Modelling plumes of overland flow from logging tracks

Abstract

Abstract:Most land-based forestry systems use extensive networks of unsealed tracks to access the timber resource. These tracks are normally drained by constructing cross-banks, or water bars, across the tracks immediately following logging. Cross-banks serve three functions in controlling sediment movement within forestry compartments:1. they define the specific catchment area of the snig track (also known as skid trails) so that the overland flow does not develop sufficient energy to cause gullies, and sheet and rill erosion is reduced; 2. they induce some sediment deposition as flow velocity reduces at the cross-bank; 3. they redirect overland flow into the adjacent general harvesting area (GHA) so that further sediment deposition may take place.This paper describes a simple model that predicts the third of these functions in which the rate of runoff from the track is combined with spatial attributes of the track and stream network. Predictions of the extent of the overland flow plumes and the volume of water delivered to streams is probabilistically presented for a range of rainfall-event scenarios with rainfall intensity, time since logging and compartment layout as model inputs. Generic equations guiding the trade-off between intercross-bank length and flow path distance from cross-bank outlet to the stream network needed for infiltration of track runoff are derived.

Quantifying diffuse pathways for overland flow between the roads and streams of the mountain ash forests of central Victoria Australia

“…In this paper we extend the previous analysis to the wet forest environments of the mountainous area of central Victoria, where the predominant species is deep-rooting mountain ash (Eucalyptus regnans). By this extension we examine the portability of the predictive capability of the model proposed by Hairsine et al (2002).…”

“…This analysis was based on the distribution of measured volumes of water entering an area of hillslope before discharge is observed at the downslope boundary of that area, termed the volume to breakthrough (Vbt). Hairsine et al (2002) considered a 5 m length of hillslope (Vbt5) receiving redirected runoff from large-scale rainfall simulation experiments on nine logged hillslopes in southeastern Australia, (Croke et al, 1999a,b). These experimental areas are characterized by a temperate climate with 600-900 mm mean annual rainfall that is evenly distributed through the year, but which can produce intense rainfall in near-coastal areas during summer.…”

“…Soils are granite derived with depths of 1-2 m, or shallower, gravelly metasediments (<0Ð75 m depth). A key question arising from the Hairsine et al (2002) paper pertains to the portability of the model, i.e. whether it is appropriate for predicting connectivity in other environments.…”

“…Croke and Mockler (2001) demonstrated an easily parameterized scheme for minimizing gully initiation. Hairsine et al (2002) proposed a probabilistic model of diffuse overland flow from roads to streams that predicts the length of hillslope required to accommodate volumes of water discharged from a drainage outlet. This analysis was based on the distribution of measured volumes of water entering an area of hillslope before discharge is observed at the downslope boundary of that area, termed the volume to breakthrough (Vbt).…”

“…It is suggested there are universal properties of pathways draining tracks and roads, with bioturbation acting to restore available pore spaces filled by antecedent plumes. Drain discharge design criteria may be developed for local conditions using the Hairsine et al (2002) model, providing a robust tool for protection of water quality in the siting of new forest roads, and maintenance of exiting roads and tracks. …”