Outline. Floods Follow Fires. Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources 9/16/2016

Transcription

1 Floods Follow Fires APWA 2016 Fall Conference Spokane, WA Katherine Rowden Service Hydrologist National Weather Service / NOAA Outline Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources 1

2 What s the Big Deal? The hydrologic response of a watershed can be changed dramatically, for years to come Loss of tree canopy & ground vegetation Upper Entiat River Valley Myrtle Lake 2

3 What s the Big Deal? The hydrologic response of a watershed can be changed dramatically, for years to come Loss of tree canopy & ground vegetation Soils can be water repellant (Hydrophobic) Hydrophobicity 3

4 What s the Big Deal? The hydrologic response of a watershed can be changed dramatically, for years to come Loss of tree canopy & ground vegetation Soils can be water repellant (Hydrophobic) Floods bring debris down: rocks, trees, BBQs Albuquerque Journal 8/23/2011 Those concrete Jersey barriers were just floating through like they were rubber duckies Typical Riverine Flooding Riverine Flooding Cataldo, ID April,

5 Carlton Complex Fire August 2014 Floods What s the Big Deal? The hydrologic response of a watershed can be changed dramatically, for years to come Loss of tree canopy & ground vegetation Soils can be water repellant (Hydrophobic) First Storms bring ash flow is more Buoyant Floods bring debris down: rocks, trees, BBQs Bottom Line Post fire peak flows can increase 5,000% or more A 5yr storm can produce a 100yr flood Assume another 25% increase in peak volume for bulking 5

6 And there s more Debris jams form and breach possibly successively in one event. R.O.T.: Outflow = 5x Inflow And there s more Debris jams form and breach possibly successively in one event. R.O.T.: Outflow = 5x Inflow Channel incision / aggradation 6

7 What s the Cause? Winter / Fall / Spring Summer Thunderstorms Flash Flood vs. Debris Flow Flash Flood: can carry rocks, branches, etc, but still behaves like water Debris Flows are like concrete much more destructive 7

8 Debris Flow Basics Rain intensity, not amount, is the trigger Can start nearly instantaneously Reach speeds > 60 mph Force can be very destructive Flash Flood: needs more water Debris Flows: needs intensity, not much water required Debris Flows Finley Canyon Carlton Complex September 2014 ~1 / 1 hour 3 dams failed, in part due to debris flows covering the spillways A fire fighter was nearly killed in the resulting dam breach flood 8

9 What s a Debris Flow? ebrisflow_southam.mp ~0.5 inches 1 hour Debris Flow? Debris Nope! Flow? Just your garden variety post fire flash flood Carlton Complex September

10 Outline Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources Vulnerability? Wenatchee 10

11 Alluvial Fans 2012 Wenatchee Complex Robinson Canyon Colockum Tarps Fire September

12 Dry washes near homes Rivers typically have enough capacity Culverts! They are almost never big enough Road fill / culvert can become dam, increasing downstream flooding if breached 8 12

13 Transportation Infrastructure Highway / 1 hour Life Safety Emergency Response Commerce Utilities Highway / 1 hour Limited Access Areas 2015 First Creek South Lakeshore Rd 13

14 Evacuation Time Short & steep drainages with homes below Wenatchee Other Vulnerabilities Municipal Water Supply Sediment delivery to surface water = Degraded Water Quality Flash floods or Debris Flows damage water intakes, treatment facilities, delivery infrastructure [*** New Mexico example of gate not closed in time**]] Dams (e.g Wenner Lake Dams in Carlton) Constrictions where Debris Dams can form: Bridges, narrow channels, Overgrown channels, etc Buried infrastructure near/below stream channels 14

15 Outline Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources Early Remote Assessment Heat Detection Maps Pre Assessment modeling techniques being developed (USACE, FEMA, USGS, NWS, WA DEM) 15

16 Windshield Surveys BAER Teams Burned Area Emergency Response Purpose Emergency Response 14 Days Determine severity of fire Identify values at risk Calc post fire flood flows & erosion Recommend mitigation actions Share findings Flavors Department of Interior: BIA, NPS, BLM USFS WA State interagency BAER = ibaer o a.k.a. Erosion Threat Assessment/Reduction Team = ETART 16

17 2014 Carlton Complex Start With Burned Area Reflectance Classification map (BARC) Derived from satellite imagery: pre vs. post fire 2015 Black Canyon BARC map Low Burn Severity Moderate Burn Severity High Burn Severity Check for: Fuel consumption Root structure Soil condition Hydrophobicity 17

18 Black Canyon BARC map Black Canyon Soil Burn Severity Map Hydrology Engineering Soils Geology Fisheries Cultural Resources Forestry Botany GIS Days of modeling & reports 18

19 USGS Debris Flow Model 19

20 Uses of BAER reports Public works & NRCS: modeling outputs for design/mitigation for infrastructure & homes NWS & Partners: Determine need for flood early warning rain or stream gages SIDEBAR Need more local input/expertise on ibaer state/private land team o Support public works staff participation in assessment efforts o Reduce unneeded conceptual design by BAER team engineers o Provide local knowledge of key areas/vulnerabilities Confusion State Team USFS Team #1 USFS Team #2 Colville Tribe / BIA Team 20

21 Outline Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources Public Awareness Outreach don t wait! Personal awareness & preparedness Flood Insurance Public meetings Door hangers PSAs Leverage Partners Efforts Conservation Districts Fire Adapted Communities NWS / NRCS / FEMA / etc Afterthefirewa.org 21

22 Other Mitigation Measures Here It Comes, Get out of the Way Greg Kuyumjian, USFS Hydrologist, Ret. Culverts Rolling/Driveable Dips Temp removal of non essential bridges Clear debris out of channels & ditches 2015 First Creek Manage the pain Storm Patrols before/after events Close minor roads Temporary bridges on hand A Tale of Two Culverts shamelessly plagiarized from Greg Kuyumjian, ret. USFS Hydrologist BAER 84 culverts at inlet, after first major storm After 2 nd major storm The Final Solution 22

23 Signage Solution Majority of flood fatalities occur in vehicles Highway 97 August 2010 Outline Define the problem Identify Vulnerabilities Determine the Risk Manage the Risk Resources 23

24 National Weather Service Resources Weather forecasts Weather station data (events) Climate data Social media NWS on Social Media All NWS Forecast Offices have Facebook and Twitter Share your local weather Interactive Photos and Video Exchange Information Situational Awareness 24

25 NWS Decision Support First, it should be understood that forecasts possess no intrinsic value. They acquire value through their ability to influence the decisions made by users of the forecast Impacts based Decision Support Services (IDSS or DSS) Work closely with key entities Forecast support as/needed Weather event support (Oso) Event Specific support (Iron Man) Post fire Flood Specific 25

26 New Post fire webpages Flash Flood Potential Index In development 26

27 Storm/Flood Detection Some areas may benefit from additional rain & stream gages for early warning New projects underway New WA State Post Wildfire Committee Facilitated by WA State Silver Jackets Chair Working collaboratively to address the gaps Currently seeking funding for suite of post fire outreach materials 27

28 Katherine Rowden (509) National Weather Service Spokane, WA 28