The geology of the Black Birch fan and catchment : Mt. Cook National Park, and its relation to proposed engineering structures.

Abstract

Pressure on tourist facilities at Mt. Cook has forced development on to the recently active Black Birch fan surface. The 479ha catchment behind the fan contributes sediment to the fan along a steep stream channel. Climatic conditions are severe; vegetation and soils reflect this. The area is within the Torlesse Group of rocks and consists of interbedded greywacke and argillite with a fault-bounded schist block. Structural features trend north-south. Pleistocene glaciations between 5,120 and 530 years B. P. deposited 150m of outwash material in the catchment which has since been dissected. Extensive areas of scree are present.

Tractive-force studies on the fan indicate a decrease in energy down-fan. The progress increase in stream sediment size downstream indicates an over-supply of sediment to the upper reaches of the stream. Hypsometric analysis shows that 50% of the area is below 40% of the total relief. Average catchment slopes have been calculated at 38°. Avalanches descend the stream channel in winter.

The concept of geological risks to the are identified. Earthquake risk is classified in a regional context, but little contemporary evidence supports the high risk classification. Flood risk is discussed in detail; approximations of stream discharge and channel capa-city are calculated using empirical formulae and the stability of the materials in the channel to these flows is calculated. Damage to the channel from a discharge with a return period of four years is suggested but because of the number of assumptions in the calculations a significantly greater discharge will probably be required to flood the fan. In social and economic terms the geological risks are probably acceptable. Continued necessary channel maintenance will be detrimental to the environment.