FLUVIAL EROSION HAZARD MAPPING GAINS MOMENTUM IN COLORADO

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Title: FLUVIAL EROSION HAZARD MAPPING GAINS MOMENTUM IN COLORADO

Author: Michael Blazewicz

Post Date: 3/25/2016

Contributed by Michael Blazewicz, Round River Design, LLC

For most early settlements in Colorado, the benefits of settling near a river to serve as a source of drinking water, irrigation, power, mineral access, and navigation often outweighed the risk posed by flooding. Over the past century, settlements alongside Colorado’s waterways have grown into large communities that have invested billions of dollars in infrastructure on or near rivers and streams. Despite warnings and destruction from periodic floods, development in stream corridors continues to grow and damages to infrastructure and economies continue to climb. In addition to thousands of highway and utility structures, the Colorado Water Conservation Board (CWCB) estimates that approximately 65,000 homes and 15,000 commercial and industrial business structures are located in Colorado’s floodplains.

The reaction to historic floods has typically been to straighten, dredge, armor, and levee streams, creeks, and rivers in an attempt to control their movement. In interim years, docile streams create a sense of security, allowing new development to creep into erodible river corridors.

During the September 2013 flooding, flood-related damage outside of the 100-year FEMA floodplain were observed throughout the Colorado Front Range in spite of many peak discharge estimates having magnitudes less than or equal to the 100-year flood. Indeed, nationally, nearly 25 percent of flood insurance claims come from areas outside the mapped 100-year floodplain.

Of particular concern for many riparian communities is that FEMA’s National Flood Insurance Program (NFIP) maps are elevation-based, and therefore only delineate flood inundation hazards by applying a water surface elevation-based standard (i.e., the 100- and 500-year base flood elevations). The hazard depicted in these maps is always approximate because of uncertainty in topographic data and flow frequency estimates. Moreover, the traditional method of mapping the floodplain assumes clear water conditions, ignoring the influences of sediment, ice, and debris transport and disregarding the fundamental, albeit complex, mechanics of physical stream change – an inherent component of these systems. NFIP maps are particularly inaccurate in Colorado’s steeper mountain valleys which do not lend themselves to precise water surface elevation modeling exercises, and where debris flows and hillslope erosion may create channel blockages. While NFIP maps are an excellent planning tool, they ultimately only represent a static, and in some cases limited, characterization of dynamic systems. This statement is evidenced by the extensive flood damage we continually observe both in Colorado and nationally to structures built outside of these mapped floodplains.

As many reading this blog already know, river change occurs as a result of the interaction among incoming flow and sediment, channel geometry, and the relative resistance of the channel boundaries. River morphology change occurs under natural conditions and can also be influenced, and in some cases exacerbated, by human actions directly on a river as well as indirectly through flow regulation and land-use change. Fluvial (river-related) erosion is an on-going natural process and even “stable” rivers shift from year to year, especially during annual or sub-annual runoff events. For human communities, this erosion can become problematic during a flood event, which may cause excessive erosion of the streambed, banks, and hillslopes or wholesale change in the location of the channel via avulsion. Fluvial erosion becomes a fluvial hazard when an adjusting stream channel threatens public infrastructure, houses, businesses, property, and other private investments.

Though recognized at the federal and increasingly local levels, fluvial erosion hazards are under-studied, under-publicized, and often not planned for or mitigated.  Broadly defined, the fluvial hazard zone (FHZ) is the area a stream has occupied in recent history, could occupy, or could physically influence as it stores and transports sediment and debris during flood events. The objective of a mapped fluvial hazard zone is to identify lands most vulnerable to fluvial hazards in the near term. 

Because of the significance of fluvial hazards, the Colorado Water Conservation Board commissioned a study in late 2015 to develop a draft protocol for mapping fluvial erosion hazard zones.  This effort is now available on CWCB’s website in draft form: http://coloradohazardmapping.com/

CWCB and the authors feel strongly that identification (through mapping) and then management of these additional flood hazards can aid in reducing flood damage to vulnerable public and private infrastructure – reducing the cost and duration of future flood recovery efforts. The FHZ delineation process and the maps it produces are intended to provide local land use and floodplain managers insight into the likely long-term behavior of their streams and serve as additional flood hazard information. They may also serve as supplemental information to landowners who may not be aware of these additional flood hazards and whose improvements may not be appropriately insured or protected. Combining flood elevation maps with fluvial hazard maps may help mitigate these hazards by guiding development, moving or protecting critical infrastructure, and encouraging long-term conversion of active river corridors. In the long term, protection of these areas should result in concomitant gains in improved water quality, recreation opportunities, and the ever-critical role of functioning riparian and floodplain ecosystems.

This summer, CWCB hopes to conduct some pilot mapping studies to further test and refine the draft protocols with the hopes of eventually expanding the mapping project to willing communities across the state.