The document discusses guidelines for determining flood hazards on alluvial fans. It focuses on Stage 2 of FEMA's process, which is determining whether parts of an alluvial fan are active or inactive. It describes identifying features that indicate whether an area is stable or prone to erosion, deposition, and shifting flood paths. Age indicators like soil development and geomorphic features can help delineate active versus inactive areas. Active areas are those still subject to flooding and deposition, while inactive areas are older surfaces no longer changing due to flooding.
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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
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