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