Project Examples

Animas River near Aztec, NM

Severe bank erosion at a meander bend of the river had taken approximately 2.5 acres of horse pasture (alluvial terrace) and washed it downstream. Left unchecked, this lateral extension of the river would eventually lead to a meander bend cutoff, causing loss of land, a large sediment load downstream, and base lowering of the river by 10 plus feet (wide spread geomorphic repercussions). The private landowner had tried building an earthen dike to divert the river away from the erosional area, but after a few years the river had washed away most of the dike. Large scale riprap was contemplated to prevent further land loss. When the landowner contacted the New Mexico Environment Department to seek funding assistance under the EPA's 319 program, NMED suggested that a Natural Channel design approach be considered instead.

Riverbend Engineering was retained to assess the river and to propose a stable river channel solution using Natural Channel Design techniques. Analysis of historic aerial photography revealed the progression of meander bend extension through time, and the relatively recent rapid erosion problem. At this particular location the river had developed a radius of curvature smaller than any other location for 5 plus miles up and downstream. It was clear that a meander bend cutoff sequence had begun, and needed to be mitigated as soon as possible. NMED's criteria for funding assistance included mitigation of in-stream deposits, so the design solution had to be hydraulically effective. After performing a detailed topographic survey of the site, and a geomorphic assessment of the rivers condition, Riverbend Engineering selected a solution based on a sustainable radius of curvature and in-stream rock structures for channel stability.

The design incorporated the reclamation of 1.5 acres of pastureland at the terrace elevation, with another 1.5 acres of active floodplain restored. Transplants of willows and Russian olive from on-site sources were used to stabilize the reconstructed floodplain. In-stream rock structures were built to prevent the floodwaters from eroding the outside bank of the river, and these structures were keyed back into the re-constructed floodplain with buried logs and old loader tires salvaged on-site. The inside bank of the river bend was graded to an appropriate point bar shape and elevation, allowing for high flows to initially flood "river right". Constructed in the winter months of 2003, this project has passed a peak flow during spring runoff approximately 20% above bankfull discharge. Nature doesn't seem to waste any time testing our restoration designs!

Riverbend Engineering's contract includes monitoring of the project for several years post-construction. Our as-built and post-flood surveys provide a detailed evaluation of how the river is adapting to the imposed form. After the first runoff season we observed fine grained sediment deposition on the constructed floodplain, no floodplain erosion, survival of 90% of the transplanted willows, stability of the constructed banks, and minor sediment deposition along the channel margins above the rock structures (as expected). The excavated pool structures below each rock structure have adjusted the locations of maximum depth and we will quantify how this changes over time with our monitoring work. It is only through monitoring can the collective wisdom of Natural Channel Design be implemented in environmentally effective and cost conscious ways.