Understanding the limits of alluvial analogues: lessons from geomorphological design of mine rehabilitation in New South Wales, Australia

Abstract

A geomorphological approach to landform design aims to pattern constructed landforms in erodible materials on natural landforms. These natural landforms represent a mature geomorphic condition subject to a slow rate of evolution. Most commonly, stable alluvial landforms in the local environment are utilised as analogues. Unfortunately, few landforms constructed in mining overburden in New South Wales, Australia, have an overall geometry that allows for alluvial analogues to be directly applied; that is, the post mining landforms are frequently too steep.To address this challenge, approaches used internationally range from very sinuous drainage lines to flatten gradients, through to wide grassed drains to reduce velocities, and the use of rock armouring in drainage lines. Each of these approaches has some challenges. Overly sinuous designs move away from a dendritic drainage layout and are overly complex to construct. They are also vulnerable to piping failures. Wide grassed drains can develop preferential flow paths and can also impact unfavourably on the overall drainage density. Rock armouring is not generally favoured by geomorphologists since armouring represents a rigid conveyance system without self-healing capabilities.The strategy adopted by the authors is to design drains using a vegetated rock matrix approach. Effectively, a dendritic drainage layout is used at a desired drainage density, and the drain widths and/or longitudinal slopes adjusted to ensure that the drains have velocities below 3 m/s even for the design event (typically the 1:100-year event). These drains are wider and shallower than a typically optimised trapezoidal drain and, for frequent flood events, the rock is primarily required for the roughness it adds to the surface.With relatively low design velocities, sediment accumulates within the rock voids and revegetation of this material significantly increases the rock stability. The embedded rock and revegetated matrix are then able to withstand extreme flood events well above the original design event. This larger flood event is more in line with that expected for the required design life of these surfaces.It is noted that, because the rock size required is proportional to the velocities, increasing the drain perimeter by designing a flatter, wider drain does not necessarily increase the cost of the lining because smaller rock sizes result in reduced thicknesses of the rock lining.