Channel Adjustment Following Culvert Removal from Forest Roads in Northern California, USA

Abstract

Forest roads can be a major contributor of sediment entering streams and estuaries. Road restoration and removal has been shown to be effective in reducing sediment delivery over decadal time spans. However, few studies have been conducted to quantify and characterize short-term (1–3 yr) channel adjustment after removal of stream crossing fill material and reconstruction of former stream channels. This evaluation is important for managers needing to assess impacts, cost, and benefits of stream crossing removal. This study examines 20 road stream crossings by surveying channel adjustments and determining rilling and gullying delivery volumes, channel substrate changes, and the role of rock placed in reconstructed channels at Redwood National and State Parks, California, USA. Erosion and channel incision were significantly higher at crossings in the 1st year following removal than at older crossings. First year crossings yielded an average of 7.71 m3 of erosion per crossing, while the 2nd- and 3rd-year crossings showed a net gain of 0.08 m3 of sediment (indistinguishable from 0). Eighty-six percent of the erosion could be attributed to channel incision and bank erosion. The remaining 14% originated from rilling, gullying, and side-slope failures. Coarsening of channel substrate was most pronounced in the 1st year after removal, and the 1st storms of the year were most important in mobilizing sediment. This study underlines the importance of excavating to bedrock or coarse material when recreating a streambed, and the identification of bulges (short convex segments) in the channel profile that are likely to erode.