1,721,099 research outputs found
Modeling and experimental analysis of peat hydrology and its relationships with the subsidence of the Zennare Basin, Venice (Italy)
No full textOptimal Design of Horizontally Framed Miter Gates
Full text linkMiter gates have been, and still are, widely used in hydraulic engineering as control devices for several types of locks (e.g., navigation, flood, tidal, etc). Since centuries, engineering practice suggests a range of optimal values for the angle that the gate leaves must form pointing upstream, facing the current. According to experience and literature, this optimal range corresponds to an angle between the leaf and the normal to the lock wall that varies between 9.5° and 33.7°. It is confirmed here that in horizontally framed miter gates, which is the most usual arrangement, these optimal values depend only on the lock dimensions (span and height) and the geometric characteristics of the girder cross-section. Using a simple dimensionless geometrical parameterization and static equilibrium considerations, a general approach for the preliminary dimensioning of miter gate leaves is developed, based on the optimization of the angle between the leaves for the three most commonly used types of girder cross-section: square, rectangular, and “I-beam” (IPE series)
A laboratory flume characterization of log jams at bridge piers
No full textIn natural rivers the transport phenomena of floating debris involve a large number of problems that are relevant to both environmental and technical aspects. A large part of the research developed has been motivated by linkages between fish habitat and geomorphologic processes and forms influenced by large woody debris, but drift reduces also the capacity of bridge openings, contributes to scour around piers and abutments, and increases lateral forces on bridges. Drift accumulates at bridges when it encounters structural components that trap it. During flood events, relevant drift accumulations can reduce the size of inadequate bridge openings and this can cause a severe backwater phenomenon and the inundation of surrounding lands. Moreover, dam break phenomena can derive from the sudden collapse of debris accumulations, increasing the power of flood with devastating effects. Spaces between piles can clog with drift, increasing flow contraction and local scour depth due to piles and bridge abutments. Drift contributes to more than one-third of the bridge failures in the United States, but methods for estimating a maximum drift accumulation size for use in bridge design have been recommended for Australia and New Zealand only. The potential scour depth as well as the backwater phenomena associated with drift depend on the maximum size that drift accumulations can reach. The features of observed accumulations give a broad idea of the maximum size related to some specific investigated sites, but studies looking for more general insights are limited. This presentation reports the evidence deduced from an extensive series of flume experiments developed to identify the general characteristics of drift accumulations at bridges in terms of size and probability of occurrence. To reach this goal a large number of experiments was developed in a flume 1.0 m wide and 21.7 m long, where the section was modeled with a central channel and two symmetrical banks for a 14 m length and slope of about 0,31%. Three different single pier shapes were considered: square, rounded and triangular nose and end, and different multiple piers configurations (two and three piers of different spacing). At each run about 500 logs of prescribed statistical distribution in terms of length and diameter were injected in the flume with a frequency of 150 - 180 logs/min, and the (possible) formation of the accumulation was video-recorded. Each run, characterized by the same discharge, geometric configuration of piers, and boundary condition (downstream water level) was repeated 10 times to obtain a broad probability of the log jams formation for a whole number of more than 500 runs. The digital analysis of the flume experiments in conjunction with the water level and discharge measurements gives a measure of both the shape and the size of accumulations at bridge piers and of their probability of occurrence in different geometric and hydrodynamic conditions
Flume experiments of log incipient motion in rivers
No full textIn natural rivers the transport phenomena of floating debris involve a large number of problems that are relevant to both environmental and technical aspects. A large part of the research developed has been motivated by linkages between fish habitat and geomorphological processes and forms influenced by large woody debris, but drifts reduce also the capacity of bridge openings, contribute to scour around piers and abutements, and increase lateral forces on bridges. Large woody debris (LWD) is defined as a log having at least 10 cm mid-point diameter, being 2 m in length while the term CWD (coarse woody debris) usually refers to smaller pieces. Key pieces are individual logs with rootwads that are less likely to move than other wood pieces during a bankfull flow. They play a relevant role in snags amassing because key pieces constitute the first step of the woody accumulation process. To identify hydraulic thresholds for movement and transport of key pieces for different log geometries we developed a series of experiments in a flume model 0.30 m wide and 5.0 m long with about 10% slope transversal to the main flow direction. The log, with and without rootwads, is assumed fallen on the sloping bank and the hydraulic threshold is analyzed as a function of log geometry and hydrodynamic action. The key pieces are simulated by wood cylinder ended with an octagon (to take into account the rootwad) of different lengths and thicknesses. Besides the acquisition of water level and discharge as usual, the video recording of the experiments permits the identification of the log position at the threshold of motion into the flume. A conceptual model, based on the 3-D stationary equilibrium of gravity, buoyancy, friction and hydrodynamic forces acting on a rigid body, was developed to allow the interpretation of scale flume model experiments. From the preliminary analysis of laboratory experiments it seems that the condition of incipient motion is mainly controlled by the ratio between the rootwad size and the log length
Numerical and physical modeling to assess landslide triggering induced by hydrological hillslope processes
No full textLandslide triggering induced by high-intensity rainfall infiltration in hillslopes is a complex phenomenon that involves hydrological processes operating at different spatio-temporal scales. Empirical methods give useful information about landslide-prone areas and rainfall intensity and duration that generate slope failures, but they do not provide the theoretical framework needed to achieve an in-depth understanding of the involved physical processes. This fact limits the predictive use of these empirical methods, which are usually site-dependent and unable to assess the landslide hazard with respect to different land uses proposed for mitigation purposes. Depending on rainfall intensity, slope geometry, and soil hydraulic properties, the runoff/infiltration process controls water pressure changes in both the saturated and unsaturated zones, affecting the behavior of shear strength and seepage forces, and, as a consequence, the slope stability. In this study, we tackle the whole process by using both numerical and physical approaches, the former being developed to design (a priori) and analyze (a posteriori) a number of experiments carried out in an ad hoc designed artificial hillslope. The maximum height of the embankment, contained in a reinforced concrete box, is 3.5 m, with length of 6 m and width of 2 m, so that a 2:3 slope can be built. On each lateral side of the box, 50 openings closed with screw caps allow the insertion on properly chosen positions of the control instrumentation (6 tensiometers and 6 TDR sensors). The monitoring network, connected to an automatic acquisition system, is completed by two piezometers, one evaporimeter, and two stream gages able to evaluate both the surface runoff and subsurface contributions to the total outflow. The numerical tool we use is a distributed physically-based catchment simulator (CATHY, CATchment Hydrology) able to model both surface routing and subsurface flow in a coupled fashion. In order to demonstrate the effectiveness of the proposed physical and numerical investigation approach, we report on the arrangement of laboratory facilities (including the non-trivial design and building of the rainfall simulator system), the theoretical design of physical experiments, and a preliminary analysis of experimental evidence
Ensemble Kalman Filter vs Particle Filter in a Physically Based Coupled Model of Surface-Subsurface Flow (Invited)
No full textData assimilation (DA) has recently received growing interest by the hydrological modeling community due to its capability to merge observations into model prediction. Among the many DA methods available, the Ensemble Kalman Filter (EnKF) and the Particle Filter (PF) are suitable alternatives for applications to detailed physically-based hydrological models. For each assimilation period, both methods use a Monte Carlo approach to approximate the state probability distribution (in terms of mean and covariance matrix) by a finite number of independent model trajectories, also called particles or realizations. The two approaches differ in the way the filtering distribution is evaluated. EnKF implements the classical Kalman filter, optimal only for linear dynamics and Gaussian error statistics. Particle filters, instead, use directly the recursive formula of the sequential Bayesian framework and approximate the posterior probability distributions by means of appropriate weights associated to each realization. We use the Sequential Importance Resampling (SIR) technique, which retains only the most probable particles, in practice the trajectories closest in a statistical sense to the observations, and duplicates them when needed. In contrast to EnKF, particle filters make no assumptions on the form of the prior distribution of the model state, and convergence to the true state is ensured for large enough ensemble size. In this study EnKF and PF have been implemented in a physically based catchment simulator that couples a three-dimensional finite element Richards equation solver with a finite difference diffusion wave approximation based on a digital elevation data for surface water dynamics. We report on the retrieval performance of the two schemes using a three-dimensional tilted v-catchment synthetic test case in which multi-source observations are assimilated (pressure head, soil moisture, and streamflow data). The comparison between the results of the two approaches allows to discuss some of the strengths and weaknesses, both physical and numerical, of EnKF and PF and to learn the implications related to the choice of the statistics used to build the ensemble of realizations
Looking below the ground: analyzing the processes that drive spatiotemporal variation of wet channels in dynamic river networks using a physics-based hydrological model
No full textCalibration of water content reflectometer sensors with a large soil sample
No full textIn situ measurement of soil water content is of fundamental importance in vadose zone processes. While time domain reflectometry (TDR) is a universally accepted technique, having been developed since the 1980s, water content reflectometer (WCR) is still a relatively new technique and does not hold a similar background. The main goal of this study was to establish an accurate calibration curve for WCR sensors to be installed in an artificial hillslope designed to study the triggering of shallow landslides. Therefore, high accuracy calibration at the high end of the water content range and for large soil volumes was necessary. An experimental device was specifically designed to provide a calibration procedure for a soil control volume commensurate with the application scale of the probes in the hillslope. A large box container (60 by 50 by 60 cm) was used, inside which three WCR probes and three tensiometers were arranged. A drip emitter produced assigned flow rates, while a load cell at the base measured the time evolution of the infiltrated water mass. Water content was evaluated at approximately steady-state flow conditions for infiltration and drainage experiments, performed with varying degrees of compaction to detect the effect of the porosity on the calibration curve. The experimental results suggest a calibration relationship linearly depending not only on the WCR output signal but also on the porosity. In addition, the calibration curve provided by the manufacturer significantly underestimates the soil water content, with a range of predicted volumetric water content values between 3 and 35%, compared with actual values varying between 19 and 61%
Hydraulic conductivity assessment via tracer test data assimilation: A comparison of updating techniques
No full textHydraulic properties of natural aquifers, such as porosity, hydraulic conductivity, and stora-
tivity, exhibit an erratic spatial variability at different scales that is difficult to recognize without
expensive in situ sampling campaigns, and laboratory analyses. Nevertheless, the relevance of
the heterogeneous structure of natural formations on solute transport is well recognized: the
variability of the hydraulic conductivity at the local scale controls the non-Fickian evolution of
contaminant plumes and the late travel time relevant dispersive phenomena. Tracer test analyses
have been widely adopted to obtain a better understanding of dispersion processes and of the re-
lated hydraulic properties of porous media. Recently, new interpretative opportunities are offered
by geophysical approaches. In particular, the borehole Electrical Resistivity Tomography seems
to be promising due to its capability to describe the spatio-temporal evolution of the injected
solute. Under the assumptions that the solute spreads as a passive tracer and with high values
of the Peclet number, the plume evolution is controlled by porosity and the spatial distribution
of hydraulic conductivity. A coupled approach based on the Lagrangian formulation of transport
and the Ensemble Kalman Filter data assimilation technique can be applied to infer the spatial
distribution of hydraulic conductivity at the local scale from a sequence of time-lapse concentra-
tion imaging. The capabilities of this approach are here investigated simulating a synthetic tracer
test in a three-dimensional finite size domain reproducing an heterogeneous aquifer. Different as-
similation scenarios are studied, varying the system state definition and the assimilated quantities.
Although the method is sensitive to the procedure initialization, and some assimilation scenarios do not work as expected, the results show that the proposed approach can represent an effective
tool for describing the hydraulic conductivity distribution at the locale scale
Insights on permeable pavement hydraulic performance from large-scale laboratory experiments and physically based modelling
Full text linkAmong other Sustainable Urban Drainage Systems, Permeable Pavements (PPs) are one can be easily retrofitted in the urban environment. However, they suffer of clogging phenomena that reduces their efficiency over time. Laboratory experiments to assess the hydraulic performance of a newly constructed PP subjected to different rainfall intensities have been conducted using a large-scale laboratory model (2x6 m2 with 1.2\% slope). The surface of the upstream portion (1.7x2 m2) is impermeable to simulate runoff generation over impermeable surfaces, while the downstream portion (4.3x2 m2) is realized with PICP. The downstream vertical side of the PP is made of permeable bricks and two gutter channels are placed crosswise to separately collect runoff and subsurface discharge. The remaining sides, as well as the bottom of the model, are impermeable. The filter package below the PICP consists of three layers: 5 cm bedding (3-6 mm gravel), a 10 cm base layer (8-12 mm gravel) and a 30 cm sub-base layer (20/40 mm gravel), which is laid on top of a 40 cm layer of native sand (silty sand with d50=0.23 mm). A geotextile separates the bedding and base layers and a 4m long drainpipe (D=150 mm) was inserted in the sub-base layer. The facility is equipped with probes on both lateral sides: 6 tensiometers in the native sand, 4 water content reflectometers in the base and sub-base layers, and 3 piezometers to record water table evolution throughout the experiments and degree of saturation of the filter layer package. Runoff and subsurface discharge are separately conveyed to two tipping bucket rain gauges. A rainfall simulator is used to generate quite uniform rainfall distribution (80 - 150 mm/h intensity) for 15 minutes or 30 minutes. Moreover, an Integrated Surface-Subsurface Hydrological model (CatHy) has been used to model the permeable pavement, assess and support data collected from the laboratory experiments.
Results from the laboratory experiments performed have proven the efficiency of a newly constructed permeable pavement to very intense rainfall events. The monitoring with spatially distributed sensors allowed to assess the evolution in time of the water table as well the “recovery” phase to pre-event conditions after the event. This is useful to assess the effect of repeated rainfall events at short distance in time. For each experiment performed, a rapid increase of subsurface discharges was recorded by the tipping bucket, whereas surface runoff occurred only for short and intense rainfall events (approximately 150 mm/h for 15 min). The system did not reach saturated conditions in any of the performed experiments due to the high permeability of the filter layer package. The monitoring with spatially distributed sensors also allowed to assess the heterogeneities of the physical processes (synthetic rainfall events, infiltration processes) as well as of the filter layer package.
Future laboratory experiments simulating clogging phenomena will be performed and compared to the results obtained from the developed experiments up to now and of the ISSH model
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