Closed Conduit System Research Articles

Transient deformation associated with explosive eruption measured at Masaya volcano (Nicaragua) using Interferometric Synthetic Aperture Radar

Deformation caused by processes within a volcanic conduit are localised, transient, and therefore challenging to measure. However, observations of such deformation are important because they provide insight into conditions preceding explosive activity, and are important for hazard assessment.Here, we present measurements of low magnitude, transient deformation covering an area of ∼4km2 at Masaya volcano spanning a period of explosive eruptions (30th April–17th May 2012). Radial uplift of duration 24days and peak displacements of a few millimeters occurred in the month before the eruption, but switched to subsidence ∼27days before the onset of the explosive eruption on 30th of April. Uplift resumed during, and continued for ∼16days after the end of the explosive eruption period. We use a finite element modelling approach to investigate a range of possible source geometries for this deformation, and find that the changes in pressurisation of a conduit 450m below the surface vent (radius 160m and length 700m), surrounded by a halo of brecciated material with a Young's modulus of 15GPa, gave a good fit to the InSAR displacements. We propose that the pre-eruptive deformation sequence at Masaya is likely to have been caused by the movement of magma through a constriction within the shallow conduit system.Although measuring displacements associated with conduit processes remains challenging, new high resolution InSAR datasets will increasingly allow the measurement of transient and lower magnitude deformation signals, improving the method's applicability for observing transitions between volcanic activity characterised by an open and a closed conduit system.

Culvert blockages in two Australian flood events and implications for design

For 25 years, the de facto code-of-practice Australian Rainfall and Runoff (ARR) has recommended that the design of roadside and property drainage systems include the design of overland floodways which come into play whenever a flood exceeds the capacity of the closed conduit system. There is no encouragement that the same ‘safe-fail’ design procedure be adopted for larger, trunk drainage systems. Floods at Wollongong in 1998 and, at Newcastle in 2007, generally exceeded all drainage channel capacities. Some engineers misinterpreted the post-flood debris in the vicinity of culvert and bridge waterways as indicating hydraulic blockage of waterways by flood-borne debris and hence as a major contributor to higher flood levels and damage. A review of the two flood events shows that heavy rainfall resulted in flood discharges which exceeded the drainage capacities such that excess water flowed through natural and artificial urban floodways. Unexpected levels of damage were not caused by culvert blockage, but by the urban floodways which had not been included in prior hydraulic designs of culvert and bridge waterways and thus not identified and planned for. The recommendation is that the ‘safe-fail’ design approach be used for all sizes of drainage systems so that the inevitable hydraulic failures of drainage works can be anticipated in terms of where and how excess floodwater is managed.

Well-Balanced Scheme for Modeling Open-Channel and Surcharged Flows in Steep-Slope Closed Conduit Systems

AbstractThe model presented in this paper preserves lake at rest conditions in sloped prismatic conduits. These schemes are based on the two-governing equation model in which open-channel flows are simulated using the Saint-Venant equations and pressurized flows using the compressible water hammer equations. The model in this paper preserves lake at rest conditions (horizontal still water) regardless of the conduit slope, resolves moving jump discontinuities over dry beds in sloped conduits, and resolves small perturbations from steady states, even when adjacent to dry regions. The preserving steady-state capability of this model is of particular importance in continuous long simulations when the conduits are relatively steep (|So|>∼0.5%. Two main contributions are presented in this paper, namely, (1) a new method for water stage reconstruction is presented that preserves lake at rest conditions regardless of the pipe slope, and (2) a horizontal system of coordinates, instead of the commonly used inclined...

Conditional Assessment of Flow Measurement Accuracy

A study conducted by the Utah Water Research Laboratory assessed the accuracies of a wide variety of flow measurement devices currently in service. During the study, a wide variety of flow measurement devices, including flumes, weirs, and rated sections in open channel systems, were evaluated; magnetic and ultrasonic meters in closed-conduit systems were also tested. The specified design accuracies for each device are presented. Actual flow measurements were determined at 70 sites and were compared with the theoretical discharges of each device. Comparison of actual and theoretical flow indicates that only 33% of the measurement devices tested currently measure flow within manufacturer-designed specifications. Field data is presented, and possible reasons for the flow measurement errors and their corrections are discussed.

Experimental study of scour rate in consolidated cohesive sediment

The erosion process of a cohesive sediment after consolidation was studied experimentally in a closed conduit system. The test samples investigated in this study are mixtures of sand and clay with variable compositions and different consolidation times. The main concern of this study is the effects of the dry density of the consolidated sediment on scour rate. A scour rate formula is derived and further interpreted based on the experimental results.

A quantitative model for the time‐size distribution of eruptions

The modeling of the statistical distribution of eruptive frequency and volume provides basic information to assess volcanic hazard and to constrain the physics of the eruptive process. We analyze eruption catalogs from volcanoes worldwide in order to find “universal” relationships and peculiarities linked to different eruptive styles. In particular, we test (1) the Poisson process hypothesis in the time domain, looking for significant clustering of events or the presence of almost regular recurrence times, (2) the relationship between the time to the next eruption and the size of the previous event (the “time predictable” model), and (3) the relationship between the size of an event and the previous repose time (the “size predictable” model). The results indicate different behavior for volcanoes with “open” conduit regimes compared to those with “closed” conduit regimes. Open conduit systems follow a time predictable model, with a marked time clustering of events; closed conduit systems have no significant tendency toward a size or a time predictable model, and the eruptions follow mostly a Poisson distribution. These results are used to build general probabilistic models for volcanic hazard assessment of open and closed conduit systems.

Response to the comments of D. R. Blackman and Y. Peleg (JRA 14, 411-14)

The comments on my article (JRA 13,104-32) by Blackman and Peleg, which are much appreciated, reflect prevailing discussions on Roman hydraulic engineering practices as well as on the relevant paragraphs of Vitruvius (8.6). Blackman focuses mainly on technical aspects, while Peleg aims at archaeological arguments.Blackman starts with a discussion of the term ‘siphon’, which he regrets being used as it could lead to misunderstanding: ‘siphon’ would represent a “real” siphon, not an “inverted” siphon, suggesting that I must have meant that there was a real siphon at Aspendos. However, in archaeology “siphon” is generally accepted to represent an inverted siphon. There is no misunderstanding: at Aspendos we have an inverted siphon, and since real siphons are not known in classical aqueduct systems, there is no objection to using ‘siphon’ and ‘inverted siphon’ for one and the same notion. Subsequently he states: “The presence of air pockets and so on is almost irrelevant to operations [of siphons] if no point in the system lies above the Hydraulic Gradient Line; were it not so, no garden hose would work reliably” This is a misconception. It is only due to the fact that our garden hoses are connected to supplies with elevated pressures that we get water from them. If we would connect the hose to a low-pressure source (e.g., to a rainwater container standing at ground level), we have to straighten out the coils before water will emerge. If the hose is coiled up, for example, on a wall support but positioned below the free water surface in the container to which it is connected, while we are holding the free end somewhere near the ground, we have nothing but an inverted siphon with high points. If air pockets are irrelevant, water should come out, but it does not if air is in the hose. For problems associated with air pockets in gravity-driven closed conduit systems (which classical siphons must be considered to be), see G. Corcos, Air in water pipes (1989) and H. T. Falvey, Air-water flow in hydraulic structures (1980).

Drainage-Subirrigation Design For Pelham Loamy Sand

ABSTRACT This project was conducted on the Bell farm located in Pierce County, GA on Pelham loamy sand soil. The study area included 40 ha of land under controlled drainage-subirrigation (CD-SI) system of which 38 ha were in blueberries. The system installation included two drain spacings of 15.3 and 20 m, and two types of control structures for the drainage system outlet water level, which were an open ditch and a closed conduit system network. The blueberry section of the field was in a closed conduit system network with lateral drains spaced at 15.3 m. Seventeen punch-tape recorders were used to measure the water table elevations in the soil profile within the field over the drain tiles, and midway between drainlines, at the open ditch and closed system control structures (one for each), and in an undrained-nonirrigated section of the farm. A punch-tape rainfall recorder was also used to measure rainfall at the site. Surface runoff, drainage effluent, or subirrigation volume was not measured. Experimental results showed that the water table and soil-water conditions could be adequately managed in the blueberry field, thus excellent crop growth and yield resulted. DRAJNMOD, a water management model for shallow water table conditions, was used to simulate the system performance for the study site. Simulation results indicated that a subsurface drain spacing of 20 m is satisfactory for controlled drainage-subirrigation (CD-SI) systems for Pelham loamy sand soils in Pierce County, GA. Simulations were done using 20 years of climatological data from Augusta, GA. Additional research is needed to develop specific design guides for other soil types in the Georgia Flatwoods region.

Test Procedures for Determining Cavitation limits in Control Valves

The design and operation of control valves in a closed conduit system require testing of the different flow characteristics of the valves under experimental conditions. Different testing standards and recommended testing procedures have been published for characteristics such as pressure loss and pressure recovery. However, discussion concerning the testing procedures for cavitation and operation limits is scarce. Detailed information is presented on testing procedures and the definitions for the different cavitation limits. The effects of the variables of pressure, size, and flow coefficients are discussed, along with design equations and data for scaling cavitation information with changes in system pressure.

Control Valve Flow Coefficients

The flow coefficient for a control valve is an important hydraulic parameter that is used to select and design the controls for a closed conduit system. The parameter is used to determine flow capacities, valve positioning, and energy losses of the system. The coefficient is also used for the analysis of the flow system for other phenomena such as cavitation and transients. It is important to understand the definition of the flow coefficient, how it is used, and how it is determined for a control valve. At present there are two sets of testing standards to determine the flow coefficient for a valve. The standards are different in the definition of how the coefficient is determined and can result in two different sets of flow coefficient data for a valve. Confusion has resulted in identifying which set of standards was used to generate the valve coefficient information, and the importance of designing a system using the same definition of the coefficient.

Needle Valves as Pressure Regulators

The potential of needle valves issuing into sudden expansions as the primary components of large closed conduit pressure regulating structures is analyzed. Results are presented of research projects conducted to evaluate the hydraulic characteristics of needle valves. The flow characteristics of the needle valve are described by presenting a submerged jet issuing from a circular orifice into an expansion chamber. The basic principles and equations of fluid mechanics are applied to qualitatively predict the cavitation, pressure fluctuation, and energy dissipation capabilities of submerged jets. A brief literature review of related studies is presented, followed by a description of the facilities used for testing and the results. These results indicate that needle valves are ideally suited for pressure regulation in closed conduit systems. Specifically they have the capability of dissipating large amounts of energy with little cavitation or pressure fluctuations. However, special attention should be given to the design to maximize the streamlining of the valve and to eliminate the possibility of local cavitation zones.

Review of Cavitaion Research on Valves

Critical cavitation indices are presented for a 12-in. ball and a 12-in. butterfly valve installed in a closed conduit system with, a sudden expansion downstream of the valves. Tests were conducted under normal operating conditions. In addition, tests were conducted to find the effect of injecting air and water at various locations on the critical cavitation index. It was found that injecting water into the sudden expansion below the valves does not affect the cavitation. However, injecting small quantities of air into the separation zones below the valves does materially reduce the cavitation intensity and hence the critical cavitation index.

Quick Design of Air Vents for Power Intakes

For closed conduit systems without surge tanks, common parameter is derived and correlated with vent diameter as used in 35 hydroelectric installations; empirical equation for solution of vent diameter size requires knowledge of only rated head, output of turbine, and length of vent; graphic solution is also given.