Falling Head Research Articles

Comparison of Hydraulic Conductivity Estimation using Falling Head and Mini Disk Infiltrometer in Gypsiferous Soil

Hydraulic conductivity (K) was calculated using falling head (FH) and Mini Disk Infiltrometer (MDI) for 15 Gypsiferous soil. In this study the hydraulic conductivity was predicted by basic infiltration rate (BIR) and using two methods, Wooding-Gardner (W-G) and van Genuchten-Zhang (V-Z). Hydraulic conductivity was measured using a falling head permeability test, and the results were compared to the expected values. A statistical analysis (t-test) was performed to show the significant differences between the two methods and the falling head. The results showed high agreement between (K) values estimated by (FH) and (BIR) for different gypsum content, the results showed that Both methods did not succeed in estimating the hydraulic conductivity for different gypsum content. Values derived from (W-G) method are exaggerated and the values derived from the (V-Z) method are very low compared to the values derived from (FH).

Hydraulic performance of horizontal constructed wetlands for stormwater treatment: A pilot-scale study in the Mediterranean

Horizontal subsurface flow (HSSF) constructed wetlands (CWs) are widely adopted as a highly effective technique for stormwater treatment. However, the cumulative clogging phenomenon in HSSF is a complex and challenging operative issue affecting removal efficiencies and the lifespan of CWs. This paper aims to evaluate the reliability of a combined approach for assessing the occurrence of clogging and its potential spatial and temporal evolution in two pilot-scale HSSF-CW. The experimental CW system was fed with stormwater runoff from a parking area during the rainy season and sequential batch reactor (SBR) wastewater (WW) during the dry season. The falling head (FH) method and a drainage equation were used to determine the hydraulic conductivity at saturation (Ks) of the porous medium and investigate the hydraulic performance of the system. A residence time distribution (RTD) analysis was also performed to evaluate the hydraulic flow dynamics and define the crucial parameters characterising the CW. The applied hydraulic measurement techniques highlighted that clogging was in the initial stages of development after 2.5 years of operation, likely associated with the presence of a pond and the partial retention of total suspended solids (TSS) by the HSSF system. Overall, the good consistency in the results of the applied methods supports their future application for predicting the occurrence of clogging phenomena in HSSF CWs.

Comparative laboratory measurement of pervious concrete permeability using constant-head and falling-head permeameter methods

The permeability characteristics is one of the most important properties of pervious concrete (PC), which represents the ability of water on the surface to drain through the pervious pavement structure. Its measuring is a sensitive key on the pervious concrete. Thus, this paper presents the experimental results of two commonly used laboratory test methods to measure the permeability of pervious concrete; namely falling-head (FH) method and constant-head (CH) method, to investigate the relationship between the examined methods and select the reliable method. Moreover, study the effect of aggregate gradation and ratio of cement to aggregate on the permeability of PC. Therefore, thirty-six groups of pervious concrete specimens with twelve different types of gradation and three kinds of ratios of cement to aggregates were prepared in this study. The comparison between the two methods to evaluate which method is more suitable includes: the precision, the cost of the experimental equipment, the convenience of the test operation process and the time required for the test. The results showed that the permeability values measured by the falling-head method were significantly higher than those measured by the constant-head method and it is more convenient compared to the constant-head method. Effect of aggregate gradation on porosity and permeability was examined in this study. Correlation between porosity and permeability was investigated, and a new model between FH and CH methods was established.

Investigation of infiltration rates under different experimental conditions

This paper presents the results of laboratory experimental studies on soil infiltration rates (IR) using constant-head (CH) and falling-head (FH) methodologies. The study also determined the initial moisture content of the soil and classified the soil in accordance with American Society for Testing and Materials (ASTM). Infiltration characteristics were measured for two different scenarios: Before Irrigation (BI) and After Irrigation (AI). The infiltration results of FH and CH showed that the initial moisture content highly affects the magnitude of the final IR as the infiltration capacity of the tested soils decreases for AI cases. CH final IRs observed were much higher than their FH analogues. This is consistent with other literature though the discrepancy here is larger than reported elsewhere. FH methodology is recommended for application to near surface processes, such as sprinkler irrigation where the measured value of IR is most representative of the actual soil IR.

A Comparison of In Situ Constant and Falling Head Permeameter Tests to Assess the Distribution of Clogging Within Horizontal Subsurface Flow Constructed Wetlands

Clogging is the main operational problem associated with horizontal subsurface flow constructed wetlands (HSSF CWs). The measurement of saturated hydraulic conductivity has proven to be a suitable technique to assess clogging within HSSF CWs. The vertical and horizontal distribution of hydraulic conductivity was assessed in two full-scale HSSF CWs by using two different in situ permeameter methods (falling head (FH) and constant head (CH) methods). Horizontal hydraulic conductivity profiles showed that both methods are correlated by a power function (FH = CH0.7821, r2 = 0.76) within the recorded range of hydraulic conductivities (0–70 m/day). However, the FH method provided lower values of hydraulic conductivity than the CH method (one to three times lower). Despite discrepancies between the magnitudes of reported readings, the relative distribution of clogging obtained via both methods was similar. Therefore, both methods are useful when exploring the general distribution of clogging and, specially, the assessment of clogged areas originated from preferential flow paths within full-scale HSSF CWs. Discrepancy between methods (either in magnitude and pattern) aroused from the vertical hydraulic conductivity profiles under highly clogged conditions. It is believed this can be attributed to procedural differences between the methods, such as the method of permeameter insertion (twisting versus hammering). Results from both methods suggest that clogging develops along the shortest distance between water input and output. Results also evidence that the design and maintenance of inlet distributors and outlet collectors appear to have a great influence on the pattern of clogging, and hence the asset lifetime of HSSF CWs.

A laboratory analysis of falling head infiltration procedures for estimating the hydraulic conductivity of soils

Falling head (FH) infiltration procedures can give rapid estimates of the field saturated hydraulic conductivity ( K fs) of soils. The objectives of this investigation were to develop a procedure for estimating both the K fs and the α ⁎ parameter of the exponential hydraulic conductivity function from a FH experiment, and to compare FH procedures differing by experimental and computational demands. A total of 33 FH experiments were carried out on repacked, sandy loam and loam soil cores. A two level (TL) analysis was applied to determine a value of α ⁎ for each soil core. K fs was then calculated by three different approaches, ranging from the fitting approach (FA) that used the measured α ⁎ value and all cumulative infiltration vs. time data pairs, to the Simplified Falling Head (SFH) approach, that used only the first and the last water level readings and reasonable estimates of α ⁎ taken from the literature. The TL analysis yielded plausible α ⁎ results, equal to 24.9 m − 1 for the initially dry sandy loam soil and to 7.0–8.1 m − 1 for the initially dry loam soil and 66.9 m − 1 for the initially wet loam soil. The mean values of K fs obtained with the different FH approaches are 120.5–159.6 mm h − 1 for the sandy loam soil and to 11.0–19.1 and 2.3–4.0 mm h − 1 for the initially dry and wet loam soil, respectively. Similar K fs results were obtained by both the SFH approach and by the more complicated approaches yielding estimates of both α ⁎ and K fs. It was concluded that (i) the SFH procedure produced results as good as the more data and time consuming procedures, and (ii) simple instrumentation and analytical methodology may be applied to obtain rapid and reasonable estimates of K fs.