USE AND EVALUATION OF THE CRITICAL-DEPTH METHOD FOR ESTIMATING PEAK DISCHARGES IN MOUNTAIN STREAMS

Fluid flow in open channels is fairly well understood, but there remain several environments where flow processes are more complex. One of these is mountain areas, which comprise about 25 percent of streams in the United States. At remote sites, during wide-spread flooding, or at those sites where debris, high velocity, or rapidly changing stage is present, peak discharge typically is computed after a flood by indirect methods, which are costly and time consuming. Although there have been numerous investigations to improve estimates of flow resistance (e.g., Manning’s n) in higher gradient (mountain) streams (slope ≥ 0.01 m/m), it remains difficult to properly quantify roughness coefficients used in indirect methods.

A critical component of flood science is rapid and reliable data collection for subsequent water-resources investigations (e.g., river restoration, sediment transport, and ecosystem studies). In mountain streams, the critical-depth (CD) method, which only uses channel geometry, avoids problems related to estimating flow resistance and has been shown to be a cost-effective method to estimate peak discharge. The CD method has been shown to be more reliable than the slope-area method, which is the most commonly used method to estimate peak discharge.

In 2013, Colorado experienced one of its worst flood seasons in history. During July and August, the 2012 Waldo Burned Area in Colorado Springs experienced three small thunderstorms that produced record flooding. Following the September 2013 Colorado Front Range Flood, emergency managers required substantial peak-discharge documentation; dissemination of results occurred within a few days of data collection. Over 150 CD measurements were obtained at a cost of approximately $250/site (vs about $15,000/site for indirect-flood methods). Over 40 CD measurements were made near where other indirect methods of peak discharge were subsequently made by other flood specialists, which presented an opportunity to compare results of these methods. The objectives of this presentation are to discuss the CD-method with application to several recent, extreme floods, and to provide a comparison of peak discharges computed with the critical-depth method versus other, indirect methods (slope-area, bridge contractions, 1- and 2-D hydraulic models, etc.).