LECTURE 7 TRIGGERING MECHANISMS OF TURBIDITY CURRENTS

1. CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS LECTURE 7 TRIGGERING MECHANISMS OF TURBIDITY CURRENTS What causes turbidity currents? Scripps and La Jolla Submarine Canyons, San Diego, California http://clasticdetritus.com/category/marine-science/

2. A PARTIAL LIST OF TRIGGERING MECHANISMS • Hyperpycnal flows • Surface waves • Breaching • Double-diffusive phenomenon • Transformation from slope failures and submarine debris flows • Internal waves • Wave-supported sheet turbidity currents

3. DENSITY OF FRESH WATER The density of (sediment-free) water varies with temperature (e.g.  C), salinity (e.g. grams/liter) and pressure (~ depth according to the hydrostatic law). Here we consider the effect of temperature and salinity. Fresh water has a maximum density of 1 ton/m3 at 4 C. Double-click to activate Excel spreadsheet.

4. DENSITY OF SEA WATER The “standard” density of sea water is ~ 1.026 tons/m3. This corresponds to a salinity of 35000 mg/liters (35 grams/liter) and a temperature of 14 C. Increasing temperature (usually) makes water lighter, and increasing salinity makes it heavier. You can find a calculator of seawater density at http://www.csgnetwork.com/h2odenscalc.html The density of seawater can, however, vary considerably. In brackish coastal waters near river mouths, the density can approach that of freshwater, i.e. 1.000 tons/m3. In nearly-enclosed seas with high evaporation rates, such as the Red Sea, the salinity can be as high as 40000 mg/liters. Assuming, for example, a water temperature of 20 C, the density is about 1.029 tons/m3

5. 1. PLUNGING AND HYPERPYCNAL FLOWS Reuss River plunging into Lake Lucerne, Switzerland: flood of summer, 2005

6. WHAT IS PLUNGING? WHAT IS A HYPERPYCNAL FLOW? Plunging is a phenomenon when sediment-laden river flow is heavier than the body of water it flows into. The sediment water immediately sinks, forming a continuous turbidity current. Plunging is usually associated with muddy flows, whereas (most of the) sand (most of the time) tends to deposit in a delta upstream. Plunging is very common in lakes and reservoirs. The resulting turbidity currents can emplace sediment over long distances. Delta of Colorado River Lake Mead

7. PLUNGING IN LAKE MEAD plunge line Logjam near plunge point Image from USBR

8. PLUNGING CREATES A HYPERPYCNAL FLOW Hyper  excess, pycnal  density. So a hyperpycnal flow is a continuous bottom turbidity created by the excess density of the river flow relative to the ambient standing water due to the presence of sediment. A (usually muddy) turbidity current is created by hyperpycnal conditions. Coarser sediment deposits in delta (topset and foreset). Finer sediment (mud) deposits in bottomset.

9. HYPOPYCNAL FLOWS A hypopycnal flow is a sediment-laden surface plume created when the sediment-laden river water is less dense than the ambient water into which it flows. Sediment gradually rains out from the surface plume (hemipelagic sedimentation) to emplace the bottomset. Surface plume of muddy water Coarser sediment deposits in delta (topset and foreset). Sediment gradually rains out to emplace the bottomset.

10. HYPOPYCNAL SURFACE PLUMES ALONG THE ADRIATIC SHORELINE OF ITALY Hypopycnal plumes along the Adriatic margin of Italy: image from J. Syvitski.

11. TURBIDITES IN LAKE MEAD EMPLACED BY PLUNGING TURBIDITY CURRENTS Twichell et al. (2006)

12. TURBIDITES IN LAKE MEAD EMPLACED BY PLUNGING TURBIDITY CURRENTS Seismic image of deposits in the west end of the Virgin Basin. Twichell et al. (2006)

13. RELATION BETWEEN SEDIMENT CONCENTRATION IN MG/L AND WATER DENSITY Let X denote the concentration in mg/l. Where f equals the density of the sediment-laden flow, rw is the density of the river water without sediment and s denotes the density of sediment. The volume concentration C is given as The density of the sediment-laden flow is thus

14. HOW EASY IS IT TO CREATE PLUNGING IN FRESHWATER? The minimum condition for plunging is: Suppose the lake has a temperature of 10 C (l = 0.9997 t/m3). We consider two cases: the river water has a temperature of 10 C (l = 0.9997 t/m3) and 20 C (rw = 0.9982 t/m3). In the latter case there is a temperature barrier to plunging). The minimum concentration is for plunging can be calculated with the spreadsheet. (Double-click to activate.) The minimum concentrations are 0 mg/l and 2413 mg/l. As shown on the next slide, even a concentration of 2413 mg/l during floods is not uncommon in mountain streams.

15. SUSPENDED SEDIMENT CONCENTRATION IN RIVERS Sediment concentrations are based on mean flows rather than flood flows. From USGS website.

16. SUSPENDED SEDIMENT CONCENTRATION IN THE MINNESOTA RIVER

17. CAN PLUNGING AND HYPERPYCNAL FLOWS OCCUR IN THE OCEAN? Let f equals the density of the sediment-laden flow, rw is the density of the river water without sediment and sea denotes the density of sea water. The minimum concentration for plunging into seawater is: If rw = 1 ton/m3 and sea = 1.026 t/m3, C must be at least 0.0158, and X must be at least ~ 40,000 mg/l in order to get plunging.

18. THIS CONDITION IS RARELY MET IN RIVERS Sediment concentrations are based on mean flows rather than flood flows. From USGS website.

19. AND IT IS ESPECIALLY RARELY MET IN LARGE, LOWLAND RIVERS FLOWING INTO PASSIVE MARGINS

20. AND SO HYPERPYCNAL FLOWS ARE NOT LIKELY TO BE RESPONSIBLE FOR THE EMPLACEMENT OF MOST LARGE SUBMARINE FANS ON PASSIVE MARGINS. Mississippi Submarine Fan meandering channel on fan

21. AND YET SOME RIVERS SOMETIMES DO FORM HYPERPYCNAL FLOWS WHEN THEY REACH THE SEA And when they do they can move enormous amounts of sediment into the sea. From International Journal of Sediment Research Plunging of the Yellow River into the Bohai Sea, China

22. NOW WHERE MIGHT RIVERS FLOWING INTO THE SEA HAVE SUCH HIGH CONCENTRATIONS OF SUSPENDED SEDIMENT? Active margins undergoing rapid uplift! 16 of the rivers listed by Mulder and Syvitski (1995) that go hyperpycnal at least once per 100 years are in Taiwan Erosion rates in mm/year for Taiwan. Image courtesy C. Stark

23. A DOCUMENTED CASE OF HYPERPYCNAL FLOW TO THE OCEAN The Eel River in Northern California carries a very high sediment load. It is estimated to become hyperpycnal once every 10 years.

24. HYPERPYCNAL EVENT ON THE EEL RIVER A hyperpycnal event was recorded and documented in the flood of 1995. From Imran and Syvitski (2000)

25. 2. TURBIDITY CURRENTS IN CANYONS GENERATED BY COASTAL SURFACE WAVE ACTION One of the first field measurements of turbidity currents in the ocean was performed in Scripps Submarine Canyon off San Diego, California (Inman et al., 1976). The turbidity currents were generated by wave action, which was in turn driven by a winter storm.

26. LITTORAL DRIFT The canyon is not located near a river mouth. The sediment (fine sand) is delivered from river mouths to the head of the canyon by littoral drift, mostly during the summer. The sediment piles up at the head of the canyon. shore Littoral drift is an along-coast flow of sediment driven by incident waves that are not parallel to the shore. Alongshore sediment transport Incident waves Reflected waves

27. STORMS, INCIDENT WAVES, AND EDGE WAVES Incident waves edge waves Incoming waves from storms oscillate in the cross-shore direction. These can generate trapped edge waves, which are standing nearshore waves that oscillate in the along-shore direction. shore The antinodes of these edge waves locate themselves at depressions, e.g. canyon heads. A’ A A’ A

28. THESE EDGE WAVES STIR UP SEDIMENT AT THE CANYON HEAD The sediment-laden seawater so created is heavier than the ambient sediment-free seawater.

29. AND THE RESULT IS A DOWN-CANYON TURBIDITY CURRENT The current so created is sustained as long as the edge waves are present and there is sediment available in the canyon head (hours, or even days). A storm immediately subsequent to one that generated a turbidity current often creates no turbidity current, because there is no longer any sediment available.

30. THESE TURBIDITY CURRENTS CAN BE QUITE STRONG!

31. 4. TURBIDITY CURRENTS CREATED BY BREACHING Breaching is the sustained, slow failure of a near-vertical subaqueous face of slightly overdensified clean, fine sand by spalling of sediment from the face. See van den Berg et al. (2002); Mastbergen and van den Berg (2003).

32. BREACHING HAS BEEN USED BY THE DREDGING INDUSTY IN THE NETHERLANDS TO INSTIGATE SELF-SUSTAINED REMOVAL BY MEANS OF SPALLING TO A TURBIDITY CURRENT

33. A SLOW BREACH FAILURE LAUNCHES A BOAT

34. BREACHING IN THE LABORATORY (courtesy T. Muto)

35. WHY THE NEAR-VERTICAL FACE? As a particle starts to fail off a modestly overdensified face of clean, fine sand, the negative pore pressure created by the gap prevents it from failing catastrophically, and instead leads to slow grain-by-grain spalling. K = hydraulic conductivity of sand [L/T] p = porosity of sand [1] cb = retreat speed of breach [L/T] Eb = volume erosion rate/surface area of sediment [L/T]

36. GENERATION OF THE TURBIDITY CURRENT

37. TURBIDITY CURRENT GENERATED BY BREACHING IN THE LABORATORY

38. SOME TURBIDITY CURRENTS IN THE MONTEREY SUBMARINE CANYON APPEAR TO BE GENERATED BY BREACHING Xu, J. P., M. A. Noble, and L. K. Rosenfeld (2004)

39. A DOCUMENTED TURBIDITY CURRENT IN THE MONTEREY SUBMARINE CANYON • Sustained event: lasted 5 - 8 hours • Max. velocity ~ 1.9 m/s • Thicker downcanyon? • Not caused by storm or hyperpycnal flow (failure of dredge spoil by breaching?)

40. 4. TURBIDITY CURRENTS GENERATED BY DOUBLE DIFFUSION Double-diffusion phenomena were discovered in the context of the ocean. As noted in a previous slide, (sediment-free) oceanic water varies in both salinity and temperature. A higher salinity makes water heavier. A higher temperature makes water lighter. In the ocean, the density of water is controlled by both factors. In oceanic waters, heat and salt can be fluxed by both convection and molecular diffusion. Here we are interested in flux by molecular diffusion. Diffusion fluxes a quantity from a zone of high concentration to low concentration. Consider any quantity per unit volume b (e.g. heat in joules per unit volume or salt in grams per unit volume. We further assume that this quantity decreases in the x direction. Diffusive flux transports a quantity from high concentration to low concentration, or in this case in the positive x direction.

41. WHAT IS DOUBLE DIFFUSION? contd. Let Fbd,x denote the diffusive flux of quantity b (quantity crossing face/time/face area) in the x direction. If b decreases in x, then Fbd > 0 Fbd,x b x Thus any quantity diffuses down its spatial gradient. Now let  denote temperature and s denote salinity.

42. WHAT IS DOUBLE DIFFUSION? contd. We denote the temperature as  and the salinity as s. The diffusive flux of heat and salt in the x-direction are denoted as Fhd, x and Fsd,x. These terms are given as where w is the density of the water, cp is the specific heat of water at constant pressure (e.g. no. of joules required to raise 1 kg by 1 C) and Dh and Ds denotes the kinematic diffusivity of heat and salt (e.g. m2/s). Typical values of cp, Dh and Ds are 4.18 x 103,1.45 x 10-7 m2/s and 1.35 x 10-9 m2/s The point to note here is: This means that in a relative sense, salt diffuses much less rapidly than heat.

43. THE FIRST CASE OF DOUBLE DIFFUSION: HENRY STOMMELS PERPETUAL SALT FOUNTAIN Warm, less salty Consider an aluminum tube that extends vertically from the deep ocean to the surface. We assume that warm, saline water at the surface overlies cold, less saline water at depth. The water is stably stratified, i.e. the deep water is denser than the surface water. But suppose we start a vertical flow in the pipe. The flow will sustain itself creating a “perpetual” salt fountain! Why? As the flow in the pipe rises, heat diffuses across the pipe walls, warming the pipe water to the temperature of the seawater surrounding it. But salt cannot diffuse in due to the walls. So the water from depth arrives at the surface with the same temperature as the surface water, but a lower salinity. Being lighter than seawater, it fountains upward! Warm, more salty Heat diffuses in, but not salt cold, less salty

44. DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS Consider hypopycnalriver freshwater entering the sea. For simplicity, we assume that both are at the same temperature. The coarse sediment deposits to form a delta, and the fine sediment, i.e. mud, forms a surface plume.

45. DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS contd. The kinematic diffusivity of mud, which is governed by Brownian motion, is far less that the kinematic diffusivity of salt. Consider a blob of mud-laden, fresh water at the interface.

46. DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS contd. Salt can diffuse into the blob much faster than sediment diffuses out. As a result, the blob can get heavier than the surrounding saltwater and sink. Parsons and Garcia (2000)

47. THE BLOBS CAN JOIN TOGETHER TO CREATE A CONTINUOUS BOTTOM TURBIDITY CURRENT Note: this mechanism is as yet unverified in the field.

48. 5. TRANSFORMATION FROM SLOPE FAILURES AND SUBMARINE DEBRIS FLOWS Submarine landslides and debris flows can generate turbidity currents as sediment is entrained into suspension from their heads (and also their bodies). The landslide/debris flow can come to rest, but the turbidity current can run out much father distances.

49. THE GRAND BANKS SUBMARINE LANDSLIDE/ TURBIDITY CURRENT http://earthnet-geonet.ca/communities/earthquake_e.php An earthquake in 1929 generated the Grand Banks failure, which produced a huge submarine landslide that devolved into a debris flow, and then into a turbidity current that ran long distances. The layer of sand deposited by the turbidity current covered a surface the size of Quebec. Maximum turbidity current velocities may have been as high as 18 m/s, as evidenced from the timing of transatlantic submarine cable breaks. This event served to catalyze interest in turbidity currents as a submarine process for the redistribution of sediment (e.g. Heezen and Ewing, 1952; Kuenen, 1952).

50. EXPERIMENTAL EVIDENCE FOR TURBIDITY CURRENTS GENERATED BY SUBMARINE DEBRIS FLOWS