Vignettes > Sediment transport (dis)connectivity in the upper Hunter catchment, NSW, Australia

Sediment transport (dis)connectivity in the upper Hunter catchment, NSW, Australia

Kirstie Fryirs
Macquarie University
Author Profile

Shortcut URL: http://serc.carleton.edu/35399

Location

Continent: Australia
Country: Australia
State/Province:New South Wales
City/Town: Hunter Catchment
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy, Chronology,


These impediments act to disconnect lateral, longitudinal and vertical linkages in catchments (source Fryirs et al. 2007a). Details


The area directly connected to the conveyor belt (switches turned on) various over different timeframes or under various flow conditions (source Fryirs et al. 2007a). Details


Note the variability in the degree to which different subcatchments are connected or disconnected (source Fryirs et al. 2007b). Details


In the west of the catchment, subcatchment such as Dart Brook are highly disconnected and contain various form of impediments (buffers). In the east of the catchment, subcatchments such as Rouchel Brook are highly connected and contain a lower distribution of impediments to bedload sediment transfer (modified from Fryirs et al. 2007b). Details




Description

Sediment flux within river systems has been described as a 'jerky conveyor belt' (Ferguson, 1981). The distribution of riverine sediment stores and sinks, and the frequency with which sediment is added or removed from these stores, reflects the degree to which a river system is (dis)connected or (de)coupled (Harvey, 2002; Hooke, 2003). (Dis)connectivity along the conveyor belt affects the extent and rate of transfer of energy and matter through catchments. Any factor that impedes sediment conveyance constrains sediment delivery to the catchment outlet. These impediments have been termed buffers, barriers and blankets (Fryirs et al. 2007 a, b).

Buffers disrupt lateral linkages within catchments (Figure 1a). They may include alluvial pockets of floodplain, fans or piedmont zones that occur at breaks in slope along valley margins, disconnecting lateral connectivity in catchments. Other buffers include features such as intact valley fills and floodouts (either terminal or intermediate) that have discontinuous or absent water courses or low slope alluvial floodplains. Elevated floodplain or terraces along trunk streams may block tributary confluences, disconnecting material supply from lower order drainage lines to the primary channel network.

Barriers most commonly disrupt longitudinal linkages through their effect on the base level or bed profile of a channel (Figure 1b). For example, bedrock steps or woody debris may locally reduce slopes by introducing a local base level control. Sediments are trapped as they backfill areas immediately upstream of the step, inducing local discontinuity in sediment transfer. Sediment slugs may act as plugs to sediment movement along channels. Over-widened channels may not have the competence to carry sediments made available to them, acting as barriers to downstream sediment conveyance. Human modification to rivers may induce considerable barriers to sediment movement. For example, dams may trap all but suspended load materials from moving through the system.

Blankets disrupt vertical linkages through their effect on surface-subsurface interactions and the entrainment of sediment (Figure 1c). They are sediments that smother other landforms, and can occur instream or on floodplains and may include features such as floodplain sand sheets or fine grained materials that infill the interstices of gravel bars. Bed armour acts as a blanket which inhibits the reworking of subsurface sediments.

These impediments to sediment movement within a catchment restrict the rate of sediment transfer from the area upstream of that point. Hence, the position of buffers, barriers and blankets determines the area or proportion of a catchment that has the potential to directly contribute sediment to, or transport sediment along, the channel network under given flow conditions. This is referred to as the effective catchment area (Fryirs et al., 2007a). Landscape (dis)connectivity, in turn, determines how geomorphic instability is manifest throughout a river system, shaping the direction and rate of geomorphic change, and lagged and off-site (propagatory) responses. However, the degree of (dis)connectivity in a catchment will change over time as different types of impediments are breached (reworked) or formed. The form, composition, scale and position of impediments, and the ease with which these features are reworked and breached allows the measurement of the effective timescale of (dis)connectivity (c.f., Harvey, 2002). The breaching capacity (c.f., Brunsden, 2001) can be considered as the magnitude-frequency characteristics of perturbations required to breach the buffer, barrier or blanket, or as the residence time of these landforms.

Switches in catchments: The importance of magnitude-frequency relationships on sediment transport

Changing landscape connectivity and effective catchment area through breaching of certain types of buffers, barriers and blankets can be considered analogous to the operation of a series of switches which determine which parts of the landscape contribute to the sedimentary cascade over different time intervals. Figure 2 conveys a conceptual framework by which these interactions may be assessed.

The spatial pattern of buffers, barriers and blankets influences the timeframe over which sediments are reworked in different landscape compartments. At low flow stages associated with frequent, low magnitude energy inputs, the capacity for slope erosion and fluvial sediment reworking is limited. Landscape disconnectivity is significant and the effective catchment area is low. At this stage, none of the buffers, barriers or blankets is breached and sediment cascading in the channel network is limited. As flow stage increases within the channel network, more readily reworked instream barriers and blankets are breached. There is sufficient energy to initiate reworking of interstitial fines that form instream blankets, and sediments are conveyed through the valley constriction, connecting the upstream channel network to the lowland plain. As connectivity increases, the effective catchment area increases. In a high magnitude-low frequency event, other buffers and barriers are breached. Reworking of alluvial fans connects the hillslopes to the channel network. The sediment slug along the lowland plain is reworked, contributing sediment to the river mouth. However, unless an extreme event occurs, the floodplains and terraces along the lowland plain are not reworked and maintain their buffering capacity. Indeed, sediments derived from upstream may be deposited on the floodplain surface forming a blanket. During high magnitude events connectivity within the system is at its highest and the effective catchment area the largest.

The degree to which certain parts of catchments are switched on, and the timeframe over which this occurs dictates how the effects of geomorphic changes are propagated through a catchment. In highly connected landscapes, the cumulative effects of alterations to forms of (dis)connectivity in upper parts of a catchment are manifest relatively quickly. In contrast, in highly disconnected landscapes, changes to the nature of (dis)connectivity in one part of a catchment are absorbed or suppressed in the system and geomorphic changes are not propagated through the catchment.

Application of the buffers, barriers and blankets framework

An application of how buffers and barriers restrict bedload supply in the upper Hunter catchment will now be presented. The upper Hunter catchment drains 4200km2 in the northern part of the greater Hunter catchment which is located just north of Sydney in coastal NSW, Australia. The catchment is composed of metasediments in the east and sedimentary rocks in the west with dissected Tertiary basalt occurring in headwater areas. These two provenances are separated by the Hunter-Mooki fault. Topography in the eastern part of the catchment is rugged and steep with little available accommodation space in valleys. The western part of the catchment is characterized by relatively subdued topography and wider, more open valleys where sediments are stored.

Under simulated conditions, effective catchment area, which is a measure of the proportion of a catchment that has the potential to contribute bedload sediment to the channel network, varies from 73 % to just 3 % of the total catchment area for differing subcatchments in the upper Hunter (Figure 3). In total, around 52 % of the catchment is disconnected and does not contribute to the overall sediment cascade as measured at Muswellbrook. This calculation reflects the role of buffers that determine lateral connectivity and does not take account of longitudinal disconnectivity induced by barriers. When the combined effect of buffers and barriers is examined, the effective catchment area of the entire upper Hunter catchment at Muswellbrook drops significantly to 334 km2 or 9 %.

These results suggest that available sediments are likely to be efficiently contributed to, and conveyed through, Rouchel subcatchment (located in the east; Figure 4) while the Dart and Lower Hunter subcatchments (located in the west; Figure 4) are extremely inefficient at contributing and conveying sediment. This variability can be explained by the spatial distribution and assemblage of buffers and barriers in each subcatchment (Figure 5). Multiple forms of disconnectivity are evident in some subcatchments (e.g. Dart Brook), such that when one buffer or barrier is breached, other features still impede sediment transfer within the system (Fryirs et al. 2007b). The importance of the position of buffers and barriers within any given subcatchment is emphasised. Spatial variability in valley width exerts a critical control on catchment connectivity, with more efficient sediment conveyance in narrow valleys relative to wider valleys characterised by piedmonts, terraces, fans and extensive floodplains in which conveyance is impeded. This variability reflects the landscape history and geological setting of each subcatchment.

Landscape change, whether natural or human-induced, may alter the extent and pattern of physical (dis)connectivity. Similarly, changes to the magnitude, frequency, duration or sequencing of flow events affect the sediment transport regime. Formation or breaching of a buffer, barrier or blanket alters the pattern and rate of biophysical linkages in a catchment (Fryirs et al. 2007a). Effective description and explanation of landscape (dis)connectivity throughout a catchment provides a basis to predict the future trajectory of geomorphic change (Brierley and Fryirs 2005). Hence, management applications must emphasize explicitly whether (dis)connectivity is 'natural' in the system and make informed decisions about whether they strive to connect, disconnect or re-disconnect biophysical relationships within any given river system (Hillman et al. 2008).

Associated References


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