A REACH-SCALE CHARACTERIZATION OF A SECOND-ORDER, TROPICAL, MONTANE STREAM: USING TERRESTRIAL LASER SCANNING TO RELATE CHANNEL MORPHOLOGY TO THE DISTRIBUTION OF STREAM POWER AND SHEAR STRESS

Steep, mountain basins present unique challenges to field research because of their limited accessibility and seasonal climatic fluctuations, yet offer great opportunities to study dynamic channel morphologies. In this study, we show that a detailed, small-scale understanding of the channel morphology/hydrology relationship can be gained using reimagined terrestrial laser scanning (TLS) techniques despite challenging topographic and climatic field conditions in a small, steep, 2^nd order, tropical mountain stream in north-central Costa Rica. A true orthographic surface is derived by suspending a Leica HDS 7000 terrestrial laser scanner upside-down from a rigid frame placed in the stream bed. The technique produces a high-resolution point cloud that accommodates dense tropical canopy, steep stream gradient (≈10%), and large boulder-bed material (D₈₄ = 60-70cm). Two sets of TLS data represent separate stream reaches. Two 1cm planimetric resolution digital elevation models were interpolated using ArcGIS 10 software after data filtering and cleaning using Leica Cyclone software V. 7.4. Analysis of these surfaces using ArcMap 10 in conjunction with the River Bathymetry Toolkit (RBT) permits the measurement of a cross-section for every meter of channel length. The streamwise distribution of stream power and average boundary shear stress at a reach-scale given user-defined bankfull conditions are calculated by measuring hydraulic geometry and calculating velocity, using the standard deviation of bed elevations (σ_z), at each cross-section. We show the partitioning of shear stress across the channel for each cross-section by assuming that the bed contributes the majority of resistance to the flow. Finally, by creating a digital overlay of the stream power and shear stress calculated for every meter section of the channel and placing it atop the digital elevation model surface, the channel hydrology can be both visually and mathematically related to the local reach morphology and distribution of large woody debris (LWD) under variable flow conditions. Our investigation shows a spatially explicit view of the variation in stream power and shear stress with channel form and finds TLS to be an effective methodology for generating ultra-high resolution datasets in remote sites where airborne LiDAR is unsuitable.