The Upper Llagas Creek watershed encompasses areas of Llagas Creek and its tributaries that run through San Martin, CA and the neighboring community of Morgan Hill. Since the 1950s, the creek has experienced flooding during major storm events, where currently over 1,000 homes, 500 businesses, and more than 1,300 acres of agricultural land in southern Santa Clara County are within the 100-year floodplain. In order to provide critical flood protection to the surrounding area, Santa Clara Valley Water District worked with Woodard & Curran to develop a more stable stream channel that provides increased flow capacities, improved storm water management, environmental habitat restoration, and substantial improvements to beneficial uses.
In the not so distant past, a flood protection project would have involved widening the creek on both sides, digging an “efficient channel shape,” straightening out the curves in the creek, eliminating maintenance issues (such as trees) and possibly even paving the creek with concrete. While these measures would help to convey the flood waters quickly downstream, this project is an example of the evolution of available approaches, as we used techniques that accomplished the same outcomes in a more ecologically responsible way.
Our design reduced the project impact by widening the creek on one side, thereby allowing swaths of native trees and habitat to remain. The team applied a modern geomorphologic analysis to the creek to determine the channel shape that would provide a more stable waterway, one that would not be prone to incising or depositing sediment. To restore the creek habitat, existing topsoil and streambed material will be harvested and stored during the widening phase, and placed back into the new channel during the later phases of construction to support the proposed native revegetation plantings. By re-using the topsoil, the natural nutrients and native bacteria will help restore the creek to a natural state. Hundreds of instream complexities such as log-rootwads, boulder clusters, and turtle perches will be constructed in the creek bed to provide habitats for fish and other animals. A 2,100-foot-long tunnel has also been designed to convey high flood flows through a section of downtown Morgan Hill, thus avoiding and minimizing impacts to the native creek channel, residences, and businesses. In addition, the project proposes to convert an existing rock quarry to wetlands and restore a remnant section of Llagas Creek to low flows for enhanced fish passage—both of which contribute to a healthier watershed and overall ecosystem.