Fluvial Erosion and Deposition

The Fluvial Hazard Zone (FHZ) is defined as the area a stream has occupied in recent history, may occupy, or may physically influence as the stream stores and transports water, sediment, and debris.

Photo: The Town of Glen Haven experienced profound damage from fluvial hazards (erosion, sedimentation, and debris accumulation) in the 2013 Colorado Flood. Image provided by the Town of Estes Park.

Until recently, stream and flood management has largely focused solely on water: where it is expected to move during a flood and how to avoid or mitigate flooding through engineering means. However, streams transport more than just water. Streams also gather, store, and move sediment and debris. Most of the time these processes are hardly noticeable but sometimes, especially during a flood event, erosion and deposition can happen rapidly, resulting in movement of the stream channel into new locations, bank retreat, and hill slope failures. This dynamism is innate to a stream. The resulting landscapes created by moving streams, scientists have found, have a positive impact on the ecology and function of a stream corridor through the creation of new channels, wetlands, and riparian habitats.

Historically, local governments have regulated stream corridors by relying on Federal Emergency Management Agency (FEMA) Guidance and Standards to create Flood Insurance Rate Maps (FIRMs) which are used to establish insurance premiums through the National Flood Insurance Program (NFIP). These maps are elevation-based, delineating only inundation hazards, as described on the Flood Page. These maps do not consider stream movement, the erosion of stream banks or hillslopes, or the impacts of sediment and debris deposition. As a result, properties located well above mapped floodplain elevations or outside FIRM floodplain boundaries may be affected by flood processes not accounted for in standard floodplain mapping.

Since 1978, approximately 49% of all NFIP claims in Colorado have come from policies written outside the high-risk area depicted on the FEMA FIRMs. The 2013 Colorado Front Range flood resulted in 52% of flood insurance claims originating outside of regulatory floodplains demonstrating that reliance on flood inundation maps alone does not provide a complete picture of flood hazards.

Fluvial Hazard Zone mapping represents a significant and necessary step forward in identifying and addressing hazards posed by flood events. Flood hazard reduction, in the long-term, will be measured primarily by our ability to solve problems at the watershed and stream corridor scale, and secondarily by how we resolve conflicts at individual sites. From a planning standpoint, this means communities should recognize that streams are naturally dynamic systems prone to move and that sediment and debris should be accounted for when decisions are made.

Because streams and waterways do not follow political boundaries, preparation for flood-related fluvial hazards requires individual, local, regional, state and federal partnerships that can work across jurisdictional boundaries in watersheds to identify these areas and develop management policies that reduce long-term threats to life and property.

Figure: The boundaries of the Fluvial Hazard Zone (FHZ) and Federal Emergency Management Agency (FEMA) floodplain generally will not coincide and should be considered independent, but complementary, of one another. Using both types of maps is critical to understanding the suite of hazards that exist in stream corridors and when taken collectively, could be used to delineate a stream corridor within which special floodplain considerations may exist. In some instances, the Fluvial Hazard Zone will be smaller than the FEMA floodplain (left) and on others the Fluvial Hazard Zone will be larger than the FEMA floodplain (right). 

Flooding is one of the three central components (along with drought and wildfire) of a complex system of interrelated natural hazards that are fundamentally tied to Colorado’s continental semi-arid climate. Drought conditions may lead to soil compaction, and wildfires may leave slopes denuded and hydrophobic (unable to absorb water). In these cases a single heavy rain event can lead to higher volumes of runoff and a correspondingly higher risk for fluvial processes of erosion and sedimentation.

In addition to the direct impacts a flood event hazard may cause, it can also trigger multiple cascading hazard events. Rising floodwaters may cause the failure of a dam, levee, or other impoundment structure resulting in the rapid inundation of locations outside of mapped special flood hazard areas.