Environmental/engineering geology of alluvial settings

Abstract

Alluvial deposits were recognized as important to mankind's welfare 2500 years ago when collector galleries (kanats) were constructed in Persia beneath alluvial slopes. Furthermore, the 4000-year history of observations along the rivers of Mesopotamia, Egypt, and China provided an early understanding of sediment loads, silting, and backfilling of river channels, elements basic to selecting a site for engineered works or locating aggregate sources. Harold Fisk's work (Fisk, 1944, 1947) in the Lower Mississippi Valley developed three-dimensional concepts relative to deposition within large stream valleys and the identification of distinctive features concerned with the origin and active processes. Utilizing Fisk's methods and techniques led to recognition of the sequential depositional history that serves to delimit and map distinctive alluvial units. This systematic unravelling of the younger alluvial history is frequently the all important factor in engineering geology practice. Moreover, Fisk's contributions on the evolution of a major river system provide guidelines to characterize a candidate site, or design and operate an engineered works. Such principles are briefly reviewed in a series of selected geological settings related to major works, such as: (1) The intermontane alluvial valleys of Cenozoic age have a complex sequence of sediments and infilling that is closely related to its structural history which enlarged the basins and accelerated erosion of surrounding terrain. Such deposits may possess substantial groundwater and other resources. (2) Construction of dams and hydrological projects within a deeply-filled alluvial valley is economical today, because of the greatly improved understanding of alluviation, characteristics of the deposits, and stratigraphic sequences common to the environs of a narrow valley. (3) An inadequate understanding of the origin, distribution, and character of alluvial features can result in geological errors of judgment. Boulders in a gravelly unit maybe interpreted as “bedrock” and the design foundation-level for works. (4) The intermixing of fluvioglacial deposits with Holocene sediments throughout 25 km of an ancient river valley resulted in physical changes that impacted tunnel construction beneath the channel. (5) Morainal openwork gravels utilized as a reservoir bank may fail with time due to liquefaction and piping action. (6) Fine-grained alluvial sediments deposited within an ancient abandoned river channel can be a deceptive and dangerous foundation material for major works. (7) Multiple alluvial units can be critical to understanding the depositional history and recency of tectonism.