Modeling Alluvial Fan Flood Hazards From Geologic Maps: Integrating Recent Quaternary Geologic Mapping With FEMA Alluvial Fan Flooding Guidelines

Hypothesis: Surficial Geologic Maps Can Be Used For Flood & Debris Flow Hazard Delineation A.What is a Surficial Geologic map? B.Where Surficial Geologic maps are available? C.How can a Surficial Maps be used for planning purposes? – Preliminary Studies – Identifying areas for Site-Specific Studies D.Prescribed FEMA analyses : – Composition/Morphology/Location – Delineate Active and Inactive – Model Design Flood Event

Old Very Young Young Qvyf, Very young, < 500 ybp Qyf, Young, 500 – 10,000 ybp (younger) Qof, Old, >10,000 ybp Qvof, Very old >>10,000 ybp

Examples of Surficial Geologic Mapping Used for Flood and Debris Flow Hazard Delineation A.Flood - Pelletier & Pearthree (2005) - Surficial Geologic Maps correlate well with probabilistic modeling, but can be overly conservative - Field & Pearthree (1997) - Geomorphic Mapping may be used to identify active and inactive surfaces B.Debris Flow - Giraud (2005) – Divide the Fan into Proximal/Medial/ Distal, Identify Holocene Debris Flow Deposits - Kellerahals & Church (1990) – Identify fan, map debris flow deposits and define their ages

Deposit Age Flood & Debris Flow Hazard

Surficial Geologic Map of the La Cresenta, La Canada, and Montrose Area

Holocene Pleistocene Geologic Age Holocene Surficial Alluvial Fan Units ≈ 2/3 of area Pleistocene Surficial Alluvial Fan Units ≈ 1/3 of area

1934 Flood - Thanksgiving 1933 fire - New Years Day 1934 storm

1934 Flood Boundaries Over Surficial Geologic Map

Holocene Pleistocene Geologic Age Flood & Debris Flow Affects by Total: Holocene = 99% Pleistocene = 1%

How Can Surficial Geologic Maps be Used for Floodplain Delineation? Delineation of Alluvial Fan Landform - FEMA Stage I Composition Morphology Location

Delineation Of Active Surfaces – FEMA Stage II D Avulsions Delineate Flood Hazard Areas On Active Surfaces - FEMA Stage III

Summary & Conclusions Surficial Geologic Maps May be used for planning purposes Surficial Geologic Maps May Help Assess FEMA Stage I and Assist in Targeting Areas for Additional Investigation Augmentation of Surficial Maps with Aerial Photo and Topographic Map Data May Help Define FEMA Stage II – Active and Inactive Surfaces using : - Historic Flooding - Avulsion Indicators - Debris Flow Deposits - Site-Specific field work Composite Map of Active Fan Surfaces for FEMA Stage III - Use to Approximate Floodplain - Use to Model 100-year/Design Flood