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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*
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
after Bull, 1991
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
Stage 2 – Tiger Wash Piedmont
Alluvial Fan Website
http://www.fema.gov/mit/tsd/FT_alfan.htmhttp://www.fema.gov/mit/tsd/FT_alfan.htm