Guidelines for Determining Flood Hazards on Alluvial Fans


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Describes updates to FEMA alluvial fan floodplain determinations

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  • * 07/16/96 * ##
  • * 07/16/96 * ## Update to show revised Stage 2 figure (rotate)
  • Guidelines for Determining Flood Hazards on Alluvial Fans

    1. 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. 2. Overview <ul><li>Emphasis is on the Lower Colorado Region </li></ul><ul><li>Alluvial surfaces differentiated by age </li></ul><ul><li>FEMA recommends a three-stage process: </li></ul><ul><ul><li>Stage 1 – Fan Identification </li></ul></ul><ul><ul><li>Stage 2 – Determining Active vs Inactive </li></ul></ul><ul><ul><li>Stage 3 – Defining the 100-yr floodplain </li></ul></ul><ul><li>Focus is on Stage 2- Determining Active vs Inactive alluvial surfaces </li></ul>
    3. 3. Background <ul><li>Variations in solar radiation caused by periodic changes in orbital pattern </li></ul><ul><li>Results in glacial/interglacial periods </li></ul><ul><li>Interglacial periods represented by aggradational deposition on alluvial surfaces </li></ul><ul><li>Aggradational/interglacial events represented by specific geomorphic features and time periods </li></ul>
    4. 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
    5. 5. after Bull, 1991
    6. 11. FEMA Guidelines <ul><li>Recent guidelines published in February 2002 </li></ul><ul><li>FEMA has a three-stage process </li></ul><ul><ul><li>Stage 1: Fan Identification </li></ul></ul><ul><ul><li>Stage 2: Determining active vs inactive </li></ul></ul><ul><ul><li>Stage 3: Defining the 100-yr floodplain </li></ul></ul><ul><li>Focus is on Stage 2: determining active vs inactive part of the fan </li></ul>
    7. 12. Defining Active vs Inactive Areas <ul><li>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 </li></ul><ul><li>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 </li></ul>
    8. 13. Identification of Stable vs Unstable Areas <ul><li>Soil development </li></ul><ul><li>Surface characteristics </li></ul><ul><li>Drainage texture </li></ul><ul><li>Topography – local relief </li></ul><ul><li>Historical flow path movement </li></ul><ul><li>Potential water and sediment delivery from basin </li></ul>
    9. 14. Identification of Active and Inactive Areas <ul><li>Active – The portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible </li></ul><ul><li>Analysis involves systematically applied judgement and the combination of hydraulic computations and qualitative interpretations of geologic evidence </li></ul>
    10. 15. Active and Inactive Areas <ul><ul><li>soils maps and reports : </li></ul></ul><ul><ul><li>historical records of flooding and deposition </li></ul></ul><ul><ul><li>field examination of morphologic features </li></ul></ul><ul><ul><li>aerial photographs </li></ul></ul><ul><li>Data Sources and Age Indicators </li></ul>
    11. 16. Defining Active and Inactive Areas <ul><li>Data Sources and Age Indicators </li></ul><ul><li>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 </li></ul><ul><li>diagnostic vegetation </li></ul><ul><li>the analysis should consider more than one source of information and cross-correlations between them </li></ul>
    12. 17. Defining Active and Inactive Areas <ul><li>Older alluvial fan surfaces are considered active if any of the following are true: </li></ul><ul><ul><ul><li>The recently active sedimentation zone is presently migrating into the older surface </li></ul></ul></ul><ul><ul><ul><li>The elevation difference between the recently active sedimentation zone and the older surface is small relative to flood, deposition and debris depths </li></ul></ul></ul><ul><ul><ul><li>Upstream of the site there is an opportunity of avulsions </li></ul></ul></ul>
    13. 19. Stage 2 – Tiger Wash Piedmont
    14. 20. Alluvial Fan Website tsd/ FT_alfan.htm