WESTSIDE ROAD: SOIL BIOENGINEERING FLOOD PROTECTION AT MOUNT RAINIER NATIONAL PARK

Recent years have witnessed consistent aggradation of the Westside Road from severe annual floods and debris flows down Tahoma Creek. Westside Road is the main access to several popular trail heads on the western face of Mount Rainier, and is no longer accessible to personal vehicles because of outburst floods born from Tahoma Glacier in response to a changing climate. These floods, along with past road construction, have led to the degradation of critical riparian gallery forests alongside Tahoma Creek. Applied geomorphic-conscious construction leave a new landscape that is barren and more difficult for natural riparian succession to begin.

In this novel experiment at Mount Rainier National Park, we seek to integrate phyto-engineering construction principles using Sitka willow (Salix sitchensis) and Black cottonwood (Populus trichocarpa) with the geologic engineering method of building Large Woody Structures (LWS) to mitigate road edge erosion. Our soil bioengineering flood protection methods include building willow wattle fences, using live willow stakes to entrain loose channel silt in the Tahoma Creek bed, and planting native species specific to erosion control. Using living structures that will continue to grow is a sustainable and more cost-effective method of construction. Our design takes advantage of the natural asexual dispersal methods, high tolerance to disturbance on site, and high flexibility of growth conditions of pioneer species, S. sitchensis and P. trichocarpa. Both species play a key role in the successional development of a riparian system on our site, and provide slope stabilization via a network of roots. Live willow stakes also contribute to the rock armament by quickly and effectively rooting throughout the LWS. This new ecological restoration approach will be a partnership of both living wood and rock flood mitigation methods, creatively seeking to restore the integrity of the Westside Road to its former popularity while repairing its crucial ecological system and creating an overall stronger solution. This study also serves as an introduction to the restoration theory of adaptive management by effectively testing our design approach on a high profile, highly volatile area of Mount Rainier National Park.