Storms vs land-use change
Windthrow damage from spring storm. Image - Alex Watson
Large, powerful infrequent storms as well as changes in land use are both major drivers altering the rates of sediment generation and delivery into rivers and the ocean.
Debate has raged over which is more damaging but there have been few studies where the impacts of the two have been compared.
Over the last few years Les Basher, with NIWA collegue Murray Hicks, has been monitoring sediment yield at seven sites in the Motueka River as part of Landcare Research’s Integrated Catchment Management (ICM) programme.
‘To measure sediment yield we continuously measured turbidity and took water samples during storms to get a time series of suspended sediment concentration, which was then combined with flow measurements to estimate sediment yield in each storm event. Longer term sediment yield was calculated by summing the storm yields,’ Mr Basher says.
At all sites there was a very strong relationship between storm sediment yield and peak flow during each storm. At all sites annual yields were dominated by a few large events that carried most of the suspended sediment load.
A significant storm in the headwaters in March 2005 dumped 160 mm of rain in 4 hours causing severe gully and bank erosion. It also increased sediment concentrations and yields by 10–20 times in the affected headwaters while almost 100 km downstream there was still an increase of 2–3 times.
Sediment yields have slowly decreased over the years since the storm – at some sites they are similar to what they were before the storm, while at others they have remained higher.
Harvesting of exotic forest produced a fi ve-fold increase in event sediment yields but the streams recovered to pre-harvesting levels in less than 5 years – although these sites did not experience any large storms during the measurement period.
Mr Basher says that land use change was an important influence at small catchment scale but the extent of the yield increase, the time to recover, and the total area affected tended to be smaller than the impact of the large storm in the headwaters.
‘The large storm was a “threshold” event that perturbed the erosion regime and activated sediment sources.
‘This caused subsequent smaller, more common runoff events from these tributaries to carry sediment loads that were over an order of magnitude larger than those streams would normally have carried. This effect extended right down the catchment to the coast. Similar results have been found in other catchment studies and have important consequences for calculating long-term sediment yield,’ Mr Basher says.