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«Ephemeral Stream Assessment Ephemeral drainage lines are subject to disturbance by climatic events coupled with management-induced run-off and ...»

Ephemeral Stream Assessment

Ephemeral drainage lines are subject to disturbance by climatic events coupled with

management-induced run-off and sedimentation. For example, beds may cut down or

walls may collapse. The procedure described here is aimed at mapping the location and

severity of impact of management on such streams. The procedure is implemented by

traversing the stream bed, beginning at the headwaters and proceeding downstream.

Boundaries between different stream conditions are recorded on a GPS (preferred) or distance measuring wheel. During this process, eight visual indicators are used to assess the erosion state of the drainage line. Indicators are assessed every 25-100m within a given stream bed condition. Some stream bed condition types will be short, others quite long. The full length of the drainage line should be assessed, preferably on foot, using a map to locate the stream type boundaries

There are four main classes of indicators are assessed to evaluate the degree of streambed condition:

A. the type and condition of the vegetation present, if any B. the shape and profile of the drainage line and type and condition of materials on the drainage line floor C. the nature of the drainage line wall materials D. the nature of the stream bank bordering flats and/or slopes and regulation of lateral flow into drainage line The indicators produce a rating which ranks each location from very actively eroding through to very stable (Table 1 in the Appendix). The main issues causing the gully erosion can also be differentiated. This differentiates gully erosion caused by i) high flow rates from upstream ii) high lateral flow rates, and iii) exposure of unstable side wall materials that slake and/or disperse. This classification can aid in defining the most appropriate form of remediation.

A. The Type and Condition of Vegetation Present Within the Drainage Line The presence of vegetation is influenced by the growing surface and therefore gives some indication of the level of activity (Landloch, 2003). Dense long-lived perennial vegetation suggests a low energy environment with little deposition, probably occurring over a long period of time. Annual vegetation may also indicate a low energy environment but this type of vegetation suggests stability has been of shorter duration.

Lack of vegetation or burying of vegetation distinguishes a more active environment (Landloch, 2003). Vegetation cover, type and density also influence the surface resistance to erosion (Hooke and Mant, 2002). No distinction is made between native and exotic vegetation as this assessment is primarily to define degree of activity within drainage line, not the extent of biodiversity.

A.1 Vegetation on Drainage Line Floor Rating Description 1 Little or no vegetation growing on drainage line floor 2 Any vegetation present is annual or short-lived: partialburial of plants by recently deposited sediment evident 3 Dense perennial plant cover, similar to vegetation on the bank of the drainage line: characteristic wetland species composition: no observable plant burial by sediment A.2 Vegetation on Drainage Line Walls Rating Description 1 Little or no vegetation growing on drainage line walls 2 Any vegetation present is annual or short-lived: partial burial of plants by recently deposited sediment evident 3 Dense perennial plant cover, similar to vegetation on floodplain/riparian zone:

characteristic wetland species composition: no observable plant burial by sediment

B. The Shape of the Drainage Line and Type of Material onFloor

Drainage line shape is influenced by the energy available to erode or deposit different materials and so differing channel shapes reflect the variation between the erosional and depositional capabilities of flows in the area (Brierley and Fryirs, 2005). The shape of the drainage line and the relationship with the wider floodplain area has further implications for the type of floodplain forming processes, that is, the balance between lateral and vertical accretion processes (Brierley and Fryirs, 2005).

B.1 Shape and Aspect Ratio of Drainage Line Cross-Section

During peak flows when gullies are being generated or enlarged, the channel tends towards a rectangular shape with steep, vertical walls (Torri and Borselli, 2003). A study by Crouch (1987) found different erosion rates for different side wall forms and illustrated that the highest sediment supply was sourced from gullies with vertical walls.

Crouch (1987) also found that when wall angles fall below 65o, wall stabilisation begins.

This discrimination of drainage line x-sectional morphology indicates the level of erosion occurring within the drainage line. This defines areas where erosion and sediment loss are greatest from those areas which are potentially stabilising and generating very little sediment.

Rating Description 1 Very actively eroding: caving, mass wasting and/or tunnelling present: depth width (aspect ratio high) 2 Actively eroding: slight undercutting, near vertical walls, alluvial fans also eroding: depth=width 3 Potentially stabilising: side walls become rounded and crusted alluvial fan at foot of side walls: width depth Stabilising: wall angle less than 65o, small inactive alluvial fan at foot of side walls: width depth 5 Stable: gently sloping walls, generally low, “S” shaped bed/bank continuum:





width depth (aspect ratio very low)

B.2 Longitudinal Morphology of Drainage Line

The characterisation of the longitudinal morphology in this assessment is used to point towards the predominant rate of flow occurring in this area and also define the amount of interaction between the drainage line and the floodplain. When assessing this indicator it is important to be aware of changes in catchment slope, an important primary control on river longitudinal morphology (Brierley and Fryirs, 2005). This indicator distinguishes between the chain-of-ponds morphology where stream bed and floodplain are connected and increased flows are dissipated onto the floodplain, through to the currently incising bed where recent high flows are evident. In this example stream bed and floodplain are disconnected and high flows lead to further channel incision instead of being dissipated over the floodplain.

Rating Description 1 Currently incising bed in pre-existing loose sediment; faceted lower wall/bed profile (benches), scour holes in bed. Morphology implies high flow rates and erosion 2 Flat, continuous, loose sediment with signs of recent/frequent movement 3 Flat with a cohesive fine textured “soil-like” bed 4 Non-cascading pools or ponds, with non-slaking, non-dispersive clay base, implying low energy flow.

B.3 Particle Size of Materials on Drainage Line Floor- material available for erosion The size of the materials on the drainage line floor can be used as an indicator of the erodibility of the surface (Hooke and Mant, 2002). Larger and/or denser rocks deposited on the drainage line floor armour the drainage line floor (Landloch, 2003) so that higher energy flows are required to mobilise this material (Brierley and Fryirs, 2005). Material that is similar or smaller in size to that on the walls require lower energy flows to be mobilised (Brierley and Fryirs, 2005) and thus indicate the drainage line floor is more susceptible to erosion (Landloch, 2003).

Rating Description 1 Material on floor is similar or smaller in particle size and/or density than material in the walls (e.g. unconsolidated fine sand deposits) 2 Material on floor is slightly larger in particle size and/or denser (more consolidated) than material on walls (e.g. well sorted gravel) 3 Material on floor is much larger in particle size and/or denser than material on walls: surface armouring (e.g. cobbles, competent country rock) C Nature of Drainage Line Wall Material Many Australian soils slake and/or disperse when wetted and these properties can have many implications for land managers (Field et al., 1997). It is especially important to identify soils with these properties in areas of gully erosion. Once these soils are no longer confined vertically or horizontally, gully erosion may occur each time the soils become wet irrespective of the flow velocity. This is particularly important when deciding the rehabilitation methods required in these situations.

Soil stability can be assessed in the laboratory and in the field. Simple tests of immersing soil aggregates in distilled water can be undertaken in the field which will clearly define the slaking characteristics of the wall material and partially define the dispersive nature of this material. Laboratory tests can be time consuming and expensive but may be required if the outcome of field tests suggest the soils will slake and/or disperse. The presence of fluting, caving, undercutting and mass wasting are good indicators of the presence of soils that slake and/or disperse (ref) and may suggest the need for further laboratory work.

C.1 Nature of Drainage Line Wall Materials

Rating Description 1 Dispersive material is exposed for greater than 1 metre of wall height 2 Materials that slake rapidly, or disperse are exposed on greater than 0.3metres and less than 1 metre of vertical wall height (the sum of multiple layers if present) 3 Materials that slake and/or disperse are exposed on less than 0.3 metre of wall height 4 Materials that do not slake or disperse are exposed on wall surface

D Nature of Bank Edge and Lateral Flow Regulation

These two indicators assess whether high water flows are the result of lateral flow from the catchment. Comparison with the drainage line shape can help assess whether the main source of water flow and the resultant erosion is coming from upstream or from the catchment area directly adjacent to this drainage line area. The identification of the source of high flows can assist in determining the types of remedial works required.

D.1 Shape of stream-bordering flats and/or slopes Rating Description Very steep slope, 30o creating high velocity flows Steep bank, 10-30o, permitting moderate to high velocity flows Moderately sloped bank, 5-10o Gently sloped bank/floodplain, laterally extensive, 5o 5 Flat bank/floodplain, laterally extensive D.2 Nature of Lateral Flow Regulation into Drainage Line Rating Description 1 Side arm channel inflow: very high inflow rates 2 Bare bank, laterally extensive 3 Sparse grassland/woodland with bare soil bank lip: moderate flow rate, some highly focused inflow locations 4 Dense grassland: low inflow rate, mostly diffuse 5 Woodland with dense litter: very low, diffuse inflow rate Important Questions to Answer During and After Assessment Is the gully erosion caused by high flow rates from upstream? (i.e. banks are well vegetated suggesting diffuse lateral flow rates but currently incising bed suggests higher flows from upstream) Is the gully erosion predominantly caused by high lateral flow rates? (i.e. are banks poorly vegetated and side arms are developing suggesting inflow is sourced laterally from catchment and is not necessarily a problem upstream) Is the gully erosion predominantly due to initial exposure of slaking and/or dispersive materials which continue to erode when exposed to low and high flow events? (i.e. banks are well vegetated, drainage line bed is continuous and not further eroding but headwall gully erosion continues unabated) The answers to these questions will be extremely important in assessing and implementing rehabilitation measures.

References

Brierley, G. J. and Fryirs, K. A., 2005. Geomorphology and River Management:

Applications of the River Styles Framework, Blackwell, Malden, MA.

Hooke, J. and Mant, J., 2002. Morpho-dynamics of ephemeral streams, in Dryland Rivers: Hydrology and Geomorhphology of Semi-arid Channels, eds L. J. Bull and M. J. Kirkby, John Wiley and Sons, West Sussex.

Landloch, 2003. A Geomorphic System for Gully Assessment, Landloch Pty. Ltd., Darling Heights.

Torri, D. and Borselli, L., 2003. Equation for high-rate gully erosion, CATENA [online], 50 (2-4):449-467.

–  –  –

B.1 Shape and Aspect Ratio of Drainage Line Cross-Section B.1.1 Very actively eroding: mass wasting following extensive caving/undercutting B1.2 Actively eroding: vertical walls, some caving/undercutting

–  –  –

B1.4 Stabilising: Wall angle 65%, inactive alluvial foot fan B1.5 Stable: Gently sloping, S-shaped bank-bed continuum B.2 Longitudinal Morphology of Drainage Line

–  –  –

B3.1 Stream bed materials similar or smaller in size than materials on walls. Easily transported, but probably low energy bed flow.

–  –  –

C.1.2 Stream bed wall with 0.3m and 1m of slaking/dispersive material exposed C.1.3 Stream bed wall with 0.3m of slaking/dispersive material exposed

–  –  –

D.2 Nature of Lateral Flow Regulation into Drainage Line D2.1 Sidearm channel inflow Very high inflow rate with high soil erosion

–  –  –

D2.3 Sparse grassland on stream bordering flat: moderate inflow rate D2.4 Dense grassland on stream bank: slow diffuse runoff into stream



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