CHAPTER 32: INTRODUCTION TO FLUVIAL FANS AND FAN-DELTAS

1. 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-deltais 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.

2. 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).

3. 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

4. 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).

5. 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

6. 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.

7. 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/mrsid.pl

8. 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.

9. FANS AND FAN-DELTAS AT VARIOUS SCALES Laboratory fan-delta, ~ 3 m. Image taken at St. Anthony Falls Laboratory, University of Minnesota USA.

10. 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.

11. FANS AND FAN-DELTAS AT VARIOUS SCALES contd. Fan in Idaho, USA created by runoff from burned hillside, ~ 50 m.

12. FANS AND FAN-DELTAS AT VARIOUS SCALES contd. Copper Creek Fan, Death Valley, USA; ~ 10 km. Image courtesy Roger Hooke.

13. FANS AND FAN-DELTAS AT VARIOUS SCALES contd. Kosi River Fan, India; ~ 125 km. Image from Internet.

14. 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

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. A FAN-DELTA CREATED BY A DEBRIS FLOW EVENT contd. Image courtesy José Lopez, Universidad Central de Venezuela, Venezuela.

21. 1938 1988 1951 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.

22. 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.

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

24. 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.

25. 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.

26. 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

27. OR DISASTERS WAITING TO HAPPEN Image from FEMA website, USA

28. 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/mrsid.pl

29. 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-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. Image courtesy C. Paola

30. 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.

31. 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

32. 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

33. 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.pl

34. 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.