Short-term changes in seafl oor character due to fl ood-derived hyperpycnal discharge: Typhoon Mindulle, Taiwan, July 2004 J.D. Milliman School of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062, USA S.W. Lin Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan S.J. Kao Research Center for Environmental Change, Academia Sinica, Taipei 115, Taiwan J.P. Liu Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA C.S. Liu J.K. Chiu Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan Y.C. Lin ABSTRACT amount of sediment, however, is signifi cant; During Typhoon Mindulle in early July 2004, the Choshui River (central-western Taiwan) since 1965 the Choshui River has discharged discharged ~72 Mt of sediment to the eastern Taiwan Strait; peak concentrations were ≥200 ~1.5–2 Gt of sediment to the Taiwan Strait. g/L, ~35%–40% of which was sand. Box-core samples and CHIRP (compressed high-intensity radar pulse) sonar records taken just before and after the typhoon indicate that the hyper- STUDY APPROACH pycnal sediment was fi rst deposited adjacent to the mouth of the Choshui, subsequently To document the impact of a hyperpycnal re suspended and transported northward (via the Taiwan Warm Current), and redeposited as event, we initiated a preliminary study of the a patchy coastal band of mud-dominated sediment that reached thicknesses of 1–2 m within seafl oor off the Choshui River from 22–26 megaripples. Within a month most of the mud was gone, probably continuing its northward June 2004. Using the Taiwanese ship RV OR-2, transit, but more mud appeared in late August in response to Typhoon Aere. Following autumn we collected 400 km of high-resolution seis- and winter storms, the entire nearshore area was again sand dominated by early spring. mic data using an Edgetech X-STAR 0512i CHIRP sonar profi ler, and collected 16 box- Keywords: hyperpycnal, Taiwan, typhoon, seafl oor. core and grab samples from nearshore waters (Fig. 1). Two days after the cruise, the approach INTRODUCTION HYPERPYCNAL EVENTS IN TAIWAN of Typhoon Mindulle provided an immedi- In recent years there has been increased inter- The best place to document hyperpycnal ate opportunity to monitor a typhoon-induced est in the potential impact of hyperpycnal events events is probably Taiwan, because of their hyperpycnal event and its impact. (wherein high concentrations of suspended frequency and severity there. Between 1979 Prior to, during, and after the typhoon (1–5 sediment, >~40 g/L, allow river water to sink and 1998, 11 Taiwanese rivers experienced at July), one of us (Kao) oversaw the sys tematic beneath ambient ocean water into which it is least 100 hyperpycnal events (Milliman and sampling of suspended sediment from the discharged) on the coastal environment. Mulder Kao, 2005), most of them typhoon related, but Choshui River at the Ziuchian Bridge gauging and Syvitski (1995) identifi ed a number of global some enhanced by the erosion of earthquake- station, 8 km from the Taiwan Strait. Depth- rivers that periodically reach hyper pycnal con- generated landslide scars and deposits (Hovius integrated water samples were obtained using centrations, but because such events may last et al., 2000; Dadson et al., 2004, 2005; Galewsky a vertically mounted 1 L bottle attached to a only a few hours, often during severe storms, et al., 2006). Because these estimates are based weighted metal frame that was gradually lowered only a few have been documented (Waananen, on infrequent observations and because many from the bridge. The U.S. Geological Survey 1969; Warrick and Milliman, 2003; Milli- events last only a few hours, the actual number DH-48/76 sampler was not used because of its man and Kao, 2005). Moreover, the impact of of Taiwan hyperpycnal events almost certainly diffi culty in sinking in turbulent fl ows >250 cm/s. hyperpycnal events on the adjacent ocean fl oor is far greater. During super Typhoon Herb Measured peak concentrations (130–200 g/L), is unclear. If fl ow is suffi ciently dense and fast (August 1996), the Choshui River (watershed however, agree with those reported by the Tai- and if the shelf gradient is suffi ciently steep, the area 3300 km2; Fig. 1) discharged an estimated wan Water Resources Agency during typhoons sediment might bypass the littoral cell, perhaps 125 Mt of sediment into the Taiwan Strait, with Herb and Toraji, suggesting that our samples eroding as it transits to deeper water (Mulder peak concentrations >150 g/L (Milliman and represent actual conditions. We collected 23 and Syvitski, 1995; Mulder et al., 2003). Sedi- Kao, 2005). An even greater event occurred samples at ~3 h intervals; except for measure- ment discharged during southern California during Typhoon Toraji (July 2001), the fi rst ments on southern California rivers in 1969 events, for example, soon reach offshore basins major typhoon following a M 7.9 earthquake (Waananen, 1969), these samples may represent (Drake et al., 1972; Gorsline, 1996), but it is not 2 yr earlier, when >200 Mt of sediment were the fi rst closely spaced samples taken during a clear whether the sediment bypasses the shelf transported, with peak concentrations probably hyperpycnal event. immediately or is temporarily stored (Warrick >200 g/L (Dadson et al., 2004, 2005). Upon return to the laboratory the water and Milliman, 2003). Although the impact of these hyper pycnal samples were passed through a 62 μm sieve, In this paper we discuss the fl ux and fate of events on the adjacent seafl oor is undocu- and total suspended sediment and suspended hyperpycnal discharge of fl uvial sediment to mented, the absence of mud throughout most sand concentrations were determined. Using the Taiwan Strait off western Taiwan. We are of the eastern Taiwan Strait (Boggs et al., 1979) discharge data (from the Ziuchian Bridge unaware of any previous study that has moni- and the general erosional character of the west- gauging station) supplied by the Taiwan Water tored a hyperpycnal event or has documented ern Taiwan coastline indicate that their cumula- Resources Agency, we derived two distinctly nearshore conditions immediately before and tive effect, if any, is short-lived. Whether fl ood- different rating curves, a steeper curve repre- after such an event, although Mailet et al. derived sediment is temporarily stored on the senting conditions up to and including the fi rst (2006) used digital terrain maps to delineate inner shelf (and whether the hyperpycnal fl ows peak discharge, and a second curve represent- morphologic changes off the Rhône River fol- might locally erode the seafl oor) or is directly ing pre-, late, and post-Mindulle conditions lowing a large fl ood. transported to deeper water is not known. The (Fig. 2A). Calculated concentrations derived © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2007; 2007 v. 35; no. 9; p. 779–782; doi: 10.1130/G23760A.1; 4 fi gures. 779 120°00′E 120°20′ 120°40′ In the early stages of Mindulle, sediment concentrations in the lower Choshui increased J 0 N -2 to 190–200 g/L (Fig. 2E); sand concentrations 0 -3 W E 0 were >40 g/L. These high concentrations may -4 0 S -5 refl ect erosion of 1999 earthquake landslide K debris. Subsequent sediment concentrations 24°20′S 24°20′ were <100 g/L, even though peak discharge was greater than during the fi rst peak; suspended Taiwan Strait Wu River sand concentrations, however, remained high N (Fig. 2B). Total sediment discharge during the 4 day period was 72 Mt, ~35%–40% of which was sand. Peak loads occurred at 7 a.m. on 3 July and 10 a.m. on 4 July, 2.6 and 1.5 Mt/h, respectively, the fi rst peak in response to high B -40 B-1 sediment concentrations and the second in response to high discharge (Figs. 2D, 2E, 2F). 0 3 - -40 Although the 72 Mt represent roughly twice 0 Fig.3A,B the Choshui’s annual sediment load, they were 24°00′ -3 24°00′ 25 26 R considerably less than the loads transported 0 -3 during Typhoon Herb (130 Mt; Milliman and Fig.3C Kao, 2005) or Toraji (200 Mt; Dadson et al., Taiwan T 2004), refl ecting Mindulle’s relatively low dis- 3 3 T1 charge (6600 m /s versus ~19,000 m /s during T2 28 27 Herb and Toraji). Choshui River SEAFLOOR IMPACT BY MINDULLE The seafl oor in the east-central Taiwan Strait V U prior to Typhoon Mindulle was much like that 23°40′ 23°40′ described by Boggs et al. (1979), with fi ne to 39 coarse sand locally punctuated by asymmetri- cal sand waves indicative of strong current and km tidal activity. Surface sediments were mostly 97%–100% sand, although sediments south of 40 0 5 10 the Choshui (station U) and north of the Wu River (stations K and J; Fig. 1) were mud domi- 120°00′ 120°20′ 120°40′ nated. At station T2, immediately north of the Figure 1. Map of southwestern Taiwan, showing tracklines and coring stations. Bold solid Choshui (Fig. 1), organic matter had C/N and lines indicate June CHIRP (compressed high-intensity radar pulse) profi les; where they dif- δ13C ratios of 7.5‰ and 23.46‰, respectively, fer substantially from the early cruise, July profi les are delineated as bold dashed lines. indicative of a marine origin. Lower right inset: Taiwan and surrounding waters. On 9–10 July, fi ve days after the passage of Mindulle, the nearshore (<20 m depth) area between the Choshui and Wu Rivers contained from the two rating curves agree closely with TYPHOON MINDULLE what appeared on CHIRP profi les as a thin layer measured values (Fig.