INTRODUCTION TO FLUVIAL FANS AND FAN-DELTAS

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CHAPTER 32 INTRODUCTION TO FLUVIAL FANS AND
FAN-DELTAS
Fluvial fans and fan-deltas form wherever rivers
deposit sediment. A fluvial fan is a completely
terrestrial feature. The fan itself may be
drained at the downstream end by a river, or may
not be drained at all. A fluvial fan-delta is a
fan that ends in standing water such as a lake, a
reservoir or the ocean.
Copper Creek Fan, Death Valley, USA. Image
courtesy Roger Hooke.
Fan-delta of the Mangoky River, Malagasy
Republic. Image from Internet.
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CHARACTERISTICS OF FLUVIAL FANS AND FAN-DELTAS
Fluvial fans and fan-deltas are depositional
zones that are larger than the rivers that create
them. Fluvial fans and fan-deltas spread out
laterally. The river(s) on them access the fan
surface by migration and avulsion (channel
jumping), so creating the characteristic
fan-shaped surface. The long profile of a river
on a fan is driven to be upward-concave by
sediment deposition. Fluvial fans and fan-deltas
tend to prograde outward in space as sediment
deposits. Fans and fan-deltas tend to form in
zones of tectonic subsidence. Subsidence creates
a hole that is filled with sediment. Fan-deltas
are strongly influenced by variations in the
level of standing water (base level).
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A LARGE FLUVIAL FAN
The Kosi River flows southward from the Himalaya
Mountains and deposits a large fan drained by the
Ganges River. The fan is located within a
subsiding foreland basin between the uplifting
Himalaya Mountains to the north and the highlands
of India to the south. Most of the sediment
carried by the Kosi River deposits on the fan and
never reaches the Ganges River.
Kosi River and Fan, India (and adjacent
countries). Image from NASA https//zulu.ssc.nas
a.gov/mrsid/mrsid.pl
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CHANNEL SHIFT ON A LARGE FLUVIAL FAN
Channel shift on the Kosi River was introduced in
Chapter 25. The map makes clear the fan-shaped
deposit created by channel shift.
Channel shift on the Kosi Fan. Adapted from Gole
and Chitale (1966).
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CHANNEL SHIFT ON AND PROGRADATION OF A FAN-DELTA
The Yellow River Delta, China, is a muddy delta
that subsides due to compaction driven by the
weight of its deposits. This subsidence acts to
limit delta progradation.
Channel shifting in the Yellow River Delta From
Sun et al. (2002) based on Pang Si (1983).
Yellow River Delta, China. Image from
NASA https//zulu.ssc.nasa.gov/mrsid/mrsid.pl
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THE FAN AND ITS RIVER(S)
At any given time a fan may contain a single
river, or multiple distributaries. These rivers
may be meandering or braided.
The Kosi River is braided in its upper reaches.
The river
The fan
The same river is meandering in its lower reaches.
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A FAN CREATED BY MEANDERING RIVER(S)
The Okavango River forms a large fan where it
flows into a graben (zone of subsidence
associated with extension of the continental
crust) in Botswana, Africa.
Meandering channel on the Okavango Fan. Image
courtesy N. Smith.
Satellite view of Okavango Fan.
Okavango Fan, Botswana, Africa. Image from Smith
et al. (1997).
Image from NASA https//zulu.ssc.nasa.gov/mrsid/m
rsid.pl
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FAN-DELTAS CREATED BY BRAIDED RIVERS
A sandur is a large fan or fan-delta created by a
braided stream carrying sediment from a glacier.
The word is Icelandic in origin. The braided
Kurobe River is confined by dikes to protect the
cultivated land on the fan.
Skeithara Sandur, Iceland. Image courtesy H.
Johannesson.
Kurobe Fan-delta, Japan. Image courtesy S. Ikeda.
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FANS AND FAN-DELTAS AT VARIOUS SCALES
Laboratory fan-delta, 3 m. Image taken at St.
Anthony Falls Laboratory, University of Minnesota
USA.
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FANS AND FAN-DELTAS AT VARIOUS SCALES contd.
Fan created by runoff from cultivated field 6
m. Image taken by author near Pigeon Point,
California.
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FANS AND FAN-DELTAS AT VARIOUS SCALES contd.
Fan in Idaho, USA created by runoff from burned
hillside, 50 m.
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FANS AND FAN-DELTAS AT VARIOUS SCALES contd.
Copper Creek Fan, Death Valley, USA 10
km. Image courtesy Roger Hooke.
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FANS AND FAN-DELTAS AT VARIOUS SCALES contd.
Kosi River Fan, India 125 km. Image from
Internet.
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BAJADAS
A bajada is a set of closely-spaced fans that
have amalgamated to form a single linear
morphology. Two examples are shown below.
Bajada in western China
Bajada in Death Valley, California, USA
Images from NASA https//zulu.ssc.nasa.gov/mrsid/
mrsid.pl
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FLOWS THAT CREATE FANS

• Fans may be created by deposition from a) debris
  flows, b) sheet flows and c) river flows.
• A debris flow is a dense flow that contains
  similar amounts by weight of water and sediment.
• A sheet flow is a broad, unchannelized open
  channel flow that may cover a significant
  fraction of the fan (e.g. 30) during a single
  flood.
• A channelized flow is within a meandering or
  braided channel.
• Debris flow and sheet flow fans tend to occur on
  slopes that are much steeper than fluvial fans
  created by channelized flows. The two do,
  however, have a range of overlap.
• Here the case of fluvial fans created by
  channelized flows are considered in detail. It
  is of use, however, to view some debris flow fans
  before proceeding.

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A DEBRIS FLOW (JAPAN)
Double-click on the image to see the video.
Video courtesy Paul Heller.
rte-bookjapandebflow.mpg to run without
relinking, download to same folder as PowerPoint
presentations.
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HARVEY CREEK FAN, PAPUA NEW GUINEA
Harvey Creek Fan, Papua New Guinea is a fan
dominated by debris flows created by the disposal
of mine waste. It grades smoothly into a braided
stream (Ok Mani) downstream.
Mine disposal site
Zone of valley wall erosion
Harvey Creek Fan
Braided Ok Mani
Image courtesy Ok Tedi Mining Ltd.
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HARVEY CREEK FAN, PAPUA NEW GUINEA contd.
While the fan is mostly formed by debris flows,
fluvial flow also plays a role. Bill Dietrich of
the University of California Berkeley serves as
scale.
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A FAN-DELTA CREATED BY A DEBRIS FLOW EVENT
A combination of debris flows and sheet flows
associated with the Vargas Disaster, Venezuela,
1999 destroyed
the town of Carmen de Uria.
December, 1999
March, 1999
Images courtesy José Lopez, Universidad Central
de Venezuela, Venezuela.
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A FAN-DELTA CREATED BY A DEBRIS FLOW EVENT contd.
Image courtesy José Lopez, Universidad Central de
Venezuela, Venezuela.
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FLUVIAL FAN-DELTAS
Fluvial fan-deltas occur where rivers meet lakes
(e.g. reservoirs) or the ocean, creating a
depositional environment. The example here is
that of a fan-delta prograding into a reservoir.
The image from 1938 is from before dam
installation. The circle denotes a fixed point
that allows tracking of progradation.
Lake Altoona, Eau Claire River, USA.
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DEPOSITIONAL STRUCTURE OF FAN-DELTAS
The deposits of fan-deltas can be divided into
three zones a coarse-grained aggradational
topset emplaced by fluvial deposition, a
coarse-grained progradational foreset emplaced
by avalanching and a fine-grained aggradational
bottomset emplaced by plunging turbidity currents
or rain from surface plumes. Subsidence may
limit or stop progradation.
The foreset may be at or near the angle of repose
(in which case it is called a Gilbert delta), but
is usually well below this angle. In a sand-bed
stream, the topset and foreset are sandy and the
bottomset is muddy. In a gravel-bed stream the
topset and foreset are often composed of gravel
and coarse sand, and the bottomset of finer sand
and mud.
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AN EXAMPLE SEDIMENTATION IN LAKE MEAD, COLORADO
RIVER, USA (based on an original from Grover and
Howard, 1937)
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EMPLACEMENT OF THE TOPSET BY BRAIDED STREAMS IN
AN EXPERIMENTAL FAN-DELTA UNDERGOING SUBSIDENCE
(Cazanacli et al., 2002)
Double-click on the image to see the video clip.
rte-bookXESbasinsurfflow.avi to run without
relinking, download to same folder as PowerPoint
presentations.
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EMPLACEMENT OF COARSE-GRAINED TOPSET AND FORESET
AND FINE-GRAINED BOTTOMSET IN A LABORATORY FLUME
(Kostic and Parker, 2003a,b)
Double-click on the image to see the video clip.
rte-bookmudsanddelta.mpg to run without
relinking, download to same folder as PowerPoint
presentations.
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DELTAS AND FAN-DELTAS ARE OFTEN THE SITES OF
RIVER DISASTERS
Approach to bridge on Boundary Creek Fan, New
Zealand, destroyed by flood. Image courtesy S.
Coleman.
Bridge on Skeithara Sandur, Iceland destroyed by
Jokullhaup flood of 1996. Image courtesy H.
Johannesson
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OR DISASTERS WAITING TO HAPPEN
Image from FEMA website, USA
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THE MISSISSIPPI DELTA PROBLEM
The Mississippi River forms a fine-grained
fan-delta as it approaches the Gulf of Mexico.
The delta subsides by compaction under its own
weight.
Image from NASA https//zulu.ssc.nasa.gov/mrsid/m
rsid.pl
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THE MISSISSIPPI DELTA PROBLEM contd.
The river has a bed of fine sand, but carries
copious amounts of mud. The river has gradually
avulsed eastward across its fan-delta since the
end of the last glaciation (Fischetti,
2001). The surface of the fan subsides under
compaction by its own weight. Without
replacement of this sediment, shoreline must
trangress, or move inland. In the fan-deltas
natural state, the sediment was replaced by
overbank deposition as the river flooded and the
channel avulsed, so that net progradation
(regression) resulted.
Image courtesy C. Paola
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THE MISSISSIPPI DELTA PROBLEM contd.
Dikes all along the Mississippi River prevent
overbank deposition of both mud and sand. As a
result, the river now aggrades within its levees,
and the surrounding fan surface is rapidly
subsiding under compaction without replacement.
Mississippi River and levees downstream of New
Orleans.
Subsiding fan-delta surface behind levees south
of New Orleans.
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THE MISSISSIPPI DELTA PROBLEM contd.
The river has aggraded to the point that it is
poised to avulse into the Atchafalaya River
through the Old River. It is prevented from
doing so by the structure shown below.
Mississippi River
Red River
Old River
Atchafalaya River
The Old River Control Structure, Louisiana
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THE MISSISSIPPI DELTA PROBLEM contd.
Subsidence rates are now so high that the
shoreline is rapidly moving landward. The entire
delta, and the city of New Orleans in particular,
are now at risk. It has been predicted that by
2090 the seashore will have advanced to New
Orleans (Fischetti, 2001). The city may be
destroyed by a hurricane well before this time.
New Orleans
Zone of rapid shoreward coastline advance
Satellite image from the Internet.
Image courtesy L. Quezergue
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A BEAUTIFUL IMAGE IN CLOSING THE FAN-DELTA OF
THE SELENGA RIVER AT LAKE BAIKAL, RUSSIA Image
from NASA https//zulu.ssc.nasa.gov/mrsid/mrsid.p
l
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REFERENCES FOR CHAPTER 32
Cazanacli, D., Paola, C. and Parker, G., 2002,
Experimental steep, braided flow application to
flooding risk on fans, Journal of Hydraulic
Engineering, 128(3), 1-9. Gole, C. V. and
Chitale, S. V., 1966, Inland delta building
activity of the Kosi River, Journal of Hydraulic
Engineering, ASCE, 92(2), 111-126. Grover, N.C.,
and Howard, C.L., 1937, The passage of turbid
water through Lake Mead, Transactions, American
Society of Civil Engineers, 103, 720-732. Kostic,
S. and Parker, G., 2003a, Progradational sand-mud
deltas in lakes and reservoirs. Part 1. Theory
and numerical modeling, Journal of Hydraulic
Research, 41(2), 127-140. Kostic, S. and Parker,
G., 2003b, Progradational sand-mud deltas in
lakes and reservoirs. Part 2. Experiment and
numerical simulation, Journal of Hydraulic
Research, 41(2), 141-152 Pang, J. Si, S., 1983,
Fluvial Process of the Yellow River Estuary,
Proceedings, International Symposium on River
Sedimentation, Beijing, China, March 24-27, 1980,
Guanghua Press, 417-425 (in Chinese). Fischetti,
M., 2001, Drowning New Orleans, Scientific
American, October. Smith, N. D., McCarthy, T. S.,
Ellery, W. N., Merry, C. L. Ruther, H., 1997,
Avulsion and anastomosis in the panhandle region
of the Okavango Fan, Botswana, Geomorphology, 20,
49 65. Sun, T., Paola, C., Parker, G. and
Meakin, P., 2002, Fluvial fan-deltas Linking
channel processes with large-scale
morphodynamics, Water Resources Research, 38(2),
doi10.1029/2001WR000284.