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Questions about the LA River and Watershed

Why is the LA River concrete?

The LA River has a long history of flooding, even before extensive settlement. When it rains, large amounts of water flow out of the mountains into the flat areas where present day cities are located. Historically this water spread out in large areas that would sometimes be miles wide. As more people settled in these areas, moving water away from homes, business, and communities became important.

To better describe the relationship between flooding and the LA River, here is an example of how water flow is measured. Flow is a measure of the velocity, or speed, of the moving water, multiplied by the cross-sectional area. For any given flow, this relationship holds true – the slower the velocity of the water the greater the cross-sectional area required to convey the flow. Conversely, the faster the velocity the smaller cross-sectional area is required. Water moves much more quickly through channels that have less friction, such as smooth concrete. Because significant development such as transportation corridors, industries, businesses, and housing, has occurred in the LA River’s floodplain and right up to it’s banks, unfortunately the cross-sectional area remaining to convey flows is very small, so the concrete is there to reduce friction, maintain higher velocities, and better manage floodwaters.

What if we put trees in the channel or have more natural areas like the part of the LA River by Griffith Park?

This is a great question, since trees, plants, and areas of native habitat can greatly improve our urban environment. This question is directly related to the discussions about concrete and flow of water. Trees and plants in the channel increase friction which slows the water down. This is why the channel capacity is lowest near the Narrows by Griffith Park. When water moves slowly, it requires the channel to be much larger to carry the same flow. However, there are limited locations where vegetation in the channel may be possible and more importantly, we need to creatively think about adding more open space where we can plant climate appropriate, non-invasive trees and other vegetation along the river, in parks, along greenways, on bridges and buildings, and even in our own yards.

Open Channel Diagram
Shown here is a stylized section of an open channel representing that total flowrate is a function of velocity multiplied by cross-sectional area.
OLIN, Geosyntec, 2019.

Can we remove the concrete?

Many have asked if we can remove the concrete from the LA River to return it to a more naturalized river. Because a more naturalized river requires a much wider flow path than the existing channel allows, it is not feasible to remove the concrete from the LA River without causing significant negative impacts to communities and local culture. Without displacing hundreds of miles of transportation routes and utility corridors, thousands of businesses, and potentially hundreds of thousands of residents, removal of concrete is difficult to accomplish. Additionally, climate change poses future uncertainty, so we need to maintain the existing capacity of the channel while also finding ways to increase capacity where the channel is undersized. As such, the LA River Master Plan does not pursue a strategy of massive concrete removal and community displacement as other goals would be supplanted by this singular strategy. Instead, the Master Plan seeks to find areas where natural ecosystems can exist while maintaining flood risk management and retaining river adjacent communities, culture, infrastructure, and amenities.

But shouldn't we try to save more water?

We should, and we are. In fact, LA County Public Works, the LA County Flood Control District, and their partners such as the Los Angeles Department of Water and Power capture and conserve enough water within the LA River watershed to supply more than 300,000 people with water each year. And, of course, more can be done, which is why significant investments in major dams and spreading grounds is ongoing. Additionally, Measure W was passed by county voters in 2018 creating the Safe, Clean Water Program. The Program includes steady funding for projects to further catch stormwater runoff as a water supply while meeting the Program’s primary objective to improve water quality.

Measure W was passed in 2018 and makes


available to the Safe Clean Water Program for projects with a focus on stormwater capture, water quality improvements and community benefits.

If we collect more water upstream in dams or greenstreets can we get rid of the concrete? Like a city of sponges?

This is an important concept to understand as it is critical that we continue to capture, collect, treat, infiltrate, and, where possible, use water closer to where the rain falls before it can reach the river. About half of the LA River watershed is undeveloped and exists in the steep mountains that surround our cities, so much of our watershed, and in particular the portions of our watershed that receive the most rain, are already undeveloped. Within the developed watershed, smaller distributed systems such as neighborhood stormwater parks, green streets, and rain gardens and cisterns improve water quality and local water supply during smaller, more frequent storm events also providing benefits when it is not raining, such as for habitat and recreation. However, these systems quickly fill up during the initial stages of larger storms and are rendered unusable when the bigger storms hit the watershed and therefore do not reduce flows to the river. Interestingly, development over the past 100 years has not caused increases in large flows to the river however development has increased peak flows from smaller storms.

Many tools work together to manage and conserve water across LA County, including dams, channels, and best management practices for local stormwater capture and water quality improvement. Source: LA County Public Works, 2019.

Large dams like Sepulveda Dam do a great job at helping reduce flood risk. Given the density of development along and near the river, there aren’t locations left to place new large dams where they would matter most, such as at the bottom of the Burbank and Verdugo Wash channels. Large areas that are available, such as old quarries, aren’t located in ideal locations for flood management and are better suited to water conservation needs. The LA River Master Plan proposes a unified strategy of stormwater management that requires distributed systems to help with water quality and conservation and centralized systems, like the LA River, to reduce flood risk.

For more information on the hydrology of the LA River Watershed or LA River hydraulics, refer to Chapter 3 in Appendix Volume II: Technical Backup Document.

What is so complicated about widening and naturalizing the river?

Large-scale widening of the existing LA River channel could provide additional flood conveyance capacity while also potentially allowing for concrete removal, but this strategy is not pursued in this Master Plan due to its serious social implications.

As described in the previous pages, vegetation and flood capacity have an inverse relationship. Adding trees and shrubs increases channel friction and slows down the water so the channel must become wider to maintain the same flood conveyance capacity. A naturalized channel for the LA River would need to be three to seven times the width of the current channel to maintain a 1% (100-year) flood capacity.

The additional space needed for channel widening would require the displacement of people, businesses, and infrastructure adjacent to the LA River. In a 3X widening scenario, this would amount to nearly 22,000 displaced residents and major impacts on government and industrial land. Over 35 miles of freeway, 60 miles of transmission lines, and 20 miles of railroad would be affected.

Vegetation and Channel Capacity have an Inverse Relationship
Vegetation and Channel Capacity Have an Inverse Relationship. Different combinations and locations of planting within the LA River channel have particular impacts on channel capacity. Whether the planting consists of grasses or trees and shrubs, and whether the planting is on the banks, on the channel bottom, or in the low flow area, are all factors that alter the channel’s ability to convey water effectively. This example shows scenarios for river mile 11.8 near the Rio Hondo Confluence.
OLIN, Geosyntec, 2020
What's At Stake With River Widening
What’s at Stake with Holistic River Widening. River widening requires property acquisition that would displace people, businesses, and infrastructure in the communities adjacent to the LA River. Between 21,000 and 107,000 people might be displaced if the river were widened three to seven times its current width. There would also be major consequences for roads, railways, transmission lines, and other public services.
OLIN, Geosyntec, 2020

In a 5X widening scenario, the number of displaced residents would rise to over 60,000. In a 7X widening scenario, the maximum widening studied, over 106,000 people would be displaced, and nearly 60 miles of freeway, 108 bridges, 90 miles of transmission lines, and 620 critical facilities would need to be relocated.

Freeway construction in LA County displaced a quarter-million people between the 1940s and 1960s. The burden of this displacement disproportionately impacted poor communities and communities of color. It is important to recognize that some channel modification strategies would disturb communities to a similar extent. The period of urban renewal is a sweeping example of how displacement in the name of “projects for the public good” carries a contentious legacy, nationally as well as locally.

While there may be strategic locations in the LA River watershed where channels can be widened into a right-of-way or an acquired mosaic of parcels, a holistic 51-mile restoration strategy is not realistic, even on a generational timeline.

For more information on the hydrology of the LA River watershed or LA River hydraulics, refer to Chapter 3 in Appendix II: Technical Backup Document.

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