The potential for extracting gravel materials from active floodplain and
channel areas has been investigated for three West Coast rivers - the Grey,
Inangahua and Whataroa. Studies have focused primarily on resource availability
and replenishment, but there are clearly also environmental management issues
associated with such a proposal. The potential uses for of extracted materials
include roading and concrete aggregates, as well as railway ballast, whilst a
major benefit would be in assisting with flood control and channel maintenance
in a naturally aggrading system.
Sediment movement in the rivers has been examined at three scales; long-term
(>100 years), medium-term (10 - 100 years) and short-term (<10 years).>
Long-term sediment movement is based on geological and geomorphological
information, such as cyclic fan development and down-cutting in the Whataroa
River where sediment input appears to be largely controlled by Alpine Fault
rupture events. Medium-term assessment relied on air-photo interpretations over
a period of about 50 years, with the mapping of channel changes and bank
migration. Short term changes were evaluated from gauging records and other
hydrological data, and took account of the development of an armour layer in
these gravel-bed river systems.
It was concluded that sediment in the lower reaches of each river was
suitable for most aggregate uses, although input of more weathered Greenland
Group materials in the upper reaches (eg of the Mawhera-iti or Little Grey)
resulted in product that was not satisfactory for sealing chip or railway
ballast. Of greater importance was the fact that short-term studies showed that
a minimum threshold of flow was required for gravel entrainment, and that
significant sediment movement only occurred above the one-year return period
flood. Ongoing monitoring of river profiles, with accompanying flow gauging,
would clearly be necessary were a substantial increase in river bed extraction
to be permitted by The West Coast Regional Council.