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Flood Risk Reduction

The LA River Master Plan is a guiding document for the LA River for the next 25 years and will likely affect the river for at least the next 50 to 100 years and beyond. Therefore, consideration of how the LA River channel continues to reduce flood risk while supporting the multi-benefit goals of the LA River Master Plan is critical.

The Master Plan’s strategic directions recommend several key factors for flood risk and resilience, such as continuing climate change research, considering flood risk to critical infrastructure, developing emergency action plans (EAP) to improve preparedness, and updating the existing LA River watershed hydrology.

Implementation of the Master Plan will increase multiuse spaces within and along the LA River including park space, ecosystems, cultural amenities, connectivity, and water quality improvements. While continuing the development of these multiuse efforts, projects should, at a minimum, maintain existing flood conveyance capacity, increase capacity in deficient reaches, and advance community resilience following extreme events, all while accounting for climate change.

Additionally, an adaptation and mitigation plan should be developed to expand regional resiliency under an uncertain future. This adaptation and mitigation plan would be developed to guide recovery efforts through a “rethink before rebuild” approach following a catastrophic flood event.

1% Flood Risk Goal Exceedance
LA County Map. Map of LA river with 1% flood exceedance
Portions of the LA River meet or exceed the 1% flood risk goal (shown in blue). Other portions (shown in pink) do not meet the 1% goal.
OLIN, Geosyntec, 2019. Source:Mapping is based on a compilation of reports--US Army Corps of Engineers (USACE) LA District. 1996a, 1996b, 1997a, 1997b, and 1999. LA County Drainage Area Improvement Projects. Design Analysis Report and Design Memoranda; USACE LA District. 1991. LA County Drainage Area (LACDA): Review, Part I Hydrology Technical Report: Base Conditions; USACE: LA District. 2015. LA River Ecosystem Restoration Integrated Feasibility Report, Final Feasibility Report and Environmental Impact Statement/Environmental Impact Report, Appendix E. Table 17: Original Design Discharge and Existing Channel Capacity; USACE. 1953. Design Memorandum No. 1 Hydrology for LA River Channel, Owensmouth Avenue to Sepulveda Flood Control Basin; Geosyntec analysis using HEC-RAS models (USACE LA District. 2005. LA County Drainage Area Upper LA River and Tujunga Wash HEC-RAS Hydraulic Models)--and approximate analyses.

System approaches for flood risk reduction include various watershed, channel, and floodplain strategies, and upstream and downstream impacts should be evaluated. Over the next century it is statistically probable that the LA River will flood and overtop its banks and levees. When this time comes, it’s important that community resilience and strategic adaptation approaches are planned well in advance. There are significant cost considerations for the flood risk reduction strategies described in this section, including channel modifications/rehabilitation, bridge modifications, and bypass tunnels, that should be balanced with the benefits of such programs.

Flood Risk Reduction Along the 51 Miles

Strategies to reduce flood risk can focus on the channel, by looking for areas to increase capacity, or on the floodplain, by providing measures for improved emergency response and resilience. Within the watershed, facilities that can capture and retain large quantities of water during peak flows when it matters most, could also reduce flood risk, however, specific studies within the LA River watershed show that limited opportunities exist for large basins in the appropriate locations to make a significant impact on reducing peak flows. Distributed watershed approaches were also investigated, and although watershed measures greatly assist with water quality and supply during the smaller, more frequent storm events, they provide little attenuation of flows during larger peak storm events.

The 1% event is used as a target in this plan because it is tied to the National Flood Insurance Program (NFIP), and studies show using this standard provides a benefit cost Ratio of 7:1 for riverine environments on average. Future local risk assessments may indicate that levels should be increased.

Channel-Based Strategies for Conveyance

Channel-based strategies should focus on improving the deficient areas of the channel that do not meet the 1% flood event capacity goal. Notable areas include intermittent locations in the West Valley between Canoga Park and Van Nuys, the Mid Valley between Studio City and Burbank, and the Glendale Narrows (Narrows). There are a range of strategies, or combinations of strategies, that may be used to improve channel conveyance capacity, including channel modifications, bridge modifications, bypass systems, and channel rehabilitation. These strategies should be assessed on a site-specific basis, while also considering system-wide flood risk reduction.

Increasing channel capacity, such as converting an existing trapezoidal channel into a rectangular channel, can result in a substantial increase in channel capacity. This approach may be suitable in the West Valley. Advantages of this approach are that the flood risk reduction goal may be achieved within the existing channel right-of-way without the need to acquire additional land. A drawback of the approach may be decreased access to the channel and the associated decreased connectivity for wildlife.

Section drawing showing transformation option for converting trapezoidal to rectangular channel
Converting an existing trapezoidal channel into a rectangular channel can result in a substantial increase in channel capacity; however, this strategy should always be combined with other multi-benefit components.
OLIN, 2020.

Trapezoidal to Rectangular Channel Modification
Given the multi-beneficial approach of the Master Plan, this approach should not be proposed without other multi-benefit additions such as terraces, park bridges, stairs, and/or wildlife access ramps. Transitions between trapezoidal to rectangular cross sections need to be designed to minimize hydraulic impacts, which can be achieved using standard hydraulic design transitions. Examples of these transitions in the LA River include either side of the rectangular channel below the 5 and 110 Freeways near Downtown LA.

Bypass Channels/Tunnels
Bypass channels or tunnels with diversions from the LA River may also be considered to increase system capacity. To be effective, these would need to be large-scale infrastructures capable of conveying a significant portion of the channel flow. The diversions could be constructed as open channels running parallel to the main channel, which would require additional land availability or acquisition. Tunnels beneath the ground may instead be used, which would enable existing land-use to be maintained. Tunnels and side-channels could also be used for temporary storage of stormwater for water supply and/or water quality benefits during non-peak flood events. In some cases, by removing flow from the main channel, a bypass channel or tunnel may allow a section of the river to have riparian vegetation or instream habitat while larger flood flows bypass the area in a pipe or channel helping reduce flood risk while maintaining other multi-benefits.

Kit of Parts axon with diversion options
Box Channel: Diversions Kit of Parts.
OLIN, 2019

Channel Rehabilitation
Channel conveyance could also be increased by rehabilitating soft-bottom portions where invasive species have become established. This approach cannot be applied system-wide but is an important strategy where applicable. This is discussed in more detail in the following system-based example below, focusing specifically on the Narrows portion of the LA River.

Bridge Modifications
More than 80 bridges cross the LA River, providing essential connectivity for cars, trains, pedestrians, cyclists, and horses. In general, bridges may cause a restriction of flow capacity due to placement of piers within the flow,
banks contracting due to bridge abutments, and bridge decks being too low. These localized constrictions may lead to flow backing up and overtopping the channel walls, potentially causing wide-spread flooding.

Many bridges were designed, or have since been modified, to enable conveyance of the 1% flood event. For example, multiple bridges in the lower river were modified as part of the LA County Drainage Area (LACDA) project in the early 2000s to convey the 0.75% (133-year) flood event.

LA County Map. Map of bridges that cross the LA River
Bridge Conditions. Bridges that cross the LA River as depicted in the National Bridge Inventory.
US Department of Transportation Federal Highway Administration, National Bridge Inventory, 2017 & US Department of Homeland Security, Homeland Infrastructure Foundation-Level Data (HIFLD), Railroad Bridges, 2009.

In that effort, bridge pier extensions were added to several bridges to force the flow velocity to increase and the flow depth to decrease (i.e., change the flow from ‘subcritical’ to ‘supercritical’) thereby passing under the existing bridge deck. This approach obviated the costly need to raise bridge decks. However, this approach only works where hydraulic conditions permit.

Other bridges, such as some in the Narrows and West Valley, do not provide enough capacity to convey the 1% event, and hydraulic retrofits are needed to meet the flood risk reduction goal of the Master Plan. This may include bridge pier extensions if hydraulic conditions permit, completely removing bridge piers and reconstructing the deck to ‘clear span’ the river and/or raising the bridge deck. Some of these options may require vertical realignment of the roadway.

Ideally, bridge retrofits or reconstructions can be carried out during other projects. An excellent example of this is the 6th Street Bridge replacement project that was initiated due to deterioration of the concrete bridge structure. The replacement design is a viaduct that clear spans the channel and enabled the large central pier to be removed, substantially improving the channel’s hydraulic performance. An inventory of existing bridge conditions could be cross-referenced with hydraulic performance to help prioritize which bridges should be rebuilt.

Opportunities for improving the hydraulics while leveraging other efforts may be during bridge seismic retrofits, bridge expansion to add traffic lanes (e.g., recently completed Spring Street Bridge), and/or bridge realignment (e.g., recently completed Riverside Drive Bridge).

In addition to the retrofit of existing bridges it is imperative that any new bridges being proposed are evaluated for capacity issues at least at the 1% flood event level.

Channel Widening
Widening the existing channel is another way to provide additional flood conveyance capacity while also potentially allowing for concrete removal. This may require additional land acquisition outside of the current channel right-of-way, which is a challenge in the heavily urbanized and developed environment. A benefit of this approach would be maintaining access to the channel and the associated connectivity for wildlife. If enough land is available, it may also be possible to convert concrete sections of the existing channel into soft-bottom portions to improve opportunities for native habitat and wildlife.

channel widening
Widening the existing channel may require additional land acquisition outside of the current channel right-of-way, which is a challenge in the heavily urbanized and developed environment.
OLIN, 2019.

This approach would still require hardened channel sides to prevent the river channel from meandering into developed areas and would also require large amounts of land outside the current river right-of-way to be obtained. For example, estimates in the West Valley indicate that the channel width of a naturalized channel would need to be increased approximately threefold to contain the 1% event. This represents a significant encroachment into residential and commercial properties resulting in displacement of residents, businesses and local infrastructure. Such actions would have significant impacts to communities and are generally not aligned with other Master Plan objective.

Floodplain-Based Strategies for Resilience

The Master Plan includes nine goals that are based on local and regional needs. One of the goals is to “Reduce flood risk and improve resiliency.” Within this goal, several actions and methods outline key steps to help meet this goal, such as increase flood capacity, reduce peak flows, use the latest climate research, improve emergency preparedness, increase public awareness, improve facility operations and maintenance, and implement consistent floodplain management practices. The goals should be considered in concert to develop multi-benefit projects and integrated infrastructure solutions. True resilience can best be achieved when water, environmental, and social priorities are balanced.

Infographic on who is affected during major storm events
Understanding who is at risk in the event of a major storm event is paramount to building resilience. Within LA County, there are nearly ten times as many people living in the 0.2% (500-year) floodplain as the 1% (100-year) floodplain.
U.S. Census Bureau, American Community Survey 2015–2019 5-Year Estimates; US Census Bureau, California Block Groups, 2019; LA County GIS Data Portal, Assessor Parcels, 2021.

In addition to consideration of the 1% event (100-year) discussed in the previous section, populations are at risk in the 0.2% floodplain (500-year) as well. Further, the population of LA County is projected to increase by over 10% by 20501California Economic Forecast. “California County-Level Economic Forecast 2017-2050,” September 2017. sea level rise is expected to claim industrial and recreation lands at the periphery, and climate change is leading to more intense storms and a hotter, drier climate. These factors are expected to stress an already at-capacity system, and land development pressures and population densification could make it more difficult in the coming decades to secure space for strategic adaptation of the integrated water systems across LA County. To continue the development of the goals, actions, and methods of the Master Plan in light of these shifting needs, this section outlines key strategies for LA River floodplain resilience and strategic adaptation, including possible strategies for building resilience in advance of, and also in the aftermath of, flood disasters.

Understanding Who is at Risk
People living within floodplains are at risk of being directly impacted by flood events. Population analysis of the floodplains indicates that many of the at-risk populations within the LA River watershed are generally poorer and more rent-burdened than the average LA County resident, particularly in the 0.2% floodplain. Unfortunately, this reveals that those most at risk of experiencing a flood event are also the least likely to have the resources to overcome significant disturbances due to flooding.

Within LA County, the 0.2% floodplain is only about 2.5X as large as the 1% floodplain, yet the magnitude of its effects is far greater in terms of the number of people and parcels that could be impacted by a flood.

Mean Household Income diagram of those living in 1% and .2% floodplains
Mean Household Income within the 1% and 0.2% Floodplains. Compared to LA County averages, those living within the floodplains tend to have lower mean household income. This analysis compares demographics across floodplains as well as across geographic boundaries including the LA River watershed ("Watershed") and LA County ("LAC") in its entirety.
U.S. Census Bureau, American Community Survey 2015–2019 5-Year Estimates; US Census Bureau, California Block Groups, 2019; LA County GIS Data Portal, Assessor Parcels, 2021.

In the 1% floodplain, one-fifth of all land is residential, and there are approximately 37,000 parcels and 90,000 people at risk. These numbers increase substantially for the 0.2% floodplain, where 30% of land is residential, and nearly 1.5 million people and over 350,000 parcels could be affected by a flood event. In relation to the population of LA County at large, this amounts to 0.9% at risk in the 1% floodplain and 14.4% at risk in the 0.2% floodplain.

There are a few demographic trends that generally hold true for both the 1% and 0.2% floodplains. Compared to LA County averages, those living within these floodplains are more often homeowners than renters. Mean household income and mortgage- and rent-based financial burdens are comparable for residents within the floodplains and residents throughout LA County at large. For example, approximately one in five owner-occupied households spends at least half of its income on mortgage expenses, and one in three renter-occupied households is severely rent burdened. These are consistent with LA County averages. LA County is already facing an ongoing housing crisis characterized by widespread displacement risk and gentrification. Within the LA River floodplains, a severe storm event could further exacerbate these issues.


Taking Action Toward Resiliency

Floodplain-based strategies should focus on improving community and critical infrastructure resiliency in the 1% floodplains where the current channel is not capable of carrying the 1% flood event and should be considered in the 0.2% floodplains along the entire river. There are several strategies that can be used separately or in combination to increase flood resiliency including strategic adaptation within the floodplains, resiliency measures for critical infrastructure, and emergency action planning. These strategies should be assessed on a local and regional basis, while also considering system-wide flood risk reduction.

Diagram for Rethinking before Rebuilding
Resilience framework for flood risk reduction and long term adaptation.
OLIN, 2020

Strategic Adaptation
The global environment is experiencing changes in climate patterns in ways that were not anticipated by the original designers and engineers of our water management systems. Patterns in rainfall intensity and duration have drastically increased the likelihood of stormwater runoff exceeding existing flood management capacities, potentially resulting in flooding. Although there is still much uncertainty, current climate change modeling for LA County indicates that the historical 1% (100-year) storm event may be more frequent, with a 1.5% probability of occurring annually (67-year).2 AghaKouchak, Amir, Elisa Ragno, Charlotte Love, and Hamed Moftakhari. (University of California, Irvine). 2018. Projected changes in California’s precipitation intensity-duration-frequency curves. California’s Fourth Climate Change Assessment, California Energy Commission. Publication Number: CCCA4-CEC-2018-005 Increased flooding and predicted sea level rise could further increase flood risk where our channels meet the ocean. While uncertainty and disasters have occurred historically, the climate has become much more volatile, and the need to adapt is clear. The LA River, which was originally engineered in the early- to mid-20th-century prior to the appreciation of multi-benefit risk reduction strategies, operates in much the same way it did when first constructed nearly a century ago. Over the next century, as watersheds continue to be impacted by catastrophic storm events and sea level rise, strategic adaptation of the system will be required to meet these future challenges.

Each segment of the LA River presents unique constraints and opportunities. Therefore, there is not a single 51-mile solution for the entire river. Strategic adaptation for the LA River involves a proactive understanding of current and future conditions to plan and design now for changes that are expected to occur in the future. Strategic adaptation is a critical component in developing floodplain, community, and infrastructure resiliency.

One of the challenges of addressing climate change for planning and design of the river and its watershed and floodplains is the uncertainty in the magnitude and timing of impacts. This uncertainty makes adaptive approaches imperative to allow for changes in conditions without the often-unmanageable costs associated with designing for the worst-case scenarios. The watershed and floodplains are impacted by catastrophic storm events, effects of sea level rise, and intensive development patterns in which the density and extent of settlements transform the surface of the land. Not all potential scenarios can be mitigated now, so plans and designs seek to lessen these impacts and provide the ability to adapt. Planning infrastructure features to increase capacity, while also identifying locations to leave space for unknown future changes, is a vital strategy.

Strategic adaptation requires a shift in thinking among planners, engineers, and political leaders, as well as in public education. It is a way of making choices to address a changing landscape, to make predictions and prepare for future challenges while continuously reevaluating.

With an eye to the future, strategic adaptation would also work in tandem with existing measures established within LA County. For example, the National Flood Insurance Program (NFIP) was created by the Federal Emergency Management Agency (FEMA) to reduce loss of life and property and meet the rising costs of disaster relief due to flooding. The program is voluntary based on a mutual agreement between the federal government and the local community. LA County entered the NFIP in 1980. Participation in the program makes flood insurance available to LA County unincorporated area residents and allows them to obtain direct federal relief loans following federally declared flood disasters.

LA County has an ongoing Floodplain Management Program, which includes mapping of flood hazard areas, adopting associated ordinances, and regulating and enforcing safe building practices. It is the combination of these activities that promotes flood management to our community and maintains LA County’s eligibility to participate in the NFIP.

Rethink Before Rebuild

Some of the best ideas for resilience are born from disaster that has already occurred. Precedents on the following pages explore a handful of lessons gleaned from major flood events that have occurred throughout the United States in recent years. However, one key lesson stands out: rebuilding without rethinking will simply result in a repeated history. With changing climate and precipitation patterns leading to more frequent flood events, simply rebuilding in the same manner repeats an endless and costly cycle of rebuild, destruction, and rebuild again. Current insurance norms also may need to be rethought to help prevent rebuilding to the same standards as before, allowing for more innovative thinking before reconstruction begins. Recent studies by the National Institute of Building Sciences show that every $1 spent on mitigation saves $6 on future disaster losses3 Multihazard Mitigation Council (2017) Natural Hazard Mitigation Saves 2017 Interim Report: An Independent Study.
with the greatest benefits realized for riverine flooding disasters. Rethinking involves a shift from reactive to proactive. Recovery begins long before a disaster. Dollars spent in advance of disasters can result in significant cost savings post-disaster and accelerated recovery time.

Extreme environmental events or disasters cause disruption but also provide an opportunity to better understand risk, allowing preparation of better approaches to respond and recover more quickly in the future. Post-disaster conditions allow actions to be taken that would be more difficult and likely not possible otherwise, such as daylighting storm drains, adjusting levees, rebuilding bridges higher or with fewer piers, or rebuilding structures within the floodplains at a higher elevation. Rather than just rebuilding, periods of recovery provide the opportunity to adapt and improve resiliency, reducing damage and recovery times following future events, while also helping meet other regional needs for healthy connected ecosystems, parks, and cleaner water.

Case Studies - Flood Risk Resiliency

Different types of storms and flood events teach different lessons. The scale of the storm, the nature of flooding, the communities and infrastructure impacted, the emergency response, and the rebuilding and adaptation following each event provide insight into better preparedness and approaches for the future. While the climates and contexts of these precedents are not directly the same as the LA River watershed and floodplain, they still demonstrate that unforeseen risks and changes in climate patterns unfortunately result in serious impacts to people, infrastructure, and ecology. As storms rise from unique conditions and pose unique threats, it is not possible to mitigate all risk for every storm. However, by founding mitigation on adaptive, imaginative strategies and by addressing the topic of recovery early—long before a disaster even occurs—responses to disasters like floods can become far more effective.

Photo of New York City during Superstorm Sandy.
Superstorm Sandy.
Reeve Jolliffe, Manhattan, Hurricane Sandy, 2012. Source License: CC BY-NC-ND 2.0.

After making landfall as a Category 4 hurricane in August 2018, Harvey’s movement slowed significantly. Over the course of four days, Harris County and the City of Houston received a record-breaking amount of rainfall totaling 26 to 47 inches. Water released from reservoirs prevented dam failure but further contributed to flooding, especially in Buffalo Bayou.

Key Takeaways

  • Storm duration is just as important as magnitude when stormwater infrastructure is overwhelmed.
  • Engineered and nature-based solutions are equally valuable but must be designed in tandem and scaled appropriately.
  • Communication and community awareness of risk must be improved; it is imperative that residents living within a floodplain are aware of the risks.
Rescue workers during Hurricane Harvey
Hurricane Harvey.
1st Lt. Zachary West, U.S. Army, Texas Army National Guard Hurricane Harvey Response, 2017.

A month after wildfires near the coastal town of Montecito had destabilized soils, heavy rainfall carried mud, large boulders, tree branches, and sediment flows from the nearby Santa Ynez Mountains to the coast. Local reservoir releases further overwhelmed the flood management system. Approximately 30,000 people were evacuated, 150 people were hospitalized, and 23 people were killed.

Key Takeaways

  • Resilient systems prepare for isolated as well as combined threats.
  • A USGS assessment conducted after the wildfires estimated debris flows in response to a design storm, yet the storm that occurred was far more severe. Planning for a range of storm intensities and impacts establishes a helpful range of responses.
  • Preparation and evacuation communications that are tailored for specific communities based on the degree and type of threat may more effectively initiate responses than widespread warnings and advisories.
Photo of rock and debris mudslide into home
Montecito Mudslides.
Los Angeles Fire Department, LAFD Assists Victims of Tragic Mudslide, 2018. Source License: CC BY-NC-ND 2.0.

In late 2016-early 2017, atmospheric rivers carried vast quantities of rainfall to California. The Oroville Dam received an entire year’s average runoff in two months. Further rainfall taxed the system, requiring use of an unlined emergency spillway that began to erode the hillside. Emergency managers proactively evacuated 180,000 people living in downstream communities in case of a dam breach or residual threats.

Key Takeaways

  • Improved climate and operations models will help predict disasters before they occur.
  • Periodically updating inundation mapping and contact lists of those within the floodplains and having pre-scripted notifications is crucial to assist in emergency response and notification and allow emergency managers to fast-track emergency decision-making processes that save lives.
  • Improved understanding of the failure mechanisms of flood risk infrastructure will allow operations and maintenance routines to focus where the risk is greatest.
Oroville Dam photo
Oroville Dam.
Cal OES, Oroville Spillway, 2017. Source License: CC BY-NC 2.0.
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