Planning the Fluvial Future: An Intro to Fluvial Hazard Zone Maps and considerations for Incorporating Geomorphology Into Stream Corridor Planning

River Network – Stream Management Planning: 8/5/21 • Kevin Houck and Chris Sturm, CO Water Conservation Board • Michael Blazewicz, Round Design • Katie Jagt, Watershed Science and Design • Joel Sholtes, University of Colorado, WASH Engineering

www.ColoradoFHZ.com

TA K E H O M E :

• Streams are dynamic, they require space

• Streams are corridors, not lines

• Fluvial Hazard Zone maps provide a cost effective w a y t o define the space streams may occupy and influence

• Identifying fluvial geomorphic processes can help communities plan with land use decisions in their stream corridors. (Fluvial Geomorphology = the study of how moving water shapes the earth) .

Flooding is the most frequent severe weather threat and the costliest natural disaster facing the nation. Ninety percent of all natural How do we disasters in the U.S. involve flooding. better capture Nationally, nearly 25% of flood insurance this existing risk claims come from areas outside of the and create a regulatory (100-year . more complete understanding From 2014 to 2018, policyholders outside of high-risk flood areas filed over 40 percent of of stream- all NFIP flood insurance claims and required related hazards? one-third of federal disaster assistance for flooding.

In Colorado, the figure is approximate 51% from the 2013 event alone, and 57% cumulatively, since 1978. Large berm that appears Berms in field to cut-off to have been used in the historic flow paths modeling to “dry up” the land to the south.

Alluvial fan The highway, with one undersized crossing - a de-facto dam.

• The First Street Foundation Flood Model (i.e., Flood Factor) is a nationwide, probabilistic flood model that shows any location’s risk of flooding from rain, , tides, and storm surge. It builds off decades of peer-reviewed research and forecasts how flood risks will change over time due to changes in the environment. First Street’s data, and other open-source datasets, can serve as important complements to existing sources of flood risk data such as FEMA maps. FEMA maps are designed to set insurance rates and for use by decision-makers, and technical experts—not as communication tools for the general public to understand actual flood risk. This often leaves average Americans navigating the alphabet soup of AE, X, and V zones and having a difficult time understanding how concerned they should be about flood risk. FEMA maps can also create a false sense that flood risk is binary by focusing on whether a property is “inside” or “outside” of a flood zone. Because this designation is binary, it does not provide any indication of risk magnitude for individual properties. This communication challenge is compounded by the fact that FEMA’s maps significantly underestimate risk. As discussed in the section on Fluvial Hazard Zones in this report, FEMA maps assume stationarity in watershed hydrology, dimensions and location, and fail to account for a host of dynamic processes inherent in fluvial systems, not to mention they are often outdated. This can lead to substantial consequences for property owners. • ↓Flood Factor map of the South Arkansas River corridor shows a possible inundation flood risk to an area that correlates well with the physical and ecological indicators that were used to define the Active Stream Corridor.

Flood Insurance Rate Maps vs. Fluvial Hazard Maps Flood Insurance Rate Maps (FIRM) vs. Fluvial Hazard Zone (FHZ) Maps

FEMA : FHZ Mapping: Captures Captures Hazards from Hazards from and Inundation Deposition Fluvial Hazard Zone

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

The primary objective of mapping the FHZ is to identify areas vulnerable to fluvial geomorphic hazards, characterize these hazards, and reduce risk to life and property through increased awareness, long-term avoidance, and mitigation. Additional Objectives of Fluvial Hazard Mapping

• Increase awareness of natural stream processes.

• Support stream planning and management on a watershed scale.

• Support long-term restoration of stream function and floodplain connection.

• Strategize opportunities for watershed-scale sediment and debris management (FOZ).

• Preserve the multitude of benefits provided by open space along stream corridors. How is FHZ mapping completed?

• Guidance provided by the Colorado Water Conservation Board (CWCB). • Methods are peer-reviewed and supported by scientific studies. • Draws on the current science related to hazard and stream management. • Mapping has been completed through out the Front Range, Denver-Metro area, and state. • For more info and FAQs, visit the website!

www.ColoradoFHZ.com Primary FHZ components

Active Stream Corridor Fluvial Hazard Buffer Lands subject to channel migration and Erosion-prone land beyond the Active Stream movement, erosion, floodplain scour and Corridor that may be susceptible to future slope deposition. It reflects both the past and the future locations where these processes may occur. failures due to toe erosion. The Active Stream Corridor is generally delineated The Fluvial Hazard Buffer is calculated based on using hydraulic, geomorphic, geologic, and biotic hydrologic and geologic characteristics of the stream data. and an analysis of historic valley and terrace erosion throughout the Colorado Piedmont. Primary Benefit: Hazard Reduction Life and Property • Provide more accurate assessments of flood hazards.

• Defensible delineations for site development and use in land planning.

• Assists in evacuation and emergency planning.

• Reduce reliance on channelization, , and bank armoring, thus decreasing the need for capital investments. Secondary Benefit: Resilient Infrastructure

• Identify at-risk infrastructure and critical facilities within FHZ

• Avoid FHZ when repairing and replacing grey infrastructure

• Site new infrastructure in less hazardous areas within river corridor

• Reduce maintenance and repair costs Secondary Benefits: Wildfire

• Provide space for erosion and sediment deposition after a wildfire.

• Provide natural fire breaks possibly aiding a community's firefighting response.

Photo: Joe Wheaton Planning the Fluvial Future: Incorporate FHZ Concepts into Community Planning

• Comprehensive (or master) plan • Local hazard mitigation plans • Pre-disaster recovery plans • Emergency response plans • Emergency response planning • Wildfire planning • Parks and open space plans • Capital improvement plans • Drainage and stormwater; infrastructure/transportation; stream corridor; water resource planning; stream management plans Incentivize Development Outside of FHZ

• Development agreements (e.g., density bonuses) • Cluster subdivisions • Transfer of development rights • Conservation easements (e.g., stream corridor easement) • Land acquisition Consider • FHZ overlay zoning Incorporation into • Comprehensive recovery ordinances Regulatory • Community rating system (CRS) Mechanisms • Development application submittal requirements • Post-disaster moratorium Enhance Local Administration & Procedures • Development application submittal requirements • Post-disaster moratorium

Aftermath of 2013 flood in Jamestown, CO. Source - Michael Rieger, FEMA Promote Actions to Mitigate Hazards

• No Adverse Impact Standards • Identify critical areas • Provide opportunities for relocation or buyouts Create Opportunities to Inform Community • Provide the FHZ as an information layer on NFIP maps • Incorporate FHZ’s into utility connections and septic permit reviews • Encourage the purchase of flood insurance in the FHZ • Incorporate fluvial hazard information onto community websites • Incorporate and hazards into school education • Partner with local watershed organizations

Defining Stream Corridors in Fluvial Hazard Mapping

Energy Flow and Dissipation Sediment Continuity ) 2

Stream Power and W

Geomorphic post

Response to Floods / W pre Unit Stream Power StreamUnit Power (W/m Distance (km)

The Active Stream Corridor (ASC) is land that has been or may be shaped by erosion and deposition under a range of conditions in the prevailing The Active Stream Corridor (ASC) flow and sediment regimes (i.e., the contemporary geomorphic floodplain). Dominant processes within this corridor include channel incision, widening, avulsion, lateral and downstream migration, aggradation, and braiding. These processes manifest over a range of time and space. The geographical extent of the riverscape is defined by the boundaries of the ASC.

← Conceptual Fluvial Hazards Map of the South Arkansas Corridor. These draft maps were developed for planning purposes only and were delineated with topographic information developed from 2011 LiDAR. The Fluvial Hazard Zone consists of two primary components: the Active Stream Corridor (described above) and the Fluvial Hazard Buffer. The Fluvial Hazard Buffer accounts for erosion prone land located beyond the Active Stream Corridor, such as hillslopes and terraces, that may be susceptible to slope failure as a result of toe erosion caused by fluvial scour. It is a buffer applied to the outer boundary of the ASC. Avulsion Hazard Zones are used to mark the area a stream may occupy and impact due to a wholesale shift in channel position on the valley floor as further defined and detailed in the Colorado FHZ Protocol. ←

See sheet 26 for a description of the relative elevation map Where to Prioritize FHZ In Your Community?

• Start with the major where LiDAR exists (public lands prob not high priority – depends) • Look at the areas where there is no FEMA mapping or where FEMA map looks highly inaccurate (and has some development pressure) - that way the FHZ can fill a full gap in knowledge rather than just supplement a tool that, while imperfect, is still out there and part of the planning process. • Get in front of development-driven floodplain modeling which can seek to minimize space for the stream. • Fully-map a few pilot reaches as demonstrations – rapid map other priority reaches as budget allows. Resources www.ColoradoFHZ.com CWCB Incentives and Support for Local FHZ Programs and Mapping The CWCB is committed to assisting communities that wish to map and acknowledge FHZs through the following actions: • Providing technical and regulatory advisory assistance to communities that wish to map and manage FHZs. • Allowing FHZ mapping and programs to be considered for competitively awarded grant funding with a 1:1 match requirement via the Colorado Watershed Restoration Grant Program. • Providing resources to and partnering with organizations and local agencies that are implementing Stream Corridor Easements. TA K E H O M E :

• Streams are dynamic, they require space

• Streams are corridors, not lines

• Fluvial Hazard Zone maps provide a cost effective w a y t o define the space streams may occupy and influence

• Identifying fluvial geomorphic processes can help communities plan with land use decisions in their stream corridors. (Fluvial Geomorphology = the study of how moving water shapes the earth) “The lingering nineteenth-century belief that the Earth changes only slowly has lulled us into thinking that it is impassive and eternal, that nothing we do could alter it significantly. That notion has also caused us to view the Earth’s intermittent adjustments—the creation of a new volcanic island, a magnitude 9 earthquake—as aberrations, when in fact these events are business as usual for the planet.

We are big enough now to scratch and dent the Earth, scar, and abrade it, but we ourselves will have to live with the damage. Earth, meanwhile, will continue to make slow repairs, punctuated by sudden renovation projects that will clear away our proudest constructions.” Sand Creek Story Map

• https://storymaps.arcgis.com/stories/0892249499894dc19a8726c41 45ddaf9

GEOMORPHICALLY SIGNIFICANT FLOWS

• An important part of river function is the movement of sediment through a river corridor and mobilization of the sediments on the channel bed.

• We did assessments to look at how much flow is needed in the Rio Grande to pick up and move the bed material and how frequently these flows occur.

• We also assessed what flows are necessary to inundate the stream’s floodplains and how frequently these occur. Rivers adjust their shape and composition in response to the At larger scales, the mobilization and deposition of bed sediments sediment and water supplied from the watershed and adjacent creates and maintains pools and . Over the long term, hillslopes and channel banks. The movement of sediment on a changes in the bed surface caused by the mobility of the sediment streambed affects instream and riparian habitat at various on the bed are necessary to maintain habitat quality in river scales: At smaller scales, a lack of bed mobility may allow the systems. Evacuating fine sediment from pools and the deposition buildup of fine particles such as sands and silts in the interstitial spaces between larger grains of sediment such as cobbles and of coarse sediment on bars may increase the quality and quantity gravels. These interstitial spaces are important for fish species of habitat used for spawning and rearing. Conversely, a lack of but also for key components to the food web such as algae, flows that trigger bed mobility will tend to cause either long-term zooplankton, phytoplankton, and macroinvertebrates. scour or aggradation and tends to simplify the channel, reduce bedform variability, and homogenize aquatic and riparian habitat. Riparian vegetation establishment and succession is dependent upon the mobilization and deposition of sediment within the stream corridor. In this reach, a flow of FLOODPLAIN ACTIVATION THRESHOLD approximately 5,600- MOBILITY THRESHOLD 6,000 cfs is necessary to activate the floodplain.

A flow of approximately 4,200-5,600 cfs is necessary to mobilize the bed. 08221500 2,500 RIO GRANDE RIVER NEAR MONTE VISTA, CO According to the analysis of historic flows, bed mobilizing flows are present in the channel at the Monte Vista 2,000 gauge for approximately 30 days during Average years and for 55 days during Wet years 1,500 and never in dry years.

However, if significant flow MOBILITY THRESHOLD (greater than approximately 1,000 200cfs) is diverted out of the channel between the gauge and the State Wildlife Area AVERAGE DAILY STREAMFLOW (CFS) 500 during peak of runoff in Average years, these flows will no longer have the strength to mobilize the channel bed. 0 1/1 2/1 3/1 4/1 5/1 6/1 7/1 8/1 9/1 10/1 11/1 12/1 Dry Average Wet RIO GRANDE STATE WILDLIFE AREA CHANNEL BED MOBILITY STUDIES An important part of river function is the movement of sediment through a river corridor and mobilization of the sediments on the channel bed. These studies look at how much flow is needed in the Rio Grande to pick up and move the bed material and how frequently these flows occur.

IMPORTANCE OF BED MOBILITY SEDIMENT CHARACTERISTICS Rivers adjust their shape and composition in response to the sediment and water supplied from the watershed and adjacent hillslopes and channel banks. The movement of Description of Pebble sediment on a streambed affects instream and riparian Size (mm) habitat at various scales: At smaller scales, a lack of bed Particle Size Count mobility may allow the buildup of fine particles such as Sand and Silts <2 0 sands and silts in the interstitial spaces between larger grains of sediment such as cobbles and gravels. These Very Fine Gravel 2 - 4 0 interstitial spaces are important for fish species but also for Fine Gravel 4 - 6 0 key components to the food web such as algae, Fine Gravel 6 - 8 0 zooplankton, phytoplankton, and macroinvertebrates. Medium Gravel 8 - 11 1 Medium Gravel 11 - 16 6 At larger scales, the mobilization and deposition of bed Coarse Gravel 16 - 22 8 sediments creates and maintains pools and riffles. Over the Coarse Gravel 22 - 32 22 long term, changes in the bed surface caused by the mobility of the sediment on the bed are necessary to Very Coarse Gravel 32 - 45 26 maintain habitat quality in river systems. Evacuating fine Very Coarse Gravel 45 - 64 27 sediment from pools and the deposition of coarse Small Cobble 64 - 90 9 sediment on bars may increase the quality and quantity of Small Cobble 90 - 128 1 habitat used for spawning and rearing. Conversely, a lack of Large Cobble 128 - 180 0 flows that trigger bed mobility will tend to cause either Large Cobble 180 - 256 0 long-term scour or aggradation and tends to simplify the channel, reduce bedform variability, and homogenize aquatic and riparian habitat. Riparian vegetation The Rio Grande channel bed at the State Wildlife Area establishment and succession is dependent upon the has an average grain size of 29mm, which is classified mobilization and deposition of sediment within the stream as a coarse gravel. corridor. The Rio Grande at the State Wildlife Area near Monte Vista, looking upstream on August 29, 2019. Approximate flow 300 cfs.

CHANNEL GEOMETRY AND HYDRAULICS

The Rio Grande State Wildlife Area has full topographic coverage by the LiDAR. This was supplemented with survey data collected by the RGHRP in the summer of 2019. Hydraulic calculations were done using the survey data. For the purposes of this study, we are concerned only with flows that remain in the main channel of the Rio Grande. Simple calculations were done using the manning’s equation to determine that the maximum capacity of the main channel is approximately 3400 cfs to 3600 cfs.

Page B-1