Olsson Associates


A review of fluvial erosion hazard setback methodologies and criteria

Wednesday, June 21, 2017

Bill Spitz, Industry Expert










The epic floods of August 2011 in Vermont, and September 2013 in Colorado, reminded people how quickly rivers and streams can change and morph into extreme storm events. Approximately half of the private structure damages and losses experienced in the 2013 Colorado flood were located outside of the regulatory floodplain, or Special Flood Hazard Area, designated by the Federal Emergency Management Agency. These flood-related risks associated with streambank erosion, sediment deposition, channel degradation, lateral channel migration, and avulsion (i.e., the rapid abandonment of a river channel and the formation of a new river channel) created disastrous outcomes in Vermont and Colorado. Those outcomes may recur in future flood events in those states and elsewhere. In the following discussion of fluvial erosion and hazards, “fluvial” refers to the processes associated with rivers and streams and the deposits and landforms created by them.

The identification of Fluvial Hazard Zones has become a high priority in several states as they recover from major floods and transition toward long-term river corridor planning. Planning for erosion hazards is an essential component of effective river corridor management and the prevention of future flood damages. Broadly defined, the Fluvial Hazard Zone (FHZ), Channel Migration Zone (CMZ), or Erosion Hazard Zone (EHZ) 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 FHZ/CMZ/EHZ is to identify lands most vulnerable to fluvial hazards in the near term.

Numerous state, county, and municipal governments have developed fluvial erosion hazard zone or channel migration zone setback criteria which have been documented by myself and others in “Fluvial Erosion Hazard Area Regulatory Guidelines Memorandum” and submitted to the Colorado Water Conservation Board. Additional documentation is provided in the “Riverine Erosion Hazards Discussion Paper” published by the Association of State Floodplain Managers Riverine Erosion Hazards Working Group. Setback criteria and methodologies have been developed in numerous locations including Albuquerque and Santa Fe, New Mexico, El Paso County, Colorado, and Maricopa County, Arizona. I have used these criteria and methodologies to delineate erosion hazard setbacks for several communities in Texas, New Mexico, Colorado, and Arizona.

As an example, I worked with the City of Austin Watershed Protection Department to develop an Erosion Hazard Zone criteria manual for the City of Austin, Texas, to control streamside development in areas that may encompass or lie outside a regulatory floodplain, but which may still face a significant risk from unstable or potentially unstable streams.

An Erosion Hazard Zone (EHZ) is a setback, or erosion risk boundary, within which development is not allowed. The methodologies described in the manual provide a quantitative approach to address the City of Austin’s requirements for property protection due to erosion. The EHZ criteria manual provides procedures for a two-level approach that is practical, yet scientifically supportable and includes comprehensible analysis procedures. Consideration has been given to both local conditions and the impacts of urbanization in the methodology. 

In addition, the Vermont Agency of Natural Resources uses FHZ delineations as a primary tool in its avoidance strategy to restore and protect the natural values of rivers and minimize flood damage. Washington state uses CMZ delineations to predict areas at risk for future channel erosion due to fluvial processes. FMZ and CMZ delineations help reduce risks to human communities by guiding development in and along river systems away from such areas. Limiting development within FHZs/CMZs also reduces the costs of repairing or replacing infrastructure and major civil works that might otherwise be threatened or damaged by channel migration.

Additionally, FHZ/CMZ delineations can provide guidance in reducing degradation and loss of critical aquatic and riparian habitats, helping assure that fluvial processes are accommodated and that the river landscape is not permanently degraded or disconnected from the river by development. The state of Colorado is currently developing protocols for delineating FHZ’s appropriate for the wide range of landscapes that are present in the state (Jagt et al. 2016).

In addition to providing services in fluvial geomorphology, I also work closely with our Water Resources, Transportation, and Environmental teams on a variety of projects involving stream channel and streambank stabilization, restoration, and rehabilitation as well as restoration, enhancement, and management of streamside and aquatic habitat.

If you have any questions, please call me at 970.635.3713 or email me at wspitz@olssonassociates.com

William Spitz - Industry Expert

William Spitz, PG

Senior Geomorphologist

Bill has 32 years of experience as a fluvial geomorphologist. He’s conducted extensive geomorphic analyses on fluvial systems throughout the United States. He has also co-authored publications related to highway research and stream stabilization.

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