Conveyance Capacity Of Channels Research Articles

Effects of sediment transport on flood hazards: Lessons learned and remaining challenges

Sediment transport (bedload and suspension) plays a relevant role in the morphological response of river channels to large floods. Sediment load controls erosion and aggradation patterns during high flows, drives the migration of macroforms and contributes to the definition of thresholds for bank erosion and channel instability. Given this influence on channel morphology, it is clear that sediment transport influences both the channel geometry and water stage reached during floods and should therefore be considered in flood hazard analysis. So far, however, legislation on flood hazard and flood management plans still continues to typically disregard sediment transport. This is due mainly to the paucity of available data on sediment transport and the lack of standardized approaches for its integration into flood hazard analysis. The present work, which aims to provide some guidance on how to advance in this issue, has undertaken a bibliometric study and thorough review of the published scientific literature that had previously addressed the implications of sediment transport on flood hazards assessment. This review showed that: i) most studies on the influence of sediment transport on flood risk have focused on mountain streams, as they are highly sensitive systems to changes in sediment supply; ii) research has also focused on explaining historical changes in channel-conveyance capacity and flood frequency based on long-term trends in sediment supply; and iii) recent developments in hydrodynamic and morphodynamic numerical models provide opportunities to explicitly incorporate sediment transport in flood hazard analysis. However, despite the recent advances, we have identified important challenges and discussed needs to better consider sediment transport in flood hazards at different spatial and temporal scales.

Flow resistance of flexible vegetation in real‐scale drainage channels

AbstractThe definition of simple and accurate methods to estimate flow resistance in vegetated channels is still a challenging issue in soil bioengineering practices and programming riparian vegetation management to control channel conveyance capacity, sediment deposition, and flooding propensity. In this paper, measurements collected by Errico et al. (2018, 2019) in drainage channels colonized by common reed (Phragmites australis) were used to study the effect of flexible vegetation and its management in flow resistance estimate. At first, a theoretical flow resistance equation, obtained applying dimensional analysis and incomplete self‐similarity condition for the velocity distribution of an open channel flow, was briefly summarized. Then, this flow resistance equation was calibrated and tested by open‐field hydraulic experiments carried out by Errico et al. (2018, 2019) at the real scale of existing vegetated drainage channels. In particular, the Γ function of the power velocity profile was empirically related to the slope energy and the flow Froude number by using the available measurements. Taking into account the hydrological regime of the flow in the investigated channels, the original data set was divided into two sub‐data sets (calibrating and testing data set) exploring the same range of measured discharges. The calibration and testing of the flow resistance equation were carried out without distinguishing measurements corresponding to different vegetation conditions (full‐vegetated, half‐vegetated, non‐vegetated, central vegetation cut, extensive vegetation cut). The analysis demonstrated that the theoretical flow resistance equation allows an accurate estimate of the Darcy‐Weisbach friction factor which is characterized by errors that are always less than 10% and less than or equal to 5% for 90.9% of the investigated cases. The finding of this study also allowed to evaluate the effects of different vegetation management scenarios on flow resistance.

Incorporating Network Scale River Bathymetry to Improve Characterization of Fluvial Processes in Flood Modeling

AbstractSeveral studies have focused on the importance of river bathymetry (channel geometry) in hydrodynamic routing along individual reaches. However, its effect on other watershed processes such as infiltration and surface water (SW)‐groundwater (GW) interactions has not been explored across large river networks. Surface and sbsurface processes are interdependent, therefore, errors due to inaccurate representation of one watershed process can cascade across other hydraulic or hydrologic processes. This study hypothesizes that accurate bathymetric representation is not only essential for simulating channel hydrodynamics but also affects subsurface processes by impacting SW‐GW interactions. Moreover, quantifying the effect of bathymetry on surface and subsurface hydrological processes across a river network can facilitate an improved understanding of how bathymetric characteristics affect these processes across large spatial domains. The study tests this hypothesis by developing physically based distributed models capable of bidirectional coupling (SW‐GW) with four configurations with progressively reduced levels of bathymetric representation. A comparison of hydrologic and hydrodynamic outputs shows that changes in channel geometry across the four configurations has a considerable effect on infiltration, lateral seepage, and location of water table across the entire river network. For example, when using bathymetry with inaccurate channel conveyance capacity but accurate channel depth, peak lateral seepage rate exhibited 58% error. The results from this study provide insights into the level of bathymetric detail required for accurately simulating flooding‐related physical processes while also highlighting potential issues with ignoring bathymetry across lower order streams such as spurious backwater flow, inaccurate water table elevations, and incorrect inundation extents.

Assessing Riverbed Surface Destabilization Risk Downstream Isolated Vegetation Elements

A few decades ago, river erosion protective approaches were widely implemented, such as straightening the river course, enhancing riverbed/bank stability with layers of concrete or riprap, and increasing channel conveyance capacity (i.e., overwidening). However, recent research has established that such practices can be tremendously costly and adversely affect the rivers’ ecological health. To alleviate these effects, green river restoration has emerged as a sustainable and environmentally friendly approach that can reduce the negative impact of the riverbed and bank destabilization and flooding. One of the typical green restoration measures, especially for instream habitat improvement, is the establishment of instream vegetation, which leads to a more diversified flow regime, increasing habitat availability and serving as refugia for aquatic species. Within the perspective presented above, flow–vegetation interaction problems for several decades received significant attention. In these studies, rigid rods have commonly been used to simulate these vegetative roughness elements without directly assessing the riverbed destabilization potential. Here, an experimental study is carried out to investigate the effect of different instream vegetation porosity on the near-bed flow hydrodynamics and riverbed destabilization potential for a range of simulated vegetation species. Specifically, the flow field downstream, four distinct simulated vegetation elements is recorded using an acoustic Doppler velocimetry (ADV), assuming about the same solid volume fraction for the different vegetation elements. In addition, bed destabilization potential is assessed by recording with optical means (a He-Ne laser with a camera system) the entrainment rate of a 15 mm particle resting on the uniform bed surface and the number of impulses above a critical value. Results revealed that the number of impulses above a critical value at the normalized distance equal to two is a good indicator for cylinder and five for other vegetation to assess the riverbed destabilization potential. The experimental findings from this study have interesting geomorphological implications regarding the destabilization of the riverbed surface (removal of coarse particles induced by high magnitude turbulent impulses) and the successful establishment of seedlings downstream of instream vegetation.

Combined effect of a mobile bed and floodplain edge vegetation on compound channel conveyance

This technical note presents results from flume experiments conducted in a straight prismatic asymmetric compound channel with two different floodplain roughnesses and rigid emergent vegetation along the floodplain edge in combination with a mobile and fixed main channel bed, respectively. It is found that the contribution of floodplain roughness and vegetation along the floodplain edge to the bulk friction factor is considerably reduced when studied in combination with a mobile main channel bed featuring bedforms. This in turn means that the effect of vegetation along the floodplain edge on the conveyance capacity and bulk friction factor may be overestimated if it is studied solely with fixed bed conditions. The presented results reveal the need to improve our understanding of the complex interplay between mobile beds and vegetation along the floodplain edge and its combined effect on compound channel conveyance capacity and hydraulics.

The Influence of Tropical Cyclones on the Evolution of River Conveyance Capacity in Puerto Rico

AbstractTropical cyclones (TCs) in Puerto Rico and other tropical environments cause some of the world's most intense rainfall rates and pose major challenges to current and future flood resilience. One potential consequence of TC‐induced floods is the reconfiguration of river channels due to sediment scour or deposition. This reconfiguration has the potential to alter subsequent flood hazard by changing channel conveyance capacity, violating statistical independence assumptions that underpin conventional recurrence interval concepts. In this study, we examine changes in channel conveyance capacity in Puerto Rico and compare these changes to streamflow trends. Results show that relatively modest long‐term changes in river channel capacity are composed of numerous short‐term transients which are of much larger magnitude. These transients are most often caused by abrupt scour or deposition during TCs and are comparable in magnitude to long‐term trends in peak streamflows. An abrupt change is typically followed by a multiyear “recovery period” as the channel reestablishes quasi‐equilibrium. Flood events with recurrence intervals of approximately 10 years and above appear to be sufficient to cause these changes; channel reconfiguration thus may be more common in Puerto Rico than in less extreme hydroclimates. Short‐term conveyance capacity changes are not considered in typical flood hazard assessments, which could substantially overstate or understate flood threat at any particular time, depending on the recent history of TC rainfall and flooding. Improving flood resiliency in Puerto Rico and elsewhere will require better understanding of rapid conveyance capacity changes, their causes, and how they influence flood hazard and risk.

River channel conveyance capacity adjusts to modes of climate variability

River networks are typically treated as conduits of fixed discharge conveyance capacity in flood models and engineering design, despite knowledge that alluvial channel networks adjust their geometry, conveyance, planform, extent and drainage density over time in response to shifts in the magnitude and frequency of streamflows and sediment supply. Consistent relationships between modes of climate variability conducive to wetter-/drier-than-average conditions and changes in channel conveyance have never been established, hindering geomorphological prediction over interannual to multidecadal timescales. This paper explores the relationship between river channel conveyance/geometry and three modes of climate variability (the El Niño–Southern Oscillation, Atlantic Multidecadal Oscillation, and Arctic Oscillation) using two-, five- and ten-year medians of channel measurements, streamflow, precipitation and climate indices over seven decades in 67 United States rivers. We find that in two thirds of these rivers, channel capacity undergoes coherent phases of expansion/contraction in response to shifts in catchment precipitation and streamflow, driven by climate modes with different periodicities. Understanding the sensitivity of channel conveyance to climate modes would enable better river management, engineering design, and flood predictability over interannual to multidecadal timescales.

Quantifying the combined effects of multiple extreme floods on river channel geometry and on flood hazards

Effects of flood-induced bed elevation and channel geometry changes on flood hazards are largely unexplored, especially in the case of multiple floods from the same site. This study quantified the evolution of river channel and floodplain geometry during a repeated series of hypothetical extreme floods using a 2D full hydro-morphodynamic model (LHMM). These experiments were designed to examine the consequences of channel geometry changes on channel conveyance capacity and subsequent flood dynamics. Our results revealed that extreme floods play an important role in adjusting a river channel to become more efficient for subsequent propagation of floods, and that in-channel scour and sediment re-distribution can greatly improve the conveyance capacity of a channel for subsequent floods. In our hypothetical sequence of floods the response of bed elevation was of net degradation, and sediment transport successively weakened even with floods of the same magnitude. Changes in river channel geometry led to significant impact on flood hydraulics and thereby flood hazards. We found that flood-induced in-channel erosion can disconnect the channel from its floodplain resulting in a reduction of floodwater storage. Thus, the frequency and extent of subsequent overbank flows and floodplain inundation decreased, which reduced downstream flood attenuation and increased downstream flood hazard. In combination and in summary, these results suggest that changes in channel capacity due to extreme floods may drive changes in flood hazard. The assumption of unchanging of river morphology during inundation modelling should therefore be open to question for flood risk management.

Supporting decision-making for channel conveyance maintenance

Flood risk management agencies spend a large amount of their budgets maintaining the conveyance capacity of river channels. Vegetation and its overgrowth is one of the main factors influencing channel capacity. Sediment deposits can also impact watercourse conveyance by reducing flow area. Maintenance works such as vegetation cutting, dredging and desilting aim to maintain or improve channel conveyance capacity. Failure to guarantee the adequate capacity of watercourses during flood events exposes communities and critical infrastructure to severe damages. Despite the undesirable consequences associated with flooding, it is rare that sufficient funds are made available to undertake all necessary activities. In the face of limited budgets, flood risk managers must take difficult decisions in order to decide how and where limited resources should be invested. This paper presents a national scale approach to identify the strategically important watercourses where conveyance related works produce the greatest benefit. This is done identifying watercourses where maintenance works may have a potential to positevely maintain or increase conveyance capacity and watercourses where the attribution of area or receptors benefiting is important. Assembling together this information produces a picture of the strategic lengths of watercourse to be maintained. For example, maintenance works may have a very positive impact in one particular watercourse, reducing effectively water levels and increasing conveyance capacity, but without modifying the flood risk of adjacent properties and thus, not producing any benefit. The paper discusses the tools and models used in the approach, the necessary data inputs, its quality requirements and the best metrics to report the maintenance work performed and its benefits. The approach has been develop for the Environment Agency in United Kingdom to inform the benefits of its annual maintenance programme so the results of the application of the approach in England are presented. The final aim of the methodology presented is to support the decision-making process of flood asset managers ensuring a proactive flood risk-based management approach.

Experimental and numerical study on hydrodynamics of riparian vegetation

Recently, many channelized rivers tend to be heavily vegetated due to regime shifts in hydrological, fluvial and ecological processes. Dense vegetation in a river frequently obstructs a flood flow and reduces conveyance capacity of channels. On the other hand, river vegetation provides various ecological services such as habitats for various species and life, natural cycle of organic and inorganic substances, etc‥ It is of engineering importance to understand vegetation hydrodynamics in order to preserve vegetation nature and keep a certain level of flow conveyance capacity. In view that willows tend to be densely vegetated along the shoreline of floodplains or sandbars, a field measurement, a physical model experiment and a numerical analysis were carried out for investigating hydrodynamics in an open channel with riparian vegetation. Discussion was made focusing on flow and shear layer structures developed around the vegetation canopy.

Optimal Hedging Rules for Reservoir Flood Operation from Forecast Uncertainties

This paper develops optimal hedging rules for reservoir flood control operation under hydrological uncertainty using hydroeconomic and mathematical analysis. The capacity to convey flood flows is sometimes a scarce resource. Hedging for flood operations uses reservoir storage to allocate the expected flood-safety margin (EFSM, i.e., the gap between expected flood volume and flood-conveyance capacity) optimally between present and future periods. Optimal flood-operation hedging falls into three cases, namely, (1) for large expected floods, all flood storage and almost all channel-conveyance capacity are used in the current period to cope with the current, more certain, and urgent flood risk; (2) for medium expected floods, the available EFSM is balanced between the current and future periods, but a larger portion of the total EFSM remains allocated to the current stage; and (3) for small expected floods, the future stage receives greater EFSM allocation by keeping reservoir space empty in the current period. Optimal hedging for flood operation is illustrated by a curve similar to that of hedging for water supply. The physical implications of hedging highlight the economic significance of this practice for balancing the marginal value of scarce flood-management resources under uncertainty.

Response to disturbance in a highly managed alluvial river: Does it conform to Le Chatelier's general law?

A magnitude 6.8 earthquake triggered a channel-damming landslide causing the lower Cedar River, Washington, to occupy an existing floodplain side channel. The channel and floodplain ecosystem initially underwent rapid geomorphic changes until the system energy approached a new system state according to Le Chatelier's general law. Multiple lines of evidence support observations that channel expansion proceeded rapidly as energy was focused on the banks of the overfit side channel until the channel's geomorphology was consistent with total system energy available for work. Large trees recruited to the channel helped create lateral channel expansion as well as bed topographic variability. One year of pre-disturbance data was compared with three years of post-disturbance data in a spatially explicit analysis of the complexity of channel form and bed surface elevations. Estimates of changes in sediment storage and erosion and, channel conveyance capacity were calculated for all time periods and interannual comparisons were carried out. Statistical analyses of aggraded and eroded areas of the channel indicated a rapid increase in complexity of the channel during the first three years and then exhibited decreasing complexity in the final year of the study.

Forest blowdown impacts of Hurricane Rita on fluvial systems

AbstractHurricane Rita, a category three hurricane which struck the US Gulf Coast near the Louisiana/Texas border in 2005, did not cause extensive river flooding. However, the storm did result in extensive forest damage and tree blowdown. High‐resolution post‐storm aerial photography allowed an inventory of river bank trees blown into the channel along the lower Neches and Sabine Rivers of southeast Texas and southwest Louisiana. Blowdowns directly into the channel averaged 9·3 per kilometer in the lower Neches and 13·4 in the lower Sabine River, but individual reaches 10 to 20 km in length had rates of 20 to 44 blowdowns per kilometer. Though large woody debris (LWD) from Hurricane Rita was widely perceived to reduce the capacity of channels to convey flow, no strong evidence exists of increased flooding or significant reductions in channel conveyance capacity due to LWD from the storm. The Rita blowdown inventory also allowed an assessment of whether similar blowdown events could account for major logjams and rafts on Red, Atchafalaya, and Colorado Rivers on the Gulf Coast, which blocked navigation from tens to hundreds of kilometers in the 1800s. Results from Hurricane Rita suggest that blowdown into channels alone – not withstanding blowdown elsewhere in the river valleys or along tributaries which could deliver LWD to the river – is sufficient to completely block channels, thus providing a plausible mechanism for initiating such (pre)historic log rafts. Copyright © 2009 John Wiley & Sons, Ltd.

VELOCITY MEASUREMENTS AROUND NON-SUBMERGED AND SUBMERGED SPUR DYKES BY MEANS OF LARGE-SCALE PARTICLE IMAGE VELOCIMETRY

Field observations on the surface flow were carried out in a straight reach of the Uji River. Velocity distributions on the water surface were obtained based on the captured video images by means of large-scale particle image velocimetry (LSPIV). Flow structures around spur dykes are studied for both non-submerged and submerged cases.It is clearly seen that, for both cases, the dykes are working as an additional roughness. Their effect on channel conveyance capacity is estimated in terms of the effective channel width calculated from the velocity distributions. It is also of interest that the maximum velocity filament appears at a different position as the water depth varies in case of the channel with single-side dykes such as studied here. In addition, large-scale boils shedding from the submerged dykes are clearly observed and their origin and advection mechanisms are also evaluated.