‘Westview’ - Leaky weir diary
Westview leaky weir two years after construction, in December 2020.
This weir at Westview was installed as part of the Mulloon Rehydration Initiative, which is jointly funded through the Mulloon Institute and the Australian Government’s National Landcare Program, with support from the NSW Government’s Environmental Trust.
Before construction
The creek was laterally unconfined and incised through to bedrock. From top bank to top bank, it was a bedload dominated, single continuous channel, flanked by sandy point bars and terraces where the fine floodplain sediments had been stripped away.
Riparian vegetation at the site was poor in density and diversity, with Crack Willow, Blackberry and some Black Wattle and River She-oak. Instream floating vegetation and emergent vegetation included: Phragmites, Cumbungi, Juncus and Marsh Wort. Riparian and instream vegetation was very sparse above this point as the creek became much shallower and livestock had easier access.
Stream condition was poor with the banks upstream and downstream of the site poorly vegetated and showing signs of recent erosion. There was poor vegetation density and species richness within both the aquatic and riparian zones.
Modelling
During the planning phase, current and post-structure modelled hydraulic conditions were calculated. This was required by the NSW Government’s Department of Natural Resources Access Regulator which granted a Controlled Activity Approval for the in-stream works. These approvals are required for in-stream works in higher order streams and help ensure that structures are designed for the stresses they will face.
Hydraulic values were also calculated for the catchment for a given point in the catchment. Point values were calculated for primary channel full, (top) bank full and post-structure (top) bank full. The calculated hydraulic parameters include:
August 2015
Discharge (Q) – Volume of water flowing past a given point in cubic metres per second (m3/s).
Average stream velocity (V) – Speed of water flowing past a given point in metres per second (m/s).
July 2017
Hydraulic radius (R) – Cross sectional area divided by wetted perimeter. The higher the R value the less flow that is in contact with the wetted perimeter of the stream, therefore, the faster the flow.
Annual exceedance probability (AEP) – a prediction of the likelihood that a given discharge will pass a given point in a catchment. For example, a 2% AEP is akin to a 1-in-50-year flood event.
October 2018
Catchment yield – Modelled average annual discharge of a catchment.
Time of concentration – measure of the response of a catchment to a rain event. Defined as the time needed for water to flow from the most remote point in a catchment to a given point.
Mean boundary shear stress (t) – Measure of the drag exerted by the flow across a channel bed – expressed as Newtons per square metre (N/m2) bearing down on the bed and banks of the stream.
Total stream power (Ω) – Measure of potential energy expenditure, expressed as watts, against the bed and banks of a stream. This reflects the total energy available to do work along a river channel.
Unit stream power (ω) – Watts of power per cross-sectional square metre of channel. The threshold for channel instability is around 35W/m2 (Fryirs and Brierley, 2013).
Froude number (Fr) – Dimensionless number to determine if stream flow is sub or super-critical. Number relates to likelihood of sediment movement. At subcritical (Fr <1), flow is relatively tranquil. At supercritical (Fr > 1), flow is high energy and turbulent.
Hydraulic modelling for this structure anticipated a significant reduction in total stream power once the structure was installed. This would be due to the proposed structure reducing the average stream gradient (slope), spreading the flow over a greater cross-sectional area of the channel, and associated riparian fencing and revegetation increasing surface roughness (Manning’s Roughness Coefficient).
During construction
November 2018
This bed control structure raised the site 700mm above the existing streambed by using a hardwood log sill and an imported rock baffle.
A hardwood log (400mm) was pinned to the bed using hardwood uprights (200mm diameter) which were keyed into each bank. Granite boulders (650mm) were also keyed into the upstream side of the existing bedrock bar to create a rock baffle.
December 2018
A bed of knitted brush mattressing and Poa tussock mulch (400mm high) was installed for undermine protection. Streambed gravel was placed against the upstream side of the structure, into which Typha and Phragmites were transplanted. For scour protection on the flanks, a deep rip rap apron (200mm) was installed on the downstream side (where bedrock is absent), extending to one metre above the low flow channel on both flanks. The banks and bed above, below and on the in-stream structure were extensively planted with native vegetation comprising reeds, sedges, shrubs and trees. This image shows stock exclusion fencing has been installed to manage livestock access.
After construction
December 2018
Flow-over, soon after construction. The width of stream inundation has been greatly increased under baseflow conditions.
February 2019
Millet was planted as a cover crop to provide rapid stability for the rock ramp. This plant will burn off with the first frost.
March 2020
The following spring, native perennial wetland vegetation had taken over, including species such as Cumbungi and Eleocharis.
October 2020
Native perennial wetland vegetation continues to take over.
December 2020
Diverse plant assemblages are colonising the in-stream works, the pond zone and the banks. The vegetation is slowing stream flow, filtering sediments and nutrients, improving water quality, moderating the micro-climate, improving the water cycle and creating a more complex ecosystem. Plants include Cumbungi, Umbrella Sedge and Marsh Wort.