Wasatch Front/Oquirrh Alluvial Fan Inventory 2018

Wasatch Front/Oquirrh Alluvial Fan Inventory Utah Division of Emergency Management JE Fuller/Hydrology & Geomorphology Study Completed in 2017 January 24, 2018

Why Utah Needed this Study • Population growth • Utah is #1 in population growth from 2015-2016 (U.S. Census)

• Not all flood risk is mapped within Utah • Zone A areas may not accurately portray true flood risk

Study Information • DEM lead • Funded through FEMA CTP Grant • Study Area: Wasatch Range from Idaho Border to Nephi; West Oquirrh’s Point of the Mountain to SE of Rush Valley

Alluvial Fans • “Fan” shaped sediment/debris deposits found at the base of mountain fronts.

• Potential high flood, velocity, scour, sediment, debris hazards • Distributary flow pattern

Death Valley

• Flow path uncertainty • Characterized as Active vs. Inactive (FEMA) • Active: younger, deposition present, no defined channel • Inactive: older, tributary development, manmade structural mitigation

Juab County

Alluvial Fans • FEMA Definitions and Criteria • “Alluvial Fan Flooding” - flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high-velocity flows; active processes of erosion, sediment transport, and deposition; and, unpredictable flowpaths.

• Active Alluvial Fan Flooding • 1. Flow path uncertainty below the hydrographic apex; • 2. Abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its ability to carry material eroded from a steeper, upstream source area; and • 3. An environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk.

• 3-Stage process for defining the 100-year SFHA

Flooding After Fires • After a wildfire, the flood risk increases significantly • FEMA Flood After Fire Toolkit www.fema.gov/flood-after-fir e

Use of Study Information • Does not map Alluvial Fans for FEMA Flood Insurance Rate Map (FIRM) purposes • Will inform future need for Alluvial Fan designations on FIRMs • Will be presented to impacted communities • Data can be made available • Can be used at the local level for planning purposes

Data Used • Geologic Mapping (USGS and UGS) • 1964 – 2013

• Over 50 maps collected and rectified in GIS • UGS identified many alluvial fan landforms, not necessarily focused on active/inactive.

Data Used • Topography • USGS Quadrangles (1:24,000) • Open Topography LiDAR (0.5 meter) • Converted to 2-ft contours

• Alluvial fans exhibit unique topographic signature • Depositional landform • Radial contour pattern

Data Used • Topography • USGS Quadrangles (1:24,000) • Open Topography LiDAR (0.5 meter) • Converted to 2-ft contours

• Alluvial fans exhibit unique topographic signature • Depositional landform • Radial contour pattern

Data Used • Aerial Photography • Historical: 2,600 digital aerial photos 1937 – 1959 • Rectified in GIS

• Development (agricultural, urban) can alter landforms • Modern (NAIP, ESRI World Imagery)

Data Used • FEMA Flood Hazard Layer • Effective flood zones on active alluvial fan landforms • Zone A • Ogden City • Kaysville • S. Weber • Fruit Heights • Alpine • Orem • Ogden City

• Zone AE – • Layton (Snow Creek) • Sandy City (Little Willow Creek)

• AO - Provo

Data Used • UGS Hazard Mapping • Flood Hazards • Geologic, hydrologic, soils data • Purpose is to aid in planning • Very Low, Low, Moderate, High

• Landslide/Debris Flow mapping

Data Used • Field Investigation • 37 sites were identified for field visit • Input from DEM staff (Jamie, Brad, Kathy) • Verify geomorphic characteristics (is it an alluvial fan?)

• Observe, record, photograph landforms: 170 photos taken • Observe and interpret geomorphic characteristics (active vs. inactive?)

…the Results

Study Process • Initial identification (desktop exercise) • Field Verification • Engineered structures?

• Active vs. Inactive

• Hazard Classification (Moderate/High) • High Hazard Classification • Downstream development • Identified on UGS Flooding Hazard (High) or Landslide/Debris Flow Maps • Field evidence indicating a high hazard

• Attributed by Municipality (City/County)

Result Examples • Unnamed Watercourse – North Ogden

Result Examples • Barrett Canyon– North Ogden

Result Examples • Big Canyon– Tooele County

FRS

Use of Study Information • Does not map Alluvial Fans for FEMA Flood Insurance Rate Map (FIRM) purposes • Will inform future need for Alluvial Fan designations on FIRMs • Will be presented to impacted communities • Data can be made available • Can be used at the local level for planning purposes

Data Contact & Questions To obtain study report and/or data please contact: Jamie Huff Risk MAP Program Manager Utah Division of Emergency Management jhuff@utah.gov 801-538-3752 For technical questions: Mike Kellogg, PG, CFM, GISP mike@jefuller.com 480-222-5712