Lower-Impact Sediment Cap Design: Using Natural “Armoring” Processes in the Design and Construction of Subaqueous Sediment Caps

摘要

Background. Subaqueous sediment containment caps are typically designed and constructedassuming multiple layers comprising specific particle sizes. For example, a capmay include one layer of sand to provide isolation of the contaminants covered by an“armor” layer of gravel/rock to prevent erosion of the isolation layer. The cap design willtypically specify a particular d50 (mean particle diameter) to be used in each layer of thecap. Obtaining the specified particle sizes requires identifying borrow sources that cansometimes be a significant distance from the project location. The quarries/sand andgravel operations used to provide capping materials consume fuel and produce emissionsduring their operations, and transporting large quantities of materials to the project sitealso consumes resources and has environmental impacts.This paper will discuss an alternative method for designing a sediment cap that relieson natural “armoring” processes and thereby allows for the use of less resource-intensivesources of capping materials. The approach is based on research by Gessler (1970) to estimatethe change of a soil/sediment grain size distribution due to winnowing of finesfrom the surface by the shear forces of flowing water. This process is known as “armoring”and Gessler found that the final particle size distribution of the soil can be estimatedbased on surface water shear stress. In principle, a sediment cap could be designed usingthe least expensive or least environmentally damaging source of capping material, ratherthan by specifying an exact grain size for each layer of the cap. Using this method toestimate the natural armoring process in a river for the design of a sediment cap canallow for:1.Reduction in costs by allowing the use of a local source with a mixed particle sizedistribution that may not otherwise be accepted2.Reduction in greenhouse gas emissions, resource consumption, and habitat destructionby reducing the reliance on distant sand and gravel operations that wouldnormally need to be used to obtain specific particle sizes3.Use of simpler construction methods, which lead to faster cap placement at a lowercost and with less fuel consumption/environmental impact4..Greater long-term sustainability of the cap because of the reliance on natural processesto establish a stable armoring layerThe authors have adapted this method for use in designing caps for tailings impoundments.The paper will present an example of a very effective cap constructed based onthis theory using a local source of fill material with a mixed particle size distributionrather than a uniform cap.