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The borehole erosion test
Get PDFSoil erosion is a major problem in civil engineering. It is involved in bridge scour, meander migration, levee and dam overtopping, internal erosion of earth dams, surface erosion of embankments, and cliff erosion. The best way to predict the erodibility of a soil is to measure it directly on a site specific basis by testing samples in the laboratory or by in-situ testing in the field. The borehole erosion test or BET is a new in-situ soil erosion test proposed to measure the erosion of the walls of a borehole while wet rotary drilling takes place. The increase in diameter of the borehole as a function of time and for a given flow velocity in the borehole is measured with borehole calipers. The result is a profile of soil erodibility as a function of depth. Tests in clay and in sand conducted at the National Geotechnical Experimentation Sites at Texas A&M University are presented
Hydrodynamics of sediment transport: grain scale to continuum scale (Keynote)
Get PDFA theory of sediment transport, describing the entrainment phenomenon from the grain scale to the continuum scale, under a steady-uniform flow over a sediment bed is presented. The sediment grains, assumed as discrete spherical grains, are subjected to turbulent wall-shear flows. At the grain scale, the forces acting on a sediment grain resting over three compact spherical grains are analysed to determine the criteria for entrainment threshold in rolling, sliding and lifting modes taking into account the turbulence effects. Comparison of the theoretical results with the experimental data shows that the entrainment threshold lies within the sliding and lifting modes. Then, at the grain scale, using the log-normal probability density function for the near-bed instantaneous horizontal velocity, the entrainment probabilities in rolling, sliding and lifting modes for a given grain size are derived. The rolling and sliding probabilities increase with an increase in Shields function and after attaining their individual maximum values, they reduce, whereas the lifting probability increases with Shields function. The maximum value of entrainment probability in rolling mode is close to the threshold Shields function in rough flow, whereas the entrainment probability in lifting mode initiates from the value of the threshold Shields function. In a continuum scale, the bedload flux is derived by hypothesising the saltating mode of sediment transport incorporating the lifting probability obtained at the grain scale
Experimental study of Scour around a complex pier with elliptical pile-cap
Get PDFA complex bridge pier is an assembly of a column and foundation such as pile foundation with a pile-cap, partially or completely exposed to the flow. Considering the interaction of various factors around the pier, the pile-cap elevation in relation to the sand bed is expected to differ with time. Therefore, it is necessary to assess scouring over various potential pile-cap elevations. In this paper, laboratory experiments were carried out to analyze scour and mean velocity profiles around complex pier under the effect of low Reynolds numbers. Three pile-cap elevation cases were studied here, viz, the sand bed depth taken above the pile-cap level; the sand bed depth taken below the pile-cap top; and the sand bed depth taken at the level of pile-cap bottom. The results showed different scour patterns in the three cases considered. It was also noted that the least scour-depth occurred in the second case and maximum in the first one
Scour hole development in river beds with mixed sand-clay-peat stratigraphy
Get PDFRiver deltas are often characterized by a heterogeneous subsoil stratigraphy, composed of layers of sand, clay and peat. This has important consequences for the riverbed morphology and in particular the formation of scour holes. When the riverbed is composed of poorly erodible clay or peat, erosion processes are retarded. However, when thinner parts of clay or peat layers erode; underlying sand patches are incised and large scour holes may develop within in a short amount of time. The unpredictability and fast development makes these scour holes difficult to manage while stability of dikes and infrastructure may be at stake. In this paper we study the scour hole formation and development in heterogeneous subsoil. The Rhine-Meuse estuary forms the ideal system for this analysis as it has been intensively measured and contains about 100 scour holes. Based on nearly 60 years of river bed topography data and data on the subsurface lithography and hydrodynamics, we present a system analysis of scour hole formation in heterogeneous subsoil stratigraphy. In addition the detailed growth of a recently formed scour hole is studied and compared to scale model tests
Numerical simulation of scour infilling in overset grid
Get PDFIn the present study, sediment transport models have been implemented in the CFD solver FANS3D to simulate scour infilling process around a circular pier. FANS3D uses the finite-analytic method, which incorporates the analytic solution of the differential equation in its linearized form in the respective small subdomain. The Reynolds-Averaged Navier-Stokes equations are solved in general curvilinear coordinate system. The Chimera overset grid technique is utilized to simulate flows in a multi-block domain. This paper presents the time development of the scour hole and the infilling process due to bed load transport of non-cohesive sediment under the three flow conditions: flood, normal, and flood-to-normal transitional. The effect of the size of the sediment particle is also discussed
Storm peak validation and analysis of uncertainty in estimates of extreme sea states
Get PDFStorm peaks are often under-estimated in numerical models, as is widely acknowledged. Yet, for many sites where estimates of extreme storm conditions are needed for engineering design, numerical models are the best – if not only – source of information. We discuss uncertainty in estimates of extreme sea states based on numerical model datasets . A method of validation for extreme conditions is presented based on matched pairs of independent storm peak events between modelled time series and buoy based observations, and found to be preferable to exceedance based validation techniques. Systematic biases in the storm peaks of the CFSR, ERA- Interim and NORA10 datasets, when compared to buoy data, are discussed. Sea states at the peaks of storms are compared and contrasted to the general population of sea states. We demonstrate a calibration scheme designed to remove bias from estimated storm peaks using a minimal set of parameters, in order that parameters may be mapped and estimated at sites where no observations are available.
Bias corrected model estimates of storm peaks have a remaining uncertainty associated with model precision. Probability distributions of extreme sea states are estimated using Markov Chain Monte Carlo and Bayesian techniques. Estimates of model precision, based on peak-focussed validation, are used to predict extreme sea states with associated uncertainty representing both model precision and sampling of the long-term distribution. The analysis allows investigation of the relative contribution to estimate uncertainty of model precision and sampling/length of record. Once uncertainty is considered in the estimation of extreme sea states, a contribution to the mean estimate from precision based uncertainty in the source data becomes apparent.
This contribution is also relevant to estimates of extreme conditions based on buoy data with measurement and short-term sampling uncertainties, and provides a baseline level of achievable uncertainty in extreme conditions.
Validating uncertainty in estimates requires analysis at a large number of sites, and depends to a great extent on the probability associated with the most extreme events in a series, the so-called plotting position. The formulation of the plotting position has been debated for many years.
Numerical experiments are presented that suggest that the correct formulation lies within a narrow subset of the debated schemes
Lessons on internal erosion in embankment dams from failures and physical models (Keynote)
Get PDFThe investigations of embankment failures reported by Erinoh R&D project, merged with the Foster & Fell Erdata and NPDP program in the USA, point out the main causes and the main situations where internal erosion could initiate and trigger the failure of dams. The investigation of physical models built in the CACOH laboratory of CNR shows under particular circumstances (piping at core base) pervious shells cannot provide enough weight or erosion resistance to resist concentrated leaks. Three different mechanisms of failure may occur: (1) general instability without hydraulic fracturing, (2) hydraulic fracturing (with or without suffusion) followed by piping, (3) backward erosion or sloughing (surface instability). Physical models help to understand the conditions and the physical situations in which each of these three modes of failure apply. This paper relates to these physical tests and presents the main results and conclusions
Effects of transitions and objects on the erodibility of grass revetments on dikes
Get PDFThis study deals with the effects of transitions and/or objects such as roads and trees in grass revetments on dikes/levees under (wave) overtopping conditions. Three types of impact categories are defined, that is 1) little impact, 2) moderate impact and 3) large impact. Based on the grass erosion model (i.e., the cumulative overload method) a relation is deduced that describes the critical overtopping discharge as function of the load–increase factor for both a sea and a river regime. This impact factor represents the load increase near transitions and objects which is here expressed by a decrease of the critical overtopping discharge. Lognormal distributions for the critical overtopping discharge are used to calculate the failure probability due to overtopping with and without transitions and objects. For several dike–sections in the Netherlands the differences are calculated with the model PC-Ring. The study shows that the differences are largest for dikes along the coast and lakes. For dikes in a river regime where the significant wave height is usually low the erosion differences are marginal
A full scale propeller wash erosion test on heterogeneous cohesive material
Get PDFAs part of a life-cycle cost analysis for the construction of a harbour basin, a full scale erosion test on a dredged slope was undertaken using a test vessel. The objective of the test was to estimate the erosion rate of the in-situ material in order to predict the final profile of the dredged slope at the end of the design life following exposure to propeller wash generated by various manoeuvring vessels. Based on theoretical formulae, a test protocol of percentages of the installed engine power to be applied was defined, aiming to generate a steady propeller flow on the slope as a function of the tide. A pre-installed anchoring point ensured the test vessel maintained a steady position during the test and the pre-defined percentages of the installed engine power were applied as a function of the tidal levels measured during the test. Several interim bathymetric surveys were undertaken in order to monitor and record the progress of the erosion over the tested slope. Orientation and location of the vessel were monitored by satellite positioning, tides were recorded by installed gauges and actual flow velocities logged by a current meter. A time stamped geo-database was developed to model the theoretical distribution of the velocities at any moment of the test. Through the analysis of the tests results, a curve defining the erosion rate as a function of the velocity at the slope was derived from the model and was verified by comparing generated cross-sections against actual surveys. Results were then extrapolated to represent the situation at the end of the design life of the harbour in order to assess whether protection of the slopes was required to preserve the integrity of the marine structures adjacent to the harbour basin. Results were also used to define an appropriate monitoring program for the dredged slopes which had potential for scour generated by propeller wash
Model tests on subsurface cavities below road pavement due to sand eruption from the liquefied ground
Get PDFA significant number of subsurface cavities was found in the liquefied ground after the 2011 Great East Japan earthquake. Using the results of the radar exploration conducted along the coastal area of Tokyo Bay, characteristics of subsurface cavities were investigated. The size and shape of the cavities are larger and thinner compared to those of cavities observed in the non-liquefied ground. A series of model tests was conducted in order to understand the mechanism of sand eruption and underground cavity formation when liquefaction occurs. It was found that the flow rate at the opening seems to be the most important factor for the sand eruption