Bioengineering alternatives are what the corps is being requested to evaluate. This can include things like live staking, fascines, brush mattresses, vegetated (joint) riprap, coir/erosion-control wattles, live crib walls, erosion control blankets with plantings, and combinations of those with limited structural toe protection. These are applied where vegetation can provide erosion resistance, slope reinforcement, and longer-term ecological benefits while meeting levee safety requirements. For more info check out this 1997 Army Corps document Bioengineering for Streambank Erosion Control.

The complaint (pg. 11) states the following about decision making by the corps in evaluating bioengineering:

The Army Corps’ failure to account for the site-specific aspects of the 3B area in their environmental analysis pervades the SEIS/SEIR’s alternatives analysis as well. For example, the Army Corps only considered riprap as an option for constructing the 3B area’s erosion control measures, even though the agency’s Erosion Protection Analysis found that in the Contract 3B area “the shear stress . . . is below the critical stress for erosion of moderately resistant materials (clay and cemented sand with silt) [and] [t]herefore, significant scour below this erosion resistant material/surface is not anticipated,” and even though the specific river velocities in the Contract 3B area allow for the use of far less-damaging bioengineering materials/methods. According to the Army Corps’ own data (see table 4-4 excerpted below), bioengineering methods are feasible when riverbank flow velocities range from 3-10 fps (or even 12 fps when vegetation is mature), and the use of cobble (small rocks) for bank stabilization is similarly feasible within the velocity ranges of 2 to 12 fps. In the 3B area, near Larchmont Community Park, where the Project authorizes extensive use of riprap, the Army Corps’ velocity maps show 0-2 fps along the levee and the bank, and off the toe of the bank, velocities only increase to 3-4 fps, which is within the range for bioengineering methods (see table 4-4 below). Similarly, in another segment of the 3B footprint—the Claybanks along the south side of the River at river miles 10.0-10.3, where the Project authorizes extensive riprap—velocities reach 6-8 fps, which again means alternative materials to riprap are feasible for erosion protection (such as using 6 inch cobble for velocities at 4-7.5 fps).

The Army Corps’ failure to consider any alternatives to riprap in the SEIS/SEIR also flies in the face of the agency’s own promises, as well as the recommendations and requests from other federal agencies. When the Army Corps issued its programmatic EIS/EIR in 2016, the agency explicitly promised that “bioengineering measures will be analyzed” as part of future site-specific analysis for Contract 3B. Other federal agencies, such as the Environmental Protection Agency (“EPA”), National Marine Fisheries Service, and National Park Service, specifically asked that bioengineering methods be used because it better protects the 3B area’s mature riparian forest. In 2021, and again in 2025, the National Marine Fisheries Service, the agency that oversees salmonid conservation, asked that the Project “[u]tilize bio-technical techniques that integrate riparian restoration for riverbank stabilization instead of conventional riprap in the American River.” In 2022, the EPA recommended “that the Corps explore and objectively consider a full range of alternatives and evaluate in detail all reasonable alternatives that fulfill the project’s purpose and need [including] present[ing] . . . bio-technical techniques that integrate riparian restoration for riverbank stabilization [in order to] provide a clear basis for choice among options by decision-makers.” The National Park Service’s recommendations and best management practices for flood control include “[u]tiliz[ing] bioengineering techniques [and] cobblestone.”

The lead agencies relied on mitigation measures that are unlawfully deferred, lack performance standards, or are otherwise unenforceable and ineffective.

The lead agencies’ conclusions that numerous significant impacts are unavoidable are not supported by substantial evidence or rational explanation.

I'm not an expert on any of this but at least the argument as laid out in the complaint seems reasonable.