Eric Mahony briefs the crew
by Dan Marshall, Coordinator Three Gullies Landcare Group
Three Gullies Landcare (3GLC), as its name implies, consists of three gullies in an area of land bordered by the railway line and Bruce Rd Glenbrook, south of Eureka Rd to number 39 Bruce Rd. The three gullies are fed by watercourses from the east and ultimately deliver the water to Glenbrook Creek via culverts under the railway corridor.
The three gullies are the intermediary from the much higher eastern side residential dwellings, road and water drainage systems and in heavy rain carry significant water flows. Over time, this water flow has caused significant erosion, particularly in Gully One.
During my time with 3GLC, we have used traditional log retaining walls to help slow down the flow. The logs were sourced on site from fallen trees and from removal of weeds. These log walls are situated at various locations along the creek and are of varying heights and widths (depending on the logs available at the time). Although these log walls worked well, retaining silt during low water flows, they were no match for the rate of flow during periods of high rain fall and more importantly didn’t stop erosion of the creek beds.
I discussed this with Tracy Williams and Eric Mahony from Blue Mountains City Council’s Bushcare program for their input. As a result, the plan for an onsite practical workshop was proposed. Timing is everything in life and presently funding is available for assistance and the main material (flat sandstone rock) is cheaply available from expanded pits at the local Blaxland tip.
So on Thursday 9th June, 2016, after initial discussions and a briefing session, and with a workforce headed by Eric Mahony and contractors from “The Bush Doctor” under Shane Grundy, Tracy Williams, the 3GLC and neighbouring Landcare groups (Bush Place, Raymond Rd and Cox Reserve), the work began. Well, mainly the “Bush Doctor” group (about 9 fit young men!) did the heavy lifting of moving the sandstone from the roadside down the gully to appropriate locations along the creek.
Of course, the starting point was at the lowest spot in the gully at the Railway built culvert and then using a step of 300mm (ruler length) the first step was installed using a bed of Bidim (Geotextile fabric). The Bidim was laid and dug in along the leading (upstream) edge, and covered by flat rock at the spill over point and underneath the water fall. The sides were lined by sandstone pieces wedged into the side of the creek wall to reinforce the wall and to direct the water over the flat rock.
Other materials were used such as skinny coir logs held in place by wooden stakes to fill undercut sections of the creek. Existing native plants in the creek bed were protected by hessian mat with appropriate cut outs for the coming plantings. Ultimately further 300mm (ruler) steps continued up the creek until a flat stretch of the creek was reached where future planting of natives will further aid in slowing the flow. Ultimately approximately half the creek was upgraded and plans were made for another day later this year. It is hoped this allows more plants to be laid and the new work to be settled in.
Lessons learnt:
- Log retaining walls are fine provided the wall is lower in the centre, otherwise the water will flow around the outside creating more paths;
- Drops should only be 300mm or less to prevent “boiling” causing undercutting;
- The base of drops must be solid impermeable material;
- Natural bends in the creek can be reinforced on the sides but its best to keep drop offs some distance away to minimise the rate of water flow entering the bends;
- Deep pools can be used or encouraged as spots for aquatic creatures (platypus would be great but unlikely)
- Silt gathered before walls needs to be monitored and removed where necessary.
Finally it was a pleasure to have a break from weeding for 3GLC and to meet the members of the nearby Landcare groups as well as to have the input from Eric Mahony and the Bush Doctor workforce. Of course the excellent morning tea supplied by Tracy Williams fully replenished us all so we could carry on working until knock off time.
The members of 3GLC look forward to the next instalment.