Source:Auckland Council Research, Investigations and Monitoring Unit, RIMU
Auckland Council (AC) has identified that harvesting of the Weiti Forestry Block at Okura, planned for the next 3 years, has the potential for adverse environmental effects on the Okura Estuary.
AC have commissioned NIWA to analyse approximately four years of available flow and suspended sediment concentration data to provide baseline sediment yield relationships, to inform AC of the minimum magnitude of sediment yields that may be expected during the harvesting period and to establish relationships between event sediment yields and rainfall-based hydrological driving parameters.
This information will help AC to forecast event sediment yields during the harvesting period from rainfall forecasts and so be able to mobilise surveys of the estuary to check for adverse environmental effects. Establishing baseline relationships will also enable AC to quantify any change in event sediment yields (at given frequency of occurrence) during the harvesting period so it can be managed appropriately. Clearly, if such changes do occur then the magnitude-frequency calculations will be updated.
Event sediment yields were calculated for the 74 storm events occurring during the 4.15-year period (2008 to 2012) of flow monitoring in the Weiti Forest catchment.
The total sediment yield from storms over the monitoring period was 209.4 t, equating to a specific sediment yield of 29.7 t/km2/yr.
Auckland Council technical report TR2013/037
NIWA Client Report No: CHC2012-115
NIWA Project: ARC13501