Stream Bank Erosion Repair: Laidley Creek Case Study
Case study location
Laidley Creek drains north from the Border Ranges for around 56km before joining the Lockyer Creek. The Lockyer catchment meets the Brisbane River below Wivenhoe Dam.
This case study outlines the steps taken in restoration activities
in Laidley Creek after the 2011 and 2013 flood events. This is a complex example involving a number of stages to address multiple threatening processes occurring simultaneously.
- Defining the problem
Laidley Valley is a highly productive horticultural area within the nationally important Lockyer Valley. Laidley Creek is degraded and unstable and, as a result, is placing production areas at risk during times of flood. The stability of the banks in 2013 was affected by the:
- Lack of riparian vegetation as a result of clearing for agriculture and recent flooding.
- Unstable banks where mass failure had recently occurred. Subtropical climate susceptible to high intensity rainfall. Relatively narrow valley.
- Modified levee system.
- Cropping system not in tune with seasonal climate risks.
- Placement of hard infrastructure and its influence on stream flow.
Consequently, stream bank mass failure occurred along with bank scour. Additionally, floodplain erosion removed valuable topsoil from paddocks, especially where paddocks had been cultivated or were only recently planted and crops were immature. Critically, the creek bed suffered from erosion and lowered in response to the flood.
Bed lowering affects not only infrastructure such as the foundation of bridges, but also the stability of the stream banks as they typically widen in response to bed lowering, potentially impacting on adjoining assets including roads and tracks; mature trees; irrigation lines; productive land; fences; buildings and yards and safety.
Understand the causes
Due to the complexity of the issue and the multiple processes occurring together, expert input was sought. After walking the reach and talking to local landholders, detailed modelling was undertaken to identify critical points for intervention in the landscape and options to increase resilience into the future. Modelled data was checked against known flood heights and community meetings were held to cross-check the information.
- Defining a solution
Expert advice and modelling results showed that it would be necessary to first halt bed lowering to successfully stabilise the stream banks in this reach.
Many of the issues ranged across a number of properties. Whilst bed lowering, or creek deepening, most likely occurred in the 2011 flood event, there was substantial widening along many parts of the creek in the 2013 event. The cross-section of the creek measured at one location shows an example of this widening (image above left). The relatively steep banks post-2011 are even steeper post-2013 with a broader base and larger distance between upper banks. Measurements show that 12,000 m3 of soil was lost from bank erosion along an 800m section of the creek, ignoring losses from topsoil scour over the floodplain. On-farm and downstream impacts were substantial.
In order to stabilise this section of creek a number of measures were needed. Several bed control structures, consisting of rock chutes, were engineered to reduce the advancing front of the stream bed erosion upstream. Battering of the banks was planned to reduce the likelihood of mass failure, provide increased channel capacity and a suitable surface for plant establishment.
For complex issues ranging across property boundaries, it is important to involve the community in developing solutions and consult with the relevant authorities for permission to implement the agreed plan of works.
Three potential management paths were identified for Laidley Creek post 2013:
Option 1. Armouring the system against further channel change using rock beaching or a similar hard engineering approach as vegetation alone would probably not be successful for the phase of creek incision at the time.
Option 2. Reducing the channel capacity and increasing floodplain inundation to reduce the energy impacting on the banks.
Option 3. Halting the incision and increasing the channel capacity by bank profiling (widening).Funds were insufficient and risk of failure was relatively high under Option 1.
Based on discussions with adjoining landholders, Option 2 was undesirable as farms have been established based on low occurrence of inundation (10 year average recurrence compared with less than a 2 year recurrence needed to reduce the stream energy to low erosion risk levels). As such Option 3 was adopted. Option 3 involved grade control structures in several locations, bank battering and revegetation.
- Implementing the design
A comprehensive set of implementation plans were developed showing engineering details and scheduled timing of work stages. On site supervision during the installation process was critical to successful implementation of the plan according to the specifications.
- Evaluating and improving
Like all farm investments, monitoring and maintenance is an important component of implementing the proposed works. Success will require regular maintenance to ensure integrity of the works is retained and longevity maximised. Scheduled inspections are especially important early on as vegetation is establishing. It is also critical in response to specific flow events. At least quarterly inspections have been conducted since works completion and more frequently after rainfall events in early 2015. After three years inspections will be event-driven to check for and repair:
- Evidence of settlement of placed rock including rills, low areas or threatened integrity.
- Significant voids, surface irregularities or loose rocks resulting from removal of smaller rock material from the chute surface by high flow events.
- Indications of tunnelling of bank material around the abutments.
- Changes in bed level immediately downstream of each chute that suggest scour at the end of the apron.
- Debris that has become lodged or any vegetation that has established on the chute in a location which may cause flow to accelerate around it or cause damage to the chute if it moves during a high flow event.
- Establishment and survival of desirable vegetation and the impact of weeds on planted and desirable regenerating plant species.
**Disclaimer – the information in this Report is provided on a general basis only. You should seek specific advice before taking any action.