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Re‐envisioning stormwater infrastructure for ultrahazardous flooding

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Abstract Ultrahazardous flooding (UHF) occurs on low relief topography at the foot of mountain catchments and is characterized by rapid‐onset, high‐velocity flood flows, large fluxes of sediment and debris, and unpredictable flow paths. 20th century stormwater infrastructure seeks to contain UHF, up to a design level, using combinations of basins, reservoirs and flood control channels. However, these flood control elements may increase the risk of disasters due to: (a) increasingly frequent and intense wildfires that amplify streamflow and debris fluxes beyond infrastructure design capacity; (b) aging and underfunded infrastructure which is susceptible to clogging and failure during extreme events; and (c) expansive urban development where communities are relatively unaware and underprepared for flooding as a consequence of the “levee effect”‐‐the false sense of safety that develops in the presence of flood defenses. 20th century stormwater infrastructure for UHF has also left communities with a legacy of social and environmental challenges including poor water quality, degraded habitats, high maintenance costs, unrealized urban amenities, and altered sediment fluxes. Adopting the Los Angeles Metropolitan Region as a type‐locality for UHF, we propose a new paradigm for stormwater infrastructure based on the concept of erodible flood corridors. Our vision aims for greater sustainability and resilience to extreme events based on congruency with natural processes, conservation of resources and associated ecosystem services, minimization of flood exposure and vulnerability, and avoidance of legacy risk and energy intensive practices. This article is categorized under: Engineering Water > Sustainable Engineering of Water Engineering Water > Planning Water Science of Water > Water Extremes
Low relief topography at the foot of mountains (a) is exposed to ultrahazardous flooding (UHF) characterized by rapid‐onset, high‐velocity flood flows, large fluxes of sediment and debris, and unpredictable flow paths. 20th century stormwater infrastructure (b) maximizes land for development with basins/reservoirs, flood control channels, urban drainage systems and groundwater recharge basins. This approach comes at the expense of riparian and stream habitats, stream health, water quality, altered sediment fluxes, increased risks of disasters, negative downstream impacts, and high management costs for water treatment, groundwater recharge, and sediment removal/disposal
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Conceptualization of stormwater infrastructure for ultrahazardous flooding including (a) reservoirs at the base of mountains with controls on the storage/release of water, sediment and debris, (b) an erodible corridor with hardened banks and a soft core that provides environmental and social benefits in addition to flood control and (c) urban drainage systems with controls to promote water conservation, reuse and water quality
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One hundred and twenty years (from January 1, 1900 to January 1, 2020) of daily average discharge upstream on the Santa Ana River (N = 43,784), in the foothills of the San Bernardino Mountains (USGS Gage 11051500). Over the entire 120‐year period of record, the 1938 March flood stands out as an exceptional event
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The LA Metro region is built on a narrow coastal plain between several mountain ranges and the Pacific Ocean. Image source: Google Earth, 2018, with data from SIO, NOAA, U.S. Navy, NGA, GEBCO, USGS, LDEO‐Columbia, and NSF
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Aerial photograph of La Paz, Mexico, in October, 1976 after precipitation and runoff from Hurricane Liza caused the Arroyo El Cajoncito to avulse through a protective levee, sending fast moving flood water, mud and debris through an urban development. This event epitomizes ultrahazardous flooding (Photo by Harry Merrick)
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McVicker debris basin in Riverside County, California. Coarse sediment and debris settle in the collection area, while flood water and suspended fines pass through a culvert into a flood control channel. A spillway is used to accommodate high flows. Image source: Google, 2018
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Science of Water > Water Extremes
Engineering Water > Planning Water
Engineering Water > Sustainable Engineering of Water

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