|
|||||
New Zealand Engineering 1999 June Infrastructure Flood Protection in the Firth of Thames Reducing environmental impact Periodic flood damage, ground settlement, a sensitive coastline, and the withdrawal of insurance cover set the stage for a somewhat unusual environmental engineering project recently completed in the Firth of Thames. January 1997 did not start well for residents of the exposed Moanataiari subdivision in the Firth of Thames. The year’s first cyclone had left a trail of devastation. Storm waves breaking over the protective sea wall embankment had flooded homes, turned roads into canals and gardens into sewage ponds. But the really depressing part was the deja vu. It mirrored a similar inundation 18 months earlier, which explained the insurance companies’ reluctance to reinstate cover. Moanataiari – everyone agreed – needed better flood protection. So the Thames Coromandel District Council appointed environmental engineering consultancy Tonkin & Taylor to investigate the problem and devise a solution. Finding one proved more of a challenge than initially envisaged. The subdivision’s flood problems were compounded by unusual factors, and required a custom-designed solution. For a start, the homes were built on land reclaimed in the 1960s. The area had been created with infill (largely spoil from the local gold diggings) dumped on top of compressible marine sediments. Over the following 30 years the land has been settling, to the point where some parts of the subdivision are now below highest sea level. The subdivision was protected from the sea by a 550 m long rockfill seawall which in severe storms proved inadequate: after marching across the length of the Firth, waves and surge overtopped the wall easily. But a graver problem was the embankment’s permeability: heavy surge simply flowed through the rockfill and quickly gave many residents a waterfront property. Poor stormwater drainage contributed to the flooding. Run-off from the hills behind the subdivision pooled against the sea wall. Drainage pipes had been fitted in the embankment, but leaking flap valves often contributed backflow of water to the landward side of the wall. The pooled water caused havoc with the sewerage system, with predictable results. Thames Coromandel District Council wanted a solution to address both the sea flooding and rainfall ponding, one capable of handling the "one-in-50 year" tempests. The budget was set at $1.6 million. At that budget, removing and replacing the sea wall with a more substantial structure was not feasible. Besides, such a step could leave the subdivision vulnerable to the sea during construction of the project. Reducing environmental impact The solution Essentially, the inundation problem was solved by upgrading the existing sea wall and making it impermeable. It has been raised 300 mm – to 3.5 m above the local mean sea level (the design level used for most stopbanks in the Hauraki Plains area). Additional pipes have been fitted to supplement the wall’s existing gravity drainage system – and have been equipped with flap or rubber Tideflex valves. The subdivision’s stormwater system has also been upgraded. It now features a pump station to handle catchment rainfall when it exceeds the capacity of the gravity drainage system – particularly at high tide. Waterproofing the wall was achieved by compacting a 1.5 m thick blanket of impermeable, local clay on the inside rather than the outside face of the wall, and carefully bonding it to the underlying subdivision fill of similar material (click to see diagram). The clay blanket has been covered with topsoil and sown with grass to inhibit erosion and enhance the wall’s aesthetic appeal. As an added precaution against wave run-up overtopping the seawall, a 600 mm heavy timber parapet wall has been erected. It is set into a new, concrete-paved walkway on the crest of the seawall (the walkway will ultimately form part of an extended coastal walkway being planned by the District Council). Any wave splash that does blow over the wall and its parapet and fall on to the road below, is now channelled to the pump station. It’s fitted with two 670 litres/second axial flow pumps that are activated automatically. Beach In addition, a graded swale was excavated behind the dune to divert local stormwater (and any seawater that did overtop the dune) to a slightly more sheltered outlet channel at the north end of the beach. Stormwater drainage previously channelled to beach outlets now exits into this swale drain, through upwelling manholes fitted with flapgates or Tideflex rubber valves. The beach is a popular spot for walkers and roosting birds, and key environmental considerations for the project focused mainly on minimising encroachment on the coastal marine area. By avoiding the migration season, the project minimised disruption to the prolific bird life. RMA procedures required application for a number of land use consents, including the source of impermeable fill for the sea wall, modifications to the beach and reserve, and a building consent for the pump station. Particular emphasis was laid on developing a memorandum of understanding with the tangata whenua, and on obtaining consent from all property owners affected by drainage works. Consent applications were accompanied by an Assessment of Environmental Effects and followed extensive public consultation by the District Council. The
outcomes The benefits and positive effects of the project are evident in the improvements to the social environment and quality of life for the households in the subdivision. Improved protection has been achieved without degradation of the natural environment - either along the sea wall or on Kuranui beach. Recreational use of the reserve has been enhanced by the coastal walkway along the sea wall. Of particular relevance to continued environmental sustainability is the establishment of an Operations and Maintenance Manual for the District Council to administer the subdivision protection works. Strong northwest winds
during the storm of Saturday 17 March produced a high spring tide and waves well above
predicted levels at Thames. Moanataiari subdivision residents were quick to acknowledge
that they would have been flooded without the sea wall. |
|||||
 |
|
|
|
