Inelastic and elastic storage properties and daily hydraulic head estimates from continuous global positioning system (GPS) measurements in northern Iran
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Keywords:
Hydraulic head
Piezometer
Specific storage
The hydraulic conductivity of submerged sediments influences the interaction between ground water and surface water, but few techniques for measuring K have been described with the conditions of the submerged setting in mind. Two simple, physical methods for measuring the hydraulic conductivity of submerged sediments have been developed, and one of them uses a well and piezometers similar to well tests performed in terrestrial aquifers. This test is based on a theoretical analysis that uses a constant-head boundary condition for the upper surface of the aquifer to represent the effects of the overlying water body. Existing analyses of tests used to measure the hydraulic conductivity of submerged sediments may contain errors from using the same upper boundary conditions applied to simulate terrestrial aquifers. Field implementation of the technique requires detecting minute drawdowns in the vicinity of the pumping well. Low-density oil was used in an inverted U-tube manometer to amplify the head differential so that it could be resolved in the field. Another technique was developed to measure the vertical hydraulic conductivity of sediments at the interface with overlying surface water. This technique uses the pan from a seepage meter with a piezometer fixed along its axis (a piezo-seep meter). Water is pumped from the pan and the head gradient is measured using the axial piezometer. Results from a sandy streambed indicate that both methods provide consistent and reasonable estimates of K. The pumping test allows skin effects to be considered, and the field data show that omitting the skin effect (e.g., by using a single well test) can produce results that underestimate the hydraulic conductivity of streambeds.
Piezometer
Hydraulic head
Slug test
Aquifer test
Water well
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Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater–surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer‐based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high‐temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human‐induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.
Piezometer
Hydraulic head
Pressure head
Pressure measurement
Elevation (ballistics)
Pressure gradient
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Internal erosion is caused by seepage body forces acting on the soil particles. One of the most dangerous modes of internal erosion at hydraulic structures is backward erosion piping, which usually initiates at the downstream end of a seepage path, e.g., at the downstream toe of the dam. The progress of backward erosion and the development of erosion pipes were tested in a newly developed laboratory device for three types of sand with grain sizes of 0/2, 0.25/2, and 0.25/1. The piezometric head along the gradually developing seepage “pipe” was observed by seventeen piezometers and seven pressure sensors. The seepage amount was measured by the volumetric method. The critical hydraulic gradient was determined and related to the soil porosity. The progression of the seepage path and relevant characteristics such as the piezometric and pressure heads and the amount of trapped sediment were observed by two synchronous cameras. Based on the analysis of the results of 42 tests, a new empirical formula for the backward erosion rate was proposed. The characteristics of lateral erosion were evaluated and compared with the available literature, which provided reasonably good agreement.
Internal Erosion
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Hydraulic head
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The results of a variable-head permeability test can be used to check whether a piezometer or sampling well has been successfully sealed in the soil and to detect hydraulic short circuits and possible cross-contamination between aquifers. An improper seal is a well-known cause of hydraulic short circuit. In some instances, also, although the tubing has been sealed over its full length, the water level in the pipe is not representative of the actual piezometric head. An example shows that the values of hydraulic conductivity determined using several test methods varied in a 1:6 ratio before detection of a piezometric error. The resulting correction reduced the variation in k to ± 12%. The causes of hydraulic short circuits have been investigated using computer simulations. It has been concluded that large errors recorded in the field are due to internal erosion of natural soils around the casing during drilling operations or development. The hydraulic damage to soils is documented. An analytical solution has been written for a simple case of hydraulic short circuit. Its theoretical predictions confirm the validity of the computer simulation. In light of experience, recommendations are proposed to reduce hydraulic damage, therefore improving the reliability of piezometric measurements and representativeness of groundwater samples. Key words: permeability, field test, piezometer, sealing, drilling, water level.
Piezometer
Hydraulic head
Permeameter
Soil test
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Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater-surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.
Piezometer
Hydraulic head
Pressure head
Pressure measurement
Elevation (ballistics)
Pressure gradient
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Piezometer
Hydraulic head
Pressure head
Pressure measurement
Hydrostatic pressure
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