence: (1) an intensive instrumentation of an upland river catchment using both traditional hydrometric and novel sediment sensing methods; and (2) a sediment delivery model that combines a treatment of sediment generation from mass failure with a treatment of the connectivity of this failed material to the drainage network. The field instrumentation suggests that the precipitation events that deliver sediment from hillslopes to the drainage network are different to those that transfer sediment within the network itself. Extreme events, that could occur at any time in the year (i.e. they are not dependent on wet antecedent conditions), were crucial for sediment delivery. However, sustained high river flows were responsible for the majority of transfer within the river itself. Application of three downscaling methods to climate model predictions for the 2050s and 2080s suggested a significant increase in the number and potential volume of delivery events by the 2050s, regardless of the climate downscaling scenario used. First approximations suggested that this would translate into annual bed level aggradation rates of between 0.10 and 0.20 m per year in the downstream main channel reaches. Second, the importance of this delivery for flood risk studies was confirmed by simulating the effects of 16 months of measured in-channel simulation with river flows scaled for climate change to the 2050s and 2080s. Short-term sedimentation could result in similar magnitude increases in inundated area for 1 in 0.5 and 1 in 2 year floods to those predicted for the 2050s in relation to increases in flow magnitude. Finally, we were able to develop an alternative approach to river management in relation to coarse sediment delivery, based upon reducing the rates of coarse sediment delivery through highly localised woodland planting, under the assumption that reducing delivery rates should reduce the rate of channel migration and hence the magnitude of the bank erosion problem. Thus, the paper demonstrates the need to conceptualise local river management problems in upland river environments as point scale manifestations of a diffuse sediment delivery process, with a much more explicit focus on the catchment scale, if our river systems are to become more insulated from the impacts of future climate changes.     

黄河内蒙古头道拐断面河道变化及其对上游水库的响应(英文)

The application of dams built upstream will change the input conditions,including water and sediment,of downstream fluvial system,and destroy previous dynamic quasi-equilibrium reached by channel streamflow,so indispensable adjustments are necessary for downstream channel to adapt to the new water and sediment supply,leading the fluvial system to restore its previous equilibrium or reach a new equilibrium.Using about 50-year-long hydrological,sedimentary and cross-sectional data,temporal response processes ...     

Do river deltas in east India retreat? A case of the Krishna Delta

The construction of multiple dams and barrages in many Indian River basins over the last few decades significantly reduced river flow to the sea and affected the sediment regime. More reservoir construction is planned through the proposed National River Linking Project (NRLP), which will transfer massive amounts of water from the North to the South of India. The impacts of these developments on fertile and ecologically sensitive deltaic environments are poorly understood and quantified at present. In this paper an attempt is made to identify, locate and quantify coastal erosion and deposition processes in one of the major river basins in India—the Krishna—using a time series of Landsat images for 1977, 1990 and 2001 with a spatial resolution ranging from 57.0 m to 28.5 m. The dynamics of these processes are analyzed together with the time series of river flow, sediment discharge and sediment storage in the basin. Comparisons are made with similar processes identified and quantified earlier in the delta of a neighboring similarly large river basin—the Godavari. The results suggest that coastal erosion in the Krishna Delta progressed over the last 25 years at the average rate of 77.6 ha yr^− 1, dominating the entire delta coastline and exceeding the deposition rate threefold. The retreat of the Krishna Delta may be explained primarily by the reduced river inflow to the delta (which is three times less at present than 50 years ago) and the associated reduction of sediment load. Both are invariably related to upstream reservoir storage development.     

River sediment supply, sedimentation and transport of the highly turbid sediment plume in Malindi Bay, Kenya

The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity and suspended sediment concentrations (TSSC) were measured in stations located within the bay using Aanderaa Recording Current Meter (RCM-9), Turbidity Sensor mounted on RCM-9, Divers Gauges and Aanderaa Temperature-Salinity Meter. The study established that Malindi Bay receives a high terrigenous sediment load amounting to 5.7 × 10⁶ ton·yr^?1. The river freshwater supply into the bay is highly variable ranging from 7 to 680 m³·s^?1. The high flows that are > 150 m³·s^?1 occurred in May during the South East Monsoon (SEM). Relatively low peak flows occurred in November during the North East Monsoon (NEM) but these were usually <70 m³·s^?1. The discharge of highly turbidity river water into the bay in April and May occurs in a period of high intensity SEM winds that generate strong north flowing current that transports the river sediment plume northward. However, during the NEM, the river supply of turbid water is relatively low occurring in a period of relatively low intensity NEM winds that result in relatively weaker south flowing current that transports the sediment plume southward. The mechanism of advection of the sediment plume north or south of the estuary is mainly thought to be due to the Ekman transport generated by the onshore monsoon winds. Limited movement of the river sediment plume southward towards Ras Vasco Da Gama during NEM has ensured that the coral reef ecosystem in the northern parts of Malindi Marine National Park has not been completely destroyed by the influx of terrigenous sediments. However, to the north there is no coral reef ecosystem. The high sediment discharge into Malindi Bay can be attributed to land use change in the Athi-Sabaki River Basin in addition to rapid population increase which has led to clearance of forests to open land for agriculture, livestock grazing and settlement. The problems of heavy siltation in the bay can be addressed by implementing effective soil conservation programmes in the Athi-Sabaki Basin. However, the soil conservation programmes in the basin are yet to succeed due to widespread poverty among the inhabitants and the complications brought about by climate change.     

Quantitative analogue flume-model study of river–shelf systems: principles and verification exemplified by the Late Quaternary Colorado river–delta evolution

ABSTRACT Physical modelling of clastic sedimentary systems over geological time spans has to resort to analogue modelling since full scaling cannot be achieved within the spatial and temporal restrictions that are imposed by a laboratory set‐up. Such analogue models are suitable for systematic investigation of a sedimentary system's sensitivity to allocyclic changes by isolating governing parameters. Until now, analogue models of landscape evolution were mainly qualitative in nature. In this paper, we present a quantitative approach. The quantitative experimental results are verified and discussed by comparison with high‐resolution data from the Colorado river–shelf system of the Texas shelf that we used as a prototype. The model's dimensions are proportionally scaled to the prototype, except for a vertical exaggeration. Time is scaled using a Basin Response factor to maintain a similar ratio between the period of change and the system's equilibrium time for model and prototype. A Basin Fill factor was used to compare the ratio between the time‐averaged sedimentation rate and the rate of change in accommodation space of model and prototype. The flume‐model results are in the form of sediment budgets that are related to shelf cannibalism and fluvial supply, which are compared with the ancestral Colorado river–delta evolution of the last 40 kyr. Model and prototype have similarities in delta evolution in response to one cycle of sea‐level change. With sea‐level change as the isolated variable, the flume model generates a significant supply pulse caused by headward erosion of the shelf in response to the sea‐level fall. This pulse adds to the yield of the hinterland. The supply induced by sea‐level change persists during the early rise, although its rate declines. A similar trend is observed on the east Texas shelf. We argue that shelfal and fluvial degradation cycles induced by sea‐level changes can significantly influence the timing and amount of sediment supply to basins and must therefore be taken into consideration.     

The impact of the Parangana Dam on the River Mersey,Tasmania

By diverting the waters of the River Mersey into a neighbouring basin, the Parangana Dam has changed the hydrological character of the downstream river. The flow records of three gauging stations, located at distances of 10, 67 and 88 km below the dam, provide the basis for identifying the changes. Intermediate discharges have decreased the most and even the furthest downstream station are 50% lower than before. The flow extremes, at both ends of the scale, have been less affected, and particularly the less frequent flood discharges which are augmented by overspill. Indeed the highest flood on record, with a recurrence interval of over 500 years, occurred in the post-dam period. The entry of unregulated tributaries in the middle section separates a much changed upstream hydrology from a downstream one able to mitigate the worst effects of the dam except at intermediate discharges.Field survey, air and ground photograph analysis are used to assess the impact of impoundment on the downstream channel. The main changes involve the expansion of existing lateral bars and the growth of new ones, but such deposition and the consequent narrowing of the channel are rather localized. The invasion of bar surfaces by vegetation could accelerate the process, but the general coarseness of the channel bed and the lack of fine material for bank building will severely delay adjustment of the Mersey channel. The chemical and biological character of the river could be adversely affected by the altered flow regime, particularly in those reaches immediately below the dam.