Stream Crossings

By Thembi Borras

Stream crossings exist where roads intersect watercourses, they include bridges, culverts, fords and rock armored fill crossings. One of the fundamentals of road management is that stream crossings be designed for large storm events. The currently accepted standard is for stream crossings to be designed for 100-year flood flows.

Stream crossings deserve considerable attention in road management because if a crossing fails the fill associated with the crossing will almost certainly enter the watercourse. Bridges with inadequately sized abutments, fords with steep dirt approaches and rock armored fill crossings with inadequately sized rock contribute sediment. However, in my experience, the highest risk of direct sediment delivery to a watercourse is from undersized, poorly designed and installed culverted stream crossings.

Culverted stream crossings have appropriately been described as a dam with a hole in it and are prone to plugging. The most common reason why culverts fail is the inlet becomes plugged with woody debris. The following design considerations will lower the risk of failure: 1) size the culvert to pass the 100-year flood flow and the wood and debris associated with that event, 2) align the culvert with the natural stream channel, 3) install the culvert at the grade of the original stream channel, 4) place the culvert in the bottom of the fill and compact the fill well, 5) install a trash rack and 6) install a diversion proof dip. The purpose of a diversion proof dip is, if the culvert should plug, the watercourse is directed back into the channel so that diversion is avoided. Finally, realize there are alternatives to culverted stream crossings that require less maintenance. For example, a rock armored fill crossing or a ford is a good alternative to a culverted stream crossing, where drivability allows.

There are a number of resources to aid you in stream crossing design and implementation. They include the Handbook for Forest and Ranch Roads by Pacific Watershed Associates and the February 2004 publication entitled Designing Watercourse Crossings for Passage of 100-year Flood Flows, Wood, and Sediment available through the California Department of Forestry and Fire Protection website. Click on Resource Management then click on Forest Practice or go directly to (http://www.fire.ca.gov/php/rsrc-mgt_forestpractice_pubsmemos.php).