The Corps has been attempting to manage flood risk on the Upper Guadalupe River in San Jose, California since the 1980s. San Jose is the 10th largest city in the country and is quickly growing as it is a hub of software engineering and innovation. The area is also diverse and home to both income inequality and an unhoused crisis, with hundreds of people living in encampments in the Upper Guadalupe riverbed and along the banks. Flooding in the river is flashy and can be hazardous to people in some places, especially where socially vulnerable communities reside; but there is also shallow widespread flooding that impacts large areas of mainly affluent populations.
The Upper Guadalupe River Flood Risk Management Project was initially authorized in 1999, reauthorized in 2007, and partially constructed. However, the Corps realized that the plan might not be justifiable nor implementable as designs advanced. While severely incised and flood prone, the Upper Guadalupe River supports Endangered Species Act threatened steelhead and is vital habitat for the recovery of this species due to the regional habitat scarcity. The without project damages can support a large and expensive project but building traditional Corps flood risk management projects is challenging in habitat for protected fish. It is an expensive place to build flood risk management projects given these sensitive habitat conditions as well as the high cost of real estate in a densely developed urban area, costs related to naturally occurring mercury in the soils, the high cost of mitigation, and the scarcity of suitable mitigation sites. These challenges prevented the project from been completed for nearly four decades.
The Upper Guadalupe General Reevaluation Study is the latest effort to plan, design, and build flood risk management measures along 5.5 miles of the Upper Guadalupe River channel between Interstate 280 to Blossom Hill Road. When the General Reevaluation was initiated, San Francisco District had recently become an EWN Proving Ground and the project team identified it was an excellent opportunity to pursue EWN designs, a technique that had not been previously used.
The real estate that the local sponsor, Valley Water, had already purchased created a ripe opportunity to explore channel widening, but in a way that enhanced habitat, rather than destroying it. By excavating an inset floodplain bench and including native plantings, the widened channel cross section could restore long lost floodplain benches and riparian forest. Though the plantings and floodplain bench add channel roughness, the added space for the river to spread and slow high flows would also benefit the velocity issues that had previously been a concern in the previous design. This reformulation also began just as the Comprehensive Benefit memo directive came out, giving the team a clear mandate to measure environmental quality (EQ) benefits that EWN would provide, and look closely at other social effects (OSE) like life safety and equity in this area where environmental justice is critical.
This project will protect communities along the Upper Guadalupe from damages and life safety risk. There is currently a high risk of flooding in the densely-populated areas surrounding the Upper Guadalupe River, and tributaries Ross Creek and Canoas Creek. In past flood events, stream flows that broke out of those channels caused deep flooding in communities with known environmental justice disparities and shallow flooding in historically more affluent communities. There are approximately 3,490 people living in residential structures that are projected to flood during a 1% annual exceedance probability (AEP) event. Just under half of those residents are considered socially vulnerable based on factors like income, race, age, and mobility. Without the project, expected annual damages from flooding are roughly $22.5 million. Critical transportation infrastructure is also at risk from flooding.
Flood depths and velocities in the channel pose a significant life safety risk to the unhoused population residing in encampments along the channel. The unhoused population in San José has increased significantly with rising home prices, and the City of San José has indicated that they do not have sufficient temporary or alternate housing to meet existing needs.
By incorporating engineering with nature, the improvements to the channel not only manage flood risk but also create a floodplain bench and valuable riparian habitat for Endangered Species Act threatened steelhead. Agriculture and development in the Santa Clara Valley have eliminated most of the riparian forest in the region. The riparian forest along the Guadalupe River and nearby creeks constitutes the last remaining areas of significant riparian forest in the valley. Along the Upper Guadalupe River, remaining riparian habitat has been reduced and degraded by channelization, gravel mining, and development. This project converts parking lots and pavement to riparian forests and reestablishes a previously urbanized floodplain. Riparian forests are among the most productive habitats for wildlife in California, providing food sources and shelter for fish and wildlife. These habitats support the most dense and diverse wildlife communities in the Santa Clara Valley, with generally the highest levels of biodiversity.
The project should result in improved aquatic habitat conditions for wildlife and federally listed salmonids. Channel widening, floodplain benches, large wood structures, and the proposed riffle creation are expected to provide more topographic complexity and support the return of the underlying channel dynamics and physical processes necessary for healthy aquatic ecosystems and the spawning and rearing of salmonids.
This project will reduce flood damages and the risk of harm to residential, commercial, public, and industrial structures, their contents, and privately and publicly owned vehicles. The project will eliminate 95% of the expected annual damages and provide $59 million in gross regional product.
Emergency costs are those costs incurred by a community during and immediately following a major storm. The cost of debris removal from inundated residential and non-residential structures was the only emergency cost reduction benefit considered for this analysis.