Incoming Sediment Load Research Articles

Formation condition for concave-bank deposition in the meanders of the Middle Yangtze River and its application

In what kind of meanders concave-bank deposition (CBD) can occur remains an open question. This study investigated the characteristics and major influencing factors of CBD in the meanders of the Middle Yangtze River (MYR), and proposed a formation condition for CBD. Concave-bank deposition occurred in 10 of 23 meanders in the MYR, including two types: counter point bar (CPB) and concave-bank bench (CBB). CPB occurred in both mildly-curved and sharp bends, while CBB only occurred in sharp bends. Hydrodynamic conditions and sediment supply are two major factors controlling the formation of CBD. The incoming sediment load remained in an amount of 0.68 × 108 t/y at Jianli and 0.77 × 108 t/y at Luoshan after Three Gorges Project operation, therefore, CBD development was not limited by a lack of sediment and instead is mainly dependent on the hydrodynamic conditions in the MYR. Flow separation zones were observed in the typical CBD-type bends of the MYR, which indicated that CBD was the result of flow separation or recirculation. An adverse longitudinal water surface gradient is the necessary condition for the formation of concave-bank flow separation. Therefore, a formation condition for CBD was proposed based on this assumption, which considered the combined effects of channel resistance, riverbed slope, channel widening, and curvature. CBD happens when the channel widening and curvature terms are greater than the channel resistance and longitudinal channel slope terms. The proposed condition was verified based on the data of the 23 bends in the MYR and 26 bends in 12 other rivers. The results indicated that more than 85% of the bends were correctly identified. Therefore, the proposed formation condition provided considerably high accuracy.

Reshuffling of the sediment suspension regime in the “starving” middle Yangtze River downstream from cascade mega-reservoirs

The impoundment of cascade mega-reservoirs in the upper Yangtze River (uYR) has exerted profound and accumulative impacts on the lower reaches—not only a drastic drop in the incoming sediment load but also overturning of the runoff-sediment relationship. Based on a 1990–2021 dataset, we examined fundamental changes in the sediment suspension regime in the Jingjiang reach, approximately 100 km below the Three Gorges Dam (TGD). The cascade mega-reservoir impacts are elucidated by three parameters: inflow water discharge and sediment concentration released to the Three Gorges Reservoir (TGR) and operating water levels in front of the TGD. The wash-load concentrations in the reach in the post-TGR period illustrate strong relationships with the flow capacity index, they, however, were actually determined by the above three parameters. The relative importance of the three parameters in determining the wash-load concentrations was evaluated by a random forest model. After upstream reservoir impoundment starting in 2003, the unsaturation degrees in the reach tended to increase, and the thresholds distinguishing wash-load and bed-material-load have risen to ∼0.125 mm and ∼0.25 mm at SS/JL and ZC after TGD impoundment, respectively. In addition, the flow capacity coefficients, i.e., m and k, in the “starving” reach became significantly lower than their equilibrium counterparts. The study aims to facilitate effective management of the uYR cascade mega-reservoirs and the experience could be drawn on for other large rivers facing hydropower development.

Characteristics of incoming runoff and sediment load in the Three Gorges Reservoir during high flood period after the operation of the cascade reservoirs in the lower Jinsha River

泥沙淤积问题直接影响着三峡水库的使用寿命及综合效益的发挥,研究其入库水沙特性对于解决水库泥沙淤积问题具有重要意义。根据水文站实测数据,分析了三峡水库入库水沙输移特性及来源组成变化,重点研究了金沙江下游梯级水库运行后的三峡水库高洪水期入库水沙特性。结果表明:2003-2021年,三峡水库入库泥沙集中于汛期的高洪水期,2013年以后该现象更为显著,泥沙来源也由金沙江为主转变为嘉陵江为主。寸滩站洪峰流量高于50000 m³/s的高洪水期三峡入库沙量显著大于30000~50000 m³/s区间的高洪水期,三峡水库泥沙调度关键在于上游发生编号洪水期间。三峡水库上游沱江或嘉陵江等支流发生流域性大洪水时,易引起高洪水期入库水沙出现“小水大沙”的特点。金沙江下游梯级水库运行后,三峡水库高洪水期入库泥沙大幅减少,中小洪水调度期间泥沙淤积量也大幅减小。研究结果可为三峡水库的泥沙精细化调度和长期高效使用提供基础数据支撑。;The problem of sediment deposition directly affects the lifespan and comprehensive benefits of the Three Gorges Reservoir (TGR). It is of great importance to explore the characteristics of incoming runoff and sediment load in the TGR, which is conducive to solving the problem of sediment deposition in the reservoir. Based on the measured hydrological data, this paper analyzes the flow and sediment transport characteristics and source composition in the TGR, with special emphasis on such characteristics during the high flood period after the operation of the cascade reservoirs in the Lower Jinsha River. The results show that: The incoming sediment load in the TGR was mainly concentrated in the high flood period from 2003 to 2021, and this phenomenon was more significant after 2013. The main source of the sediment load has also changed from the Jinsha River to the Jialing River after 2013. Besides, for the high flood period in which the peak discharge at Cuntan Station was higher than 50000 m³/s, the sediment load in the TGR was significantly greater than those in the peak discharge intervals of 30000-50000 m³/s. The key periods for the sediment regulation of the TGR was during the numbered flood periods in the Upper Yangtze River. Additionally, the basin floods occurred in the tributaries upstream of the TGR, such as the Tuo River or the Jialing River, may cause the phenomenon of “little water carries massive sediment” happened in the TGR during the high flood period. After the operation of the cascade reservoirs in the Lower Jinsha River, the sediment load in the TGR during the high flood period has been significantly decreased, which has greatly reduced the amount of sediment deposition during the middle and small flood operation. This study can provide technical support for the refined sediment regulation of the TGR, and provide a solid guarantee for the long-term use of the TGR.

High-efficiency sediment-transport requirements for operation of the Xiaolangdi Reservoir in the Lower Yellow River

Abstract Sediment accumulation is severe in the Lower Yellow River. Improving sediment-transport efficiency is an important way to save water resources. In this study, we analyzed the sediment-transport efficiency at different sediment concentrations and discharge levels using data from 306 non-overbank floods in the lower reaches of the Yellow River from 1960 to 2016. When the sediment concentrations were below 20 kg/m3, 60 kg/m3, and greater than 100 kg/m3, the clear water volumes for transporting sediment were above 50 m3/t, approximately 16 m3/t, and less than 10 m3/t, respectively. Given current scheduling, the Xiaolangdi Reservoir can lower its water level and increase non-overbank large water flows with an outflow discharge of >3,000 m3/s when the incoming water flow has a sediment concentration of >60 kg/m3. Mathematical modeling indicates that, for an incoming sediment load of 600 million tons in the Middle Yellow River, the average sediment-transport water volume at the downstream Huayuankou Station decreased by 27%, Optimized scheduling at the Xiaolangdi Reservoir could reduce the sedimentation in the downstream channel by 1.1 billion tons more than current scheduling, and extend the reservoir's sediment-trapping period by ten years. These findings can be applied widely in sandy rivers.

Using systems thinking to study the coordination of the water\u2013sediment\u2013electricity coupling system: a case study on the Yellow River

Joint operation of the Longyangxia and Liujiaxia reservoirs (Long-Liu operation) is of great significance for water and sediment regulation in the Yellow River. The water–sediment–electricity coupling system is a giant system with complex nonlinear relationships. A reliable Long-Liu operation scheme facilitates maximization of the benefits of the water–sediment–electricity system. Based on systems thinking, this paper quantitatively evaluated the reliability of different Long-Liu operation schemes and coordination of the water–sediment–electricity coupling system through the entropy weight method and dissipative structure model. The results indicated that the current operation scheme is more reliable than the adjusted scheme at the inter-annual scale and during the summer-autumn flood season and ice flood season within a year. However, the operation scheme should be improved during the spring irrigation period. The key factors influencing the quality of the water–sediment–electricity system include the outflow of the Liujiaxia reservoir, incoming sediment load into the Yellow River at Toudaoguai, sediment inflow-outflow difference in the Ningxia-Inner Mongolia Reach, water flow at Lanzhou and power generation upstream of Toudaoguai. The water–sediment–electricity system under the current Long-Liu operation scheme is more coordinated than that in the adjusted state, but the overall coordinated development of the system remains at a low activity level.

Prediction of Reservoir Sedimentation: A Review

Reservoirs are storage structures for water and are useful for irrigation, navigation, hydroelectric power, water supply for drinking, flood control, etc. But reservoir sedimentation is the gradual accumulation of the incoming sediment load from rivers and causes serious problem on the environment, flood management, economy and the long term efficiency of reservoirs also gets reduced. So, we have to aware of these critical complications and for that the causes of sedimentation, calculation of existing sedimentation volume, estimation of possibilities of sediment increment in future and remedial measures are some of considerable key parameters in reservoir sustainability. From the literature collection, some of important models and techniques are discussed for prediction of reservoir sedimentation in this review paper.

Sedimentation and its response to management strategies of the Three Gorges Reservoir, Yangtze River, China

The Three Gorges Dam (TGD), on the upper valley of the Yangtze River, China, is the largest water conservancy and power generation project in the world. Problematic sedimentation in the Three Gorges Reservoir (TGR) is related to the reservoir lifetime, hydropower generation, riverbed erosion, and sustainable development of ecosystems. In this study, measures to holistically and systematically address sedimentation in the TGR are -discussed and demonstrated, including upstream sediment trapping, regulatory optimisation with respect to sediment peaks and reservoir tail sedimentation reduction, and sediment dredging. Owing to upstream reservoir sediment trapping, the incoming sediment load of the TGR has decreased markedly. Results showed that the annual incoming sediment was only 164 million tons, nearly 70% lesser than the preliminary design value. By intercepting the sediment in the upstream reservoirs and conducting sediment peak regulation, the volume and rates of sedimentation in the TGR decreased significantly. Meanwhile, through sediment peak regulation during the flood season, increased volumes of sediment could be discharged downstream, thereby relieving sediment starvation downstream of the TGD. Overall, through adoption of the related sedimentation management strategies, sediment deposition decreased considerably, to approximately 33% of the preliminary design value, thereby successfully addressing the sediment problems in the TGR. Useful sedimentation management strategies learned from the TGR experience can be applied elsewhere to ensure the sustainable use and management of reservoirs.

Idealized Model for the Deflection of Sediment Into Lateral Branches of Lowland Rivers

AbstractThe division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers and distributary delta channel networks. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for man‐made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents cause especially water from the lower parts of the water column to be diverted into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. However, a systematic mathematical analysis of the parameter space has not yet appeared in the literature. Here, we present a comprehensive analysis by way of an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability.

Responses of channel morphology to flow-sediment variations after dam construction: a case study of the Shashi Reach, middle Yangtze River

AbstractUpstream damming has profoundly impacted downstream channel morphology by altering inflowing water and sediment conditions, which can mostly be ascribed to variations in the flow hydrograph and sediment supply regime. In this paper, channel erosion and deposition during different flow-sediment processes are quantified using a 2D hydro-morphodynamic model. Our results revealed that the net erosion mainly occurred during the flood season when the flow discharges were above 15,000 m3/s. Together, larger peak discharges and less sediment supply could produce greater net erosion. Flow hydrograph variations could alter the inundation extent, thus creating a more widespread redistribution of channel deposition and erosion and possibly causing a shift in the active channel adjustment area, where more channel scouring and siltation occurred. The channel adjustments caused by the sediment supply regime variations underwent a gradual downward propagation process, and most of the riverbed thalweg profile variations could first be observed at a very short distance from the studied reach entrance. A larger cross-sectional area and channel depth as well as a lower width-depth ratio could result from larger floods and less incoming upstream sediment load. We found that a comprehensive flow-sediment combination coefficient with a value of m ranging from 2 to 4 most appropriately reflected the post-dam flow-sediment imbalance regime at the studied reach, which implied the leading role of flow hydrograph variations in shaping channel morphology. In summary, the combined results presented herein for the Shashi Reach of the Yangtze River can provide a better understanding of the downstream morphological impacts of different flow-sediment processes caused by dam operation.

Sediment Management Strategies for Hydropower Reservoirs in Active Agricultural Area

Ringlet, Jor and Mahang reservoirs are part of Cameron Highlands – Batang Padang Hydroelectric Scheme. Conversion of forest to agricultural and urban area within the catchment has caused Ringlet Reservoir to suffer severe sedimentation problem and waste dumping. This has caused operational difficulties to the hydropower operator. Based on estimation, sediment inflow into Ringlet Reservoir has increased multiple folds from 25,000 m3/year in 1960s up to between 120,000 m3/year to 200,000 m3/year in 2010. This reduces the total storage capacity of Ringlet Reservoir to almost 50% of its original design value, and subsequently affects Jor and Mahang Reservoirs. Bertam Intake is often choked by the sediment built up within the area, thus limiting the running hours of the plant to generate electricity. Without sediment management strategies, the incoming sediment load into Ringlet Reservoir would increase tremendously and can cause the hydropower scheme to cease operation faster that its design life expectancy. Various mitigation strategies have been implemented such as dredging, construction of check dams and settling basins and flushing from the bottom outlet, resulting to an increase in storage. Despite these efforts which focus within the reservoir, the best solution is by control the sediment and waste at source, through the concept of an integrated catchment management. This requires concerted effort from the local authorities and public to ensure successful implementation. This paper outlines the methods, analyses and results of various mitigation strategies.

Estimation of Sediment Deposition at Nausehri Reservoir by Using Multiple Linear Regression and Assessment of its Effect on Hydropower Generation Capacity

This article presents investigation of the inflow of sediments depending upon the stream flow, density and kinematic viscosity of water. The process involves Multiple Linear Regression (MLR) technique for estimation of sediment load and volume, by using UNISTAT software (Version 5.6). A general equation is developed for a specific flow rate while considering density and kinematic viscosity of water as other parameters. The developed equation can be applied in future for determining the sediment concentration when the flow rate is known. The daily stream flow and suspended sediment data belonging to the station Nausehri operated by the Water and Power Development Authority, Pakistan is used as case study. The results indicate that the dam site experiences a mean annual sediment load of 3.74mst. With this much incoming sediment load the reservoir life comes out to be 45 years when no flushing is done. The impact of this sediment deposition on hydro-electric energy generation is also determined, which shows a decrease of 113MW (12%) in the production capacity of the power plant over the life of the reservoir.

Wide river or narrow river: Future river training strategy for Lower Yellow River under global change

The choice of a river training strategy is extremely important for the Lower Yellow River (LYR). Currently, the wide-river training strategy applies in the training of the LYR. However, remarkable changes in the hydrological processes in the Yellow River basin, as well as immediate pressure from socio-economic development in the Yellow River basin, make it necessary to consider if there is a possibility to change the river training strategy from wide-river training to narrow-river training. This research investigates the impacts of different river training strategies on the LYR through numerical simulations. A one-dimensional (1-D) model was used to simulate the fluvial processes for the future 50 years and a three-dimensional (3-D) model was applied to study typical floods. The study focused on river morphology, the results show that if the present decreasing trend in both water discharge and sediment load persists, the deposition rate in the LYR will further decrease no matter what strategy is applied. Especially, narrow-river training can achieve the aim to increase the sediment transport capacity in the LYR compared with wide-river training. However, if the incoming water and sediment load recovers to the mean level of the last century, main channel shrinkage due to sedimentation inevitably occurs for both wide-river and narrow-river training. Most importantly, this study shows that narrow-river training reduces the deposition amount over the whole LYR, but it provides little help in alleviating the development of the “suspended river”. Instead, narrow-river training can cause aggradation in the transitional reach where the river pattern changes from highly wandering to meandering, further worsening the “hump deposition” there. Because of uncertainty regarding future changes in hydrological processes in the Yellow River basin, and the lack of feasible engineering measures to mitigate “suspended river” and “hump deposition” problems in the LYR, caution should be exercised with respect to changes in the river training strategy for the LYR.

Full Spectrum Analytical Channel Design with the Capacity/Supply Ratio (CSR)

Analytical channel design tools have not advanced appreciably in the last decades, and continue to produce designs based upon a single representative discharge that may not lead to overall sediment continuity. It is beneficial for designers to know when a simplified design may be problematic and to efficiently produce alternative designs that approximate sediment balance over the entire flow regime. The Capacity/Supply Ratio (CSR) approach—an extension of the Copeland method of analytical channel design for sand channels—balances the sediment transport capacity of a design reach with the sediment supply of a stable upstream reach over the entire flow duration curve (FDC) rather than just a single discharge. Although CSR has a stronger physical basis than previous analytical channel design approaches, it has not been adopted in practice because it can be a cumbersome and time-consuming iterative analysis without the use of software. We investigate eighteen sand-bed rivers in a comparison of designs based on the CSR approach and five single-discharge metrics: the effective discharge (Qeff) or discharge that transports the most sediment over time; the 1.5-year recurrence interval discharge (Q1.5); the bankfull discharge (Qbf); and the discharges associated with 50th (Qs50) and 75th (Qs75) percentiles of the cumulative sediment yield curve. To facilitate this analysis, we developed a novel design tool using the Visual Basic for Applications (VBA) programming language in Excel® to produce stable channel slope/width combinations based on the CSR methodology for both sand- and gravel-bed streams. The CSR Stable Channel Design Tool’s (CSR Tool) code structure was based on Copeland’s method in SAM and HEC-RAS (Hydrologic Engineering Center’s River Analysis System) and was tested with a single discharge to verify outputs. The Qs50 and Qs75 single-discharge designs match the CSR output most closely, followed by the Qbf, Qeff, and Q1.5. The Qeff proved to be the most inconsistent design metric because it can be highly dependent on the binning procedure used in the effectiveness analysis. Furthermore, we found that the more rigorous physical basis of the CSR analysis is potentially most important in designing “labile” channels with highly erodible substrate, high perennial flow “flashiness”, low width-to-depth ratio, and high incoming sediment load. The CSR Tool provides a resource for river restoration practitioners to efficiently utilize design techniques that can promote sediment balance in dynamic fluvial systems.

Estimation of Sediment Deposition at Nausehri Reservoir by Using Multiple Linear Regression and Assessment of its Effect on Hydropower Generation Capacity

This article presents investigation of the inflow of sediments depending upon the stream flow, density and kinematic viscosity of water. The process involves Multiple Linear Regression (MLR) technique for estimation of sediment load and volume, by using UNISTAT software (Version 5.6). A general equation is developed for a specific flow rate while considering density and kinematic viscosity of water as other parameters. The developed equation can be applied in future for determining the sediment concentration when the flow rate is known. The daily stream flow and suspended sediment data belonging to the station Nausehri operated by the Water and Power Development Authority, Pakistan is used as case study. The results indicate that the dam site experiences a mean annual sediment load of 3.74mst. With this much incoming sediment load the reservoir life comes out to be 45 years when no flushing is done. The impact of this sediment deposition on hydro-electric energy generation is also determined, which shows a decrease of 113MW (12%) in the production capacity of the power plant over the life of the reservoir

Assessment of Clogging Dynamics in Permeable Pavement Systems with Time Domain Reflectometers

Infiltration is a primary functional mechanism in green infrastructure storm water controls. This study used time domain reflectometers (TDRs) to measure spatial infiltration and assess clogging dynamics of permeable pavement systems in Edison, New Jersey, and Louisville, Kentucky. In 2009, the U.S. Environmental Protection Agency constructed a 0.4-ha parking lot surfaced with three permeable pavement types (permeable interlocking concrete pavers, pervious concrete, and porous asphalt). Paired TDRs were installed at two locations in each permeable pavement type and at a depth of 0.4 m below the driving surface. The relative volumetric water content (RVWC) prior to an event had a significant negative correlation to antecedent dry period, and the peak RVWC during an event had a significant positive correlation to the peak 5-min rainfall intensity. The TDRs measured a significantly different response when water was presumably infiltrating as direct rainfall compared to rainfall combined with runoff from a contributing drainage area. The results indicated clogging progressed from the upgradient edge. Based on the lessons learned at Edison, TDRs were installed in permeable pavement strips in Louisville, Kentucky, during December 2011. The TDR placement strategy was selected to understand the spatial infiltration of runoff and to document clogging and infiltration dynamics. As contributing drainage area size and condition impacts incoming sediment load, and the runoff infiltrates along the upgradient edge of the permeable pavement surface, the ratio of drainage area to working width of permeable surface is an important design parameter to predict the rate of clogging. At Edison, the design ratio of contributing drainage area to permeable pavement width at the upgradient edge was 7.62 m2/m. In Louisville, the ratio was about 3,829 m2/m. Because of the much larger ratio, clogging was expected to occur rapidly at the Louisville site. Responses from rainfall events during the first three months at Louisville supports the hypotheses related to surface clogging mechanisms. This paper highlights evaluation techniques, placement locations, and techniques to use TDRs to remotely monitor surface clogging, which can be used to provide guidance for maintenance scheduling.

Bed load transport under different streambed conditions – a field experimental study in a mountain stream

Bed load transport in mountain streams is closely linked to streambed structures. Strambed structures are arrangements of boulders and cobbles deposited during extreme floods, in a stable configuration exhibiting high dissipation of flow. Field experiments were carried out in a mountain stream in Yunnan, southwestern China, studying bed load movement on three typical streambeds, i.e., with well developed, partially developed, and no structures. An underwater observation and video-capturing system was designed to observe and measure the movement of bed load particles. The initiation mode, trajectory, velocity, and acceleration of bed load particles under the three conditions were observed and analyzed. Results showed that the bed load movement was highly associated with streambed condition. With well-developed structures, bed load particles moved intermittently through saltation and the bed load transport rate was very low. For partially-developed structures most bed load particles moved through saltation but a portion of sediment moved in sliding and rolling. In the case with no streambed structure (plane bed) contact load motion (sliding and rolling) gradually became dominant. Moreover, laminated load motion occurred and became the main component of bed load transport when the flow discharge and incoming sediment load were very high. Laminated load motion was a special form of bed load motion with an extremely high intensity. Bed load transport and streambed structure both acted to dissipate flow energy and were mutually constraining. High rates of bed load transport occurred in the streams with no or poor bed structures, and low bed load transport was associated with well developed structures. The bed load transport rate was inversely correlated to the degree of streambed development.

A study of sedimentation rate in the lower Yellow River based on cross section measurement data

Using cross section measurement data, the sedimentation rate of the lower Yellow River was determined. There are two methods to determine channel sedimentation rate, the method of sediment budget using data of suspended sediment loads, and the method using data of cross-section measurement. The latter has the advantage of avoiding the error that may be resulted from sediment load measurement, which is incapable of measuring the near-bottom suspended sediment and bed-load. Moreover, in so-doing, when relating annual channel sedimentation rate to the incoming sediment load, the demand of independence between the independent and dependent variables can be satisfied. Based on annual data from the period 1952-1999, correlations between channel sedimentation rate and a number of flow and sediment indices have been established, all significant at a level of p <0.01. Then, some multiple equations have been established, which can be used to estimate channel sedimentation rate based on a number of influencing variables including suspended sediment load and its grain size compositing, water discharge and, hyperconcentration flow frequency. These equations may provide some useful information for better environment management in general, and for sediment management in particular, in the Yellow River basin.

Status and Potential of Spate Irrigation in Ethiopia

This article discusses spate irrigation in Ethiopia and aims to take stock of the current status of spate irrigation development. It summarizes experiences so far and formulates a number of recommendations on the development of this upcoming resource management system. It argues that raised weirs are useful mainly in areas where a large head for spate flow diversion is required, but that traditional earthen structures with conical stone/gabion reinforcements are cost-effective and technically adequate for floodwater distribution and management. It contends that the practical successes of sediment settling ponds (gravel traps) are at best mixed. Even where a small basin is justified, it can only ever trap a small proportion of the incoming sediment load. It is better to allow large floods with excessive sediment load (>100,000 ppm) to by-pass upstream intakes to be diverted lower down the system, and provide additional livelihood opportunities in downstream areas in the form of rangeland and agro-forestry development. This is particularly useful in the lowlands of Ethiopia with large pastoral community where spate irrigation is yet to make in-roads. The article further explains that water rights in spate are different from the sharing and allocation of perennial flows—they are dynamic and respond to a situation that differs from year to year as well as within a year and that a certain degree of inequity among users is inevitable. Keeping the command area compact can ensure two or more irrigation turns and this can highly increase productivity as crops are no longer in the ‘stress zone’. To transform spate irrigation in Ethiopia from subsistence to a business-oriented production system, the article proposes the promotion of cash crops including pulses and oil seeds as well as encouraging investors to go for bio-fuel development and agro-forestry in the lowland areas where huge potential exist. To avoid vehement conflicts that may arise among the settled agricultural and pastoral communities due to the implementation of the business-oriented production system, the traditional rights of the pastoral communities must be respected and their rangeland and water resources be safeguarded. For maximum yield, soil moisture conservation measures such as pre-irrigation land preparation, deep ploughing and mulching are essential as is the conjunctive use of spate flow and groundwater. Last, but most important, the article emphasizes that farmers need to be placed at the heart of any spate irrigation development programme as primary beneficiaries, managers and operators as well as part of the decision making institution.

Effect of incoming sediment on the transport rate of bed load in mountain streams

Field experiments were conducted on bed load transport in the Diaoga River, a mountain stream in southwest China, to study the variation of bed load transport with varying sediment supply. The rate of bed load transport is greatly affected by incoming sediment (load and size). Under the same flow conditions, bed load transport rates may differ by three orders of magnitude depending on whether measurements were taken before or after the first flood of the year. The relation of the “bed load transport rate versus flow intensity” appears to have similar characteristics as a clockwise looped-rating curve. Experiments also were conducted during the non-flood season to study bed load transport processes with different incoming load from an upstream section. Bed load with different sizes can be grouped into two types: traveling bed load and structural bed load. Traveling bed load is composed of sediment finer than a critical size, D c, and its transport rate depends mainly on the incoming sediment rate. The incoming sediment rate can alter the rate of bed load transport by three orders of magnitude. Structural bed load is composed of coarser sediment and its transport rate closely relies on the flow intensity.

Response of bankfull discharge to discharge and sediment load in the Lower Yellow River

This paper presents a study on the response of bankfull discharge to incoming discharge and sediment load based on long-term hydrological and bankfull discharge data measured on the lower reaches of the Yellow River. The analysis indicated that the bankfull discharge was a function of the preceding 5 to 6 years' flow regimes, revealing that bankfull channel dimensions were a result of the accumulative effect of several consecutive years' flow discharge and sediment load conditions. In other words, the mean relaxation time was about 5 to 6 years for channel adjustment. Theoretical methods for the prediction of bankfull discharge were developed, in which the response time or the relaxation time was fully considered. Testing of the methods using data observed at five hydrologic stations in the lower reaches of the Yellow River from 1960 to 2003 showed that the proposed methods can predict the variation of bankfull discharge in response to changes in the incoming discharge and sediment load. The proposed methods hold promise for predicting the magnitude and trend of channel response to other rivers undergoing aggradation or degradation from changes in hydrologic or sediment regime.