An Evaluation of the Stream Simulation Culvert Design Method in Washington State

Abstract

Stream simulation has become an increasingly common culvert design method around the world. It is based on the assumption that geologic and hydraulic conditions in natural channels define passage characteristics for migrating fish and that water crossing structures that imitate these conditions can then achieve those same passage characteristics. This study expands on an initial evaluation of 19 culverts in 2003 to 50 culverts and includes methods and analyses comparing hydraulic characteristics based on cross sections, profile variation, and bed texture between each culvert and its paired reference reach situated in an adjacent section of the natural channel of each stream. Taken as a group, these culverts simulate bed texture, 100‐year recurrence interval flood velocity and 2‐year flood width but did not simulate thalweg complexity or other hydraulic metrics. Culvert span, relative to the bankfull width of the stream, does not by itself determine whether the culvert simulates the reference reach. Of the 50 culverts, many of which experienced record floods, only one showed significant bed degradation. Copyright © 2014 John Wiley & Sons, Ltd.

Providing Aquatic Organism Passage in Vertically Unstable Streams

“…SSD holds the assumption that if the stream channel of a road/stream crossing can be constructed to sustain similar geometric, geomorphic, and hydraulic conditions of the reference channel, no additional unnatural barriers to passage are created through the crossing [6]. This concept has been developed and applied successfully in stable steep coarse alluvial channels of both confined and moderately confined morphologies [8]. SSD may have further limitations to its applicability when: (1) the project slope becomes too dissimilar to the upstream or downstream channel; (2) bankfull indicators are absent, conflicting or non-diagnostic; (3) sediment transport is degraded or fragmented through the project reach; or (4) where existing channel hydraulics do not represent reference conditions.…”

“…The design process is iterative with feedback loops refining the design geometry, spacing, and roughness to provide morphology supporting AOP and structural integrity of the constructed channel. Engineering guidance has been applied to the design of morphologic simulations with varying degrees of success [8,10,[30][31][32][33][34].…”

“…This fact has resulted in many road crossing structures, especially culverts, to be replaced with bottomless structures. While replacing a culvert with a geomorphic-based solution may be a vast improvement for AOP at the crossing itself, it does introduce the risk of upstream migration of vertical channel instability [7,8].…”

“…Understanding and analyzing these linkages informs the extent and magnitude of the various processes occurring in the vicinity of a road crossing structure. Improperly installed culverts may compromise or eliminate AOP and can alter the quantity or quality of stream corridor habitat, not to mention that they may not perform as expected under a wide range of flows [8]. Because culverts are static structures in dynamic systems, even properly designed and installed structures can become passage barriers if channel incision is initiated downstream and then migrates upstream.…”

“…During the intervening years, various survey and analysis protocols have been developed to aid in the identification of vertical instability and selection of treatment alternatives for road crossings. There are a variety of culvert design manuals available [6,8,10], however, there are very few guidance documents that bring the engineering and geomorphology together in the design process, which is the intent of this document.…”