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Investigation of sediment accumulation in Nubia Lake, using RS/GIS
Get PDFThe main focus of this paper is to detect the changes in AHDL bed surface (sedimentation and erosion) in the period 2000-2012 in the active sedimentation zone of the Lake which is located in Nubia Lake (Sudanese part of AHDL). To enable this detection, the remote sensing (RS) and (Geographic Information Systems) GIS techniques are used to build the 3-D profile of the Lake portion where most of the sediment accumulate. Also, the accumulated sediment during this period is computed using the developed 3-D profile and using the cross section method as conducted by Aswan High Dam Authority (AHDA). Also, the surface areas of sedimentation and erosion are mapped and computed. Results indicated that sedimentation are dominant in years with high flood while erosion occurs when the incoming flow to the lake is low. Moreover, results indicate that the present approach overestimate the sedimentation amount by about 4% compared to the results of the method used by AHDA
Mechanics and simulations of the groundsill damage due to the impacts from pebbles/cobbles
Get PDFA pebble/cobble passing a head drop tends to carry a large impact momentum when it hits a groundsill; the heavy impact may result in the breakage of concrete. This study explores the mechanics and the principles of this issue by computational fluid dynamics with particle tracking. The results reveal that the kinematics of a single particle passing a head drop is primarily affected by the size of particles. A small particle tends to travel along the flow lines and brings relatively low impact energy. A large particle is more affected by gravity; saltation is more likely and will carry relatively high impact energy. The major role of the discharge magnitude is just to enable the transportation of larger particles to the downstream. With the simulation, it is possible to evaluate the impact position, load and kinetic energy. The locations prone to damage and failure potential can then be identified
Causal loop analysis of coastal geomorphological systems
No full textAs geomorphologists embrace ever more sophisticated theoretical frameworks that shift from simple notions of evolution towards single steady equilibria to recognise the possibility of multiple response pathways and outcomes, morphodynamic modellers are facing the problem of how to keep track of an ever-greater number of system feedbacks. Within coastal geomorphology, capturing these feedbacks is critically important, especially as the focus of activity shifts from reductionist models founded on sediment transport fundamentals to more synthesist ones intended to resolve emergent behaviours at decadal to centennial scales. This paper addresses the challenge of mapping the feedback structure of processes controlling geomorphic system behaviour with reference to illustrative applications of Causal Loop Analysis at two study cases: (1) the erosion–accretion behaviour of graded (mixed) sediment beds, and (2) the local alongshore sediment fluxes of sand-rich shorelines. These case study examples are chosen on account of their central role in the quantitative modelling of geomorphological futures and as they illustrate different types of causation. Causal loop diagrams, a form of directed graph, are used to distil the feedback structure to reveal, in advance of more quantitative modelling, multi-response pathways and multiple outcomes. In the case of graded sediment bed, up to three different outcomes (no response, and two disequilibrium states) can be derived from a simple qualitative stability analysis. For the sand-rich local shoreline behaviour case, two fundamentally different responses of the shoreline (diffusive and anti-diffusive), triggered by small changes of the shoreline cross-shore position, can be inferred purely through analysis of the causal pathways. Explicit depiction of feedback-structure diagrams is beneficial when developing numerical models to explore coastal morphological futures. By explicitly mapping the feedbacks included and neglected within a model, the modeller can readily assess if critical feedback loops are included
A possible characterization of suffusion susceptibility independent of the hydraulic loading history?
Get PDFSuffusion is a complex phenomenon which involves selective erosion of fine particles under the effect of seepage flow in the matrix of coarser particles. With the objective to characterize suffusion susceptibility, a series of downward seepage flow tests was realized with a triaxial erodimeter developed in our laboratory. Three different cohesionless soils were tested under controlled hydraulic gradient or under controlled flow rate. This study shows the significant effect of hydraulic loading history on the value of critical hydraulic gradient. Moreover, method characterizing the erosion susceptibility based on rate of erosion doesn’t lead to a unique characterization of suffusion process for different histories of hydraulic loading. The new analysis is based on energy expended by the seepage flow to characterize the hydraulic loading and the cumulative eroded dry mass to characterize the soil response. The results demonstrate that this approach is effective to characterize suffusion susceptibility for cohesionless soils
Prediction of scour in overtopped floodwalls using JET and EFA
Get PDFErosion due to water overtopping of floodwalls is one of the main causes of levee failures during storms events. Prediction of scour rate and depth in fine grained material has always been a critical issue for engineers. Performing full scale erosion tests on geo-structures like levees can be difficult and expensive. However, JET and EFA test are alternative tests to determine the scour potential of a soil. JET can be conducted both in the lab and in-situ, while EFA is conducted in the laboratory. In this study, a series of laboratory JET, EFA, and lab-scaled levee-floodwall erosion tests have been conducted on material retrieved from Mississippi River bank. The results of erosion rate on these soils have been compared. In addition, a correlation is proposed to estimate the erosion rate that occurs during floodwall overtopping in crest of levees based on JET and EFA results for soils with various compactions and plasticity indices
Scour properties of Mono Bucket foundation
Get PDFField experience proved that the Mono Bucket foundations (MBFs) have good response against scour development. Moreover, the ratio between large diameter (bucket lid) and the small diameter (shaft tower) is the driving parameter for the process of erosion/backfill, like scour protection diameter in the case of scour protected monopiles. However, the structural design to reduce the scour development for MBFs is still open to optimization. The influences of parameters that generate backfill and scour, the transfer load webs and the misalignment with seabed, have not been systematically studied until now. Thus, an experimental analysis was carried out to quantify the influence of webs, the misalignment parameters and combination of the two. The physical analysis uses the flume facility at Aalborg University. The test conditions are irregular waves superposition with co-directional or opposite tidal flow. Three structural models have been tested in several tidal cycles with a variation of the current intensity. Three levels of alignment with seabed are taken: one flushing and two more with different levels of misalignment. Field results for installation stage have been collected from the North Sea and compared with lab tests. The results showed a sensibility to the misalignment height comparable with the berm height of the scour protection on traditional monopiles leading to edge scour. Design improvements to limit the scour and increase backfill have been found. The experimental analysis compared with real surveys and existing studies showed good agreements. Scour protection based on collar solution shows high efficiency when scour protection should be required. The paper demonstrates good agreement between field measurements and small-scale studies. The unique value of the field measurements increases confidence in small-scale studies which are subject to scale and lab effects
Comparison between methods for creating DEMs of physical models
Get PDFWithin physical modelling, it is often necessary to create DEMs (digital elevation models) when testing the stability of rock structures or the filter layers and scour protection around foundations and other marine structures. These DEMs are used to detect changes in the position of the structure or surrounding protective material. Several methods are available to create these models, yet no one technique has been selected as an industry standard. A comparison between three widely used methods – terrestrial laser scanner (TLS), combined laser scanner (CLS) and structure from motion (SfM) – are presented within this paper. The CLS in underwater mode gave low measurement errors and can be deployed without having to drain the facility but requires a traverser system. An area of approximately 7 m by 4 m can be measured in half an hour. The TLS can survey a much larger area in the same time, but requires the facility to be drained. SfM is cheapest method, but struggles to create a full shape and more care must be taken. The CLS in underwater mode has been chosen for use in scour studies in the Fast Flow Facility, with high volumes of water but a relatively limited area
Simulating mesoscale coastal evolution for decadal coastal management: a new framework integrating multiple, complementary modelling approaches
No full textCoastal and shoreline management increasingly needs to consider morphological change occurring at decadal to centennial timescales, especially that related to climate change and sea-level rise. This requires the development of morphological models operating at a mesoscale, defined by time and length scales of the order 101 to 102 years and 101 to 102 km. So-called ‘reduced complexity’ models that represent critical processes at scales not much smaller than the primary scale of interest, and are regulated by capturing the critical feedbacks that govern landform behaviour, are proving effective as a means of exploring emergent coastal behaviour at a landscape scale. Such models tend to be computationally efficient and are thus easily applied within a probabilistic framework. At the same time, reductionist models, built upon a more detailed description of hydrodynamic and sediment transport processes, are capable of application at increasingly broad spatial and temporal scales. More qualitative modelling approaches are also emerging that can guide the development and deployment of quantitative models, and these can be supplemented by varied data-driven modelling approaches that can achieve new explanatory insights from observational datasets. Such disparate approaches have hitherto been pursued largely in isolation by mutually exclusive modelling communities. Brought together, they have the potential to facilitate a step change in our ability to simulate the evolution of coastal morphology at scales that are most relevant to managing erosion and flood risk. Here, we advocate and outline a new integrated modelling framework that deploys coupled mesoscale reduced complexity models, reductionist coastal area models, data-driven approaches, and qualitative conceptual models. Integration of these heterogeneous approaches gives rise to model compositions that can potentially resolve decadal- to centennial-scale behaviour of diverse coupled open coast, estuary and inner shelf settings. This vision is illustrated through an idealised composition of models for a ~ 70 km stretch of the Suffolk coast, eastern England. A key advantage of model linking is that it allows a wide range of real-world situations to be simulated from a small set of model components. However, this process involves more than just the development of software that allows for flexible model coupling. The compatibility of radically different modelling assumptions remains to be carefully assessed and testing as well as evaluating uncertainties of models in composition are areas that require further attention
Experimental study of scour around bridge piers of different arrangements with same aspect ratio
Get PDFScour around bridge piers is a challenging problem faced by bridge engineers. Scour is caused by the horse-shoe vortex formed due to the presence of the pier obstructing the flow. The behaviour of horse-shoe vortex differs according to the arrangement of piers. The flow pattern is different for a group of piers and a solid pier thereby creating different scour patterns. In this paper, the scour around bridge piers of different arrangements with same aspect ratio is investigated in a laboratory flume. All three arrangements studied have an aspect ratio (L/B) of 5.The experimental flume is 7.5m long,0.3m wide and 0.6m deep with a re-circulating facility. The results obtained showed that scour volume is considerably reduced around a single solid pier compared to the combinations of piers. Further, the flow field becomes complex due to the combination of piers