chapter 32: introduction to fluvial fans and fan-deltas

1D SEDIMENT TRANSPORT MORPHODYNAMICSwith applications to

RIVERS AND TURBIDITY CURRENTS

CHAPTER 32:INTRODUCTION TO FLUVIAL FANS AND FAN-DELTAS

Fan-delta of the Mangoky River, Malagasy Republic.

Image from Internet.

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.



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).



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.nasa.gov/mrsid/mrsid.pl



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).



CHANNEL SHIFT ON AND PROGRADATION OF A FAN-DELTA

Channel shifting in the Yellow River Delta

From Sun et al. (2002) based on Pang & Si (1983).

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.

Yellow River Delta, China.Image from NASA;

https://zulu.ssc.nasa.gov/mrsid/mrsid.pl



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 same river is

meandering in its lower reaches.

The river

The fan



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.

Okavango Fan, Botswana, Africa.Image from Smith et al. (1997).


Satellite view of Okavango

Fan.



FAN-DELTAS CREATED BY BRAIDED RIVERS

Skeithara Sandur, Iceland.Image courtesy H. Johannesson.

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.

Kurobe Fan-delta, Japan.Image courtesy S. Ikeda.



Laboratory fan-delta, ~ 3 m.Image taken at St. Anthony Falls Laboratory, University of Minnesota USA.

FANS AND FAN-DELTAS AT VARIOUS SCALES



Fan created by runoff from cultivated field; ~ 6 m.Image taken by author near Pigeon Point, California.

FANS AND FAN-DELTAS AT VARIOUS SCALES contd.



Fan in Idaho, USA created by runoff from burned hillside, ~ 50 m.




Copper Creek Fan, Death Valley, USA; ~ 10 km.Image courtesy Roger Hooke.




Kosi River Fan, India; ~ 125 km.Image from Internet.




BAJADASA 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



FLOWS THAT CREATE FANS

Fans may be created by deposition from a) debris flows, b) sheet flows and c) river flows.

a) A debris flow is a dense flow that contains similar amounts by weight of water and sediment.

b) 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.

c) 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.



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.



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.



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.



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

Images courtesy José Lopez, Universidad Central de Venezuela, Venezuela.

the town of Carmen de Uria.

March, 1999December,

1999



Image courtesy José Lopez, Universidad Central de Venezuela, Venezuela.

A FAN-DELTA CREATED BY A DEBRIS FLOW EVENT contd.



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.

1988

1938

1951

Lake Altoona, Eau Claire River, USA.

FLUVIAL FAN-DELTAS



DEPOSITIONAL STRUCTURE OF FAN-DELTAS

topset

foreset

bottomset

antecedent bed

coarse-grained

fine-grained

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.



AN EXAMPLE: SEDIMENTATION IN LAKE MEAD, COLORADO RIVER, USA(based on an original from Grover and Howard, 1937)



EMPLACEMENT OF THE TOPSET BY BRAIDED STREAMS IN AN EXPERIMENTAL FAN-DELTA UNDERGOING SUBSIDENCE (Cazanacli et al., 2002)

rte-bookXESbasinsurfflow.avi: to run without relinking, download to same folder as PowerPoint presentations.

Double-click on the image to see the video clip.



EMPLACEMENT OF COARSE-GRAINED TOPSET AND FORESET AND FINE-GRAINED BOTTOMSET IN A LABORATORY FLUME (Kostic and Parker, 2003a,b)

rte-bookmudsanddelta.mpg: to run without relinking, download to same folder as PowerPoint presentations.

Double-click on the image to see the video clip.



DELTAS AND FAN-DELTAS ARE OFTEN THE SITES OF RIVER DISASTERS

Bridge on Skeithara Sandur, Iceland destroyed by Jokullhaup flood of

1996.Image courtesy H. Johannesson

Approach to bridge on Boundary Creek Fan, New

Zealand, destroyed by flood. Image courtesy S. Coleman.



OR DISASTERS WAITING TO HAPPEN

Image from FEMA website, USA



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.



THE MISSISSIPPI DELTA PROBLEM contd.

Image courtesy C. Paola

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-delta’s natural state, the sediment was replaced by overbank deposition as the river flooded and the channel avulsed, so that net progradation (regression) resulted.



Mississippi River and levees downstream of New Orleans.


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.

Subsiding fan-delta surfacebehind levees south of New

Orleans.



The Old River Control Structure, Louisiana


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.

Red River

Old River

Atchafalaya River

Mississippi River



Image courtesy L. Quezergue

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.

Satellite image from the Internet.

New Orleans

Zone of rapid shoreward coastline advance



A BEAUTIFUL IMAGE IN CLOSING: THE FAN-DELTA OF THE SELENGA RIVER AT LAKE BAIKAL, RUSSIA




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), doi:10.1029/2001WR000284.

chapter 32: introduction to fluvial fans and fan-deltas

Documents

kosi fan

large fan

okavango fan

fan surface

plthe fan

fluvial fandelta

fandeltas form

fandeltasfluvial fans