Coastal erosion and protection talk
Professor Bob Kirk explains erosion processes at a U3A talk. Photo: Ina Holst
It was standing room only at talk organised by U3A about coastal erosion and coastal protection on Monday.
The speaker, Bob Kirk, Emeritus Professor in the Department of Geography at the University of Canterbury, is a specialist in physical coastal science and management, and was elected onto the Canterbury Regional Council (Environment Canterbury) in 2004 and 2007. His academic and applied research includes processes and management of lake shores and ocean coasts, with a particular focus on shore platforms and mixed sand and gravel beach systems and lagoons.
The professor discussed the effects of different land uses on the coast, various forms of erosion, and coastal structures that failed to provide protection from or enhanced inundation and erosion.
He defined erosion generally as the “net landward retreat of stated coastal contours with or without net loss of sediments,” and said that it affected perhaps as much as half of New Zealand coasts. Today, long-term erosion is a problem occurring on most coasts around the world, due to natural processes as well as human uses and intervention with natural processes.
He said that erosion was always an expression of an imbalance and required careful analysis of a particular coastal system and knowledge of land-forming processes and human uses prior to selecting appropriate protection measures.
“Erosion is always the expression of a sediment deficit budget,” he said. By shifting material around, the coastal system tried to make up the deficit and to recreate a balance.
“We are dealing with a dynamic system. It is never static, with constant gains and losses of material. If the losses exceed the gains then the system is in deficit and the shoreline erodes. It is like a household account and we have to investigate what the state of balance is and where are the boundaries to that system? We need to look at flows of material across those boundaries and where is the input?”
Professor Kirk called coastal protection a “bit of a misnomer” because the concern was primarily with the protection of assets such as roads, real estate and other structures threatened by inundation and scouring instead of protecting the foreshore. Yet, healthy foreshores, and especially dune systems, act as buffers to the highly variable and continual energies of wind by adapting to the most favourable shapes to recreate balance. They should not, however, be regarded as “an insurance for asset protection” said the professor.
“The foreshore dune is the active part of the beach system to be eroded in times of storm, and the shock absorber in the system. If we remove it or replace it with roads and houses, these become the shock absorbers.”
The role of the foreshore dunes was toprovide a barrier to inundation and dump sand into the waves to offset potential storm energy, he continued, and to substitute natural foreshore with poles and car tyres, sand-filled oil drums or trip walls and rock revetments was ineffective, ugly and often expensive and “commonly results in the structures becoming the coastlines.”
According to a handout he provided, successful protection work needs to fulfil all of the following six rules: adequate toe protection of any structure so it will not be undermined; securing both ends of the works against flanking; check for foundation conditions, as soft foundation material may adversely affect structures; use of material heavy and dense enough so it will not move individual pieces of the protection works; building structures high enough to avoid overtopping; and making sure that cavities between layers of protection material are small enough so that underlying material is not washed out by the waves.
Having imparted all the best advice, however, Professor Kirk concluded that the best protection measure consisted of not intruding on the active part of the beach in the first place.
Ina Holst
