By Bill Clayton
Chairman, Flood District 10
The Boise River is the lifeblood of the Treasure Valley, providing irreplaceable natural, social, recreational and economic value to our community. We each benefit from the river every day.
It is also powerful and unpredictable. Flooding in 2017 eroded banks, carried large objects downstream, formed gravel bars, and inundated farms, neighborhoods and commercial properties. Historically, our response to this ever-present risk has been reactive and dependent upon an incomplete understanding of the natural forces and human activities that affect the river. While we will never have a complete understanding of forces impacting the river, we are moving to a new era of proactive, informed and collaborative river management.
Sustainable management of the Boise River and of land use development within the Boise River corridor require ongoing, in-depth assessments of the riverine environment. Local communities, governmental agencies and other stakeholders will soon have modern data-gathering techniques and analytical tools to inform this planning and decision making.
Using water-penetrating Light Detection and Ranging (LIDAR) technology, water management agencies and Treasure Valley water user groups (led by Flood Control District No. 10 and the University of Idaho, Center for Ecohydraulics Research) recently collaborated to map the Boise River channel and floodplain from Diversion Dam to Parma.
Last November, an aircraft carrying a “green” LIDAR device captured nearly one million pulses per second of green-wavelength laser light. The light penetrated through water to the bottom of the river channel, measuring each square foot of the channel and surrounding topography. Once foliage and buildings were removed, images revealed elevations and contours of the channel that could not otherwise be seen.
(The Lidar data will be publicly available through the Idaho Lidar Consortium’s website, idaholidar.org)
The U.S. Army Corps of Engineers is using the LIDAR data to develop of a two-dimensional (2-D) hydraulic model of water flows called the Boise River Management Tool (BRMT). The BRMT will enable us to model, understand and predict the changing hydraulics of the river.
Yellow and orange areas in the Lidar images highlight the channel-changing “stream power” of rapidly flowing water, which can carry boulders and tree stumps miles downstream. We can evaluate how the force of this water erodes stream banks and impacts highway bridges over time. As we increase flows in the 2-D model tool, we see where the river will flood the greenbelt, roads, parking lots and buildings.
With this tool, we will better anticipate how and where evolving hydraulics could lead to floods, damaged infrastructure, irrigation problems, degraded water quality, harmed aquatic life, and impacts on recreation and aesthetic beauty. We will be positioned to design projects to repair and prevent bank erosion, gravel bar formation and sediment transport and accumulation. Users can use the BRMT to guide “flood-wise” development of land and to prioritize projects that reduce flood risk.
The BRMT will be ready this fall. Developing it has brought a wide range of stakeholders interested in the Boise River together – and we want to thank all our partners who helped make this project possible, especially those who provided financial support: the Idaho Water Resource Board, Army Corps of Engineers, Natural Resources Conservation Service, Cities of Boise, Caldwell, Eagle, Middleton and Garden City, Ada County Highway District, Eagle Sewer District, Treasure Water Users Association, and Pioneer Irrigation District.