Floodplain management challenges following rapid landscape transformation

Philip Wallace

Friday 3 july 2015

14:30 - 14:45h at Africa (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Adaptation measures

Parallel session: 16K. Managing deltas - Adaptive delta management

The Rangitaiki and Tarawera Rivers drain a combined catchment of 4000 km2 in the eastern Bay of Plenty of New Zealand. The rivers rise in the Volcanic Plateau and hill country in the before flowing out over the Rangitaiki Plains and into the sea. This alluvial floodplain covers an area of about 250 km2. Until the end of the 19th century, the floodplain was largely undeveloped, much of it being a vast wetland ,“a road-less wilderness”, with myriad channels and with only limited outlets through sand dunes to the sea. Large-scale drainage and flood protection works on the Plains commenced in the early part of the 20th century, allowing the establishment of wide-scale dairy farming. Today a system (Rangitaiki-Tarawera Rivers Scheme) of stopbanks (levees), floodwalls, pumps, floodgates and bank protection measures is in place. However the Scheme faces a number of challenges in maintaining design standards, including financial stresses from the cost of repairs following repeated flood events in recent years, continual subsidence of the floodplain and stopbanks, poor geotechnical conditions, climate change (sea level rise and increased flood flows) and ecological concerns. A number of potential measures to deal with these challenges are being considered. Given the parallels between some of the challenges identified and those being faced by The Netherlands, elements of the latter’s “Room for the River” programme may be appropriate. Indeed, several such measures have been identified and one that has been adopted at this stage is a programme of investigations and works to increase the capacity of a bypass spillway. A history of the drainage and flood protection works, and the assessment tools used, is presented, along with discussion of possible futures for the floodplain.