ºÝºÝߣ

ºÝºÝߣShare a Scribd company logo
Ataul Hannan, P. E., CFM  - Douglas B. Blatchford,  P.E. Guidelines for Determining Flood Hazards on Alluvial Fans Floodplain Management Association Newport Beach, California  September 9-12, 2003
Overview Emphasis is on the Lower Colorado    Region Alluvial surfaces differentiated by age FEMA recommends a three-stage process: Stage 1 – Fan Identification Stage 2 – Determining Active vs Inactive Stage 3 – Defining the 100-yr floodplain Focus is on Stage 2- Determining Active  vs Inactive alluvial surfaces
Background  Variations in solar radiation caused by  periodic changes in orbital pattern Results in glacial/interglacial periods  Interglacial periods represented by  aggradational deposition on alluvial  surfaces Aggradational/interglacial events  represented by specific geomorphic  features and time periods
Ages of Alluvial Surfaces*  * after Bull, 1991 Epoch  Estimated Age Range (ka)  Geomorphic Surface  Holocene Late  0 Q4b 0.1 – 2  Q4a Middle  2 – 4  Q3c 4 -- 8 Q3b Early 8 -- 12 Q3a Pleistocene Late  12 -- 70 Q2c 70 -- 200 Q2b Middle  400 --730 Q2a Early  >1200  Q1
after Bull, 1991
Ìý
Ìý
Ìý
Ìý
Ìý
FEMA Guidelines  Recent guidelines published in February  2002 FEMA has a three-stage process Stage 1: Fan Identification Stage 2: Determining active vs inactive  Stage 3: Defining the 100-yr floodplain Focus is on Stage 2: determining active vs  inactive part of the fan
Defining Active vs Inactive Areas Seeks to delineate areas of the alluvial fan  that are active or are inactive in the  deposition, erosion and unstable flow path  flooding that builds alluvial fans This stage attempts to narrow the area of  concern for Stage 3, which is the specific  identification of the extent of the 100-year  flood
Identification of Stable vs Unstable Areas Soil development Surface characteristics Drainage texture Topography – local relief Historical flow path movement Potential water and sediment delivery from basin
Identification of Active and Inactive Areas Active – The portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible Analysis involves systematically applied judgement and the combination of hydraulic computations and qualitative interpretations of geologic evidence
Active and Inactive Areas soils maps and reports :   historical records of flooding and deposition field examination of morphologic features aerial photographs Data Sources and Age Indicators
Defining Active and Inactive Areas Data Sources and Age Indicators  weathering characteristics such as desert   pavement, rock varnish, B-horizon development in the soil profile, calcic-horizon development, and pitting and rilling of clasts may also provide relative age information diagnostic vegetation the analysis should consider more than one source of information and cross-correlations between them
Defining Active and Inactive Areas Older alluvial fan surfaces are considered active if any of the following are true: The recently active sedimentation zone is presently migrating into the older surface The elevation difference between the recently active sedimentation zone and the older surface is small relative to flood, deposition and debris depths Upstream of the site there is an opportunity of avulsions
Ìý
Stage 2 – Tiger Wash Piedmont
Alluvial Fan Website http://www.fema.gov/mit/ tsd/ FT_alfan.htm

More Related Content

Guidelines for Determining Flood Hazards on Alluvial Fans

  • 1. Ataul Hannan, P. E., CFM - Douglas B. Blatchford, P.E. Guidelines for Determining Flood Hazards on Alluvial Fans Floodplain Management Association Newport Beach, California September 9-12, 2003
  • 2. Overview Emphasis is on the Lower Colorado Region Alluvial surfaces differentiated by age FEMA recommends a three-stage process: Stage 1 – Fan Identification Stage 2 – Determining Active vs Inactive Stage 3 – Defining the 100-yr floodplain Focus is on Stage 2- Determining Active vs Inactive alluvial surfaces
  • 3. Background Variations in solar radiation caused by periodic changes in orbital pattern Results in glacial/interglacial periods Interglacial periods represented by aggradational deposition on alluvial surfaces Aggradational/interglacial events represented by specific geomorphic features and time periods
  • 4. Ages of Alluvial Surfaces* * after Bull, 1991 Epoch Estimated Age Range (ka) Geomorphic Surface Holocene Late 0 Q4b 0.1 – 2 Q4a Middle 2 – 4 Q3c 4 -- 8 Q3b Early 8 -- 12 Q3a Pleistocene Late 12 -- 70 Q2c 70 -- 200 Q2b Middle 400 --730 Q2a Early >1200 Q1
  • 10. Ìý
  • 11. FEMA Guidelines Recent guidelines published in February 2002 FEMA has a three-stage process Stage 1: Fan Identification Stage 2: Determining active vs inactive Stage 3: Defining the 100-yr floodplain Focus is on Stage 2: determining active vs inactive part of the fan
  • 12. Defining Active vs Inactive Areas Seeks to delineate areas of the alluvial fan that are active or are inactive in the deposition, erosion and unstable flow path flooding that builds alluvial fans This stage attempts to narrow the area of concern for Stage 3, which is the specific identification of the extent of the 100-year flood
  • 13. Identification of Stable vs Unstable Areas Soil development Surface characteristics Drainage texture Topography – local relief Historical flow path movement Potential water and sediment delivery from basin
  • 14. Identification of Active and Inactive Areas Active – The portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible Analysis involves systematically applied judgement and the combination of hydraulic computations and qualitative interpretations of geologic evidence
  • 15. Active and Inactive Areas soils maps and reports : historical records of flooding and deposition field examination of morphologic features aerial photographs Data Sources and Age Indicators
  • 16. Defining Active and Inactive Areas Data Sources and Age Indicators weathering characteristics such as desert pavement, rock varnish, B-horizon development in the soil profile, calcic-horizon development, and pitting and rilling of clasts may also provide relative age information diagnostic vegetation the analysis should consider more than one source of information and cross-correlations between them
  • 17. Defining Active and Inactive Areas Older alluvial fan surfaces are considered active if any of the following are true: The recently active sedimentation zone is presently migrating into the older surface The elevation difference between the recently active sedimentation zone and the older surface is small relative to flood, deposition and debris depths Upstream of the site there is an opportunity of avulsions
  • 18. Ìý
  • 19. Stage 2 – Tiger Wash Piedmont
  • 20. Alluvial Fan Website http://www.fema.gov/mit/ tsd/ FT_alfan.htm

Editor's Notes

  • #2: * 07/16/96 * ##
  • #20: * 07/16/96 * ## Update to show revised Stage 2 figure (rotate)