Levee Breach Analysis for King County Rivers Prepared for: King County Department of Natural Resources and Parks Water and Land Resources Division River and Floodplain Management Section 201 South Jackson Street, Suite 600 Seattle, Washington 98104 Prepared by: Watershed Science and Engineering, Inc. 506 2nd Ave, Suite 2700 Seattle WA 98104 May 8, 2019 KING COUNTY FLOOD CONTROL D I S T R. I C T I King County Levee Breach Analysis for King County Rivers May 8, 2019 Table of Contents 1 Introduction .......................................................................................................................................... 1 1.1 Purpose ............................................................................................................................................... 1 1.2 Background and Objective .................................................................................................................. 1 1.3 Study Area ........................................................................................................................................... 2 1.4 Approach/Methods ............................................................................................................................. 2 1.5 General Overview of Levees ............................................................................................................... 3 1.5.1 Functions ............................................................................................................................... 3 1.5.2 Risks....................................................................................................................................... 4 1.5.3 Levee Certification and FEMA Accreditation ........................................................................ 4 1.5.4 Rehabilitation and Inspection Program (PL 84-99) ............................................................... 5 1.5.5 King County Flood Protection Facility Inspections ................................................................ 6 1.5.6 Flood Warning Program ........................................................................................................ 7 1.5.7 Effects of Dams ..................................................................................................................... 8 2 Lower Tolt River levees from RM 2.2 (Holberg levee) to the confluence with the Snoqualmie River . 9 2.1 Available Information ......................................................................................................................... 9 2.1.1 Survey/topography ............................................................................................................... 9 2.1.2 Geotechnical and Geomorphic Assessments ........................................................................ 9 2.1.3 Hydrogeologic/Seepage analysis .......................................................................................... 9 2.1.4 Hydrologic data (Peaks, durations, ramping rates)............................................................... 9 2.1.5 Hydraulic modeling and Levee Failure Analysis .................................................................. 11 2.1.6 Inundation and Breach Progression Mapping .................................................................... 11 2.1.7 Recent CIP Projects ............................................................................................................. 11 2.1.8 PL84-99 Status..................................................................................................................... 12 2.2 Site Visit/Inspections/Pertinent Observations .................................................................................. 12 2.3 Data Gaps .......................................................................................................................................... 13 2.4 Levee and Channel Characterization (Physical Properties) .............................................................. 14 2.5 Land-Use ........................................................................................................................................... 14 2.6 Hazards .............................................................................................................................................. 14 2.7 Risks .................................................................................................................................................. 15 3 Lower Raging River levees from RM 1.5 (328th Way SE) to the confluence with the Snoqualmie River ............................................................................................................................................................ 17 Page i Levee Breach Analysis for King County Rivers May 8, 2019 3.1 Available Information ....................................................................................................................... 17 3.1.1 Survey/topography ............................................................................................................. 17 3.1.2 Geotechnical and Geomorphic Assessments ...................................................................... 17 3.1.3 Hydrogeologic/Seepage analysis ........................................................................................ 17 3.1.4 Hydrologic data (Peaks, durations, ramping rates)............................................................. 17 3.1.5 Hydraulic modeling and Levee Failure Analysis .................................................................. 18 3.1.6 Inundation and Breach Progression Mapping .................................................................... 18 3.1.7 Recent CIP Projects ............................................................................................................. 18 3.1.8 PL84-99 Status..................................................................................................................... 19 3.2 Site Visit/Inspections/Pertinent Observations .................................................................................. 19 3.3 Data Gaps .......................................................................................................................................... 19 3.4 Levee and Channel Characterization (Physical Properties) .............................................................. 19 3.5 Land-Use ........................................................................................................................................... 20 3.6 Hazards .............................................................................................................................................. 20 3.7 Risks .................................................................................................................................................. 20 4 South Fork Snoqualmie River levees from RM 5.4 (McConky and Holstein Ext. Levees) to RM 2.1 (Snoqualmie Trail Crossing) ........................................................................................................................ 21 4.1 Available Information ....................................................................................................................... 21 4.1.1 Survey/topography ............................................................................................................. 21 4.1.2 Geotechnical and Geomorphic Assessments ...................................................................... 21 4.1.3 Hydrogeologic/Seepage analysis ........................................................................................ 21 4.1.4 Hydrologic data (Peaks, durations, ramping rates)............................................................. 21 4.1.5 Hydraulic modeling and Levee Failure Analysis .................................................................. 22 4.1.6 Inundation and Breach Progression Mapping .................................................................... 23 4.1.7 Recent CIP Projects ............................................................................................................. 23 4.1.8 PL84-99 Status..................................................................................................................... 24 4.2 Site Visit/Inspections/Pertinent Observations .................................................................................. 24 4.3 Data Gaps .......................................................................................................................................... 24 4.4 Levee and Channel Characterization (Physical Properties) .............................................................. 24 4.5 Land-Use ........................................................................................................................................... 25 4.6 Hazards .............................................................................................................................................. 25 4.7 Risks .................................................................................................................................................. 26 5 Lower Cedar River levees from RM 1.23 to the mouth ...................................................................... 28 Page ii Levee Breach Analysis for King County Rivers May 8, 2019 5.1 Available Information ....................................................................................................................... 28 5.1.1 Survey/topography ............................................................................................................