Existing Flood Risk Reduction
Although channelization of the LA River began as early as the 1800s, the channelization of the LA River as we know it today occurred under the direction of the US Army Corps of Engineers (USACE) and began in 1936 with the passage of the Flood Control Act following destructive flood events in the late 1930s.
Additional phases of construction continued up until 1959. Motivated by the February 1980 flood, channel improvements on the lower LA River were implemented in the late 1990s to early 2000s as part of the LA County Drainage Area (LACDA) project1USACE 1991. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Final Feasibility Report. December. 2USACE 1996a. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Los Angeles River Improvements Project Including Rio Hondo and Compton Creek. Design Analysis Report No. 1. December. 3USACE 1996b. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Los Angeles River Improvements Project Including Rio Hondo and Compton Creek. Design Memorandum No. 2. June. 4USACE 1997a. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Design Memorandum for Los Angeles River Improvements Century Freeway to Willow Street. Final Draft Report. July. 5USACE 1997b. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Los Angeles River Improvements Project Including Rio Hondo and Compton Creek. Design Memorandum No. 3. October. 6USACE 1999. U.S. Army Corps of Engineers, Los Angeles District: Los Angeles County Drainage Area Los Angeles River Improvements Project Including Rio Hondo and Compton Creek. Final Design Memorandum No. 5. June. to increase the channel capacity in the lower 12 miles of the river. These efforts have been largely successful in managing flood risk, but there are problematic reaches along the river and flooding remains a threat.
Annual Chance of Exceedance
The level of flood capacity of the LA River indicates the size of the flood in terms of annual exceedance probability, at which the LA River can safely convey flows. The “Annual Chance of Exceedance” map and ruler presents the estimated current level of flood capacity along the LA River in terms of the annual chance of exceedance, and shows that in general, capacities in the lower river are greater than those throughout the City of Los Angeles.
The annual chance of exceedance is the probability that the channel capacity will be exceeded in a single year. For example, a flood event with an annual chance of exceedance of 1% has a 1 in 100 probability of being exceeded in any given year and a flood event with a 2% annual chance of exceedance has a probability of 1 in 50. On average, the return period for a 1% event is 100 years, and as such the 1% event is often referred to as a “100-year flood event.”
The “Annual Chance of Exceedance” map and ruler were developed using the most current models and information available, and there are recommendations to update the hydrologic and hydraulic models in the master plan. These mappings indicate that the channel upstream of Sepulveda Basin (RM 51 to RM 46) mostly has a mixture of 2% (yellow) and 1% (green) flood capacity levels, with a few locations with worse than 2% (orange) channel capacity level, likely due to local constrictions from bridges. The channel in this reach is concrete-lined, trapezoidal in shape, and increases in width from 125 feet to 200 feet.
Channel Type, Width at Top of Channel, Levees, and Tributaries
From Sepulveda Basin to Tujunga confluence (RM 38), the channel generally has better than 1% flood capacity with the exception of a short segment upstream of the Tujunga confluence, where worse than 10% (red) flood capacity level is estimated. This may be caused by additional inflows, but it is noted that the hydrologic analysis used to derive the inflows may be at a coarser scale than the hydraulic model (i.e., more detailed analysis may be warranted in this region). The channel in this reach is concrete-lined and rectangular. Notably, the channel width is only 50 feet, compared with 200 feet above Sepulveda Basin, indicating the effectiveness of Sepulveda Basin in attenuating peak flows in the river.
From Tujunga confluence to the Narrows (RM 33), the level of flood capacity is generally better than 2%. The channel is concrete-lined and rectangular, with widths ranging from 125 feet to 130 feet.
The Narrows reach (RM 33 to RM 22) has known deficiencies that are exacerbated by the heavy vegetation that has established itself in the soft bottom of the trapezoidal channel. Despite the presence of levees along portions of this reach, the flood capacity level is worse than 2%, with many regions having worse than 10% flood capacity and as low as 25% flood capacity.
Further downstream, between the Arroyo Seco (RM 24) and Rio Hondo confluences (RM 12), the flood capacity level is mostly better than 1%, although just downstream from Arroyo Seco and in Vernon (RM 18), the flood capacity level is worse than 2%. The channel in this reach is concrete-lined, mostly trapezoidal, and has top width varying from 225 feet to 415 feet.
Following the LACDA improvements, where the levees were raised and parapet walls were added, the river downstream of Rio Hondo confluence has better than 0.75% (133-year) flood capacity. Flows greater than the 0.75% event are designed to overtop two weirs located downstream of Imperial Highway on the east bank (near RM 11.4) and near the 105 Freeway on the west bank (near RM 10.7). The channel in this reach is concrete-lined (apart from the lower 3 miles in the tidally influenced portion of the river), trapezoidal, leveed, and with top width varying from 400 feet to 585 feet.
Combined Flood Hazard
Additional information is available to indicate the extent to which the water may inundate surrounding neighborhoods. The “Combined Flood Hazards within LA County” map above shows the 1% (100-year) and 0.2% (500-year) floodplains as determined by the Federal Emergency Management Agency (FEMA) and USACE and areas near the coast that are at risk of inundation due to tsunami and sea level rise.
The extents of the floodplains vary considerably due to differences in surrounding topology. The 1% floodplain along the Narrows (RM33 to RM22) is confined within a relatively narrow corridor near the river due to the terrain rising to the east and the west of the Elysian Valley. It is estimated that approximately 3,300 parcels will be impacted by a 1% flood event.7USACE 2016. U.S. Army Corps of Engineers, Los Angeles District: Hydraulics Report, Floodplain Analysis, Los Angeles River: Barham Boulevard to First Street, floodplain Management Services Special Study, Los Angeles, California, October 2016. By contrast, the alluvial floodplain in the lower river (RM16 to RM0) covers a vast expanse due to the largely flat terrain formed by deposition of sediment along the LA, Rio Hondo, and San Gabriel Rivers over a long period of time. The floodplain also includes water from the San Gabriel River to the east. This larger floodplain corresponds to a 0.2% event with the risk level being reduced to better than 1% by the LACDA efforts in the late 1990s and early 2000s.
The 1% floodplain that would be expected along the channel in the upper river (e.g., intermittently between RM51 and RM45 and between RM38 and RM33) is not mapped, but would be anticipated to remain relatively close to the main river channel.
Critical Facilities Within Flood Hazard Area
|Facility Descriptions||Within Flood Hazard Area||Total Facilities|
|FEMA 100-Yr Floodplain||FEMA 500-Yr Floodplain||Tsunami Inundation||1.41-Meter Sea Level Rise with 100-Yr Storm Event||Any of the 4 Flood Hazard Area|
|Emergency Operations Facilities||2||12||1||1||12||105|
|Medical Care Facilities||37||752||16||12||757||5,754|
|Hazardous Material Sites||311||2,836||243||210||2,910||18,667|
Residents and infrastructure within the floodplains may be substantially impacted by flood events. Of paramount importance during such emergencies are critical facilities where emergency operations are conducted, including police and fire stations, medical care facilities, and schools that may be used as evacuation centers. Also of importance are hazardous materials sites, that may pose a significant threat to public safety and health and the environment should they become inundated with water. Analysis of critical facilities and hazardous material sites indicates that there are 404 total facilities/sites within the 100-year floodplain, and 4,359 total facilities/sites within the 500-year floodplain.