PROJECT:

Healthy Land Water SEQ published a Gully Erosion Repair Case Study where they used our Flexible concrete mats. Click to download and the full case study here:

LOCATION:

Near Rosevale SEQ

CONTRACTORS

KLAN BROS EARTHMOVING – Mark Klan 07 5467 2737
www.KlanBrosEarthmoving.com.au

AIRBORN INSIGHT – Environmental Engineers
www.airborninsight.com.au  Nat Parker M. 0490 325 261 E. nat@airborninsight.com.au

This project was supported by Healthy Land & Water through funding from the Australian Government’s National Landcare Program and the Queensland Government Department of Environment & Science’s Healthy Catchments Program. For more information: Healthy Land & Water    Email: info@hlw.org.au    www.hlw.org.au

 

Case Study: Gully Erosion Repair – Using a Concrete Mats

Defining the problem

Gully erosion was affecting the overflow area of a small farm dam on a grazing property near Rosevale in South East Queensland. The gully had cut into the dam wall and without intervention would soon cause the dam to fail. The area had been used as a cattle pad, which in combination of fast-flowing water from a nearby poorly designed spillway, wore away the thin topsoil and exposed the sodic subsoil.

Sodic soil disperses or ‘melts’ very quickly when it gets wet, meaning that once the few centimetres of topsoil are gone, gully erosion can quickly take hold. About 360 tonnes of soil has been lost from this gully over the last ten years, which is 36 tonnes per year. Runoff from the adjacent pasture was causing further damage (Inset A).

Designing a solution

Fixing the dam wall and spillway

The two main options considered were:
a) Traditional rock chute.
b) Concrete matting chute supported by rock edging.

The concrete matting option was chosen for this site because it is a more cost effective and environmentally friendly option that is easy for land managers to implement themselves. The concrete mats weigh about one tonne and can be transported in the back of a utility vehicle. Cement chutes also allow vegetation to grow through the mats, which can be mowed over by a commercial mower.

Essential design features:
Gully head reprofiling 1:4 batter slow velocity is ideal (15° or 25%).
Rock edging to secure the matting.
Overhang Concrete matting into trenching (0.5 m at the top end and 0.4 m at the sides).
Lining of gully with geofabric of an adequate grade.
Stilling well at the base of the structure.
Contour bank to redirect runoff away from gully sides.
Vetiver hedges to slow water before it enters the chute.

Refer Figure 4

Diverting runoff from the adjacent pasture

To ensure runoff from the pasture upslope from the site didn’t continue to cause ongoing problems, a contour bank was included in the design to redirect runoff to the catchment above the dam. This not only protects the erosion repair works, but also helps keep water in the landscape for longer by delivering it higher up into the catchment to flat areas not prone to erosion.

Installing vegetative infrastructure

With appropriate species selection and planting design, vegetation can be used to slow, control and redirect both overland and subsurface flow of water across the landscape. In this instance, vetiver hedges and tree plantings were used to support and enhance the functionality of the cement mat chute.

Implementing the design

After careful planning and design, a cement mat chute supported by rock edging and stilling well (apron) was installed.

The restoration process:
Batter gully sides and remove the bank confining flows.
Reprofile the head cut and gully sides using clay fill (from dam) and subsoil from reshaping the gully to create a 1:4 batter.
Create trenching to secure the chute and profile the stilling well at the base of the chute to allow for a minimum of 0.5m from the top of the rock to natural ground level (refer to figure).
Overlay geofabric across the entire chute and trenching.
Overlay concrete mats ensuring a minimum of two rows of cement blocks over the sides.
Place rocks into trenches surrounding the concrete mat chute, ensuring that rock entry is flush with ground at the top of the chute.
Revegetate the batter with grass cover across all exposed soil and plant vetiver hedges at the base of the chute.
Restore native shrubs and trees back into the surrounding landscape, away from dam wall, spillway and gully.

Refer to figure 6/7/8/9

Contour bank construction an essential design features:

• 1:10 fall ensuring topsoil is separated and put back on top.
• Compacted every 400 mm.
• Push up the hill from below contour.
• Ending the contour directing the water to a wide open, flat area above the dam.
• Seed the contour and the exposed soil below it to establish groundcover.
• Vetiver hedges installed where water is entering the gully above dam to slow velocity of runoff to prevent future erosion.

The materials used for this project included:

Rocks for chute and stilling well D50 Ensure rock is sized appropriately for your catchment size. If it is too small it will fail/wash away.
Geofabric A44 Bidum minimum spec Under all surfaces of rock
Flexible Concrete Mats 2.4 x 10m Refer to manufacturer for installation guide.
Ubar anchors   Refer to Concrete mat manufacturer.
Vetiver sedge 3 x 10 m long hedges Hedges to be width of channel intercepting flow at 90 d, with rows 1-1.5 m apart and spacing of 25 cm between plants.
Grass seed Kikuyu and Rhodes grass Stoloniferous grasses and season appropriate cover crops.

Evaluating and Improving

Challenges and Key Learnings

• The concrete matting came with its own geofabric lining which was too thin and broke down during a big rain event. This caused some erosion to occur underneath the matting. A44 bidum is the recommended minimum specification geofabric needed to protect the integrity of the chute and underlying soil.
• Don’t step down from a two mat width to a one mat design. The excavation and boxing was more time consuming and expensive and did not deliver additional benefits.

Avoiding future erosion problems when installing a farm dam:

Pre-construction Ensure dam is well planned and takes into account the location and size in relation to catchment, soils and safe disposal of water (considering neighbouring properties and roads).
• Correctly design a wide and stable bywash to gently spread overflows.
• Factor in adequate freeboard to prevent dam failure.
Construction Compact wall every 400 mm.
• Set topsoil aside and redistribute after construction.
Post-construction • Immediately establish desirable perennial grasses (preferably stoloniferous) and a cover crop (appropriate for time of planting) to revegetate and protect the soil.
• Install fencing to protect the establishing grasses and existing vegetation around the dam from concentrated livestock grazing.
Ongoing monitoring & maintenance • Establish a photo monitoring point and keep a record of observations.
• Identify, seek expert advice and act on potential problems early.

This project was supported by Healthy Land & Water through funding from the Australian Government’s National Landcare Program and the Queensland Government Department of Environment & Science’s Healthy Catchments Program For more information: Healthy Land & Water    Email: info@hlw.org.au    www.hlw.org.au

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