PRESENTATION FOR THE POTENTIAL ADDITION OF SPECIAL FLOOD HAZARD AREAS

Transcription

1 PRESENTATION FOR THE POTENTIAL ADDITION OF SPECIAL FLOOD HAZARD AREAS Part 1: Summary of Floodplain Analysis for Los 0 Angeles River Part 2: Los Angeles County Drainage 130 Area 48 (LACDA) study results Prepared by Rene Vermeeren, P.E., D. WRE. Adam Bier, P.E. 17 October 2016 The views, opinions and findings contained in this report are those of the authors(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation.

2 PART 1: SUMMARY OF FLOODPLAIN ANALYSIS FOR LOS ANGELES RIVER 2 Prepared by Adam Bier, P.E. Senior Hydraulic Engineer Photo: Los Angeles River showing flows near the top of the channel at Los Feliz Boulevard with debris on the railing (January 26, 1969)

3 PRESENTATION OVERVIEW Study Overview and Project Area Los Angeles River Channel Existing Federal Emergency Management Agency (FEMA) Floodplain Hydraulic Modeling Overview 100-Year Floodplain Changes in Floodplain Mapping Results and Summary 3

4 STUDY OVERVIEW AND PROJECT AREA 4 Study Overview Flood Plain Management Services (FPMS) Study Prepared 100-Year and 500-Year Floodplain Maps Analysis completed for the City of Los Angeles Los Angeles River Study Reach Barham Blvd to First St Approximately 13 miles Main Tributaries: Burbank Western Verdugo Wash Arroyo Seco

5 LOS ANGELES COUNTY DRAINAGE AREA 5

6 LOS ANGELES RIVER CHANNEL 6 Topographic Data Sources Overbanks: LARIAC (2006) Provided by City of Los Angeles Channel (Concrete): As-built drawings Channel (Earth bottom): 2008 Survey Corps Topographic Survey Levees Levees are within study reach Levees extend intermittently from Burbank Western Confluence south past Glendale Blvd LARIAC: Los Angeles Region Imagery Acquisition Consortium

7 EXISTING FEMA FLOODPLAIN (100-YEAR) 7 Special Flood Hazard Area (SFHA) Study area currently has 874 parcels in SFHA in City of Los Angeles 100-Year Flood FEMA Classification Zone A In channel - west of I-5 FEMA Classification Zone AE In Channel from Los Feliz Blvd to Arroyo Seco Confluence Overbank flooding south of Interstate 5 (Arroyo Seco) FEMA Flood Insurance Rate Maps Effective date: September 26, 2008 Panel Numbers in Study Area 1339F 1345F 1610F 1626F 1628F 1636F FEMA Flood Map Service Center

8 HYDRAULIC MODELING OVERVIEW Hydraulic Modeling Software Hydrologic Engineering Center River Analysis System (HEC-RAS) Models Current Condition of Channel Dense vegetation and sediment in earth bottom channel reaches Temporary flood barriers are not modeled Hydraulic Model Models how water flows down the river and how it overtops out of the Los Angeles River channel into the Floodplain One Dimensional (1D): Channel Two Dimensional (2D): Overbank areas 27 Bridges in study reach

9 100-YEAR FLOODPLAIN 9 Google Earth Video, 2016

10 FLOODPLAIN MAP EXAMPLE 10

11 CHANGES IN FLOODPLAIN MAPPING Existing Flood Insurance Rate Maps (100-Year Flood) Show; 874 City of Los Angeles parcels currently in Special Flood Hazard Area (SFHA) 11 Difference from the FEMA FIRM and this 2016 Corps FPMS (100-Year Flood) Existing FIRM 373 City of Los Angeles parcels are no longer shown in Floodplain 501 City of Los Angeles parcels are still shown in the Floodplain 2016 Corps FPMS Study Adds: 2,574 City of Los Angeles parcels to the Floodplain Adds: 271 Parcels outside of the City of Los Angeles (Glendale & Burbank) Net Total: 3,075 City Of Los Angeles parcels Parcel Count: City of Los Angeles Estimates

12 RESULTS Number of Parcels shown Floodplain (100-year) 3,075 City of Los Angeles 271 City of Glendale & City of Burbank Average Flood Depths 5-10 feet in the floodplain Approximate Maximum Flood Depth in Overbank 18 feet (Griffith Park) Approximate Area of Inundation: 1,000 acres Flow is contained in channel downstream of Arroyo Seco Confluence 12

13 SUMMARY Corps Study identifies Flood Risk to the public Potential Flooding shown in Bette Davis Park, Griffith Park, Atwater Village, Elysian Valley & other areas 13 Next steps Corps study can be used to help develop evacuation plans & for the public s consideration in purchasing flood insurance Corps will coordinate with local flood control agencies in reducing the flood risk hazard

14 PRESENTATION FOR THE POTENTIAL ADDITION OF SPECIAL FLOOD HAZARD AREAS PART 2: LOS ANGELES COUNTY DRAINAGE AREA STUDY RESULTS Prepared by Rene Vermeeren, P.E., D. WRE. Chief, Hydrology and Hydraulics Branch 17 October 2016 The views, opinions and findings contained in this report are those of the authors(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation.

15 PRESENTATION OVERVIEW Los Angeles County Drainage Area Feasibility Study Floodplain Mapping LACDA Study Study Results 15

16 LOS ANGELES COUNTY DRAINAGE AREA 16 Review Feasibility Study (1992) Source LACDA Feasibility Study (1992)

17 FLOODPLAIN MAPPING 17 Hansen Dam Los Angeles River Sepulveda Dam Glendale Narrows Downtown Los Angeles Source: LACDA Feasibility Study (1992)

18 LACDA STUDY LACDA study evaluated flooding for entire LACDA system Extended from San Gabriel Mountains to the ocean Purpose: propose modifications to reduce flood risk The flood damage reduction general measures evaluated Reduce inflow to the system (detain water) Convey more water in the system (increase channel capacity) Damage management (floodfighting, floodproofing, etc.) Alter the reservoir s current operating regulations Several measures were considered as solutions to the flooding problem Deepen/Widen channels, alter channel shape Raise channel walls Modify bridges Divert flows into tunnels or detention facilities Source: LACDA Feasibility Study (1992)

19 STUDY RESULTS 19 Project Justification Average Annual Benefits must exceed the Average Annual Cost of the measures considered (B/C ratio) Benefits are derived from reduction of floodplain damages when evaluating measures. Results of study found: Evaluation of measures indicate low benefit to cost ratio Measures not economically justified Report recommended no action for this reach Source: LACDA Feasibility Study (1992)