FLOOD01621

• County: Statewide
• Community: State of Colorado
• Title: Flood Hazard Delineation on Alluvial Fans and Urban Floodplains
• Date: 1/1/2001
• Prepared For: State of Colorado
• Prepared By: J.S. O'Brien
• Floodplain - Doc Type: Educational/Technical/Reference Information

I . IN'l'RODUCTION

This report introduces a compui:er model, FLO-2D, for routing
water floods and mudflows over unconfined surfaces i.n two-
dimensions. It was initially desigl1E!d for simulating flood hazards
on alluvial fans but can also simulate overbank river flooding in
urban areas. The focus of simulating alluvial fan and floodplain
flows is to predict a range of hydraulics for various flow
scenarios. It will prove valuablE" to floodplain managers coping
with the problems of floodplain zoning and regulations, flood
insurance and the design of flood con1:rol measures.

FLO-2D has been used on a variElty of hydraulic projects that
include: flood delineation on coalescing alluvial fans; design of
a flood containment wall; water flooding on an urban floodplain;
channel obstruction at the apex of an alluvial fan; and mudflow
inundation of an urbanized alluvial fan (Table 1).

Table 1. FLO-2D Proj,~t Applications

PrOject Name and location Date of
CompletIon
L Flood Insurance Study; 3/89
Cornet Creek, Telluride, CO
2. Letter of Map Revision 8/89
Hik.o Springs Wash, Laughlin NV
3. Flood Inundation Study, 6189
Unnamed Wash,
Laugh lin, NV
4. Mudflow Inundation Study 11/88
Rudd Creek
Cavia County, UT
5, Flood Insurance Study, 1/90
Detailed Supplemental Invllstlgatlon
Cornet Creek, TelluTlde, CO
6, Poudre River Flood Inundation Study 11/89
Fort Colhns, CO

7. Letter of Map Revision, Submittal to FEMA tO/89
San Seval"" and Etiwanda Alluvial Fans
San Bernardino, CA

Scope of Project

DelineatIon of mudflow and water ftood hazard on a developed allUVial fan. PredIcted
flow velOCIty and depth for ovorbank f100dmg due to Obstructed chann"l.

PredictIOn of hydr.lulic flow properties for the design of a flood conta.lI1ment wall in
an alluvial wash, 1=loorl flows were rout"d three mIles over 1111 alluvi<t.l wasn.

Delineation of flooding hazard of split flows and cnannel overbank llooding through
a subdiviSion on an alluvial wash. Failure of a concrete channel reqUired lllmulatlon of
flows split by a sn,aU I,m and diverted through a housll1g development.

Prediction of l1lul:Ulow nydraullc.."l origll18tll1g from a landslide that culminated In a mudflow
over a developel:l alluvilll fan. Field data and .erral photos wer" used to verify FLQ-2D
model. Simul.t.t~ld the C....tlon of VI.COUS mudflow 111 the r"sldentlal .re..

Detailed analysi,!l of flow hydraulics at the fan apex using 25 ft squaro grid elements.
ThIS detailed ar>alysis mmulatec! levee fallure and predicted flow direction at the fan
apex.

Delineation of an ~Irea of inundation along a two-mile reach of the Poudre River.
Compared With HE,C-2 analYSIS. Simulation was used for vsnflcatlOn of the FLQ-2D
model for water fll)oding In relatively flal, urban arllas.

Analysis of two".dlm.n$lonal flooding, hydraulics and inundation of coalsscll1g allUVIal
fans. PrlldlCtod hydravllcs for the deSign of flood mitigation, Several flow scenarios
werll analyzed tel evaluato hydrauliCS for dosign of a flood channel and flood contall1ment
levees.

The report is organized into ei'ght chapters. Following brief
introductory and background chaptEn"s, Chapter III discusses the
physical processes of flooding on alluvial fans. The Federal
Emergency Management Agency (FEMA) method for predicting flow
hydraulics on alluvial fans is presented in Chapter IV. Chapter V
contains the technical background and constitutive equations for
the FLO-2D model. Verification of the model using field data is
discussed in Chapter VI. Chapter VII describes two applications of
the model and conclusions are presentnd in Chapter VIII.

1