Use of living and inert materials: a rebar stake with willow "whips"
Bioengineering in the sense used here refers to the use living and inert materials to achieve an engineering goal. It is a rough translation of the German term "Ingenieurbiologie" (Engineering Biology). The systematic development of Bioengineering as a body of knowledge was pioneered in the mountainous areas of Austria and southern Germany in the 16th century and the initial terms reflect this heritage.
These days, to avoid the confusion noted previously, Gray and Sotir (1996) advocate the use of the terms Soil Bioengineering and Biotechnical Stabilization, each of which is defined below:
Biotechnical Stabilization: The use of natural inclusions, living or inert to reinforce soil and stabilize slopes.
Soil Bioengineering: The use of live plants, and plant parts alone to reinforce soil and stabilize slopes.
Techniques falling under under these definitions are mainly useful to reduce surface erosion, reduce streambank erosion, and to promote slope stabilization.
Surface erosion is reduced by several processes. The first is interception of rainfall by foliage which reduces the velocity of the droplets and minimizes their impact. The second is restraint- as roots anchor soil particles and reduce the likelihood of their movement. The third is retardation - the presence of vegetation slows the movement of runoff across a surface, decreasing soil movement. Finally, infiltration - the roots of plants open up many paths of entry into soil thereby increasing opportunities for water to soak in rather than runoff.
Stream bank erosion is reduced above ground by vegetation slowing the velocity of water flowing over it, and below ground by roots which anchor soil in place.
Slope stabilization is promoted by vegetation. Roots provide reinforcement and buttressing, particularly when there is bedrock that they can penetrate within the rooting zone. Additionally, transpiring plants remove excess water from soil, reducing the weight and improving the strength of soil.