Backwater Rise Research Articles

Backwater rise upstream of a stream-crossing culvert due to large wood accumulation and sediment deposition

Backwater rise upstream of a stream-crossing culvert due to large wood accumulation and sediment deposition

Analysis of hydrodynamic behavior in response to diverse pile arrangements adjacent to an impermeable dike

ABSTRACT Intense energy flow near the dike head significantly influences the development of the dike field zone (DFZ), momentum exchange zone (MEZ), and mainstream zone (MSZ), leading to potential partial or full failure of the dike in the MEZ. To address this, laboratory experiments were conducted to observe flow changes and rate of energy reduction around a single impermeable dike, with alterations made to the pile group length (HL: half-length = 11.5 cm and FL: full length = 23 cm), location (U: upstream, D: downstream), and shape (C: Circular, DV: Delta Van, ST: Streamlined tapered, APF: Angled Plate footing). Modifications in the MEZ flow structure were studied to mitigate concerns about intense energy vortices and dike head flows. These piles helped to assess factors like flow deflection, backwater rise, energy reduction rate, velocity fluctuations, and discharge distribution percentages across the zones. The data revealed that when the pile group length was increased from HL to FL, there was an inverse effect on depth-averaged velocity, MEZ discharge distribution percentage, and flow deflection toward the DFZ. Conversely, there was a direct influence on the rise in backwaters, rate of energy reduction, and discharge distribution percentages in the DFZ. The U-FL-APF pile dike showcased optimal results, displaying a reduction in maximum energy by 52% and streamwise velocity by 96%, while also increasing the backwater rise and discharge distribution by 22% and of only 5% in the MEZ, respectively, compared to a single impermeable dike. During flood events, these suggested measures can mitigate the intensified swirls formed in the DFZ, protect the dike head from direct momentum exchanges in the MEZ, and enhance flow deflection into the MSZ.

A review of flash flood hazards influenced by various solid material sources in mountain environment

Solid material sources, such as sediment, large wood, and vehicles, intensify flash flood hazards. This paper provides a detailed review of processes involving the recruitment, entrainment, transport, and blockage dynamics of various solid material sources. Results indicate that sediment supplied by processes like landslides and debris flows can obstruct river channels, leading to a sudden increase in flash flood levels. The failure of a barrier dam results in an expansion of downstream inundation areas. Large wood and floating vehicles transported by flash floods and debris flows may directly impact and destroy built structures or form blockages at built structures. Blockages lead to a backwater rise, and the sudden amplification of flow during the failure of these blockages causes more severe disasters. Based on these analyses, the paper proposes future research directions primarily focusing on the changes in sediment burial processes caused by the sheltering effects of building groups. Furthermore, the study aims to investigate the flow amplification effects of large wood and vehicle blockage.

Forecasting of sub-surface flow velocity through a temporarily open/close estuary under closed condition using machine learning

TOCE or temporarily open/closed estuary is a complex estuary system created by interaction of longshore transport and river outflows. Under the closed condition when an estuarine sandbar is formed, river flow is impeded creating backwater rise that could result in floods. At present, there are no hydrological models that could cater for river flows under closed estuary conditions. This paper proposes the application of a machine learning (ML) approach to model flow velocity in an estuarine sandbar which is crucial for flow estimations. A TOCE located at Mengabang Telipot, Terengganu, Malaysia was chosen for model development where river water and groundwater levels (inside the sandbar) were measured and tidal levels from an existing station were used. Average flow velocities were calculated using Darcy’s equation to represent observed data. Multilayer perceptron (MLP) neural networks were trained by the Levenberg-Marquardt algorithm and developed concerning the parameters above to predict groundwater average velocity. In this study, the optimal model was a neural network with four neurons in the hidden layer and the degree of influence of each parameter was determined by the results of the sensitivity analysis, where the R2 was obtained to be about 97–99% when predicted data was compared to observed data. Hence, the results indicated that neural networks were able to predict groundwater mean velocity quite well. The developed model was further verified using data from another TOCE from a different time. The results were very good having a R2 of 0.990. Hence, it was concluded that the ML approach was able to model flow in a TOCE and has great potential application in such an environment seeing that it is simpler than complex physics-based models.

Comment on “Momentum and Energy Predict the Backwater Rise Generated by a Large Wood Jam” by Follett, E., Schalko, I. and Nepf, H.

AbstractFollett et al. (2020a, https://doi.org/10.1029/2020gl089346) developed an analytical model to predict backwater rise by log jams, using the size and packing density of logs and the jam length, as well as river slope and bed roughness. We show that the model formulas can be rewritten using the Froude number instead of river slope and roughness, thus improving their applicability in engineering practice. The equation terms and results of Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) are found to be similar to those of the empirically derived formula by Schalko et al. (2018, https://doi.org/10.1061/(asce)hy.1943‐7900.0001501). However, some differences are identified, calling for further study. Most notably, these distinctions pertain to the effect of accumulation porosity, with additional minor differences in the exponent of the Froude number. Lastly, model implications for some broader applications are explored, showing a methodology to calculate the representative log size for log mixtures, and the expected effect of log orientation on backwater rise.

Reply to Comment by Poppema and Wüthrich on “Momentum and Energy Predict the Backwater Rise Generated by a Large Wood Jam”

AbstractFollett et al. (2020, https://doi.org/10.1029/2020gl089346) demonstrated that a large wood jam can be modeled as a porous obstruction with momentum loss proportional to the number, size, and packing density of the logs and jam length. Poppema and Wüthrich (2024, https://doi.org/10.1029/2023gl106348) incorporated uniform flow Froude number, broadening the scope of our work. Here, we demonstrate that Froude number can be directly introduced to equations in the main body of Follett et al. (2020, https://doi.org/10.1029/2020gl089346), without requiring uniform flow. Based on this, we show that a managed increase in upstream depth is possible for conditions below a critical discharge, in which equilibrium upstream depth over uniformly distributed jams can be adjusted with inter‐jam spacing. This design could retain water in low flow conditions, allowing jams to act independently above critical discharge. Finally, we suggest that log orientation can be included in our model by varying both drag coefficient and frontal area perpendicular to the flow.

Formation and growth of wipe blockages due to a vertical obstruction in sewers: Experimental analysis to policy and management strategies

Improper disposal of wipes results in significant sewer blockages, especially during the COVID-19 pandemic. However, the formation and growth of wipe blockages and their backwater effect in sewers have not been studied. This study employed a vertical rod to simulate sewer obstructions such as tree root intrusion in sewers. Stochastic interactions among wipes, turbulent flow, and the obstruction were experimentally studied in a circular pipe (diameter D = 25 cm), with systematic variations in flow Froude number (Fr), wipe length (L), flow depth (H), submerged rod length (hrod), and rod diameter (drod). It was found that the mean area ratio of wipes (ratio of projected area in water to original surface area of the wipe) ranged from 0.14 to 0.30, indicating the distorted shapes in water. The entrapment probability P (ratio of entrapped to released wipes) for a single wipe was closely correlated with Fr, H/D, L/H, drod/H, and hrod/H, with a maximum value of 0.90. With the increase in the number of wipes released, the influence of the obstruction on wipe accumulation processes became negligible. New equations were developed for characterizing entrapment probability, blockage length, and backwater rise, which can be used to predict the development of wipe blockages and sewage levels. Based on the research findings, policy and management strategies were proposed for wipe blockage reduction in sewers.

A review of large wood dynamics relevant to hazard characteristics for built structures

Large wood exerts an influence on the topography and geomorphology of channels in areas of good vegetation development, and it is also an important component of riverine ecosystems. However, large wood may impact and damage buildings. The formation and failure of logjams can cause backwater rise, local scour, and the amplification of flow discharge, leading to more serious disasters. Therefore, quantifying the processes of large wood recruitment, entrainment, transport, deposition, accumulation and logjams failure is highly significant for the prevention of large wood hazards in channels. Based on existing research on large wood, this review provides a comprehensive analysis of large wood dynamics and hazards. First, the processes involved in large wood recruitment, entrainment, transport, and deposition are reviewed. Second, the results of large wood accumulation probability and the backwater rise, local scour, critical conditions of logjams failure and magnitude amplification of the flow discharge caused by large wood are analyzed comprehensively. Finally, large wood-related hazard mitigation measures are also introduced. Future research directions to improve the mitigation of hazards from large wood should concentrate on exploring quantification models for understanding how large wood impacts and accumulates on various built structures.

Blockage slope and ratio effects on box culvert hydraulics

Culvert blockage at the inlet can dramatically alter the local hydraulic characteristics that often increase backwater rise, change the flow patterns, and thereby increase the risk of upstream flooding. The impact of culvert blockage is not usually included in the initial hydraulic design, although it can cause significant losses once the culvert becomes blocked. Culvert hydraulics including backwater rise with respect to blockage slope and ratio has been neglected in the studies conducted so far. In this study, we conducted a series of narrow flume tests to investigate the effects of four bottom blockage slope angles and five blockage ratios on the hydraulic characteristics of box culverts, including backwater rise, and upstream mean velocity at various Froude numbers. The results showed that Froude number, blockage ratio, and slope angle were regarded as the controlling variables for culvert backwater rise. Backwater rise increased dramatically with increasing blockage ratio and Froude number. Compared to blockage ratio B = 0.20, ΔH/hu increased by 2, 5, 9, and 13 times for B = 0.40, 0.60, 0.80, and 1, respectively. However, when the blockage slope angle decreased, the backwater rise reduced and upstream mean velocity increased in compared with vertical blockage. The relative backwater rise reduced from 0.32 to 0.29, when blockage slope angle decreased from 90° to 18.44° at B = 1 and Fo = 0.10. In addition, the length and maximum depth of water surface depression inside the culvert itself were larger with increasing blockage ratio and Froude number. For Fo = 0.10, d/ho = 0.13 and L/ho = 0.94 for B = 0.20 compared to d/ho = 0.32 and L/ho = 4.34 for B = 1. The results were also combined to develop an equation to predict backwater rise in the range 0.10 ≤ Fo ≤ 0.25, 0 ≤ B ≤ 1, and 18.44° ≤ α ≤ 90°. To enhance the accuracy, it is recommended to apply the equation for ΔH/hu ˃ 0.10. It is useful and practical in applications to add the probability of backwater rise or losses owing to culvert blockage within accounting of losses during the hydraulic design of culverts, especially in channels exposed to debris.

Impact of Lateral Gap on Flow Distribution, Backwater Rise, and Turbulence Generated by a Logjam

AbstractLogjams may form at natural obstructions and are also used as nature‐based solutions for river restoration and natural flood management. Previous research has described backwater rise due to logjams that span the full channel cross‐section and logjams with a gap between the lower edge of the logjam and the bed. Logjams that fill the channel depth, but not its width, leaving a lateral gap between the logjam and the channel bank, are also common natural formations and the focus of this study. The flow distribution between the logjam and the lateral gap, backwater rise, and wake turbulence are key factors in determining the ecologic and flood risk impact of a logjam. Specifically, relative to a channel‐spanning logjam, the introduction of a lateral gap can reduce backwater rise and increase the potential for trapping particles, such as nutrients or microplastics, within the wake region, but may also promote erosion in the gap. The choice of logjam and gap widths can be used to maximize flow and habitat diversity in rivers, while reducing erosion risk. We present experimental results demonstrating that the flow distribution between the logjam and the lateral gap can be predicted by assuming equal resistance through the logjam and gap sections. Further, we show that backwater rise can be determined from the predicted discharge through the logjam using a momentum balance developed for channel‐spanning logjams. Finally, turbulence generated within the jam was observed directly downstream of the logjam, and, for the densities considered, increased with jam density.

A momentum-based prediction model of backwater rise within and downstream of an emergent canopy in nonuniform flow

Vegetation is widely used to restore waterways and wetland ecosystems. However, emergent canopies create a nonuniform water surface, and predicting the backwater rise is necessary to evaluate the flood hazard. This paper investigates the water surface elevation from downstream to upstream of an emergent canopy in a steady nonuniform flow and predicts the upstream backwater rise. Laboratory flume experiments are conducted on rigid vegetation with varying coverage exposed to different flow rates. The backwater rise and the nonuniformity of vegetated flow are explored based on a momentum equation, and a general formula for backwater rise within and downstream of an emergent canopy is proposed for nonuniform flow. The results reveal that a unified phase diagram of backwater rise can be derived from the momentum model, which presents the variation of both the upstream and downstream water surface elevations and acquires a threshold to identify strong or weak nonuniformity in vegetated flow. The upstream water depth H1 exhibits a nonmonotonic variation, first decreasing and then increasing with water depth H2 at the trailing edge of the emergent canopy. Two partitions of subcritical flow and supercritical flow for the downstream water depth H3 are quantified by the experimental data. The proposed model can be used to accurately predict the backwater rise based on a dimensionless vegetated parameter Cveg and unit discharge q, as demonstrated by both the present and previous experimental results. This work improves the understanding and predictability of the nonuniformity and backwater rise generated by emergent canopies in natural or ecology-restored rivers.

Laboratory study on relative energy loss and backwater rise at bridge piers and abutment

Laboratory study on relative energy loss and backwater rise at bridge piers and abutment

Field-based monitoring of instream leaky barrier backwater and storage during storm events

Engineered leaky barriers are increasingly used as natural flood management methods providing ecosystem and water quality benefits in addition to flood attenuation, complementing hard engineering flood defences. Field-based monitoring of a natural flood management site, Wilde Brook in the Corvedale catchment, England (UK) studied the rainfall-runoff relationship for a 5.36 km reach with 105 leaky barriers over two years. Paired pressure transducers were placed upstream and downstream of three channel spanning leaky barriers, allowing evaluation of upstream backwater rise relative to rainfall intensity, storm magnitude, and frequency. By increasing backwater rise, the leaky barriers caused overbank flows, resulting in a reduction in the cross-sectional area velocity after the event. The incidence of overbank flow depended on the local stream cross-sectional profile, barrier properties, location in the reach, and storm magnitude. Barrier operational flow conditions were classified into five modes according to relative bank inundation and barrier submergence extent. The backwater rise magnitude depended on barrier physical properties and evolution over time through cycles of accretion and build-up of brash and leafy material in the barrier, in addition to local bedload sediment transport dynamics, where instances of scour around the barriers were observed. Backwater rise and net volume hydrographs showed rapid filling up behind the barriers on the rising limb and slower water release on the falling limb. For a ∼ 4 yr return period storm event, results indicated that one leaky barrier increased storage volume by up to 102 m3, which translates to an overall net volume increase of ∼ 10,700 m3 for the full reach. These new findings provide quantitative evidence of leaky barrier backwater and storage performance, and leaky barrier design recommendations for storms up to a 4 yr return period. This evidence can be used to develop and validate flood modelling generalised approaches for smaller, more frequent storms, and work towards the development of an approach for modelling leaky barriers for larger storm magnitudes.

Investigation of flow characteristics in open channel with leaky barriers

• Backwater rise and flow characteristic of open channel with leaky barrier were experimentally investigated. • Theoretical equation of the water level upstream the leaky barrier was presented. • The equation was easy to use and agreed well with the experimental results. • Solid volume fraction of the leaky barrier was the primary factor affecting backwater rise. Traditional flood control projects, such as dams and gates, may disrupt river connectivity, resulting in the fragmentation of river habitats. Leaky barriers, as a new method of flood defense, are gradually used in flood management especially in small rivers due to functions of the engineering in ecological and connecting rivers and regulating floods. Understanding the hydraulic effects and especially the backwater rise caused by leaky barriers is necessary to assess its impact and optimize the design. This paper investigates the influence of the leaky barrier on hydrodynamics through a designed flume experiment. A formula that reveals the backwater effects of the barrier was obtained by solving the momentum and continuity equations. Comparing the predicted and measured values of upstream water depth, the difference was within a reasonable range. Based on the results, the factors affecting the backwater rise are the Froude number of the approaching flow, downstream water depth, and the longitudinal length, drag coefficient, the solid volume fraction of the leaky barriers. Among the characteristics of leaky barrier, the solid volume fraction is the main factor affecting the backwater effect. The findings are helpful to understand the hydraulic influence of leaky barriers and can provide references for practical project.

Laboratory flume experiments on the characteristics of large wood accumulations from debris flow and the backwater rise at slit-check dams

Laboratory flume experiments on the characteristics of large wood accumulations from debris flow and the backwater rise at slit-check dams

Countermeasure against local scouring and tsunami damage by landward forests behind a coastal embankment

In the Great East Japan Tsunami, many coastal embankments collapsed due to excessive local scouring at the embankment toe. Thus, as a countermeasure against local scouring and tsunami damage, the present study performed a series of flume experiments with landward forests models (the combination of a short frontal forest (SF) and a landward tall forest (TF)) behind a coastal embankment model (EM) on a gravel bed. The damming of washed-out trees as debris occurs due to the excessive local scouring around the embankment; if they are properly managed, they could also contribute to reducing the tsunami energy. Thus, the effect of damming by tsunami-borne driftwood on scouring was also investigated. The maximum reductions of the scour depth at the EM toe, maximum scour depth, and scour length were found to be approximately 44.1–65.2%, 30.2–58.6%, and 21.9–41.9% compared with the single embankment system when the SF was placed between the EM and TF (i.e., Case 3). Moreover, damming (i.e., Case 5) also effectively reduced the local scouring around the EM toe. However, at the highest overflow depth, significant scouring was observed beneath the formed driftwood dam, which implies the washout of TF. Both Case 3 and the damming case increased the energy reduction due to the greater shift of the position of a submerged hydraulic jump toe toward the EM crest and an increased backwater rise in front of TF. However, considering the vulnerability of TF, Case 3 is recommended to mitigate a large tsunami.

Numerical Investigation of the Effectiveness of Vegetation-Embankment Hybrid Structures for Tsunami Mitigation Introduced after the 2011 Tsunami

As a mitigation measure against a tsunami inundation, vegetation-embankment hybrid structures received attention after the 2011 Great East Japan Tsunami, and some structures have already been constructed or are under construction in Japan. The present study conducted a series of numerical simulations using a hybrid system comprised of an artificial structure (an embankment, moat) and a natural component (vegetation) that was experimentally proposed in previous studies as an effective structure for tsunami mitigation. After validating the numerical model using published data, this study investigated differences in the performance of the hybrid system by changing the tsunami period and height characteristics of the tsunami-like surge-type flow. As a result, the delay in tsunami arrival time (ΔT) was not affected by the tsunami wave period for the investigated hybrid structures. Among the investigated structures, Case Ve40ME (where Ve40, M, and E represent vegetation, moat, and embankment, respectively, in that order from seaward) showed the maximum performance of ΔT. The reductions of overflow volume (ΔQ), fluid force index (RFI), and moment index (RMI) declined during the tsunami period. The tsunami mitigation effect is closely related to the relationship between the development times of backwater rise, hydraulic jump, and the tsunami period. Case Ve40ME was effective for ΔT, ΔQ, and RMI. Case EMVe40 was especially effective for RFI. When the tsunami period is short, the water level at the shoreline starts to decrease before full development of the hydraulic jump generated in the hybrid system. Thus, overflow volume to landward decreases, and the mitigation effects increase. When the tsunami period is long, the receding phenomenon at the peak water level does not affect the maximum values, thus the mitigation effects become smaller compared with the short period. However, the superiority to other structures is maintained in Case Ve40ME and Case EMVe40 with seaward vegetation and landward vegetation, respectively.

Logjams With a Lower Gap: Backwater Rise and Flow Distribution Beneath and Through Logjam Predicted by Two‐Box Momentum Balance

AbstractLogjams with a gap at the bed form naturally in small channels and are used in engineering practice to maintain river connectivity at base flow. Limited understanding of a jam's effect on backwater rise and flow velocity limits assessment of geomorphic and ecological impacts of natural logjams, design of river restoration interventions, and representation in flood models. The distribution of flow through and beneath the jam satisfies a two‐box, momentum‐based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. The model was validated with 68 flume experiments. Backwater rise is predicted from discharge beneath the jam following established models for solid sluice gates. As a result, backwater rise increases with jam resistance, which forces a greater discharge beneath the jam. Below‐jam velocity and Shields parameter increased with ratio of friction coefficient to slope and decreasing gap height.

Wood Retention at Inclined Bar Screens: Effect of Wood Characteristics on Backwater Rise and Bedload Transport

In forested mountain catchment areas, both bedload and large wood (LW) can be transported during ordinary flows. Retention structures such as sediment traps or racks are built to mitigate potential hazards downstream. Up to now, the design of these retention structures focuses on either LW or bedload. In addition, the majority of LW retention racks tend to retain both LW and bedload, while bedload transport continuity during ordinary flows is an important aspect to be considered in the design. Therefore, a series of flume experiments was conducted to study the effect of LW accumulations at an inclined bar screen with a bottom clearance on backwater rise and bedload transport. The main focus was put on testing different LW characteristics such as LW size, density, fine material, and shape (branches and rootwads), as well as a sequenced flood. The results demonstrated that a few logs (wood volume of ≈ 7 m3 prototype scale with a model scale factor of 30) are sufficient to reduce the bedload transport capacity to below 75% compared to the condition without LW. Fine material and smaller wood sizes further reduced bedload transport and increased backwater rise. In contrast, LW density and LW shape had a negligible effect. The test focusing on a sequenced flood highlighted the need for maintenance measures to avoid self-flushing of the bed material. The results of this study further indicate that an inclined bar screen may need to be adapted by considering LW characteristics in the design of the bottom clearance to enable bedload continuity during ordinary flows.

Local geomorphic effects in the presence of accumulations of different densities

Mobile components of dynamic river systems are varied, and more than fundamental water and sediment interactions must be considered to assess morphodynamic changes. Here, the effects of instream wood and plastic accumulations on the riverbed are modelled and assessed in the presence of a hydraulic structure. We observe a recent tendency towards considering instream wood and plastics as integral riverine system components. Such shift is driven by a multidisciplinary interest in instream wood and riverine plastic dynamics, including those addressing flood hazards and risk assessments. Like sediment, instream wood and plastics can be transported along the river channels during floods, increasing flooding risks and hazards by accumulating near hydraulic structures. Collection racks, a commonly encountered hydraulic structure, are used for trapping and removing various accumulated material, mitigating any potential risks. However, accumulations at collection racks can cause other risks because of potential backwater rise and local scour development. Accumulations of different densities, such as a combined presence of instream wood and plastics, add further complexity to the process understanding. In this study, a physical model is used to explore the geomorphic effects of accumulations of two different densities at a collection rack. Local geomorphic changes (i.e., scour and deposition), their evolution and the spatial extent around a vertical collection rack were analysed with Structure from Motion photogrammetry. The results show a strong relationship between the accumulation density and the hydraulic and geomorphic changes. Light floating material, forming accumulations closer to the water surface, are associated with the largest geomorphic changes, in contrast to dense material, forming accumulations close to the riverbed. In the presence of different densities, intermediate geomorphic changes were observed. Finally, to discuss the observations, we present a conceptual flow field for different scenarios of accumulation densities. Ultimately, outcomes from this work are essential to improve design practices for river managers.