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Erosion and Avulsion Hazard Mapping and Methodologies for Use in the Nooksack River Channel Migration Zone Mapping

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Erosion and Avulsion Hazard Mapping and Methodologies for use in the Nooksack River Channel Migration Zone Mapping By Paul Pittman LEG, Whatcom County Public Works and Peter Gill, Whatcom County Planning and Development Services Final Internal Draft th September 15 , 2009 5/26/2015 - 0 - Erosion and Avulsion Hazard Mapping and Methodologies for use in the Nooksack River Channel Migration Zone Mapping 1.0 Purpose The purpose of mapping the erosion and avulsion hazards of the Nooksack River was necessitated by both riverine hazard planning through the Comprehensive Flood Hazard Management Plan and recent updating of the Shoreline Master Plan and Critical Areas Ordinance. The intent of this study is to provide a technical background document and set of maps to help guide decision makers in adopting a channel migration zone and developing comprehensive flood hazard and ecological planning. The combination of the historic, erosion and avulsion hazard mapping can be the foundation for delineating Channel Migration Zones (CMZ) (Rapp et al, 2003). 1.1 Background The Shoreline Master Plan (SMP) and Critical Areas Ordinance (CAO) updates of 2004 and 2005 are the catalyst for mapping a Channel Migration Zone (CMZ). However, the concept of a CMZ has its roots in multiple management plans, acts, and regulations: • Shorelines Management Act (SMA): “Applicable shoreline master programs should include provisions to limit development and shoreline modifications that would result in interference with the process of channel migration that may cause significant adverse impacts to property or public improvements and or result in a net loss of ecological functions associated with the rivers and streams” (Chapter 173-26 WAC, 58). • Growth Management Act (GMA): Critical Areas - RCW. 36.70A.030(5). • ESA: Limit 12 of the 4(d) Rule requires the delineation of a CMZ. • National Flood Insurance Act (NFIA) supports the delineation of a CMZ to manage flood hazards and reduce flood damages. Department of Ecology encourages development of a “meander belt” delineation in Comprehensive Flood Hazard Management Plans. The Whatcom County Lower Nooksack River Comprehensive Management Plan (1999) states that the Channel Migration Limits will be mapped. 1.2 Definitions The “historic migration zone” is a composite of the historic locations of the river as determined from historic information and interpretation, over some length of historic record. The term “erosion hazard” in this analysis refers to the hazards resulting from the natural process of lateral migration of a channel through bank erosion that occurs from channel expansion, channel meandering, channel course changes, or channel bank and fluvially related slope failures. 5/26/2015 - 1 - The term “avulsion hazard” describes a multiple set of hazards associated with rapid channel course changes or temporary channelization of flow that in addition to having flooding hazards, instantaneously becomes an erosion hazard. The combination of the historic migration zone, erosion hazard area and avulsion hazard zone, in general terms, is “the geographic area where a stream or river has been and will be susceptible to channel erosion and/or channel occupation” (Rapp et al, 2003). The State of Washington, in [WAC 173-26- 221(2)(c)(iv)(3)(b)] describes this concept further as: “The dynamic physical processes of rivers, including the movement of water, sediment and wood, cause the river channel in some areas to move laterally, or "migrate," over time. This is a natural process in response to gravity and topography and allows the river to release energy and distribute its sediment load. The area within which a river channel is likely to move over a period of time is referred to as the channel migration zone (CMZ) or the meander belt.” The State further establishes recommendations on how to delineate the historic and hazard areas in [WAC 173-26-221(2)(c)(iv)(C)(3)(b)]: “For management purposes, the extent of likely migration along a stream reach can be identified using evidence of active stream channel movement over the past one hundred years. Evidence of active movement can be provided from historic and current aerial photos and maps and may require field analysis of specific channel and valley bottom characteristics in some cases. A time frame of one hundred years was chosen because aerial photos, maps and field evidence can be used to evaluate movement in this time frame.” In addition to the State recommendation of using the 100-year migration potential, FEMA also recommends developing channel migration zones that predict 100-year migration potential. The CMZ can be used to plan for or assess public safety (risk to life/property), economic costs (cost – benefit), and ecological function (salmon recovery). The CMZ can also be used for regulatory purposes (CAO and SMP) to lessen future risk. The State describes this concept in [WAC 173-26- 221(2)(c)(iv)(C)(3)(b)]: “Scientific examination as well as experience has demonstrated that interference with this natural process often has unintended consequences for human users of the river and its valley such as increased or changed flood, sedimentation and erosion patterns. It also has adverse effects on fish and wildlife through loss of critical habitat for river and riparian dependent species. Failing to recognize the process often leads to damage to, or loss of, structures and threats to life safety.” 2.0 Hazards mapped on the Nooksack River 2.1 General Methodology The Washington State Department of Ecology Publication (#03-06-027) “A Framework for Delineating Channel Migration Zones” was the general guideline used to develop the Historic Migration Zone (HMZ), Erosion Hazard Area (EHA), 5/26/2015 - 2 - and Avulsion Hazard Zone (AHZ) areas. A summary of DOE Methodology used to delineate Channel Migration Zones (CMZ) follows: CMZ = (HMZ + EHA + AHZ) – DMA The HMZ (Historic Migration Zone) is created using historical spatial information (maps, air photos, survey records). Whatcom County has this data going back to 1880’s. The HMZ for the Nooksack River was mapped (Collins and Sheikh, 2004; Appendix A) and modified by Whatcom County staff. The EHA (Erosion Hazard Area) is predicted horizontal channel migration potential through fluvial erosion moving laterally by eroding bank material (Figure 2-1). Lateral erosion is not necessarily limited to the floodplain or areas inundated during the 100-year flood event. For this study the fluvial-related geotechnical hazards are incorporated into the EHA, but are shown as an overlay on the maps. The AHZ (Avulsion Hazard Zones) are predicted locations of rapid channel change by capture of a relict channel or topographic low within the floodplain, or of temporary channel-like conditions that can exist and create erosion hazards (for example, a levee break). The erosion and avulsion hazards analysis “takes into account trends in channel movement, context of disturbance history and changes in boundary conditions, as well as topography, bank erodibility, hydrology, sediment supply and woody debris loading” (Rapp et al, 2003). It predicts possible hazard areas based on existing and historic process observations. This document incorporates the information from the Historic Channel Locations of the Nooksack River (Collins, et al, 2004; Appendix A) to complete the background scientific and historical mapping efforts as a component in mapping a CMZ or as a stand-alone document to assess erosion and avulsion hazards. The hazards mapped in this analysis do not suggest a level of acceptable risk. The mapping should be considered a coarse, base level assessment. Site-specific analysis should be performed for site level projects or more detailed assessment needs. 2.2 Study Area The study area includes the Nooksack Valley and all areas potentially impacted by or adjacent to the Nooksack River channel migration (Figure 2-2 - vicinity and study area). 2.3 Base maps and existing information Information, mapping and documented studies were consulted whenever possible in order to build upon existing information. In particular, the Historic Migration Zone mapping effort was conducted by Collins and Sheikh and is presented in Appendix A. Changes to the maps or methods presented in our document may be necessary as new information becomes available. 5/26/2015 - 3 - 2.4 Mapping Efforts Paul Pittman of Whatcom County Public Works River and Flood took the lead on mapping the EHA and AHZs of the Nooksack River. Informal technical review and assistance was provided by: Tim Hyatt and Alan Soicher, LEG, of Nooksack Tribe Natural Resources; Michael Maudlin, LEG of Lummi Nation Natural Resources; John Thompson, LEG and Steve Fox, LG of Whatcom County Public Works; Doug Goldthorp, LEG, LHG of Whatcom County Planning and Development Services; Roger Nichols, LEG of the United States Forest Service; Patrica Olsen, LHG of the Department of Ecology; and Barry Wenger of the Department of Ecology. Oversight, review, and mapping assistance was provided by: Jeff Chalfant, of Whatcom County Planning and Development Services (SMP Update Coordinator); and Peter Gill of Whatcom County Planning and Development Services; Paula Cooper, PE of Whatcom County Public Works; the Whatcom County Flood Control Zone District Advisory Committee (Land Use/Meander Subcommittee). Input, comments and local knowledge from the community was also gathered at public meetings. 2.5 Limitations of this study Flood hazards have historically been focused on rising water levels resulting from overbank flooding.
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