Local Hydraulic Conditions Research Articles

An improved small-scale test setup for assessing backward erosion piping

Backward erosion piping is one of the erosion mechanisms that lead to embankment dam incidents, whereby foundation granular materials are transported to the downstream toe leaving a shallow pipe in place. Most previous studies have focused on secondary erosion as the responsible process for the pipe progression. On the other hand, a few experimental investigations have considered primary erosion by restricting the pipe pathway within a confined channel for easier monitoring of local gradients. This paper presents a modified small-scale setup with the hole-type exit configuration to capture local hydraulic conditions associated with the progression of the pipe in a realistic and unconstrained pathway. Horizontal compaction and a more accurate measurement and acquisition system are other improvements added to this setup. The test outcomes of the two fine sands have revealed that the scale factor of Sellmeijer’s model can be used to predict the critical global hydraulic gradient for the specific geometry of the small-scale setup with the hole-type exit. Moreover, the progression of the pipe remains unaffected by loading history. Results also confirm the previous findings regarding the mutual influence of local gradient and pipe tip progression.

Geomorphological controls on estuary hydrodynamics with implications for diatom blooms in deglaciated coastal areas

Understanding local hydraulic conditions is imperative to coastal harmful algal bloom (HAB) monitoring. The research summarized herein describes how the locations and tidal phases selected for coastal hazard sampling can influence measurement results used to guide management decisions for HABs. Our study was conducted in Frenchman Bay, Maine, known for its complex deglaciated coastline, strong tidal influence, and shellfishing activities that are susceptible to problematic HABs such as those produced by some species (spp.) of the diatom genus Pseudo-nitzschia. In-situ measurements of current velocity, density, and turbulence collected over a semidiurnal tidal cycle and a companion numerical model simulation of the study area provide concurrent evidence of two adjacent counter-rotating sub-mesoscale eddies (2–4 km diameter) that persist in the depth-averaged residual circulation. The eddies are generated in the wake of several islands in an area with abrupt bathymetric gradients, both legacy conditions partly derived from deglaciation ∼15 kya. Increased concentrations of Pseudo-nitzschia spp. measured during the semidiurnal survey follow a trend of elevated turbulent dissipation rates near the water surface, indicating that surface sampling alone might not adequately indicate species abundance. Additional measurements of Pseudo-nitzschia spp. from two years of weekly sampling in the region show that algal cell abundance is highest where residual eddies form. These findings provide incentive to examine current practices of HAB monitoring and management by linking coastal geomorphology to hydraulic conditions influencing HAB sampling outcomes, coastal morphometric features to material accumulation hotspots, and millennial time scales to modern hydraulic conditions.

Reliability of flood marks and practical relevance for flood hazard assessment in southwestern Germany

Abstract. Flood marks are rarely utilized in hazard assessment, mainly because of a lack of data availability and accessibility and mistrust in their reliability. Challenging these common assumptions, we present an approach for evaluation and practical utilization of flood marks by the example of the Kinzig River, a Rhine tributary from the Black Forest with a history of severe floods. We combined written documents describing flood marks with field mapping at three study sites and collected information relating to 89 marks – about 50 % of them still preserved – which refer to ≥15 large floods between 1824 and 1991. The inclusion of a detailed historical-mark survey enabled an assessment of changes through time for each flood mark: they extend from small (±15 cm) imprecisions in mark heights to considerable uncertainties in position, height, and displayed date for some modified marks. Plausibility checks with further data nevertheless demonstrated good overall consistency. We then juxtaposed these marks with the current, modeled flood hazard maps. A wide agreement is apparent, in that the large majority of marks are situated at probable heights and within the modeled flooding area associated with extreme floods. For the few exceptions, we see plausible and historically sound reasons in changed local hydraulic conditions by flood protection walls, exceptional processes during a massive ice jam, and possibly also a local underestimation of hazard along Kinzig River tributaries. Overall, this study highlights (1) the broad availability of flood mark data, both on a larger spatial scale and with regard to already vanished marks, and (2) the significance of the marks, verified by further data, and also demonstrates (3) the possibility of a straightforward inclusion in hazard assessment. We thus encourage the systematic collection, maintenance, and integration of flood marks as responsible risk management, not least regarding their value in the wider context of risk awareness and memory.

Temporal evolution of backward erosion piping in small-scale experiments

Backward erosion piping (BEP) is a form of internal erosion which can lead to failure of levees and dams. Most research focused on the critical head difference at which piping failure occurs. Two aspects have received less attention, namely (1) the temporal evolution of piping and (2) the local hydraulic conditions in the pipe and at the pipe tip. We present small-scale experiments with local pressure measurements in the pipe during equilibrium and pipe progression for different sands and degrees of hydraulic loading. The experiments confirm a positive relation between progression rate and grain size as well as the degree of hydraulic overloading. Furthermore, the analysis of local hydraulic conditions shows that the rate of BEP progression can be better explained by the bed shear stress and sediment transport in the pipe than by the seepage velocity at the pipe tip. The experiments show how different processes contribute to the piping process and these insights provide a first empirical basis for modeling pipe development using coupled seepage-sediment transport equations.

Wide Green Dike (WGD) concept for grass revetment under coastal conditions

The Dike along the Dollard estuary in the northeast of the Netherlands has a grass cover. This revetment is not sufficiently strong for the local hydraulic conditions. Traditional dike strengthening leads to the replacement of grass with a hard revetment. An alternative under investigation is the “Wide Green Dike” (WGD) concept: a grass-covered dike with a gentle slope of around 1:7, naturally merging into the salt marsh located in front. The WGD concept is easily adaptable to future challenges, such as sea level rise. Furthermore, sediment of the estuary is currently being ripened to clay near the dike, potentially saving numerous truck movements needed for strengthening. The WGD is therefore the example of a circular, innovative and sustainable concept. However, the WGD needs to comply with the national flood safety standards to make it a feasible alternative, while the current assessment methods and instruments are not yet suited for this particular type of dike. During normative conditions, the grass cover is allowed to fail and the underlaying clay experiences erosion. A clay erosion model is used to design the thickness of the clay layer and proof that the WGD has sufficient residual strength to meet the norms of flood risk.

Floodplain losses and increasing flood risk in the context of recent historic land use changes and settlement developments: Austrian case studies

AbstractFloodplains play a central role in flood risk management since they function as retention areas which attenuate and decelerate flood waves. However, during the last decades land use has changed distinctly on floodplains which has led to a change in topography due to the construction of levees and dykes. Using geographic information system analysis we assessed floodplain developments over 60 years for five Austrian rivers. We used these findings as input for hydrodynamic‐numerical modelling. A comparison of computations of current and historic floodplain topographies demonstrated the complex impacts that changes on floodplains have on catchment level flood risk. Results showed that the losses of floodplains were in general linked to a deterioration in hydrological (flood peaks and travel times) and hydraulic (water level) parameters. In rare cases the unintentional overtopping of dykes resulted in an improved reduction of the peak of the flood wave, but included a worsening of local hydraulic conditions. Hence, this study demonstrates that general conclusions about an alteration of flood risk cannot be easily reached, with a demand for further site‐specific assessment. This novel way of investigating the trends of flooding characteristics by including the historic development within a catchment offers valuable information to planners for a future flood risk management.

Habitat‐specific invertebrate responses to hydrological variability, anthropogenic flow alterations, and hydraulic conditions

Abstract Quantifying ecological responses to river flow regimes is a key scientific approach underpinning many environmental flow (e‐flow) strategies. Incorporating habitat‐scale influences (e.g. substrate composition and organic matter cover) within e‐flow frameworks has the potential to provide a broader understanding of the causal mechanisms shaping instream communities, which may be used to guide river management strategies. In this study, we examined invertebrate communities inhabiting three distinct habitat groups (HGs—defined by coarse substrates, fine sediments, and the fine‐leaved macrophyte Ranunculus sp.) across four rivers (each comprising two study sites) within a single catchment. We tested the structural and functional responses of communities inhabiting different HGs to three sets of flow‐related characteristics: (1) antecedent hydrological (discharge—m3/s) variability; (2) antecedent anthropogenic flow alterations (percentage of discharge added to or removed from the river by human activity); and (3) proximal hydraulic conditions (characterised by the Froude number). The former two were derived from groundwater model daily time series in the year prior to the collection of each invertebrate sample, while the latter was collected at the point of sampling. While significant effects of hydrological and anthropogenic flow alteration indices were detected, Froude number exerted the greatest statistical influence on invertebrate communities. This highlights that habitat‐scale hydraulic conditions to which biota are exposed at the time of sampling are a key influence on the structure and function of invertebrate communities. Mixed‐effect models testing invertebrate community responses to flow‐related characteristics, most notably Froude number, improved when a HG interaction term was incorporated. This highlights that different mineralogical and organic habitat patches mediate ecological responses to hydraulic conditions. This can be attributed to HGs supporting distinct taxonomic and functional compositions and/or providing unique ecological functions (e.g. flow refuges), which alter how instream communities respond to hydraulic conditions. While the individual importance of both flow and small‐scale habitat effects on instream biota has been widely reported, this study provides rare evidence on how their interactive effects have a significant influence on riverine ecosystems. These findings suggest that river management strategies and e‐flow frameworks should not only aim to create a mosaic of riverine habitats that support ecosystem functioning, but also consider the management of local hydraulic conditions within habitat patches to support specific taxonomic and functional compositions.

Morphodynamic model of the lower Yellow River: flux or entrainment form for sediment mass conservation?

Abstract. Sediment mass conservation is a key factor that constrains river morphodynamic processes. In most models of river morphodynamics, sediment mass conservation is described by the Exner equation, which may take various forms depending on the problem in question. One of the most widely used forms of the Exner equation is the flux-based formulation, in which the conservation of bed material is related to the stream-wise gradient of the sediment transport rate. An alternative form of the Exner equation, however, is the entrainment-based formulation, in which the conservation of bed material is related to the difference between the entrainment rate of bed sediment into suspension and the deposition rate of suspended sediment onto the bed. Here we represent the flux form in terms of the local capacity sediment transport rate and the entrainment form in terms of the local capacity entrainment rate. In the flux form, sediment transport is a function of local hydraulic conditions. However, the entrainment form does not require this constraint: only the rate of entrainment into suspension is in local equilibrium with hydraulic conditions, and the sediment transport rate itself may lag in space and time behind the changing flow conditions. In modeling the fine-grained lower Yellow River, it is usual to treat sediment conservation in terms of an entrainment (nonequilibrium) form rather than a flux (equilibrium) form, in consideration of the condition that fine-grained sediment may be entrained at one place but deposited only at some distant location downstream. However, the differences in prediction between the two formulations have not been comprehensively studied to date. Here we study this problem by comparing the results predicted by both the flux form and the entrainment form of the Exner equation under conditions simplified from the lower Yellow River (i.e., a significant reduction of sediment supply after the closure of the Xiaolangdi Dam). We use a one-dimensional morphodynamic model and sediment transport equations specifically adapted for the lower Yellow River. We find that in a treatment of a 200 km reach using a single characteristic bed sediment size, there is little difference between the two forms since the corresponding adaptation length is relatively small. However, a consideration of sediment mixtures shows that the two forms give very different patterns of grain sorting: clear kinematic waves occur in the flux form but are diffused out in the entrainment form. Both numerical simulation and mathematical analysis show that the morphodynamic processes predicted by the entrainment form are sensitive to sediment fall velocity. We suggest that the entrainment form of the Exner equation might be required when the sorting process of fine-grained sediment is studied, especially when considering relatively short timescales.

A novel laboratory test for backward erosion piping

A new type of laboratory test has been developed for measuring the local hydraulic conditions required for the progression of backward erosion piping, an internal erosion mechanism for water-retaining structures. The device consists of clear acrylic cylinders in which soil samples can be subjected to unidirectional, horizontal flow. The cylindrical sample shape ensures that the eroding pipe passes along the top of the sample allowing for pressure, pipe flow velocity and pipe geometry to be accurately measured in time. Experimental results are presented for tests conducted on two uniformly graded sands. The test results indicate that the critical hydraulic conditions for progression of backward erosion piping remain constant for all scales tested when defined over a distance of 10 cm upstream of the pipe tip. Additionally, the rate of pipe progression was found to be proportional to the seepage velocity upstream of the eroding pipe.

Status, sources, and composition of fine sediments in Upper Austrian streams

High loads of fine sediments are considered one of the most important causes of river ecosystem degradation worldwide. Especially in intensively used catchment areas changes in the sediment regime must be regarded as a key factor that prevents the achievement of the objectives of the European Water Framework Directive. To work on these problems, two comprehensive studies were carried out in Upper Austria to get a wider understanding of the causes of siltation, its current status, and its effects. The results revealed that fine sediments considerably degrade about one-third of the examined river sites in the pre-alpine foothills in Upper Austria and, only about a fifth of the stretches exhibit a natural sediment composition. It turned out that (A) human land uses and their changes over the last decades, (B) increased entry paths, (C) climate change, (D) local hydraulic conditions, and (E) lateral connectivity are key issues for siltation. Durable and valuable river restoration measures are determined by the consideration of these drivers. Moreover, pursuing investigations emphasized the determining processes and related chemical alterations, with further implications for aquatic biota.

A lacustrine record of East Asian summer monsoon and atmospheric dust loading since the last interglaciation from Lake Xingkai, northeast China

AbstractA 336-cm-long sediment core spanning the last 130 ka was recovered from Lake Xingkai on the northeastern margin of the East Asian summer monsoon domain to reveal the linkage between lacustrine depositional processes and environmental changes. Bayesian end member modeling analysis was conducted to partition and interpret the grain-size distributions of Lake Xingkai sediments. Our results suggest that the sedimentary system is characterized by three end members (EMs). EM1 and EM2, with a modal value of 13 and 10 μm, respectively, indicate the variation of local hydraulic conditions. EM3, with a modal value of 5 μm, reflects the background atmospheric dust loading. High atmospheric dust concentration generally occurred during Marine Isotope Stage (MIS) 5d, MIS 4, and early MIS 3, when the climate in the Asian dust source region was cold and dry. In contrast, low dust concentration prevailed during MIS 2, likely due to the southward shift of the westerlies driven by maximum ice volume in the high latitudes.

Numerical Issues Arising in Determination of Interlayer Conductivities in Layered Unsaturated Soils

AbstractThe one-dimensional (1D) Richards equation is used to model infiltration flow in an unsaturated two-layer soil. Imposing continuities of both flux and pressure head at the interface yields a nonlinear equation for determining interface conductivities. The authors show that multiple solutions of this nonlinear interface equation may exist if the spatial discretization is not fine enough around the interface, in particular as sharp wetting fronts pass through the interface, or for flow across highly dissimilar materials. Three hydraulic models, the Gardner model (G), the Mualem-van Genuchten model (MvG), and the Fredlund-Xing-Leong-Rahardjo model (FXLR), are investigated to demonstrate the nonuniqueness of solutions to the interface problem. For the simplest G model, a full mathematical analysis of the interface problem in terms of two parameters depending on local hydraulic conditions and mesh size is possible. For more advanced models, the interface equation can only be analyzed numerically. In al...

The grain-size depositional process in wetlands of the Sanjiang Plain and its links with the East Asian monsoon variations during the Holocene

Compared to the well-studied the grain-size depositional process in hydraulic and aeolian sediments, the process in wetlands is still poorly understood. In this study, the grain-size components of a well-dated mud/peat profile (SJ2) in the Sanjiang Plain were partitioned using a lognormal distribution function. The results suggest that the lacustrine deposits contain four distinct unimodal grain-size distributions, C-1 through C-4, with their modal sizes ranging 0.7–1.6, 2.4–8.7, 15.7–27.8, and 380.2–529.6 μm respectively. Only the C-1 and C-3 were identified in the peat layers. A grain-size depositional modal was introduced to explain the environmental meanings of the four components. C-3, as a nearshore suspension component, was the most sensitive component to the variation of local hydraulic conditions. Based on the variation of C-3 percentages of the profile SJ2, the local hydraulic conditions were tentatively reconstructed, and the results showed a close relation with the monsoon variations during the Holocene. Accordingly, we suggest the relative coarse component of the polymodal peat deposits in monsoonal regions can be used as a sensitive indicator for the hydraulic conditions and the monsoon circulations in the geological past.

Environmental variability of Macaronichnus ichnofabrics in Eocene tidal-embayment deposits of southern Patagonia, Argentina

The Middle Eocene Man Aike Formation of southern Patagonia, Argentina, offers the opportunity to study the palaeoecological and palaeoenvironmental significance of Macaronichnus ichnofabrics in a tide-dominated, coastal embayment compound-dune complex. Four recurrent Macaronichnus ichnofabrics are recognized: Macaronichnus IF-1 characterized by Macaronichnus segregatis segregatis; Macaronichnus IF-2 with M. segregatis segregatis, Ophiomorpha nodosa, Skolithos linearis and rare Palaeophycus tubularis; Macaronichnus IF-3 typified by M. segregatis segregatis, O. nodosa, Asterosoma radiciforme and rare S. linearis; and Macaronichnus IF-4 consisting of M. segregatis segregatis, Nereites missouriensis, A. radiciforme, O. nodosa, Rosselia socialis, S. linearis, P. tubularis, Planolites isp. and Chondrites isp. Variations in these ichnofabrics are linked to changes in local hydraulic conditions in the upper and lower trough areas of the compound dunes. In high-energy, shallow and marginal marine environments, the Macaronichnus tracemaker is commonly the first to colonize the shifting, sandy substrates. Overprinting by other ichnoguilds occurs only after the physiochemical conditions became favourable and the colonization window length increases.

Forensic Diagnosis on Flood-Induced Bridge Failure. II: Framework of Quantitative Assessment

ThefailureoftheShuang-YuanBridgeinTaiwancausedby floodsassociatedwithTyphoonMorakotinAugust2009promptedthis forensic investigation into its causes. The investigation found multiple factors for the bridge failure: (1) flood flows, (2) river-bend-induced skewed flow, and (3) the effects induced by extended foundation caps and rafted woody debris. A qualitative assessment framework, which includes hydraulic computation, a fluid-solid-coupled model, and foundation stability analysis, is introduced to develop scenario simulations and forensic diagnosis to identify the main causes of the bridge failure. The watershed-wide and local hydraulic conditions are reconstructed using hydraulic computation. The flow field, dynamic water-pressure distributions, scour formation, flood resistance, and flood thrust are ef- ficiently simulated using a fluid-solid-coupled model and foundation stability analysis around the bridge foundations under different hydraulic and environmental conditions. The forensic diagnosis results show that river flooding, extended foundations, skewed flow, and rafted wood aggravated the scour depths and undermined the flood-resistant capacity of the bridge at different severities. In particular, rafted wood contrib- utedtoindividualfracturingofthebridgepiles.DOI:10.1061/(ASCE)CF.1943-5509.0000393. © 2014 American Society of Civil Engineers. Author keywords: Scenario simulation; Forensic diagnosis; Hydraulic computation; Fluid-solid-coupled model; Foundation stability analysis; Scour; Overflood; Skewed flow; Rafted wood debris.

Numerical Modeling Study of Hydraulic Impact of a Large Sediment Capping Facility

AbstractAn engineered containment facility (ECF) that would cap and contain some of the most contaminated sediments in Hamilton Harbour has been proposed. Because of the size and shape of this structure, the ECF will alter local flow hydraulic conditions and isolate local discharges and intakes from free interaction with the open water of the harbour. To investigate the impact and functionality of the ECF, a three-dimensional (3D) hydraulic model was adopted in this study. The model was verified against vertical velocity profiles measured with an acoustic doppler current profiler in the study region. Results of the verification reveal that under the driving force of wind, the best agreement between simulated and measured currents occurs in the surface layer. Considering the complex nature of the hydraulic conditions in the harbour, the model performs reasonably well. Overall, the model should provide a useful tool considering and comparing hydraulic conditions under different ECF configurations and the ex...

Numerical Modelling Approaches for Assessing Improvements to the Flow Circulation in a Small Lake

Kamaniskeg Lake is a long, narrow, and deep small lake located in the northern part of Ontario, Canada. The goals of this paper were to examine various options to improve the water quality in the northern part of the lake by altering the local hydraulic flow conditions. Towards this end, a preliminary screening suggested that the flow circulation could be increased around a central island (Mask Island) in the northern part of the lake by opening up an existing causeway connecting the mainland and central island. Three‐dimensional (3D) hydraulic and transport models were adopted in this paper to investigate the hydraulic conditions under various wind forces and causeway structures. The modelling results show that opening the causeway in a few places is unlikely to generate a large flow circulation around the central island. Full circulation only appears to be possible if the causeway is fully removed and a strong wind blows in a favourable direction. The possible reasons for existing water quality variations at the intake of a local WTP (water treatment plant) are also explored in the paper.

Subcooled critical heat flux: an assessment of the risk to front-end and beamline components of synchrotron light sources

X-ray absorbers in the front ends and beamlines of synchrotron light sources are exposed to very high thermal loads. Many facilities, such as the Advanced Photon Source, are investigating upgrades that will further increase the thermal load. The likelihood of exceeding the limit of subcooled critical heat flux (CHF) in these components was examined. The assessment was performed for both currently possible off-normal operational conditions, such as might occur in the event of a failure of multiple safety interlocks, and the anticipated operating conditions that may result from future upgrades. The subcooled CHF values were calculated using empirical equations frequently cited in the literature and then compared with the computed values of the heat flux at the walls of the component cooling channels in cases where the cooling wall temperature exceeded the water saturation temperature at local hydraulic conditions. Having in mind that the great majority of the available empirical correlations were developed for the conditions characteristic for the operation of heat exchangers in the nuclear power industry, the limitations of this approach are discussed and an experimental study of the subcooled CHF values in the conditions similar to those expected in the front-end and beamline components is proposed.

The lentic–lotic character of Mediterranean rivers and its importance to aquatic invertebrate communities

Hydromorphological features are crucial in structuring habitats for freshwater organisms. The quantification of these variables is often performed through accurate measuring or detailed estimation, but their assessment is not always feasible for river management purposes. Economic and time constraints often lead to difficulty in creating simple summaries of collected data for practical use. The Lentic–lotic River Descriptor (LRD) was developed to identify the character of a river site in terms of local hydraulic conditions. Information about the presence of flow types, channel substrates, in-stream vegetation, organic debris and artificial features is included in its calculation. The main aim of this paper is to investigate whether the lentic–lotic character of a river site, as summarized with the LRD descriptor, is relevant to aquatic invertebrate communities in nearly natural river sites. Invertebrate data were collected with multi-habitat, proportional sampling and hydromorphological information was gained by applying the CARAVAGGIO method (river habitat survey technique) in the field. The dataset was generated from High or Good ecological status river sites located in Mediterranean areas of Italy. Correspondence Analysis was performed to relate the invertebrate community structure to a set of catchment-scale, reach-scale and chemical environmental variables. The results of the multivariate analysis indicate that LRD provides a persuasive explanation of the most important axis of variation in benthic data. This paper also presents the optimal LRD range for a set of invertebrate taxa, accompanied by a short discussion of their potential use in conservation issues.

Does the lentic-lotic character of rivers affect invertebrate metrics used in the assessment of ecological quality?

The importance of local hydraulic conditions on the structuring of freshwater biotic communities is widely recognized by the scientific community. In spite of this, most current methods based upon invertebrates do not take this factor into account in their assessment of ecological quality. The aim of this paper is to investigate the influence of local hydraulic conditions on invertebrate community metrics and to estimate their potential weight in the evaluation of river water quality. The dataset used consisted of 130 stream sites located in four broad European geographical contexts: Alps, Central mountains, Mediterranean mountains and Lowland streams. Using River Habitat Survey data, the river hydromorphology was evaluated by means of the Lentic-lotic River Descriptor and the Habitat Modification Score. To quantify the level of water pollution, a synoptic Organic Pollution Descriptor was calculated. For their established, wide applicability, STAR Intercalibration Common Metrics and index were selected as biological quality indices. Significant relationships between selected environmental variables and biological metrics devoted to the evaluation of ecological quality were obtained by means of Partial Least Squares regression analysis. The lentic-lotic character was the most significant factor affecting invertebrate communities in the Mediterranean mountains, even if it is a relevant factor for most quality metrics also in the Alpine and Central mountain rivers. Therefore, this character should be taken into account when assessing ecological quality of rivers because it can greatly affect the assignment of ecological status.