Reclaiming the floodplain can create space for the river where the hydrologic relationship between a river and its floodplain can be reconnected. Strategic and opportunistic buyback of parcels within the floodplain or transitioning adjacent rights-of-ways or public lands into floodable areas can begin to allow for this reconnection.
Historically, the LA River had a vast floodplain and the river would commonly shift its course after major floods. As the area’s population grew, these floods increasingly caused damage to life and property. The LA River was being manipulated as early as the mid-19th-century, and likely long before that. In the 1930s, a decision was made by the US Army Corp of Engineers to channelize the river and replace the shifting floodplain in order to prevent existing communities from further flooding. This ultimately allowed for future development and urbanization. Currently, the historic floodplain of the LA River is almost entirely developed.
Trapezoidal Channel Cross Section
Trapezoidal Channel: Floodplain Reclamation.
OLIN, 2019.
Box Channel Cross Section
Box Channel: Floodplain Reclaimation.
OLIN, 2019.
Benefits
Reclaiming the floodplain will reconnect the hydrologic relationship between the river and its floodplain, which has the potential to enhance ecological function, promote biodiversity, create park space, and improve water quality among other benefits.
Considerations
As a response, floodplain reclamation, if completed at very large scales, could contribute to the overall flood risk reduction system. It should be noted that reclamation of singular parcels or short channel lengths typically does not help reduce flood risk, and, in fact, has the potential to increase flood risk in localized areas. One exception is that the reclaimed parcel itself, which was intentionally designed to be floodable, is no longer a flood risk. Currently, there are a limited number of opportunities along the LA River for floodplain reclamation at any scale, and all the opportunities identified in the opportunity analysis only allow for small scale reclamation that will not have a role in flood risk reduction but could have significant benefits for ecosystem function. Due to development and urbanization in the watershed, large scale floodplain reclamation is not currently feasible without significant impacts to existing residents, businesses, transportation corridors, and other vital infrastructure. It is therefore not a recommended action at this time to address flood risk. Any floodplain modification requires hydraulic analysis to ensure flood risk is not increased.
