Waihi Beach to the future

An objective review

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Abstract

Waihi Beach is a village consisting out of 2300 inhabitants, located in the Bay of Plenty on the North Island of New Zealand. It can be seen as a 9 km long tombolo beach. Dunes used to protect most of the land, but have decreased in size through natural erosive processes and through urban development in the dune areas. The long and short-term coastal processes result in erosion and flooding problems. At some places dunes are partly eroded, while elsewhere they don't exist at all and a seawall protects the properties (over a total distance of about 2,3 km). The natural process of accretion and erosion can't take place here. Two and Three Mile Creek are the two biggest creeks that flow through the village. During heavy rainfall some properties suffer storm water flooding. This flooding is divided in two appearances: flooding of the creeks (due to blocking) and secondly flooding because of a lack of proper drainage to the creeks. The creeks have outlets in the sea, which meanders over the beach. This causes erosion problems over about 500 m of beach per creek. In calmer weather the discharge of the creeks is little and the waves are small, which causes accretion of the beach. This causes blocking of the creeks and by that flooding of the nearby properties. In New Zealand the Central Government, the Regional Councils and the District Councils are the three levels of government. They all have their own policies and responsibilities. Waihi Beach is part of the Western Bay of Plenty District (Regional Council), which is part of the Environmental Bay of Plenty Region (District Council). The institutional and decision-making system has been looked at. Recommendations have been made, how to make improvements to the system. Coastal hazard problems are often difficult to solve, because they exceed the Districts borders or the Districts institutional measures. A national Integrated Coastal Zone Management Committee could help with collecting data, knowledge and experiences from other coastal projects, in order to ease a process for the District Council. At this moment the WBOP District Council has decided to design a seawall to address the erosion problem. This was the cheapest option that provided direct safety to the nearby properties. A lot of local interest groups and residents don't agree with this solution. A good cost-benefit analysis has to be made. Increasing benefits for more people could create financing options. Besides that communication with the community (including holiday residents) to create commitment and awareness is important. The main problem for the beach is the erosion problem near the seawall and the creeks. The seawall isn't a very satisfying solution for this problem. Beach stabilisation options could be. These options result in wider beaches, which increase the amenity values. Two beach stabilisation options have been looked at more closely. One option is executing nourishment and keeping the shoreline in place by renourishing every 5 years. First nourishment is needed to create a healthy beach profile, and after that for widening the beach. Due to the longshore and crosshore transport, about 125.000m3 of renourishment is needed every 5 years. The creeks are trailed by constructing groynes through the surf zone (225 m). These groynes (made from geotextile bags) also decrease the required amount of 5-year renourishment. The second option is executing nourishment in combination with breakwaters. Offshore breakwaters composed out of geotextile tubes are required at Waihi Beach according the Multi Criteria Assessment between eleven possible options of breakwaters. A preliminary design is made for this breakwater option. With different amounts of nourishment and the number of breakwaters, the shoreline response is determined. Renourishment is needed to compensate the sand transport induced by the breakwaters from the adjacent areas to the leeside. Also to address the long-term erosion trend periodically renourishment is required (50.000m3 10-yearly). The creeks are trailed by constructing geotextile bags on the beach, until the shoreline. Some additional sand removal to prevent blocking is probably needed. The nourishment option and the breakwater option are compared in a Multi Criteria Assessment. The breakwater option is the most favourite, mainly because of the lower costs, $5,5 mln instead of $7,4 mln (NPV over a 50-year period). The breakwater option also creates more amenity values (no hard construction can be seen and natural processes are restored) and protects a bigger part of the beach. At the end the breakwater option is compared to the seawall in a cost-benefit analysis. To quantify the benefits the increase of value of the beachfront properties are examined. The conclusion of this analysis is that an extra increase of value of 1.6% in a time span of 5 years will completely compensate the difference of costs between the breakwater option and the seawall. This percentage seems reasonable so the design of the breakwaters is the most preferable solution for the erosion problem in Waihi Beach.