presentation for the potential addition of special...
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“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.”
Prepared by
Rene Vermeeren, P.E., D. WRE.
Adam Bier, P.E.
PRESENTATION FOR THE POTENTIAL ADDITION OF SPECIAL FLOOD HAZARD AREAS
1
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17 October 2016
Part 1: Summary of Floodplain Analysis for Los Angeles RiverPart 2: Los Angeles County Drainage Area (LACDA) study results
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2
PART 1: SUMMARY OF FLOODPLAIN ANALYSIS FOR LOS ANGELES RIVER
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)
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3PRESENTATION 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
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4STUDY OVERVIEW AND PROJECT AREA 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
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5LOS ANGELES COUNTY DRAINAGE AREA
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6LOS ANGELES RIVER CHANNEL 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
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7EXISTING FEMA FLOODPLAIN (100-YEAR) 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 https://msc.fema.gov/portal
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8HYDRAULIC MODELING OVERVIEW Hydraulic Modeling Software Hydrologic Engineering Center River Analysis System (HEC-RAS) 5.0.1
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
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9100-YEAR FLOODPLAIN
Google Earth Video, 2016
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10FLOODPLAIN MAP EXAMPLE
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11CHANGES 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)
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
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12RESULTS 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
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13SUMMARY Corps Study identifies Flood Risk to the public Potential Flooding shown in Bette Davis Park, Griffith Park,
Atwater Village, Elysian Valley & other areas
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
217217217
200200200
255255255
000
163163163
131132122
2396553
110135120
1129256
62102130
1025648
130120111
237237237
8011927
252174.59
“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.”
Prepared by
Rene Vermeeren, P.E., D. WRE.
Chief, Hydrology and Hydraulics Branch
PRESENTATION FOR THE POTENTIAL ADDITION OF SPECIAL FLOOD HAZARD AREAS
PART 2: LOS ANGELES COUNTY DRAINAGE AREA STUDY RESULTS
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17 October 2016
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15PRESENTATION OVERVIEW Los Angeles County Drainage Area Feasibility Study Floodplain Mapping LACDA Study Study Results
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16LOS ANGELES COUNTY DRAINAGE AREA Review Feasibility Study (1992)
Source LACDA Feasibility Study (1992)
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17FLOODPLAIN MAPPING
Downtown Los Angeles
Hansen Dam
Sepulveda Dam
Glendale Narrows
Los Angeles River
Source: LACDA Feasibility Study (1992)
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18LACDA STUDY
Source: LACDA Feasibility Study (1992)
1992 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
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19STUDY RESULTS
Source: LACDA Feasibility Study (1992)
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