Debris flow velocities are commonly back-calculated from superelevation events which require subjective estimates of radii of curvature of bends in the debris flow channel or predicted using flow equations that require the selection of appropriate rheological models and material property inputs. This research investigated difficulties associated with the use of these conventional velocity estimation methods. Radii of curvature estimates were found to vary with the extent of the channel investigated and with the scale of the media used, and back-calculated velocities varied among different investigated locations along a channel. Distinct populations of Bingham properties were found to exist between those measured by laboratory tests and those backcalculated from field data; thus, laboratory-obtained values would not be representative of field-scale debris flow behavior. To avoid these difficulties with conventional methods, a new preliminary velocity estimation method is presented that statistically relates flow velocity to the channel slope and the flow depth. This method presents ranges of reasonable velocity predictions based on 30 previously measured velocities.

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Debris flows are hazardous because of their poor predictability, high impact forces, and ability to deposit large quantities of sediment in inundated areas. To minimize the risk to developments on alluvial fans, debris-flow mitigation structures may be required. This study reviewed the state of practice for the design of two types of debris-flow mitigation structures: basins and deflection berms. Published guidelines for these structures are rare, and there appears to be little standardization. Recommended design improvements, particularly for fire-related debris flows, include incorporating several recent developments in debrisflow mitigation design, reducing subjectivity, and enhancing the technical basis for the designs. Specific shortcomings of existing design methodologies include techniques for predicting debris-flow volume, specifications for berm geometry, impact loading considerations, and lack of flexibility in outlet works design, among others. Proposed solutions and guidelines for these issues are presented.

Estimation of the impact forces from boulders within a debris flow is important for the design of structural mitigation elements. Boulder impact force equations are most sensitive to the inputs of particle size and particle velocity. Current guidelines recommend that a design boulder should have a size equal to the depth of flow and a velocity equal to that of the flow. This study used video analysis software to investigate the velocities of different sized particles within debris flows. Particle velocity generally decreased with increasing particle size, but the rate of decrease was found to be dependent on the abilities of particles to rearrange within debris flows.

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