Possible Points: 150 points for the element.

Purpose of Element: Some communities are subject to additional natural hazards that exacerbate the effects of or cause flooding. For example, coastal communities in the Pacific Northwest region, central and southern California, Alaska, Hawaii, and the South Pacific are vulnerable to tsunamis. Inland communities can also be vulnerable to flood-related hazards like ice jams, mudflows, and land subsidence. In communities that are subject to special flood-related hazards, open space preservation can be a vital tool for preventing future flood damages. As a result, the CRS rewards communities that preserve areas that are subject to flood-related hazards as open space. In the table below is the full list of hazards credited under this CRS element.

Communities rarely receive credit for preserving open spaces that are impacted by hazards other than channel migration. As a result, this element profile will limit its discussion to the requirements and benefits of this element as they pertain to channel migration. For more information, reference page 420-19 of the CRS Coordinator’s Manual.

Special Flood Related Hazard Hazard Definition and  Associated Vulnerabilities
Tsunamis A Tsunami is defined as "set of ocean waves caused by any large, abrupt disturbance of the sea-surface" (NOAA, no date). Typically this so called "abrupt disruption" is caused by powerful earthquakes. The major risks associated with tsunamis include loss of life and property due to magnitude and strength of its waves.
Ice Jam Flooding Ice jams occur when the water level within a completely or partially frozen river or stream rises and breaks up the ice, creating an ice floe that piles up behind a blockage, and subsequently damming the channel (Perry, 2000). The movement of ice floes can damage channel-adjacent structures. In addition, ice jams can result in or exacerbate upstream flooding (Perry, 2000).
Closed Basin Lake Flooding Lakes levels in lakes without outlets can change rapidly and remain elevated for extended periods of time causing localized flooding.
Mudflows Mudflows are "are rivers of rock, earth, and other debris saturated with water. They occur when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snowmelt, changing the earth into a flowing river of mud" (DHS, no date; FEMA, 2013). Mudflows move rapidly and can travel several miles from their sources. Loss of life and property are the most common risks associated with mudflows, which can pick up large objects such as trees and cars if they have enough energy (FEMA, 2013).
Land Subsidence/ Sinkholes Sporadic (sinkholes) or gradual collapse of the land surface as a result of the "subsurface movement of earth materials" (USGS, 2000). A report by the USGS estimates that 80% of the identified land subsidence in the U.S. is caused by humans (USGS, 2000; Galloway et al., 1999). Land subsidence can increase flood risk especially in low-lying areas. In coastal communities, land subsidence can exacerbate the effects of sea level rise. While in riverine areas, land subsidence can "alter the topographic gradient that drives the flow of the river" which can contribute to flooding (Eggleston and Pope, 2013).
Alluvial Fan Flooding Alluvial fans are commonly found in the western, mountainous region of the United States (Conrad et al., 2012). These geologic formations are formed when floodwaters move through steep canyon channels picking debris along the way (Conrad et al., 2012). When these channels open up and spread out, the water within them spreads out in a fan-like fashion, dropping the debris it was carrying along the way (Conrad et al., 2012; USGS, 2016). Alluvial fan flooding can be very powerful and is associated with erosion, scour, channel migration, and debris and mudflows (Conrad et al., 2012). Loss of life and property are the greatest risks to fan-adjacent communities.
Stream Channel Migration, Erosion, and Deposition Channel migration is a by-product of two inherently linked in-stream processes; erosion and deposition. Rivers and streams are constantly moving sediment. Areas in which sediment from the streambed and channel walls is lost are said to be eroding. Conversely, areas of the stream in which sediment is accumulating are said to be subject to deposition. Over time this movement of sediment can cause the channel to migrate laterally (Legg and Olson, 2014; "Shoreline Management," no date). Channel migration can occur very slowly or abruptly (Legg and Olson, 2014; "Shoreline Management," no date). Migrating channels can damage adjacent structures and put their owners at risk.

Impact Adjustment: Yes. The impact adjustment for this element is calculated differently for each special flood related hazard. For specific information on how to calculate impact adjustments for this element, reference the CRS Coordinator’s Manual.

Potential to Double Count Credit: Yes. The impact adjustment for this element is calculated by taking the ratio of the area of land that is prone to one of the above listed special flood-related hazards to the area of the Special Flood Hazard Area. For specific information on how to calculate the impact adjustment for this element, reference the CRS Coordinator’s Manual.

Degree of Difficulty - Documentation: Low. The required documentation for this element is minimal. A community must assemble copies of some of its ordinances, create an impact adjustment map of areas that are vulnerable to special flood-related hazards, and provide evidence that credited areas meet the requirements of element 422.a. Open Space Preservation.

Degree of Difficulty - Implementation: High. In order to obtain credit for this element a community must develop and adopt a map designating the channel migration zone that will be protected as open space. Often times a licensed geologist, surveyor, or engineer with experience in hydrologic modeling is required in order to create these maps. Many communities will not have staff capable of creating these maps alone and as a result will need to hire a contractor to assist with this task. As a result, the amount of effort associated with earning credit for this element is high.

Flooded Puyallup River in Pierce County, WA. Image courtesy of Dennis Dixon, CFM; Pierce County Public Works – Surface Water Management.
Flooded Puyallup River in Pierce County, WA. Image courtesy of Dennis Dixon, CFM; Pierce County Public Works – Surface Water Management.

Tip for Success:

  1. Contact your state’s National Flood Insurance Program Coordinator or Floodplain Manager to determine if your state maps channel migration zones and/or other flood hazard areas. For example, the state of Montana has mapped several major channel migration zones. While other states, like Washington, have laws in place that require communities to study channel migration zones. Because mapping channel migration zones and other flood hazard areas is a prerequisite for earning CRS credit under this element, communities in which these maps have already been created are at a significant advantage.

Co-Benefits Associated with this Element: Preserving high-risk areas as open space has several obvious benefits including reducing loss of life, property, and infrastructure when flooding occurs (Legg and Olson, 2014). Preserving land in high risk areas can also help to protect the integrity of the physical processes and defining characteristics of inland waterways and water bodies. For example, protecting open space adjacent to channels that are known to migrate allows rivers to function as they would have prior to development, creates diverse habitats for fish and wildlife, improves water quality, and protects the historic floodplain of the river (Montana Aquatic Resources Services, no date; Legg and Olson, 2014).