Rangitata River – Peel Forest Stopbank Erosion Control

SCOPE:

• Site establishment and erection of temporary fencing.
• Removal and disposal of unsuitable material
• (e.g. organic material not suitable as topsoil).
• Diversion of the right bank braid of the Rangitata River to provide access to the adjacent beach for the source of fill.
• The construction of an approximately 30,000 m3 compacted gravel embankment tied into the existing terrace and having an open end on the beach immediately in front of the Peel Forest Closed landfill.
• Topographic survey of the site to determine as-built quantities for payment at the following stages;

o Stripped surface

o As Built (compacted gravel) completed surface

Following an extensive flood in 2019 which caused extensive erosion to the Rangitata Riverbank Grant Hood Contracting guided the river away and undertook further work to avoid potential landfill entering waterways.

A flood event in December 2019 caused erosion of the right bank of the Rangitata River in a number of locations including at the Peel Forest closed landfill located at the end of Dennistoun Road. Some waste from the perimeter of the landfill entered the river and was subsequently cleaned up. There was concern that further erosion of the right bank could result in parts of the main landfill falling into the river.

Grant Hood Contracting was engaged to guide the river away from the landfill with a large sacrificial gravel embankment constructed in front of the terrace on which the landfill is located. The bank is to be approximately 250 m long and 5 m high and will be constructed of river gravel extracted from the central island directly adjacent to the site. 38,000 m3 was used for the construction of the embankment.

During the pre-start process, a bird survey was undertaken before any machinery movements or earthworks were begun. Site Fencing was set up to protect the public from accessing the work area.

The first step of the project was to divert the river channel away from the work area to construct a haulage cycle area. This was followed by building up a diversion bund to cut off existing channel in the work area. Importantly a culvert system and raised gravel crossing were constructed to ensure that heavy vehicles were at no stage working within any residual flows that might permeate through the diversion bund. The 45T Excavator removed some excess vegetation which was carted away to appropriate fill site. The Engineer undertook a site inspection of the proposed stopbank foundation setout as a hold point in construction. A Plateau Density Test (PDT) was carried out on the first layer which determined a reference Maximum Dry Density for the remaining stopbank construction with additional NDM testing as per specified frequency.