Field research on the performance of various drainage materials in Lithuania
Abstract Field evaluation of the performance of a drainage system or of various drainage materials is a long-lasting research, very often with disappointing results. These disappointing results stem either from a too wide a spread of the field data, or one or more necessary parameters which were not measured, or an insufficient knowledge of the flow occurrence in the vicinity of the drain resulting in a wrong interpretation and evaluation of the research findings. Yet, a correct evaluation of the performance of a drainage system is of primary importance to ascertain the real value of a drainage system or to compare various drainage materials. As a result of the wide spread of field data within a measuring season, a large amount of data from several seasons is required to obtain reliable results. From discharge and total head loss measurements a second-degree polynomial results which only permits a determination of whether the design criteria are met. The similarity with Hooghoudt’s equation allows the determination of soil hydraulic conductivity and the equivalent depth of the impervious layer. With this data the drain spacing can be checked. Too wide a spacing can be the cause of malfunctioning of a drainage system, but the drainage materials can also affect its performance. To evaluate or compare drainage materials it is necessary to measure the so-called approach-flow head loss in the vicinity of the drain. The relationship between discharge and approach-flow head loss should theoretically be linear and the regression coefficient gives the approach-flow resistance. However, the relationship depends to a large extent on the approach-flow occurrence in the vicinity of the drain. For non-linear relationships, the approach-flow resistance can be estimated from the slope of the tangent line, which tends to a constant value at greater discharges for flow patterns covering the whole drain circumference. The approach-flow resistance depends on the hydraulic conductivity of the soil and does not permit a direct comparison of various drainage materials. Therefore, the approach-flow constant or the dimensionless approach-flow resistance is calculated. This parameter is related to the material itself and to the installation conditions reflected in the hydraulic conductivity of the trench backfill. Consequently the comparison of the various drainage materials includes the trench backfill. The approach-flow head loss fraction is that part of the total head loss which is dissipated by the approach-flow. The relationship between approach-flow head loss and total head loss should theoretically be and is, indeed, linear so that the slope of the regression line gives the approach-flow head loss fraction. This parameter combines the influence of drainage material and flow pattern so that real flow situations in the vicinity of the drain, which are not necessarily the same for all materials, can be compared.
Design and performance of materials for subsurface drainage systems in agriculture
During the second half of the 20th century, numerous land drainage systems using new materials for the drain channels that often function inadequately due to biochemical and mechanical clogging, were developed. The design of drainpipes and envelope materials used for these land drainage systems was based on both theoretical and experimental investigations. This contribution reviews the simultaneous development of theory and practical experience in Europe and North America. The results of the effect of perforation shape and pattern in drainpipes and of envelope materials on drainage performance are summarized and the latest design criteria of granular materials and synthetic envelopes presented. Theoretical investigations are associated with field data on the performance of drainage materials. A thorough knowledge, not only of the flow conditions, but also of the physical soil properties in the vicinity of the drains is imperative for a correct interpretation of field data. This contribution represents the main features on drainpipes and envelope materials, and the physical soil processes that occur in the vicinity of the drain. It gives an overview of the research work done referring to numerous publications where relevant and detailed information can be found.
O2 constraints from Paleoproterozoic detrital pyrite and uraninite
Redox-sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the Archean and early Paleoproterozoic atmosphere. Today, these minerals are rapidly chemically weathered within short transport distances. Prior to the rise of oxygen, low O2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes. After the rise of oxygen, redox-sensitive detrital grains effectively vanish from the sedimentary record. To get a better understanding of the timing of this transition, we examined sandstones recorded in a scientifi c drill core from the South African 2.415 Ga Koegas Subgroup, a mixed siliciclastic and iron formation–bearing unit deposited on the western deltaic margin of the Kaapvaal craton in early Paleoproterozoic time. We observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section, indicating the rise of oxygen is younger than 2.415 Ga. To better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain Earth surface redox conditions, we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental O2 concentrations. Even conservative model calculations for deltaic depositional systems with suffi cient transport distances (approximately hundreds of kilometers) show that redox-sensitive detrital grains are remarkably sensitive to environmental O2 concentrations, and they constrain the Archean and early Paleoproterozoic atmosphere to have <3.2 × 10 –5 atm of molecular O2. These levels are lower than previously hypothesized for redox-sensitive detrital grains, but they are consistent with estimates made from other redox proxy data, including the anomalous fractionation of sulfur isotopes. The binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric O2 concentrations rose substantially at this time and were never again suffi ciently low (<0.01 atm) to enable survival and preservation of these grains in short transport systems.