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Stream Channel Stability, Assessment, Modeling, and Mitigation

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Hydraulics and Hydrodynamics".

Deadline for manuscript submissions: closed (30 November 2016) | Viewed by 52229

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Dear Colleagues,

Unstable conditions in a stream channel can result in bed degradation and aggradation, channel widening, and rapid lateral migration at engineering time scales. Unstable channels can negatively impact infrastructure, property, water quality, and ecosystems. Long-term degradation of channel systems can be challenging to mitigate. Particularly in urban areas, predicting responses of stream channels to direct modification and land use and climatic changes can be critically important to the safety and well being of both infrastructure and environment. Assessment methods and models are needed to determine the current condition of the channel, as well as the impacts and responses of the channels as they continue to evolve. Mitigation against unstable channels can be challenging as the cause of the problem may be well outside of the unstable reach. Modeling these unstable systems is highly complex; thus, uncertainty and risk are implicit in predictions. In this Special Issue, we address the problems, challenges, and solutions for dealing with stream channel instability and its impacts.

Prof. Dr. Peggy A. Johnson
Guest Editor

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Keywords

stream channel stability
unstable channels
urban channels
channel stability assessment
bank stabilization
erosion
in-stream structures
sediment transort
sedimentation
channel instability impacts
uncertainty
risk

Published Papers (7 papers)

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Research

2420 KiB

Open AccessFeature PaperArticle

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

by Travis R. Stroth, Brian P. Bledsoe and Peter A. Nelson

Water 2017, 9(4), 271; https://doi.org/10.3390/w9040271 - 12 Apr 2017

Cited by 6 | Viewed by 5449

Abstract

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 (Q_eff) or discharge that transports the most sediment over time; the 1.5-year recurrence interval discharge (Q_1.5); the bankfull discharge (Q_bf); and the discharges associated with 50th (Q_s₅₀) and 75th (Q_s₇₅) 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 Q_s₅₀ and Q_s₇₅ single-discharge designs match the CSR output most closely, followed by the Q_bf, Q_eff, and Q_1.5. The Q_eff 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. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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6325 KiB

Open AccessFeature PaperArticle

Quantitative Spatio-Temporal Characterization of Scour at the Base of a Cylinder

by Pol Bouratsis, Panayiotis Diplas, Clinton L. Dancey and Nikolaos Apsilidis

Water 2017, 9(3), 227; https://doi.org/10.3390/w9030227 - 20 Mar 2017

Cited by 19 | Viewed by 4940

Abstract

The measurement of the morphologic characteristics of evolving sediment beds around hydraulic structures is crucial for the understanding of the physical processes that drive scour. Although there has been significant progress towards the experimental characterization of the flow field in setups with complex geometries, little has been done with respect to the quantitative investigation of dynamic sediment bed geometry, mainly due to the limited capabilities of conventional instrumentation. Here, a recently developed computer vision technique is applied to obtain high-resolution topographic measurements of the evolving bed at the base of a cylinder during clear water scour, without interrupting the experiment. The topographic data is processed to derive the morphologic characteristics of the bed such as the excavated volume and the slopes of the bed. Subsequently, the rates of scour and the bathymetry at multiple locations are statistically investigated. The results of this investigation are compared with existing flow measurements from previous studies to describe the physical processes that take place inside a developing scour hole. Two distinct temporal phases (initial and development) as well as three spatial regions (front, side and wake) are defined and expressions for the statistical modelling of the bed features are derived. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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1704 KiB

Open AccessArticle

Quantifying River Channel Stability at the Basin Scale

by Philip J. Soar, Nicholas P. Wallerstein and Colin R. Thorne

Water 2017, 9(2), 133; https://doi.org/10.3390/w9020133 - 17 Feb 2017

Cited by 28 | Viewed by 7749

Abstract

This paper examines the feasibility of a basin‐scale scheme for characterising and quantifying river reaches in terms of their geomorphological stability status and potential for morphological adjustment based on auditing stream energy. A River Energy Audit Scheme (REAS) is explored, which involves integrating stream power with flow duration to investigate the downstream distribution of Annual Geomorphic Energy (AGE). This measure represents the average annual energy available with which to perform geomorphological work in reshaping the channel boundary. Changes in AGE between successive reaches might indicate whether adjustments are likely to be led by erosion or deposition at the channel perimeter. A case study of the River Kent in Cumbria, UK, demonstrates that basin‐wide application is achievable without excessive field work and data processing. However, in addressing the basin scale, the research found that this is inevitably at the cost of a number of assumptions and limitations, which are discussed herein. Technological advances in remotely sensed data capture, developments in image processing and emerging GIS tools provide the near‐term prospect of fully quantifying river channel stability at the basin scale, although as yet not fully realized. Potential applications of this type of approach include system‐wide assessment of river channel stability and sensitivity to land‐use or climate change, and informing strategic planning for river channel and flood risk management. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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43185 KiB

Open AccessArticle

Effects of Different In-Stream Structure Representations in Computational Fluid Dynamics Models—Taking Engineered Log Jams (ELJ) as an Example

by Yuncheng Xu and Xiaofeng Liu

Water 2017, 9(2), 110; https://doi.org/10.3390/w9020110 - 10 Feb 2017

Cited by 16 | Viewed by 6206

Abstract

In-streamstructurescontributegreatlytothebiodiversityinstreamsandplayanimportant role in restoring and protecting rivers. They usually have complex geometries. To evaluate their impact and effectiveness, computational models are increasingly used. However, how to faithfully represent them in computer models remains a challenge. Often, simpliﬁcations have to be made. This work evaluated the effects of geometric simpliﬁcation of an example in-stream structure, an engineered log jam (ELJ), in computational models. Three different representations were considered, namely full resolution, the porous media model and the solid barrier model. The turbulent ﬂow was resolved with large eddy simulation (LES). First, the simulations were validated with a physical experiment in a ﬂume. Then, the results from the three models were comparedandanalyzedonvariousaspectsrelatedtothestabilityandfunctionalitiesofthestructures. Unsurprisingly, it is found that the porous media model and the solid barrier model, which are computationally economic, can describe the ﬂow dynamics only to some extent. From the calibration ofdragforceandwakelength,wefoundthattheequivalentgrainsized50 intheporositymodelshould scale as the key element diameter for the simulated ELJ. A wake length scale analysis was performed for the semi-bounded ﬂow around this in-stream structure near the bank. The length estimator in the literature for unbounded vegetation patches can be used with modiﬁcations. The results also show that the ﬂow passing through the porous in-stream structure has a signiﬁcant impact on mean velocity, turbulence kinetic energy, sediment transport capacity and integral wake length. Since geometrically-fully-resolved simulations are not currently feasible for engineering practices, the following suggestions are made based on this study. If the near-ﬁeld and wake are important for the purpose of the structure, the well-calibrated porosity model seems to perform better than the solid barrier model. However, care needs to be taken when interpreting the results because this work also identiﬁed substantial loss of physical information with the porosity model. When the emphasis is the far ﬁeld away from the structure, both the porosity model and the solid barrier model give comparable results Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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9987 KiB

Open AccessArticle

Flow Patterns and Morphological Changes in a Sandy Meander Bend during a Flood—Spatially and Temporally Intensive ADCP Measurement Approach

by Elina Kasvi, Leena Laamanen, Eliisa Lotsari and Petteri Alho

Water 2017, 9(2), 106; https://doi.org/10.3390/w9020106 - 10 Feb 2017

Cited by 45 | Viewed by 13826

Abstract

The fluvio-geomorphological processes in meander bends are spatially uneven in distribution. Typically, higher velocities and erosion take place near the outer bank beyond the bend apex, while the inner bend point bar grows laterally towards the outer bank, increasing the bend amplitude. These dynamics maintain the meander evolution. Even though this development is found in meandering rivers independent of soil or environmental characteristics, each river still seems to behave unpredictably. The special mechanisms that determine the rate and occasion of morphological changes remain unclear. The aim of this study is to offer new insights regarding flow-induced morphological changes in meander using a novel study approach. We focused on short-term and small-spatial-scale changes by conducting a spatially and temporally (daily) intensive survey during a flood (a period of nine days) with an ADCP attached to a remotely controlled mini-boat. Based on our analysis, the flood duration and the rate of discharge increase and decrease seems to play key roles in determining channel changes by controlling the flow velocities and depth and the backwater effect may have notable influence on the morphological processes. We discuss themes such as the interaction of inner and outer bend processes and the longer-term development of meander bends. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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6567 KiB

Open AccessArticle

Channel Planform Dynamics Monitoring and Channel Stability Assessment in Two Sediment-Rich Rivers in Taiwan

by Cheng-Wei Kuo, Chi-Farn Chen, Su-Chin Chen, Tun-Chi Yang and Chun-Wei Chen

Water 2017, 9(2), 84; https://doi.org/10.3390/w9020084 - 30 Jan 2017

Cited by 24 | Viewed by 7018

Abstract

Recurrent flood events induced by typhoons are powerful agents to modify channel morphology in Taiwan’s rivers. Frequent channel migrations reflect highly sensitive valley floors and increase the risk to infrastructure and residents along rivers. Therefore, monitoring channel planforms is essential for analyzing channel stability as well as improving river management. This study analyzed annual channel changes along two sediment-rich rivers, the Zhuoshui River and the Gaoping River, from 2008 to 2015 based on satellite images of FORMOSAT-2. Channel areas were digitized from mid-catchment to river mouth (~90 km). Channel stability for reaches was assessed through analyzing the changes of river indices including braid index, active channel width, and channel activity. In general, the valley width plays a key role in braided degree, active channel width, and channel activity. These indices increase as the valley width expands whereas the braid index decreases slightly close to the river mouth due to the change of river types. This downstream pattern in the Zhuoshui River was interrupted by hydraulic construction which resulted in limited changes downstream from the weir, due to the lack of water and sediment supply. A 200-year flood, Typhoon Morakot in 2009, induced significant changes in the two rivers. The highly active landscape in Taiwan results in very sensitive channels compared to other regions. An integrated Sensitivity Index was proposed for identifying unstable reaches, which could be a useful reference for river authorities when making priorities in river regulation strategy. This study shows that satellite image monitoring coupled with river indices analysis could be an effective tool to evaluate spatial and temporal changes in channel stability in highly dynamic river systems. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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6470 KiB

Open AccessArticle

Generation of a Design Flood-Event Scenario for a Mountain River with Intense Sediment Transport

by Alessio Radice, Laura Longoni, Monica Papini, Davide Brambilla and Vladislav Ivov Ivanov

Water 2016, 8(12), 597; https://doi.org/10.3390/w8120597 - 16 Dec 2016

Cited by 24 | Viewed by 6077

Abstract

International directives encourage the incorporation of sediment transport analyses into flood risk assessment, in recognition of the significant role played by sediment in flood hazard. However, examples of risk analysis frameworks incorporating the effect of sediment transport are still not widespread in the literature, resulting in a lack of clear guidelines. This manuscript considers a study site in the Italian Alps and presents a hydro-morphologic model for generation of flood scenarios towards hazard assessment. The analysis is concentrated on a design flood event with 100-year return period, for which an outflowing discharge is computed as a result of the river modeling. However, it is also argued how suitable model input parameter values can be obtained from analyses of river flows in a yearly duration curve. Modeling tools are discussed with respect to their capabilities and limitations. The results of the analysis are site-specific, but the proposed methodology can be exported to other hydro-graphic basins. Full article

(This article belongs to the Special Issue Stream Channel Stability, Assessment, Modeling, and Mitigation)

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