Reduce flood risk and improve resiliency.
The LA River did not always look like it does today. In the mid 1800s, the LA River was a braided stream that, during wet weather events, spread out over vast amounts of flat land.1Blake Gumprecht, The Los Angeles River: Its Life, Death, and Possible Rebirth, 2001, pp 9-15.As agricultural diversions, transportation infrastructure, and cities grew around the river, this vast floodplain was encroached upon by buildings and roads. After increasingly devastating floods, it was engineered into a concrete channel with basins, dams, levees, and floodwalls to move stormwater as quickly as possible to the Pacific Ocean to reduce flood risk to these communities. Not all areas of the river have equal conveyance capacity. In some areas, low channel capacity makes the probability of flooding of the river adjacent communities in any given year as high as 25%.2USACE. 2015. Los Angeles River Ecosystem Restoration Feasibility Study, Appendix E, Hydrology and Hydraulics, US Army Corps of Engineers, Los Angeles District. September. There will always be financial and physical limits to flood risk infrastructure. Therefore, we must strive for resilient communities that can respond to extreme flood events that exceed the river channel’s capacity. With the threat of a changing climate, the importance of reducing flood risk increases as the frequency and intensity of extreme storms change.
Steps that should be taken to achieve the goal
LA River Flood Risk Reduction Need
Needs: Flood Risk Reduction
Flood risk is related to both the capacity of the LA River channel to convey water in large storms and the area outside of the channel impacted by flooding.
To evaluate need related to flooding along the LA River corridor, the level of existing channel capacity was analyzed and combined with the floodplains directly associated with the LA River. Areas that may be subjected to sea level rise inundation and areas with high amounts of critical infrastructure and facilities in the floodplain were also assessed.
The “Level of Channel Capacity” refers to the statistical return period that channel capacity is exceeded. Locations in the river with capacities to convey storm events with a greater than the 1% (100-year) flood event should be assessed for improvements. Areas with a very high need have capacity to convey no more than a 10% (10-year) flood event. Areas with a general need fall between the 10% (10-year) and 1% (100-year) conveyance capacities.
Floodplains are the lowland areas that border a river and though usually dry are subject to flooding. Floodplains are most commonly mapped where models indicate a 1% annual chance of flooding (100-year floodplain) or a 0.2% annual chance of flooding (500-year floodplain) in any given year (i.e., areas with a flooding recurrence interval of 500 years, on average). Areas within the 1% floodplain were identified as very high need and require flood management improvements. A degree of risk should be considered for the 0.2% floodplain, which was identified as general need. Areas not in a 1% or 0.2% floodplain were considered to have no need.
Areas subject to sea level rise, including approximately the lower 3 miles of the channel, have a higher need for flood risk reduction.
Critical infrastructure and facility types such as emergency facilities, evacuation routes, and wastewater treatment plants were included based on facility types identified in the 2016 LA County Comprehensive Floodplain Management Plan, and were collected from various sources. Given the lack of detail about the size of specific facilities, the relative density of facilities was used. Areas that had the highest density qualified as very high need, and areas with the lowest density qualified as general need. All areas outside of the floodplain were considered to have no need.