Stream power framework for predicting geomorphic change: The 2013 Colorado Front Range flood

doi:10.1016/j.geomorph.2017.03.004

Geomorphology

Volume 292, 1 September 2017, Pages 178-192

https://doi.org/10.1016/j.geomorph.2017.03.004 Get rights and content

Abstract

The Colorado Front Range flood of September 2013 induced a diverse range of geomorphic changes along numerous stream corridors, providing an opportunity to assess responses to a large flood in a semiarid landscape. We defined six classes of geomorphic change related to peak unit stream power and valley confinement for 531 stream reaches over 226 km, spanning a gradient of channel scales and slope. Geomorphic change was generally driven by erosion of channel margins in confined reaches and by a combination of deposition and erosion in unconfined reaches. The magnitude of geomorphic change typically increased with unit stream power (ω), with greater responses observed in unconfined channels. Cumulative logit modeling indicated that total stream power or unit stream power, unit stream power gradient, and valley confinement are significant predictors of geomorphic response for this flood event. Based on this dataset, thresholds for geomorphic adjustment were defined. For channel slopes < 3%, we noted a credible potential for substantial channel widening with ω > 230 W/m² (16 lb/ft-s; at least 10% of the investigated sites experienced substantial channel widening) and a credible potential for avulsions, braiding, and loss of adjacent road embankments associated with ω > 480 W/m² (33 lb/ft-s; at least 10% of the investigated sites experienced such geomorphic change). Infrequent to numerous eroded banks were very likely with ω > 700 W/m² (48 lb/ft-s), with substantial channel widening or major geomorphic change shifting from credible to likely. Importantly, in reaches where there were large reductions in ω as the valley form shifted from confined to relatively unconfined, large amounts of deposition-induced, reach-scale geomorphic change occurred in some locations at relatively low ω. Additionally, alluvial channels with slopes > 3% had greater resistance to geomorphic change, likely caused by armoring by larger bed material and increased flow resistance from enhanced bedforms. Finally, we describe how these results can potentially be used by practitioners for assessing the risk of geomorphic change when evaluating current or planned conditions.

Introduction

Predicting the settings where geomorphic change can be expected as a result of flooding is valuable for informing environmental management and policy, as well as specific project designs. Human encroachments into stream corridors, including roadway and railroad alignments, commercial developments, and private residences, are key drivers of flood hazards. Geomorphic change and subsequent flood hazards include localized streambank erosion, hillslope and terrace failure, reach-scale channel widening, sediment deposition and associated loss of channel and floodplain capacity, rapid downstream meander migration, and channel avulsions and braiding. Where allowed by the valley form, sufficient floodplain extent and connectivity is needed to diminish the effects of erosive forces on streambanks, floodplain and terrace surfaces, infrastructure and property, and riparian and aquatic habitats. The identification of hydraulic thresholds beyond which a substantial potential exists for geomorphic change of channel and floodplain form are consequently quite valuable, but hindered by variability in driving mechanisms (peak discharge, flow duration, channel and floodplain slope, stream power, etc.) and resisting mechanisms (flow resistance, bank composition, vegetation type and extent, rip rap, etc.).

The 2013 Colorado Front Range flood provides an opportunity to assess geomorphic changes associated with a large flood in a semiarid landscape. This event, which impacted a substantial geographic extent and numerous streams in and adjacent to the Colorado Front Range foothills, allows us to relate diverse geomorphic changes to unit stream power (ω) and other variables. Observed responses ranged from undetectable or minimal impacts upon cross sections and planforms to major changes including widening, braiding, overwhelming erosion of road embankments, hillslopes and terraces, and extensive aggradation. Numerous avulsions were also observed. Additionally, a large number of landslides and debris flows occurred (> 1100; Coe et al., 2014), contributing large point sources of sediment and water to many of the most-impacted streams. The largest rainfall accumulations were observed on the mid-elevation foothills (elevations primarily from 2000 to 3000 m; Fig. 1). The flood response extended from confined, high-gradient reaches within canyons of the foothills to wide and lower-gradient high plains stream valleys; thus, the geomorphic variability and extent of affected reaches provide a valuable opportunity to explore relationships between hydraulic descriptors such as ω and observed geomorphic changes.

Given the limited available guidance for predicting varying types and degrees of geomorphic change resulting from large floods, we initiated a project with the following objectives:

•
develop an ordinal classification scheme describing diverse types of geomorphic change and apply it to stream reaches impacted to varying degrees by the 2013 Front Range flood;
•
assess reach-specific hydraulic variables that quantify driving mechanisms of geomorphic change and assess their effectiveness for predicting observed responses; and
•
identify ω thresholds associated with specific types of geomorphic change and compare these results to previously published findings from floods in other regions.

Section snippets

Background

Floodplain mapping based on flood-frequency relationships and hydraulic modeling of inundation extent and elevation is the standard method for characterizing flood hazards and informing floodplain management (e.g., Federal Emergency Management Agency, FEMA, floodplain planning and mapping). However, the area within stream corridors subject to flood hazards caused by geomorphic change typically is not considered. Flood hazard maps based on inundation alone can underestimate compounding threats

Study area

During this flood, large portions of the Colorado Front Range foothills (Fig. 1) received heavy rainfall, with up to 460 mm falling in 10 days. The majority of the precipitation in and along the Front Range fell during 36 h on 11–12 September 2013. Rain gauge data from the most severely affected areas of the foothills indicate that up to 380 mm fell in Larimer County, 460 mm fell in Boulder County, and 410 mm fell in El Paso County (Community Collaborative Rain, Hail and Snow Network (CoCoRaHS), 2013

Results

A total of 531 reaches were assessed on 226 km of streams impacted to various extent by the 2013 flood event in the Cache la Poudre River, Big Thompson River, Little Thompson River, Saint Vrain Creek, Left Hand Creek, Boulder Creek, and Coal Creek watersheds, which are all located in the South Platte River basin in and adjacent to the Colorado Front Range foothills. Reach lengths varied from 71 to 500 m, with an average of 426 m. A variety of stream channel scales and types were included in the

Discussion

The Colorado Front Range flood impacted streams along a substantial extent of the foothills and high plains in September 2013, from Fort Collins south to Pueblo. Infrastructure, homes, businesses, stream functions, and ecosystem services were disturbed by the flooding and negatively impacted by emergency response measures within the riparian corridors. Owing to the large spatial extent of flooding induced from this event, from narrow foothill valleys to wide high plains stream corridors,

Summary and conclusions

This study has utilized an extensive data set from the 2013 Colorado Front Range Flood to relate channel adjustments to descriptors of driving processes and geomorphic setting. We defined six classes of geomorphic change related to stream power and valley confinement for 531 stream reaches over 226 km, spanning a gradient of channel scales, slopes, and watershed areas. The geomorphic change classes (and median ω values) were: (1) no detected geomorphic change; (2) infrequent eroded streambanks

Acknowledgements

We thank Sara Rathburn and John Moody, as well as an anonymous reviewer, for constructive reviews that substantially improved the manuscript. Additional review and editorial work by Richard Marston, Gene Bosley, and David Levinson is also greatly appreciated.

The many hydrologists and hydraulic engineers who collected the peak flow data utilized in this analysis are greatly appreciated. This includes individuals with the U.S. Geological Survey and the Colorado Division of Water Resources, for

References (79)

C. Cunnane
Unbiased plotting positions — a review
J. Hydrol.
(1978)
I.C. Fuller
Geomorphic impacts of a 100-year flood: Kiwitea Stream, Manawatu catchment, New Zealand
Geomorphology
(2008)
H. Hajdukiewicz et al.
Impact of a large flood on mountain river habitats, channel morphology, and valley infrastructure
Geomorphology
(2016)
J.M. Hooke et al.
Geomorphological impacts of a flood event on ephemeral channels in SE Spain
Geomorphology
(2000)
A.J. Lee et al.
Velocity and flow resistance in step-pool streams
Geomorphology
(2002)
F.J. Magilligan
Thresholds and the spatial variability of flood power during extreme floods
Geomorphology
(1992)
F.J. Magilligan et al.
The efficacy of stream power and flow duration on geomorphic responses to catastrophic flooding
Geomorphology
(2015)
J.A. Ortega et al.
Geomorphological and sedimentological analysis of flash-flood deposits: the case of the 1997 Rivillas flood (Spain)
Geomorphology
(2009)
N. Surian et al.
Channel response to extreme floods: insights on controlling factors from six mountain rivers in northern Apennines, Italy
Geomorphology
(2016)
C. Thompson et al.
Geomorphic effects, flood power, and channel competence of a catastrophic flood in confined and unconfined reaches of the upper Lockyer valley, southeast Queensland, Australia
Geomorphology
(2013)

M.R. Tye et al.

A spatial model to examine rainfall extremes in Colorado's Front Range

J. Hydrol.

(2015)

J. Aguilar

Colorado re-emerging from $2.9 billion flood disaster a year later

American Association of Floodplain Managers (ASFPM)

Riverine Erosion Hazards White Paper

(2016)

S.W. Anderson et al.

Exhumation by debris flows in the 2013 Colorado Front Range storm

Geology

(2015)

V.R. Baker

Stream-channel response to floods, with examples from central Texas

Geol. Soc. Am. Bull.

(1977)

V.R. Baker et al.

Flood power

G. Benito

Energy expenditure and geomorphic work of the cataclysmic Missoula flooding in the Columbia River Gorge, USA

Earth Surf. Process. Landf.

(1997)

P.M. Biron et al.

Freedom space for rivers: a sustainable management approach to enhance river resilience

Environ. Manag.

(2014)

S. Bizzi et al.

The use of stream power as an indicator of channel sensitivity to erosion and deposition processes

River Res. Appl.

(2013)

G.L. Bodhaine

Measurement of peak discharge at culverts by indirect methods

G.R. Brooks et al.

The drainage of the Lake Ha!Ha! reservoir and downstream geomorphic impacts along Ha!Ha! River, Saguenay area, Quebec, Canada

Geomorphology

(1999)

W.B. Bull

Threshold of critical power in streams

Geol. Soc. Am. Bull.

(1979)

E.M. Buraas et al.

Impact of reach geometry on stream channel sensitivity to extreme floods

Earth Surf. Process. Landf.

(2014)

E.A. Carlson

Fluvial riparian classification for National Forests in the Western United States

D.A. Cenderelli et al.

Geomorphic effects of large debris flows on channel morphology at North Fork Mountain, Eastern West Virginia

Earth Surf. Process. Landf.

(1998)

D.A. Cenderelli et al.

Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal

Earth Surf. Process. Landf.

(2003)

R.H.B. Christensen

A Tutorial on Fitting Cumulative Link Models with the Ordinal Package

R.H.B. Christensen

Ordinal — Regression Models for Ordinal Data. R Package Version

(2015)

M. Church

Geomorphic thresholds in riverine landscapes

Freshw. Biol.

(2002)

J.A. Coe et al.

New insights into debris-flow hazards from an extraordinary event in the Colorado Front Range

GSA Today

(2014)

Community Collaborative Rain, Hail & Snow Network (CoCoRaHS) 2013. Data accessed 9/13/2013....

R. Core Team

R: A Language and Environment for Statistical Computing

(2015)

J.E. Costa

Rheologic, geomorphic, and sedimentologic differentiation of water floods, hyperconcentrated flows, and debris flows

J.E. Costa et al.

Geomorphically effective floods

T. Dalrymple et al.

Measurement of peak discharge by the slope-area method

C. Daly et al.

Physiographically-sensitive mapping of temperature and precipitation across the conterminous United States

Int. J. Climatol.

(2008)

G.C.L. David et al.

Controls on spatial variations in flow resistance along steep mountain streams

Water Resour. Res.

(2010)

O.J. Dunn

Multiple comparisons among means

J. Am. Stat. Assoc.

(1961)

M.V. Ferencevic et al.

Creating and evaluating digital elevation model-based stream-power map as a stream assessment tool

River Res. Appl.

(2012)

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