Urban Stream Restoration - Erosion Control and Fish Habitat Engineering

The Mangakotukutuku Gully is located on the south western side of the Hamilton urban area and represents one of six urban gully systems with a connection to the Waikato River within Hamilton City.

The Mangakotukutuku Stream Care Group (MSCG) was established in 2006 in response to community concerns about the poor health of Mangakotukutuku Stream and the lack of attention being given to its ecological values. Tonkin & Taylor (T&T) has worked with MSCG since 2008 in developing restoration demonstration sites aimed at improving the health of the stream whilst engaging with and educating the local community.

T&T has developed four individual reach restoration plans and implemented physical works for two of those plans. This paper presents engineering design and construction issues associated with physical works for an offline mini wetland intended as native mudfish habitat, three different stream bank re-grading options incorporating soil filled containers, woody weirs for flow diversity, log overhangs, and novel stream bank fish refuge habitats targeting tuna (eel) and giant kokopu.

The paper presents key learnings including: soil container construction, advantages and disadvantages of the three bank re-grading options used, potential for community construction initiatives, construction of fish refuge habitats, woody weirs and log overhangs, and effects of sedimentation on those features. The performance of the stream bank erosion mat product used and associated variable vegetation uptake on the stream banks is also discussed. A design calculation method for anchored woody instream structures is also presented.