Draft Evaluation of the Stream Simulation Culvert Design Method in Western Washington, a preliminary study.

More than 50 stream simulation culverts have been constructed in Washington State since 1995. This paper summarizes monitoring conducted on 19 of these culverts in various settings. The monitoring goal was to compare the physical characteristics of the adjoining upstream channel with those of the culvert bed. The premise of stream simulation design is that similar physical characteristics imply similar passage conditions. Field parameters included channel geometry (channel width, slope and cross section, pool spacing, and residual pool depth) and sediment size distribution. Mathematical modeling using field data compared culvert and channel hydraulic performance including inlet contraction and depth distribution (quantification of shallow water habitat). Standard statistical tests were used to evaluate individual parameters, unfortunately the sample size was too small to perform multivariate analysis. Results show that when designed and constructed according to stream simulation design criteria (Culvert bed width = 1.2(Channel width)+2 feet, and slope of culvert < 1.25(Channel slope)), stream simulation culverts are reliable and create similar passage conditions compared to the adjoining channel.