The ratio of measured to total sediment discharge

It is important to examine the ratio of measured to total sediment discharge to determine the error in measured sediment transport rates from depth-integrated samplers. The ratio of measured to total sediment discharge as well as the ratio of suspended to total sediment discharge are examined based on the Modified Einstein Procedure. Both ratios reduce to a function of the ratio of shear velocity, $u_{*}$, to the fall velocity, $\omega$, of suspended material, $u_{*}/\omega$, and the ratio, $h/d_s$, of flow depth, h, to the median grain size of bed material, $d_{50}$. In rivers transporting fine material (such as silt or clay), the ratio of suspended to total load is a function of the ratio, $h/d_{50}$. In this study, it is found that the ratio of measured to total load becomes a simple function of flow depth. For fine sediment transport, with a Rouse number (Ro) $<0.3$, at least 80% of sediment load is in suspension when $h/d_{50}>15$, and at least 90% of sediment load is measured from depth integrating samples when $h>1\mathrm{m}$. Detailed measurements from 35 river stations in South Korea demonstrate that sand sizes and finer fractions predominantly are transported in suspension. Also, at least 90% of sand and finer fractions are transported in suspension in gravel and sand bed rivers when the discharge is larger than the mean annual discharge.     

Estimation of debris flow discharge coefficient considering sediment concentration

A sabo dam has a purpose to block the path of debris flow. However, when overflow occurs, a sabo dam works as a weir, a vertical obstruction, where the fluid must flow over. Many empirical formulas and discharge coefficients for weirs relating flow depth to discharge have been proposed to calculate overflow discharges. However, only a few studies about overflow discharge coefficients are available in the case of debris flow. In this paper, experiments and numerical simulations were done to estimate debris flow discharge coefficients by considering the sediment concentration. In the numerical simulation, a complete overflow equation and a free overfall equation were implemented to calculate debris overflow discharges at a sabo dam. To determine the discharge coefficients for each equation, single factor regression analysis was used. Laboratory experiments were done to calibrate and to compare with the simulation. Study results showed that the discharge coefficients increase as the sediment concentration increases. This finding suggests debris flow discharge coefficients are derived to calculate the debris overflow discharges at a sabo dam.     

Impact of climate change on stream discharge and sediment yield in Northern Viet Nam

The purpose of this paper is to apply “Soil and Water Assessment Tool (SWAT)” model to assess the impacts of climate change on stream discharge and sediment yield from Song Cau watershed in Northern Viet Nam. Three climate change scenarios B1, B2, and A2; representing low, medium, and high levels of greenhouse gas emission, respectively, were considered in this study. The highest changes in stream discharge (up to 11.4%) and sediment load (15.3%) can be expected in wet season in 2050s according to the high emission scenario (A2), while for the low emission scenario the corresponding changes equal to 8.8% and 12.6%. The results show that the stream discharge is likely to increase in the future during the wet season with increasing threats of sedimentation.     

Flood-duration-integrated stream power and frequency magnitude of >50-year-long sediment discharge out of a hyperarid watershed

Determining sediment discharge out of watersheds is a global, long-term challenge. In the vast, usually data-poor, hyperarid regions of the world, this is a greater challenge. Here, we present a unique, decades-long dataset of individual floods and their respective sediment discharge out of Nahal Yael, an experimental, well-instrumented, hyperarid (~25–30 mm year⁻¹) watershed in southern Israel. The high correlation between directly measured sediment yield by discrete individual floods and their respective total energy, represented by flood-integrated stream power (FISP), serves here as a rating curve. Using this rating curve, the 51-year-long series of FISP in Nahal Yael, calculated from the detailed individual flood hydrographs, was converted into a series of sediment yield by these floods. This, in turn, allows determining the long-term frequency-magnitude of sediment exported out of this hyperarid basin. This can assist in landscape evolution modeling, in testing impacts of flood frequency changes enforced by altered regional climatology, and hint at changes needed in forming the observed alluvial fans. We conclude that, at the decadal scale, moderate floods are the most effective in terms of total sediment transport. However, the recurrence intervals of these moderate hyperarid floods are longer than in temperate regions and reach 5–10 years.     

Continuous measurement of sediment transport in the Erlenbach stream using piezoelectric bedload impact sensors

We report on bedload transport observations using piezoelectric bedload impact sensors (PBIS), an indirect method of estimating the volume of bedload transport of coarse sediment. The PBIS device registers vibrations produced by bedload (particle diameter >～20 mm) and records the signal as a sum of the number of impulses per time. Sediment transport at the Erlenbach stream has been continuously monitored with a PBIS array starting in 1986. The sensor array spans the width of an entire cross‐section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin. We compare PBIS data with long‐term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin, with artificial sediment input under controlled conditions in the field, and also with laboratory experiments. The rate of bedload transport is proportional to the number of impacts on the sensor per unit time. The reliability of the calibration relationship increases with the length of the observation period, e.g. for higher numbers of impacts and larger bedload volumes. Sediment volumes for individual flood events estimated with the PBIS method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water, the peak water discharge, and the critical discharge for the onset of sediment transport. Copyright © 2007 John Wiley & Sons, Ltd.     

Estimation of suspended sediment loads in a seasonal stream in the wet‐dry tropics,Northern Territory,Australia

One year of instantaneous suspended sediment concentration, C, and instantaneous discharge, Q, data collected at Ngarradj downstream of the Jabiluka mine site indicate that the use of a simple C–Q rating curve is not a reliable method for estimating suspended sediment loads from the Ngarradj catchment. The C–Q data are not only complicated by hysteresis effects within the rising and falling stages of individual events, but also by variable depletion of available suspended sediment through multipeaked runoff events. Parameter values were fitted to an event‐based suspended sediment load–Q relationship as an alternative to the C–Q relationship. Total suspended sediment load and Q data for 10 observed events in the Ngarradj stream catchment were used to fit parameter values to a suspended sediment load–Q relationship, using (a) log–log regression and (b) iterative parameter fitting techniques. A more reliable and statistically significant prediction of suspended sediment load from the Ngarradj catchment is obtained using an event‐based suspended sediment load–Q relationship. Fitting parameters to the event‐based suspended sediment load–Q relationship using iterative techniques better predicts long‐term suspended sediment loads compared with log–log regression techniques. Copyright © 2004 John Wiley & Sons, Ltd.     

Study of deposition of fine sediment within the pores of a coarse sediment bed stream

Elaborate experiments were performed in a 30 m long, 0.5 m deep and 0.2 m wide laboratory flume to study the process of infiltration of fine sediment into the pores of coarse sediment forming the channel bed material. Different concentrations of suspended load of fine sediment of size 0.064 mm were passed over the channel bed made up of three different types of coarse sediments; two uniform and one nonuniform. The proportion of fine sediment infiltrated into the pores of bed material for each equilibrium concentration of suspended load of fine sediment in the flow was studied during several experimental runs. The proportion of fine sediment within the pores of bed material increased with an increase in the equilibrium concentration of suspended load of fine sediment in the flow. This process continued till the pores within the coarse sediment bed were filled up to the capacity with the fine sediment transported by the flow in suspension. The theoretical value was identified for limit for maximum proportion of fine sediment that can be present within the pores of bed material. On further increase in the concentration of suspended load of fine sediment in the flow, deposition of fine sediment occurs on the surface of the flume bed in the form of ripples of the fine sediment. This condition is defined as 'depositional condition'. Experimental observations on these and related aspects are presented herein.     

Estimation of instantaneous peak flows from maximum mean daily flows using the HBV hydrological model

The record length and quality of instantaneous peak flows (IPFs) have a great influence on flood design, but these high resolution flow data are not always available. The primary aim of this study is to compare different strategies to derive frequency distributions of IPFs using the Hydrologiska Byråns Vattenbalansavdelning (HBV) hydrologic model. The model is operated on a daily and an hourly time step for 18 catchments in the Aller‐Leine basin, Germany. Subsequently, general extreme value (GEV) distributions are fitted to the simulated annual series of daily and hourly extreme flows. The resulting maximum mean daily flow (MDF) quantiles from daily simulations are transferred into IPF quantiles using a multiple regression model, which enables a direct comparison with the simulated hourly quantiles. As long climate records with a high temporal resolution are not available, the hourly simulations require a disaggregation of the daily rainfall. Additionally, two calibrations strategies are applied: (1) a calibration on flow statistics; (2) a calibration on hydrographs. The results show that: (1) the multiple regression model is capable of predicting IPFs with the simulated MDFs; (2) both daily simulations with post‐correction of flows and hourly simulations with pre‐processing of precipitation enable a reasonable estimation of IPFs; (3) the best results are achieved using disaggregated rainfall for hourly modelling with calibration on flow statistics; and (4) if the IPF observations are not sufficient for model calibration on flow statistics, the transfer of MDFs via multiple regressions is a good alternative for estimating IPFs. Copyright © 2015 John Wiley & Sons, Ltd.     

Estimation of soil erosion and sediment yield using GIS

Abstract

A Geographical Information System (GIS) based method is proposed and demonstrated for the identification of sediment source areas and the prediction of storm sediment yield from catchments. Data from the Nagwa and Karso catchments in Bihar (India) have been used. The Integrated Land and Water Information System (ILWIS) GIS package has been used for carrying out geographic analyses. An Earth Resources Data Analysis System (ERDAS) Imagine image processor has been used for the digital analysis of satellite data for deriving the land cover and soil characteristics of the catchments. The catchments were discretized into hydrologically homogeneous grid cells to capture the catchment heterogeneity. The cells thus formed were then differentiated into cells of overland flow regions and cells of channel flow regions based on the magnitude of their flow accumulation areas. The gross soil erosion in each cell was calculated using the Universal Soil Loss Equation (USLE) by carefully determining its various parameters. The concept of sediment delivery ratio (SDR) was used for determination of the total sediment yield of each catchment during isolated storm events.     

Evaluation of total load sediment transport formulas using ANN

The calculated results from various sediment transport formulas often differ from each other and from measured data. Some parameters in the sediment transport formulas are more effective than others to estimate total sediment load. In this study, an Artificial Neural Network （ANN） model is trained using four dominant parameters of sediment transport formulas. ANN models are able to reveal hidden laws of natural phenomena such as sediment transport process. The results of ANN and some total bed material load sediment transport formulas have been compared to indicate the importance of variables which can be used in developing sediment transport formulas. To train ANN, average flow velocity, water surface slopes, average flow depth, and median particle diameter are used as dominant parameters to estimate total bed material load. Two hundreds and fifty samples are used to train the ANN model. Twenty-four sets of field data not used in the training nor calibration of ANN are used to compare or verify the accuracy of ANN and some well-known total bed material load formulas. The test results show that the ANN model developed in this study using minimum number of dominant factors is a reliable and uncomplicated method to predict total sediment transport rate or total bed material load transport rate. Results show that the accuracy of formulas in descending order are those by Yang （1973）, Laursen （1958）, Engelund and Hansen （1972）, Ackers and White （1973）, and Toffaleti （1969）. These results are similar to those made by ASCE （1982） based on laboratory and field data not used in this paper. Study results also show that the formulas based on physical laws of sediment transport, like those formulas that were developed based on power concept, are more accurate than other formulas for estimating total bed material sediment load in rivers.     

Morphological controls on the downstream passage of a sediment wave in a gravel-bed stream

Sediment waves in river systems have been widely reported, although few studies have examined the interaction between these waves and the morphology of the reaches through which they pass. This interaction determines how waves are modified as they propagate downstream. This study documents the origin and downstream passage of an avulsion-generated sediment wave through a 374 m study reach of the Allt Dubhaig, Scotland. A nested survey framework was adopted, with volumes calculated from cross-sections spaced between 10 and 40 m apart documenting the origin and downstream passage of the wave. The wave moved through an intensively (c. 1 m cross-section spacing) monitored 120 m stretch (Reach A) within the study reach, allowing assessment of sediment exchanges between the incoming wave and the local morphology. Successive surveys show the movement of the wave through and out of the reach, and also that areas where wave sediment was deposited did not always correspond with areas of subsequent erosion. Reach A was divided into three morphologically distinct sub-reaches (1A, 2A and 3A) within which sediment fluxes and the three-dimensional distribution of erosion/deposition were estimated. Sediment wave input into 1A and 2A (relatively stable sub-reaches) caused forced bar aggradation and erosion of sediment from elsewhere within the reach, which then became part of the wave. The downstream transfer of this sediment into unstable 3A caused aggradation and, in response, widespread erosion which increased the magnitude of the sediment wave as it exited reach A. Sediment exchange between the recipient reach and the wave depends upon local morphological stability and is a crucial process affecting wave magnitude and attenuation. The macroscale sediment wave interacted with, rather than overwhelmed, the recipient morphology. © 1998 John Wiley & Sons, Ltd.     

Developing a new stream metric for comparing stream function using a bank–floodplain sediment budget: a case study of three Piedmont streams

A bank and floodplain sediment budget was created for three Piedmont streams tributary to the Chesapeake Bay. The watersheds of each stream varied in land use from urban (Difficult Run) to urbanizing (Little Conestoga Creek) to agricultural (Linganore Creek). The purpose of the study was to determine the relation between geomorphic parameters and sediment dynamics and to develop a floodplain trapping metric for comparing streams with variable characteristics. Net site sediment budgets were best explained by gradient at Difficult Run, floodplain width at Little Conestoga Creek, and the relation of channel cross‐sectional area to floodplain width at Linganore Creek. A correlation for all streams indicated that net site sediment budget was best explained by relative floodplain width (ratio of channel width to floodplain width). A new geomorphic metric, the floodplain trapping factor, was used to compare sediment budgets between streams with differing suspended sediment yields. Site sediment budgets were normalized by floodplain area and divided by the stream's sediment yield to provide a unitless measure of floodplain sediment trapping. A floodplain trapping factor represents the amount of upland sediment that a particular floodplain site can trap (e.g. a factor of 5 would indicate that a particular floodplain site traps the equivalent of 5 times that area in upland erosional source area). Using this factor we determined that Linganore Creek had the highest gross and net (floodplain deposition minus bank erosion) floodplain trapping factor (107 and 46, respectively) that Difficult Run the lowest gross floodplain trapping factor (29) and Little Conestoga Creek had the lowest net floodplain trapping factor (–14, indicating that study sites were net contributors to the suspended sediment load). The trapping factor is a robust metric for comparing three streams of varied watershed and geomorphic character, it promises to be a useful tool for future stream assessments. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.     

Possible effect of ENSO on annual sediment discharge of debris flows in the Jiangjia Ravine based on Morlet wavelet transforms

The multi-time-scale structures of an annual sediment discharge series of debris flow in the Jiangjia Ravine and the Southern Oscillation index are analysed using the method of Morlet wavelet transformations. The possible effects of E1 Nirio episodes on the annual sediment discharge are discussed by comparing the period variations of ENSO and the discharge. The results show that the annual sediment discharge series of debris flow is related to E1 Nifio episodes. Generally, the annual sediment discharge of debris flow is less than usual during an E1 Nifio episode and debris flow is less active. On the contrary, the annual sediment discharge of debris flows is greater than usual during a La Nifia episode and debris flows are more frequent. There is a relationship between the annual sediment discharges of debris flow in the Jiangjia Ravine and the summer Southern Oscillation index, with both having quasi-periodic variations of 2 and 5-6 years.     

Predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion

In this paper, a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed. The method is based on the criterion suggested by Gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture. In particular, a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed; then, a simple two‐steps criterion is suggested to estimate the safety factor required by the proposed approach. The method is applied in three different Italian regions (Calabria, Basilicata, and Tuscany) and required several field campaigns involving the survey of 251 stream reaches upstream of grade‐control structures. The area including five Calabrian streams is firstly used to test the suitability of the two‐steps approach. In this area, considering the detailed information about the particle size of bed material, the effect of using a simplified grain size distribution for each torrent is also checked. The method is then applied within the two additional areas (Basilicata and Tuscany) in order to check the geographic influence on the prediction of the stable longitudinal profile. A comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the Calabrian streams, where the number of the investigated stream reaches is greater, and satisfactory results for the torrents located in Basilicata and Tuscany where the field data set is more limited. The overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams. Copyright © 2011 John Wiley & Sons, Ltd.     

Two-dimensional numerical modeling of dam-break flows over natural terrain using a central explicit scheme

A two-dimensional (2D) numerical model has been developed to solve shallow water equations for simulation of dam-break flows. The spatial derivatives are discretized using a well-balanced explicit central upwind conservative scheme. The scheme is Riemann solver free and guarantees the positivity of the flow depth over complex topography if the Courant number is kept less than 0.25. The time integration is performed by Euler’s scheme. The model is verified against analytical results for water surface elevation and discharge for three benchmark test cases. A good agreement between analytical solutions and computed results is observed. The property of well-balancing in still water over an uneven bottom is also confirmed. The model is then validated by simulating a laboratory experiment in which a dam break flow propagates over a triangular obstacle. The model performance was found to be satisfactory. A dam break laboratory experimental test case on a frictionless horizontal bottom is also simulated for 2D validation of the model, and good agreement between simulation and the experimental data is observed. The suitability of the proposed model for real life applications is demonstrated by simulating the Malpasset dam-break event, which occurred in 1959 in France. The computed arrival time of the flood wave front and the maximum flow depths at various observation points matched well with the measurements on a 1/400 scale physical model. The overall performance indicates that this model can be applied for simulation of dam-break waves in real life cases.     

Estimate of long-term sediment discharge at tidal inlets using water level data from a tide gauge

A simple approach to the estimate of long-term sediment discharge through an entrance to a tidal basin is described. Using the method, cross-sectional mean current speeds are derived, on the basis of the definition of an ‘apparent tidal basin area’, from records of water level data from a single tide gauge. The obtained time-series of current speeds are then used to define ‘local’ current speeds, through the use of a sectional distribution function for the currents. Sediment transport formulae are then applied, using the obtained speed data and other relevant parameters contained within the formulae and frequency distribution functions with regard to wave and tidal current characteristics, to estimate sediment discharge through the entrance. Analytical procedures of the method are described in detail, in an example from Christchurch Harbour (southern England).     

Estimation of suspended sediment concentration in the Saint John River using rating curves and a machine learning approach

ABSTRACT

Sedimentation in navigable waterways and harbours is of concern for many water and port managers. One potential source of variability in sedimentation is the annual sediment load of the river that empties in the harbour. The main objective of this study was to use some of the regularly monitored hydro-meteorological variables to compare estimates of hourly suspended sediment concentration in the Saint John River using a sediment rating curve and a model tree (M5?) with different combinations of predictors. Estimated suspended sediment concentrations were multiplied by measured flows to estimate suspended sediment loads. Best results were obtained using M5? with four predictors, returning an R² of 0.72 on calibration data and an R² of 0.46 on validation data. Total load was underestimated by 1.41% for the calibration period and overestimated by 2.38% for the validation period. Overall, the model tree approach is suggested for its relative ease of implementation and constant performance.

EDITOR M.C. Acreman; ASSOCIATE EDITOR B. Touaibia     

Critical assessment and validation of a time‐integrating fluvial suspended sediment sampler

Delivery of fine sediment to fluvial systems is of considerable concern given the physical and ecological impacts of elevated levels in drainage networks. Although it is possible to measure the transfer of fine sediment at high frequency by using a range of surrogate and automated technologies, the demands for assessing sediment flux and sediment properties at multiple spatially distributed locations across catchments can often not be met using established sampling techniques. The time‐integrated mass‐flux sampler (TIMS) has the potential to bridge this gap and further our understanding of fine sediment delivery in fluvial systems. However, these devices have undergone limited testing in the field. The aim of this paper was to provide a critical validation of TIMS as a technique for assessing fluvial fine sediment transfer. Fine sediment flux and sediment properties were assessed over 2 years with individual sampling periods of approximately 30 days. Underestimation of sediment flux ranged between 66% and 99% demonstrating that TIMS is unsuitable for assessing absolute sediment loads. However, assessment of relative efficiency showed that six of seven samplers produced statistically strong relationships with the reference sediment load (P < 0.05). Aggregated data from all sites produced a highly significant relationship between reference and TIMS loads (R² = 0.80; P < 0.001) demonstrating TIMS may be suitable for characterizing patterns of suspended sediment transfer. Testing also illustrated a consistency in sediment properties between multiple samplers in the same channel cross section. TIMS offers a useful means of assessing spatial and temporal patterns of fine sediment transfer across catchments where expensive monitoring frameworks cannot be commissioned. Copyright © 2013 John Wiley & Sons, Ltd.     

Scenario analysis for assessing the impact of hydraulic fracturing on stream low flows using the SWAT model

Scientists and water users are concerned about the potential impact on water resources, particularly during low-flow periods, of freshwater withdrawals for hydraulic fracturing (fracking). Therefore, the objective of this paper is to assess the potential impact of hydraulic fracturing on water resources in the Muskingum watershed of Eastern Ohio, USA, especially due to the trend of increased withdrawals for hydraulic fracking during drought years. The Statistical Downscaling Model (SDSM) was used to generate 30 years of plausible future daily weather series in order to capture the possible dry periods. The data generated were incorporated in the Soil and Water Assessment Tool (SWAT) to examine the level of impact due to fracking at various scales. Analyses showed that water withdrawal due to hydraulic fracking had a noticeable impact, especially during low-flow periods. Clear changes in the 7-day minimum flows were detected among baseline, current and future scenarios when the worst-case scenario was implemented. The headwater streams in the sub-watersheds were highly affected, with significant decrease in 7-day low flows. The flow alteration in hydrologically-based (7Q10, i.e. 7-day 10-year low flow) or biologically-based (4B3 and 1B3) design flows due to hydraulic fracking increased with decrease in the drainage area, indicating that the relative impact may not be as great for higher order streams. Nevertheless, change in the annual mean flow was limited to 10%.