- Oregon has thousands of culverts along mountainous roads that interact with perennial streams, intermittent streams, and intercepted subsurface flow from hill slopes. Culverts installed at stream crossings are designed to move water under the road and avoid failure of the fill. Similarly, ditch relief culverts transfer water through a road prism.
There are two main issues that are often considered when designing a stream crossing culvert: fish passage and peak flow. For example, Oregon Department of Fish and Wildlife (ODFW) has developed fish passage guidelines that apply to any stream utilized by fish (ODFW, 1997). In addition, Oregon's Forest Practice rules require culvert drainage structures in forest roads to pass a peak flow that at least corresponds to the 50- yr return interval (ODF, 1995).
The inlet area of installed culverts is sometimes damaged or reduced in size after installation in forest roads. Reductions in the cross sectional area of inlets is usually related to factors such as sediment deposition, debris blockage, and structural damage. The major concern with partially plugged culvert inlets (reduced flow area) is that the reduction of flow area of the culvert inlet increases the possibility of failure during a peak flow. Piehl (1987) made a random sample selection of stream crossing culverts and ditch relief culverts on the Oregon Coast Range. He found that the average inlet cross-sectional area of stream crossing culverts was 88% of the original end area. Denting and sediment blockage were the most frequent factors causing reduction. Understanding the hydraulic conditions of partially plugged culverts is fundamental evaluating their effect on peak flow events. Depth of flow, flow velocity, and available area for flow are critical factors considered by the guidelines and regulations of the ODFW and Oregon Department of Forestry (ODF). With a better understanding of culvert inlet conditions water resource managers will be able to establish priorities of maintenance, estimate risk of failure, predict implications relative to high flows, and identify culverts that need to be replaced.
The overall purpose of this project is to evaluate potential effects of inlet constrictions on culvert discharge. Analysis and recommendations for peak discharges will be presented after evaluating the effects of inlet conditions on flow capacity.