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Diversions include elements such as tunnels, pipes, pumps, and weirs. These components remove wet or dry weather flows from the river to increase overall system capacity during larger storm events and when flows are reduced and/or composed with treated wastewater and/or other non-stormwater urban flows. These diversions can also be used to treat and reuse water for other benefits.
Historically, water flow in the LA River has varied greatly based on seasonal rainfall and groundwater conditions, and diversions for flooding and irrigation were common. Today, water flows in the LA River are highly engineered with dams, reservoirs, and spreading grounds regulating wet weather events, while dry weather flows consist mostly of treated wastewater discharged from water reclamation plants.
Trapezoidal Channel Cross Section
Box Channel Cross Section
In addition to reducing flood risk and benefiting local water supply reliability, diversions can also provide opportunities for treatment and reuse of water for groundwater recharge, habitat features, or recreational opportunities during smaller storm events, or in the dry season when flows are reduced.
Any modification to the LA River channel or its water flow requires hydraulic analysis to ensure flood risk is not increased and to consider the impacts of altering the flow rate on other uses of the water such as ecosystem function. Diversions that can utilize nature-based solutions, such as a vegetated side channel, are ideal. Where gray infrastructure, such as pipes or tunnels, are needed to meet needs, designs should balance the negative impacts of adding concrete or underground infrastructure with the benefits afforded by the design. In some cases, such as the Narrows, a pipe or tunnel may allow a section of the river to have riparian vegetation while larger flood flows bypass the area in a pipe and manage flood risk.