IMPLICATIONS OF HYDROCLIMATIC DIFFERENCES FOR SEDIMENT TRANSPORT IN STEEP CHANNELS

This research uses nine step-pool channel segments from six regions to examine the magnitude, frequency, and duration of flows hypothetically capable of mobilizing the D50 and D84 of streambed sediment. Each channel segment is located close to a gage with a record of discharge in 10- or 15-minute increments for the period 1994-2003. Channel geometry along approximately 50 m of channel was surveyed during base flow conditions, and streambed grain-size distribution was measured using a grid spaced at half the largest clast size. Field and gage data were used with the Manning equation to estimate shear stress as a function of discharge at a representative cross section. Critical shear stress was estimated using equations developed by Buffington (D50) and Komar (D84). Critical shear stress was equated to a threshold discharge for sediment mobilization, and stream data were filtered to determine the exceedance of the threshold discharge during the period analyzed. Magnitude was analyzed as the ratio of maximum discharge each year to threshold discharge. Frequency was number of events per year during which threshold discharge was exceeded. Duration was number of minutes per event and total number of minutes per year during which threshold discharge was exceeded. A cumulative mobility variable was developed from the product of the sum of the discharges exceeding the threshold and the duration of those discharges for each year of record. The six regions examined represent different hydroclimatic regimes: Switzerland (snowmelt runoff and high-intensity rainfall during summer); Puerto Rico (runoff from high-intensity, short duration rainfall throughout the year); Colorado (snowmelt runoff); Oregon and northern California (low-intensity winter rainfall runoff); and Idaho (snowmelt and rain-on-snow). Puerto Rico ranked highest for the majority of variables related to magnitude, frequency, duration, and cumulative mobility of D50 and D84. These results suggest some of the geomorphic implications of the highly peaked runoff events characteristic of many tropical montane regions.