Flood Risk Reduction in the Narrows

Back to: Design │ Project Examples │ System-Based Examples

Flood Risk Reduction in the Narrows

The Narrows offers multiuse opportunities not found elsewhere along the LA River.

The Narrows planning frame of the LA River presents a specific set of issues because the channel has limited capacity to convey flows greater than the 2% event (50-year), and in some locations the levels are as low as the 10% (10-year) or even the 25% (4-year) events.¹ At the same time, the Narrows offers multiuse opportunities not found elsewhere along the LA River, including ecosystems with opportunities for ecological improvements and recreational attributes such as kayaking and birdwatching. There are several strategies available to improve the flood conditions in the Narrows that have the capability to increase conveyance to as high as the 2% (50-year) or even the 1% (100-year) flood events. Depending on the goals, these strategies need to be explored in concert to develop the best project available for the LA River system.

Soft-Bottom Channel Sections

Channel Rehabilitation at the Narrows

In addition to documented areas of willow, cottonwood, and other native vegetation,² large woody and non-native invasive species, along with mass sediment accumulation in the soft bottom reaches of the LA River, specifically in the Narrows, restrict flows during larger events (2%, 1%, and 0.2%), which would cause the river to overtop its banks. The larger, non-native and invasive species (Arundo donax, jubata [Cortaderia jubata], Mexican fan palm [Washingtonia robusta], Canary Island date palm [Phoenix canariensis]) have become overgrown with only intermittent maintenance for the past several decades, and in combination with the sediment buildup along the channel bottom, flood risk has increased significantly. Rhizomatic root systems of species such as Arundo trap sediment and create large hummocks within the channel, often 10 feet high, restricting flows and creating low value habitat when compared to native plant species. Invasive species such as Arundo also thrive in the year-round dry weather flows in the Narrows which is rich in nitrogen from treated effluent from upstream wastewater treatment facilities that discharge into the LA River.

A channel rehabilitation program could reduce flood risk in several stretches along the Narrows. If the rehabilitation removes sediment and replaces existing vegetation with native grasses, capacities in some reaches may increase from below 35,000 cubic feet per second (cfs) to the original design discharge of 78,000 cfs, more than doubling the carrying capacity of the current channel itself, from the 25% (4-year) event to greater than the 2% (50-year) event.³ A combination of this approach with other flood risk reduction strategies, including bridge modifications and a bypass tunnel, could potentially bring the LA River in the Narrows up to the 1% flood event capacity goal.

increasing native vegetation along the channel could be followed. Through this approach, biodiversity of native mammals, avian, and insect species that rely on native vegetation would be increased. There would also be a decreased need for the installation of unsightly temporary flood barriers, which are often installed by jurisdictional agencies to reduce flood risk and block access to the river. This approach, while not strictly meeting the freeboard requirements ⁴ throughout the Narrows, could enable the 4% (25-year) event to be mostly contained within the channel, except for a few locations where overtopping may be expected.

In a multi-beneficial channel rehabilitation program, the ideal resulting river cross section would include native grasses, species such as willows that “lay down” during flood events (such as, but not limited to, arroyo willow [Salix lasiolepis], black willow [Salix gooddingii], red willow [Salix laevigata], sandbar willow [Salix exigua]), and some native riparian trees (such as, but not limited to, Fremont cottonwood [Populus fremontii], coast live oak [Quercus agrifolia], California sycamore [Platanus racemosa], California walnut [Juglans californica]) along with a reduction of sediment mounding on the channel bottom. Further detail on recommended native species and plant communities can be found in Appendix Volume I: Design Guidelines, Chapter 5. Natural sediment transport processes will still allow some accumulation of sediment, and the removal of the large piles of sediment and the Arundo rhizome root hummocks will reduce the large piles that exist within the flood channel. Considering that smaller and larger storm events will continue, the implementation of a long-term adaptive management approach is important. Future storm events will continue to shape and contour the channel, and maintenance will help support a healthy viable ecosystem that can co-exist with decreased flood risk to the community.

Systematically Removing INVASIVE VEGETATION AND SEDIMENT PILES, WHILE also MAINTAINING REFUGE HABITAT, WILL INCREASE CHANNEL CAPACITY AND SPECIES DIVERSITY

Process
Despite the advantages of channel rehabilitation for flood risk reduction, it is critical that this type of project be implemented in an environmentally responsible way that identifies, creates, and maintains refuge habitats for keystone species during periods of invasive species and sediment removal. Invasive species removal must be carried out by a trained team of landscape maintenance workers with specialized heavy equipment that can identify species and selectively remove invasive vegetation and their root systems. This method would be a patchwork removal process by first identifying and protecting critical habitat zones, then sequentially removing invasive species in the areas outside of the protected zones and installing native plant species. Only once the installed native habitat is established should the careful removal of the invasive species in the protected zones be completed. Ecologists, arborists, and other vegetation specialists should consult and supervise the patchwork removal process.

All native plant species should be installed and maintained through establishment, following requirements set forth in Appendix Volume I: Design Guidelines, Chapter 5. The process of channel rehabilitation is not a singular 11-mile process to be carried out once and left alone for 20 years. Instead, this will require an ongoing multi-year adaptive management strategy that includes measures such as consistent monitoring and removal of any reintroduced non-native invasive species and native plant species replacement as needed while the habitat becomes established. In association with this program, a concerted effort should be made to clear the upper tributaries and watershed of the highly invasive non-native vegetation species to reduce the chances of recolonization.

Refuge Habitats
Prior to beginning channel rehabilitation, it is necessary to identify native, endemic keystone wildlife, insects, and invertebrate species along the channel that should be maintained and determine their maximum range of habitation. The LA River Master Plan biodiversity profiles indicate desirable species ranging from large fauna to insects that can guide this process. The LA River Design Guidelines plant lists specify native plant communities and key indicator species within each community. At a minimum, one to two species in each category should be selected to serve as target species to determine an appropriate refuge habitat area.

Overlapping the range of target species will assist in determining the maximum distance that a refuge habitat can be from an area of invasive species and/or sediment removal. This patchwork pattern would define the ongoing process of adaptive vegetation and habitat management. A refuge habitat should not be disturbed until the adjacent rehabilitated area can meet the same habitat needs, allowing wildlife or other species to migrate to the rehabilitated area. It is expected that rehabilitated areas can meet habitat needs within the first few years after rehabilitation, so the process of channel rehabilitation will be ongoing.⁵ ⁶

Hydraulic Considerations
The process of creating refuge habitats will result in a patchwork pattern of invasive species and sediment removal so each section of channel rehabilitation undertaken would be studied for specific hydraulic effects. As the invasive species and sediment removal process is planned, and the adaptive management program is developed, consideration would be made to create passageways for large volumes of water during times of high flows.

It is necessary to identify keystone wildlife, insects, and invertebrate species along the channel that should be maintained and determine their range

Adaptive Management
Ongoing observation and management of habitat areas should be continuously carried out over time by a team of specialized scientists, ecologists, plant specialists, and environmental engineers. Wildlife monitoring should begin prior to any channel rehabilitation work. Any changes observed can be compared to the initial baseline ecosystem function.

Monitoring of ecosystem function not only includes observations of keystone species but also includes allowances for the dynamic biological, geochemical, and physical processes that occur within the riparian habitat. Ecosystem functions such as nutrient cycling, providing connected shade, or filtering pollutants should be encouraged in the adaptive management work. This requires that a trained team of landscape maintenance workers engage in practices contrary to typical landscape maintenance. For example, organic matter and debris from native vegetation should not be cleared, and refuge habitat should be left undisturbed. As invasive species are removed, constant monitoring and maintenance is required to ensure that invasive species do not encroach into recently cleared areas.

Adaptive management practices should be flexible enough so that they can be adjusted over time as scientific observations are made and the ecosystems themselves change. Practices may also vary throughout different times of the year to best react to the varying conditions of the river.

Hydraulic performance should be monitored over time to determine which species have the greatest impact on the capacity of the channel.

Education and Engagement
The Narrows provides a unique opportunity in LA to study, learn, and experience native ecosystems if properly managed and maintained. Local schools as well as colleges and universities could benefit from learning about the adaptive management process, native plants, native wildlife, and hydraulics. School curriculum for nearby elementary or secondary schools could help provide much needed education on the importance of native ecosystem adaptive management, native plant communities, and native wildlife.

Local communities could also be engaged through wildlife monitoring programs that highlight specific native keystone species. Programs might include wildlife cameras, educational exhibits about the adaptive management process, or tours and nature walks.

Green Jobs/Local Jobs/Youth Internship Potential

The labor-intensive process of selective invasive species removal and adaptive management could provide a local jobs opportunity, job training for working with native plant systems, or a teen internship program for local high school students. Another opportunity would be the integration of native plant and ecosystem job training with criminal justice reform initiatives or jobs programs for persons experiencing homelessness.

Planning for workforce development is essential to this process as typical vegetation removal processes will not meet the needs of a nuanced program for invasive species and sediment removal along with strategic adaptive management.

Bypass Tunnel

Currently, the 10%, 4%, 2%, 1%, and 0.2% flood events are shown to cause varying levels of flooding along the Narrows. Channel rehabilitation in the Narrows to remove invasive species and replace with a range of native species including grasses and some riparian trees could lower the water surface elevation throughout the Narrows improving capacity. The 4% (25-year) flood event would generally not meet the freeboard requirements, but the flow would be largely contained within the riverbanks, except at a few locations.

Addition of a bypass tunnel could further improve the capacity. For example, a large bypass tunnel, diverting water from the channel upstream of the Headworks property at river mile 33 and returning it back into the channel downstream of Piggyback Yard at river mile 22, could provide approximately 20,000 cfs of additional capacity. This could result in the 2% (50-year) flood event being largely contained within the riverbanks, although the freeboard requirements would not be met, and overtopping would be expected in some locations. The bypass tunnel may enable the 1% (100-year) flood event conveyance goal in the Narrows to be achieved, but this would also require rehabilitation of the channel to add native grasses and modification of several bridges to clear span.

Size, Hydraulic Considerations, and Multi-Benefits
The concrete tunnel would be approximately 40 feet in diameter and nine miles long, with a 0.6% slope. The inlet may consist of a lateral weir on the existing channel approximately 1,000 feet long leading to a forebay and tunnel entrance. Additional hydraulic considerations for the inlet and outlet would have to be evaluated during design. This tunnel could provide some of the needed relief required during very large flood events while also providing much needed storage within the system, allowing for multiple benefits (i.e., water supply and water quality) to be
accrued even during smaller storms. Multiple precedents for this type of intervention exist.

Bridges in the Narrows

The following is an overview of the hydraulic impacts of the bridges in the Narrows, assuming channel rehabilitation has been completed. The overview is generalized based on one-dimensional (1-D) hydraulic modeling using different values of hydraulic roughness to represent different levels of channel rehabilitation and a range of different flows. This screening level modeling identifies several bridges that should be prioritized for hydraulic retrofits. In these cases, any design modifications should always consider a bridge’s historic significance. In addition to analyzing existing bridges, all new bridges should be assessed from a hydraulic perspective to make sure the 1% flood event level can be met.

Riverside Dr: The bridge deck is close to the top of the channel banks. The four bridge piers result in some backing up of the flow and a slightly raised water surface elevation (WSE). Retrofitting the bridge (e.g., raising and modifying to clear span) will only provide minimal hydraulic benefits due to the low channel capacity.

Highway 134: The bridge deck is well elevated above the channel banks. The three sets of piers that span the confluence with Verdugo Wash do not appear to have a substantial impact on the hydraulics.

Los Feliz Blvd: The bridge deck sits low at the top of the banks and is supported by five long bridge piers/walls and is accompanied by a grade break (i.e., local steepening) that were designed to force shallow critical flow under the bridge. The 1-D modeling indicates that the bridge deck will be impacted and possibly overtopped during the 4% flood event, although additional analyses may be needed to confirm this. Modeling indicates that better than 1% flood event capacity can be achieved through modifying the bridge to be clear span, although channel capacity either side of the bridge may not be able to convey the 1% flood. For example, if the channel is rehabilitated with native riparian habitat including trees, then the channel upstream of Los Feliz Boulevard can only convey approximately the 4% flood event without meeting freeboard requirements. The benefit of the bridge modification would primarily be to protect the bridge infrastructure and reduce the flooding localized around the bridge.

The grade break at the bridge location results in locally shallow flow, and as such the bridge soffit (i.e., invert elevation of the bridge) may not need to be raised when modifying to clear span. However, clear span may require thicker deck girders thereby raising the elevation of the roadway.

Glendale Blvd: The bridge deck is slightly elevated above the top of the banks and is supported by five long bridge piers/walls that are also used to support the Red Car rail trolley and Red Car Pedestrian Bridge. The 1-D modeling indicates that the bridge causes the flow to back up and increase the WSE upstream of the bridge. This may cause the freeboard requirement to be exceeded during the 4% or 2% flood event, depending on the level of channel rehabilitation, and the banks to be overtopped during the 2% or 1% events. Modeling indicates that better than 1% event capacity may be achieved, depending on the level of channel rehabilitation, through modifying the bridge to be clear span. For example, if the channel is rehabilitated with native riparian habitat including trees, then the channel upstream of Glendale Boulevard will be able to convey the 4% flood event, including freeboard requirements, and likely convey the 2% flood event while not meeting freeboard requirements.

The bridge soffit (i.e., invert elevation of the bridge) does not need to be raised when modifying to clear span. However, clear span may require thicker deck girders thereby raising the elevation of the roadway.

Taylor Yard Bicycle and Pedestrian Bridge: This pedestrian and bicycle bridge is currently under construction and will have one pier within the river. This bridge was not modeled. This stretch of the river may be able to meet the 1% event level, depending upon the level of rehabilitation.

Interstate 5: The bridge deck is elevated above the channel banks. The two bridge piers result in some backing up of the flow and a slightly raised WSE. Retrofitting the bridge (e.g., modifying to clear span) will only provide minimal hydraulic benefits due to the low channel capacity.

Colorado St: The bridge deck is close to the top of the channel banks. The two bridge piers result in some backing up of the flow and a slightly raised WSE. Retrofitting the bridge (e.g., raising and modifying to clear span) will only provide minimal hydraulic benefits due to the low channel capacity.

North Atwater Bridge: (Also known as LA Kretz Crossing) This multi-modal bridge was recently constructed and has one large pier within the river. This bridge was not modeled as part of this effort and is in a stretch of the river with generally deficient channel capacity. During design it was determined that 1.5 to 2.0 foot-high flood walls were required along the top of the levee, both upstream and downstream of the bridge, to increase local capacity.

Sunnynook Pedestrian Bridge: The deck of this pedestrian bridge is close to the top of the banks. The six bridge piers result in some backing up of the flow and a slightly raised WSE, and sometimes a hydraulic jump depending upon conditions. The bridge does generally meet the 1% event level, provided the Los Feliz bridge (upstream) and Glendale bridge (downstream) are both modified.

Fletcher Dr: The bridge deck is elevated above the top of the banks and is supported by six bridge piers. The 1-D modeling indicates that the bridge causes the flow to back up and increase the WSE upstream of the bridge. This may cause the banks to be overtopped during the 4% or 2% flood event, depending on the level of channel rehabilitation. Modeling indicates that better than 1% flood event capacity may be achieved, depending on the level of channel rehabilitation, through modifying the bridge to be clear span. For example, if the channel is rehabilitated with native riparian habitat including trees, then the channel upstream of Fletcher Drive will be able to convey the 4% flood event and be close to meeting freeboard requirements, and likely convey the 2% flood event while not meeting freeboard requirements.

The clear span may require thicker deck girders, but these are likely able to be accommodated without changes in road elevation through utilizing the vertical space that exists between the bottom of the current bridge deck and the top of banks.

Highway 2: The bridge deck is well elevated above the channel bank elevations of the surrounding reaches. The four bridge piers result in some backing up of the flow and a slightly raised WSE. These local increases in WSE are generally contained within the levees within the vicinity of the bridge and as such retrofitting the bridge will only result in minimal improvements in hydraulics.

Case Study: Tunnel and Reservoir Plan, Chicago, IL

The Tunnel and Reservoir Plan (TARP) is a large-scale engineering project that is designed to reduce flooding and combined sewer overflows in the Chicago River watershed. The regional plan was approved in 1972 and began construction in 1975. Currently, 109 miles of tunnels that are 30 feet in diameter and 300 feet below ground have been constructed. The system of tunnels divert excess rainfall and combined stormwater and sewage from local waterways to large holding reservoirs, where it is held to be treated before being released back into the water system. This plan has reduced flooding throughout Cook County and has eliminated a large amount of pollutants being released directly into Lake Michigan during periods of heavy rainfall.⁷

Lessons Learned

Large investment in engineering and design projects yielded extensive returns for cities and counties and was possible to complete.
Water quality improvements have brought waterside economic drivers and interest throughout the region.

Back to: Design │ Project Examples │ System-Based Examples