South Fork Inundation River The South Fork Inundation River at Floodville typically exceeds flood stage at least once each winter. Flood stage in the Coastland area is higher than in the areas downstream because of a natural constriction in the flood plain immediately downstream of the confluence of the North and South Forks of the Inundation River. In December 1964, the Spruce Street Bridge staff gage at Coastland, indicated that the South Fork Inundation River crested at approximately 11 feet above flood stage (bankfull discharge) with an estimated discharge of 100,000 cfs. This flow has a return period greater than 500 years. Stream gage No. 19999999 on the South Fork Inundation River at Floodville recorded a peak flow of 48,900 cfs . This flow has a return period of about 500 years. Table 7 contains information about historic flood elevations in the communities within Flood County. Table 7: Historic Flooding Elevations Flooding Source Location Historic Peak (Feet NAVD88) Event Date Approximate Recurrence Interval (years) Source of Data Inundation River Outlet of Inundation River at Big Ocean 19.8 1986 80 USGS gage South Fork Inundation River 700 feet upstream of Fulton Road 18.8 2007 50 NRCS high water marks 4.3 Non-Levee Flood Protection Measures Table 8 contains information about non-levee flood protection measures within Flood County such as dams, jetties, and or dikes. Levees are addressed in Section 4.4 of this FIS Report. Table 8: Non-Levee Flood Protection Measures Flooding Source Structure Name Type of Measure Location Description of Measure Big Ocean A.B. Smith Jetty Jetties At entrance channel Constructed by USACE in 1929 Big Ocean N/A Berms and riprap Floodville, along the coast of the Big Ocean Several property owners in this city have placed berms and riprap to protect homes Big Ocean N/A Tidal flooding warnings Low-lying coastal areas Flood Weather Forecast Office issues storm tide warnings
Table 9: Levees Community Flooding Source Levee Location Levee Owner USACE Levee Levee ID Covered Under Program? FIRM Panel(s) Flood County, Unincorporated Areas Inundation River Left Bank Flood County Water Supply Yes 1234545362 Yes 12345C0234X Flood County, Unincorporated Areas Inundation River Right Bank Flood County Water Supply Yes 1354212346 Yes 123450C234X Floodville, Town of Inundation River Left Bank Floodville Waterworks No 1901990990 No 12345C0245X
Table 10: Summary of Discharges Peak Discharge (cfs) Flooding Source Location Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance Culvert Creek Just downstream of Smith Lane 1.0 130 * 170 190 * 240 Inundation River Confluence with Big Ocean 1,058 77,200 * 107,000 122,000 132,000 143,000 Inundation River 1.2 miles downstream of US Highway 27 980 73,100 86,800 101,000 116,000 119,000 136,000 Inundation River 2,000 feet downstream of 3rd Avenue 930 70,500 82,360 97,100 111,000 115,000 130,000 Inundation River 500 feet upstream of Main Street 902 69,000 81,100 95,000 109,000 113,000 128,000 Inundation River Confluence with North Fork Inundation River and South Fork Inundation River 879 67,700 * 93,200 107,000 114,00 125,000 North Fork Inundation River Just upstream of State Highway 42 137 18,100 * 24,000 27,000 * 31,600 South Fork Inundation River Confluence with North Fork 598 51,100 * 69,700 79,600 * 93,300 *Not calculated for this Flood Risk Project
Table 13: Summary of Hydrologic and Hydraulic Analyses Flooding Source Study Limits Downstream Limit Study Limits Upstream Limit Hydrologic Model or Method Used Hydraulic Model or Method Used Date Analyses Completed Flood Zone on FIRM Special Considerations Culvert Creek Confluence with South Fork Inundation River 2.3 miles upstream of confluence of Ripple Creek 1994 State Regression Equations – Region 3 HEC-2 4.6 03/22/1997 AE Ice jam analysis evaluated by Modified Indirect Method (CRREL 2004). Flood Profile reflects results of ice jam analysis. Inundation River Confluence with Big Ocean Approximately 500 feet upstream of State Highway 999 2004 State Regression Equations – Region 3 HEC-RAS 3.1 06/30/2007 AE w/ Floodway Levee analysis and mapping procedures were applied to Levee IDs 1354212346 and 1234545362. Inundation River Approximately 500 feet upstream of State Highway 999 Confluence of N. Fork Inundation River and S. Fork Inundation River 2004 State Regression Equations – Region 3 HEC-RAS 3.1 06/30/2007 A Effects of hydraulic structures were not considered in the model. Lily Pond Pear Tree Circle Westwood Lane ICPR 2.20 ICPR 2.20 05/28/2002 AE Elevations determined using ICPR. Survey data utilized in model was based on county information collected in 2008. North Fork Inundation River Confluence with Inundation River 0.7 miles upstream of confluence of Lilac Stream Log Pearson Type III Frequency Analysis HEC-RAS 4.0 12/12/2010 AE Gage No. 19999998 was used in hydrologic analysis. Hydraulic models incorporated field measured bridge and culvert data. South Fork Inundation River Confluence with Inundation River 3.2 miles upstream of confluence of Culvert Creek HEC-HMS 3.4 Unsteady HEC-RAS 4.0 12/12/2010 AE w/ Floodway Hydraulic model was calibrated to high water marks collected for flood of 2007, which was estimated to be the 2% annual chance flood.
Table 17: Coastal Transect Parameters Flood Source Coastal Transect Starting Wave Conditions for the 1% Annual Chance Starting Stillwater Elevations (ft NAVD88) Range of Stillwater Elevations (ft NAVD88) Significant Wave Height Hs (ft) Peak Wave Period Tp (sec) 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance Big Ocean 1 27.2 13 5.6 5.6-5.6 * * 10.6 10.1-10.9 15.7 15.2-15.8 19.6 18.6-19.8 *Not calculated for this Flood Risk Project
Table 18: Summary of Alluvial Fan Analyses Flooding Source Location From (apex) Location To (toe) Drainage Area above Apex (sq mi) Model(s) Used Date Analysis was Completed Method Description Culvert Creek Fan From apex of fan Highway I-10 24.2 N/A 2005 Geomorphic Data, Post Flood Hazard Verification, and Historical Information Mountain Wash Fan Apex of fan Stan Road 54.5 FLO-2D, version 2006.07 2006 Risk-Based Analysis Petal Creek fan From apex of fan Tangerine Road 15.8 FLO-2D version 2007.06 2009 Composite Methods Valley Creek Fan Apex of N. Fork Inundation River Fan Maple Lane 44.7 FAN Computer Program 1993 Areas identified with historical aerial photos. FAN analysis used for flood in active areas. HEC-2 4.6 was used in inactive areas, where incised networks and little risk of avulsion observed. Table 19: Results of Alluvial Fan Analyses Location From (apex) Location To (toe) 1% Annual Chance Peak Flow at Fan Apex (cfs) Flood Zones and Depths (ft) Minimum Velocity (fps) Maximum Velocity (fps) Flooding Source Culvert Creek Fan From apex of fan Highway I-10 1,750 AO 1-2', AE 1 6 Mountain Wash Fan From apex of fan Stan Rd 2,140 AO 1-3' 2 6 Petal Creek Fan From apex of Petal Creek fan Tangerine Rd 880 AO 1-3', A 1 7
LOCATION FLOODWAY 1% ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION (FEET NAVD88) CROSS SECTION DISTANCE2 WIDTH (FEET) SECTION AREA (SQ. FEET) MEAN VELOCITY (FEET/SEC) REGULATORY WITHOUT FLOODWAY WITH FLOODWAY INCREASE A1 12,930 * * * 11.4 11.4 * * B 13,165 25 98 4.5 12.2 12.2 13.2 1.0 C 13,315 47 210 2.1 12.8 12.8 13.5 0.7 D 13,835 71 279 1.6 12.9 12.9 13.7 0.8 E 14,345 29 85 4.7 14.1 14.1 14.4 0.3 F1 14,425 * * * 14.6 14.6 * * G1 14,695 * * * 15.5 15.5 * * H 14,985 53 144 2.8 16.2 16.2 16.3 0.1 I 15,785 28 98 2.2 17.2 17.2 17.4 0.2 J 16,465 22 80 2.7 18.4 18.4 19.3 0.9 K 17,965 19 69 3.2 19.8 19.8 20.3 0.5 1Floodway not computed/shown for this cross section 2Feet above mouth TABLE 24 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA FLOOD COUNTY, STATE FLOODING SOURCE: PETAL CREEK AND INCORPORATED AREAS
Table 33: Bibliography and References Citation in this FIS Publisher/ Issuer Publication Title, “Article,” Volume, Number, etc. Author/Editor Place of Publication Publication Date/ Date of Issuance Link ABC Eng, 1978 ABC Engineers, Inc. Flower Creek Water Supply, Coastland Water Board, City of Coastland, State, C10933.00 City of Coastland, State April 1978 City of Coastland Water Board Coastland 1977 City of Coastland Inventory of Coastal Resources for the 1990 Comprehensive Plan December 1977 City of Coastland library Coastland 1978 City of Coastland 1990 Comprehensive Plan September 1978 City of Coastland library FEMA 1989 Federal Emergency Management Agency Flood Insurance Study, Flood County, State, and Unincorporated Areas Washington, D.C. 1989 FEMA Flood Map Service Center FEMA 1996 Federal Emergency Management Agency Flood Insurance Study, City of Floodville, Flood County, State Washington, D.C. 1996 FEMA Flood Map Service Center FIA 1977 U.S. Department of Housing and Urban Development, Federal Insurance Administration Flood Hazard Boundary Map, Flood County, USA, Community-Panel Numbers 410042 0001 through 0021 Sidney McFlood Washington, D.C. September 1977 FEMA Flood Map Service Center State CES 1967 State University, Resource Development Section, Cooperative Extension Service Resources Analysis, Flood County, State Dave Waters and Gary Mapper City of Coastland, State December 1967
USGS 1988 U.S. Department of Interior, Geological Survey 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 10 Feet. Coastland, ST (1984, revised 1988) Washington, D.C. Various USGS 2008 U.S. Department of Interior, Geological Survey LiDAR Data, Scale 1:4,800, Contour Interval 2 Feet. Washington, D.C. 2008