Two mitigation measures have previously been identified: source reduction and flow interception. The amount of water that reaches an aquifer or water-bearing strata is a function of the permeability of the material overlying the aquifer and the extent to which water is allowed to accumulate on the surface. Source-reduction objectives must then typically focus on improving the surficial flow of water that would otherwise reach the aquifer and, where necessary, introducing an impermeable barrier.
Surficial flow can be improved by grading and the installation of drainage swales. This objective is contrary to that associated with erosion control, since the velocity of the surficial water must be maintained to minimize the rate of absorption. The appropriate slope will be a function of the soil characteristics and ground cover. Where the slope is not sufficient, concrete or a less permeable channel must be created to rapidly convey most of the water to the valley floor, thereby reducing the amount of water that will flow through the aquifer.
The permeable characteristics of the surficial soils can be altered chemically or through the introduction of geotechnical products such as geodrain or geomat. These are generic names for a particular type of subsurface drain and an impervious polymer sheet, respectively, which create a barrier and reduce the velocity of flow that might otherwise be required to effectively remove the water at the surface. Geodrain is a fabric that captures and channels water to an included pipe, while geomat is essentially an impermeable barrier. The various approaches to improving surficial flow can be combined as shown in Figures 8 and 9. Before localized drainage solutions such as these are attempted, site geology and hydrology characteristics must be studied to ensure that the source of water reaching the aquifer has been identified.
Figure 9
Schematic section of a proposed drainage-control system, Yungang.
Figure 8
Plan of proposed surface drainage-control system, Yungang.
Water interception can be an effective means of diverting the flow of underground water. This procedure involves the construction of a major access tunnel or tunnels from which a peripheral drainage system may be constructed. The major access tunnel usually serves as the drainage channel, collecting water from the peripheral tubes and discharging the flow beyond the area of concern. Advantages associated with a water-interception program include the ability to more accurately locate the aquifer and alter the intercept field until the objectionable flow has been eliminated. The conceptual development of a water-interception program is described in Figure 10, with a typical detail connection of a collector tunnel shown in Figure 11.
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