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Hyperpycnal flows at the Mouth of Huanghe (Yellow River): Past, Present and Future Houjie WANG 1, 2 , Naishuang BI 1, 2 , Z uosheng Y A NG 1, 2 , Y oshiki SAI T O 3 1 C ollege of M ar ine G eosciences, Ocean Univer sity of C hina, 238 Songling R d., Qingdao 266100, C hina 2 K ey L abor ator y of Submar ine Science and Pr ospecting T echniques, Ocean Univer sity of C hina, Qingdao 266100, C hina 3 A I ST , G eological Sur vey of J apan, I G G , C entr al 7, H igashi 1-1-1, T sukuba 305-8567, J apan A G U C hapman C onfer ence Jan. 24-27, 2011 Oxnard, USA Poster Number: T 4 Suppor ted by M OST , C hina (2010C B 951202) and NSF C (40876019) R apid incr ease in the H uanghe (Y ellow R iver ) sediment yield fr om the L oess Plateau at 3000 yr B P was caused by the human inter ventions. A s a r esult, the sediment load deliver ed to the sea incr eased to appr oximately ten folds of the pr istine level (100 M T /yr ) with suspended sediment concentr ation (SSC ) high up to 25 kg/m 3 at the r iver mouth that is favor able for the for mation of hyper pycnal flows in the coastal envir onment. Obser vations fr om cr uises in the 1980s and 1995 suggest that hyper pycnal flows off the H uanghe mouth ar e maintained by high concentr ation of r iver sediment and modulated by tides. T he hyper pycnal flows star t at the slack befor e high water , and dur ing the developing stage the bottom suspended sediment concentr ation incr eases r apidly while the salinity dr astically decr eases and the median gr ain size of suspended par ticles within the hyper pycnal layer incr eases, cr eating a sediment- stratified water column due to the straining effect from tides. Because of the momentum dissipation, they begin attenuating at the slack before low water while the stratification of the water column becomes collapsed owing to the enhanced tidal mixing. A s a r esult, both the sediment concentr ation and median gr ain size of suspended par ticles within bottom layer decr eases. A s coar ser sediment par ticles ar e dumped on the seafloor , the hyper pycnal flows ar e no longer maintained because of density loss. Near ly 90% of the r iver -laden sediment is deliver ed to the sea dur ing the per iod when the hyper pycnal flows ar e pr ominent. G iven the extr emely high SSC at the r iver mouth dur ing the past thousands of year s, hyper pycnal flows have been a dominant patter n for the ter r estr ial sediment disper sal in the coastal ocean. H owever , constr uction of lar ge r eser voir s and soil-conser vation pr actices within the r iver basin dur ing the past sever al decades have r educed the sediment flux to the sea by ~90% and incr eased the gr ain size of suspended sediment deliver ed to the sea (30 um now versus 18 um before in median grain size). Scouring of the channel in the lower reaches has added a new sediment sour ce to those der ived fr om the loess r egion of the middle r eaches. T hose conditions ar e unfavor able for the for mation of hyper pycnal flows at the r iver mouth. Obser vations fr om cr uises after the oper ations of the X iaolangdi R eser voir , suggest that buoyant hypopycnal plumes, r ather than hyper pycnal plumes, have occur r ed at the r iver mouth since the dr amatic changes in concentration and grain size of suspended sediment discharged to the sea. C limate change (E NSO events impacted r egional pr ecipitation) and human activities in the r iver basin have alter ed the 'Sour ce-to-Sink' patter n of the H uanghe sediment as indicated by the infr equency of hyper pycnal flows at the r iver mouth. T he Huanghe has becoming an artificially regulated river, and the delta, similar to the example of the Nile, will be starved. As a r esult, the delta er osion by monsoon activities will pr obably be a dominant sour ce for sediment expor ting to the distal mud deposition on the continental shelf. T he perturbations from climate change and human activities propagate throughout the sediment disper sal system fr om sour ce to sink. ABSTRACT: 95 o 97 o 99 o 101 o 103 o 105 o 107 o 109 o 111 o 113 o 115 o 117 o 119 o 32 o 34 o 36 o 38 o 40 o 42 o 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 GC ASH LJ TNH LYX LJX LZH TDG SMX XLD HYK LM TG Loess Plateau Qinghai-Tibet Plateau Bohai Sea Maowusu Desert Longitude Latitude Gauging Station TNH: Tangnaihai; LZH: Lanzhou; TDG:Toudaoguai; LM: Longmen; TG: Tongguan; HYK: Huayuankou; GC: Gaocun; ASH: Aishan; LJ: Lijin Reservoir LYX: Longyangxia; LJX: Liujiaxia; SMX: Sanmenxia; XLD: Xiaolangdi elevation (m) Lanzhou Toudaoguai Huayuankou Lijin 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 Water discharge Sediment load Water discharge (km 3 /yr) Sediment load (10 9 t) 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 0 4 8 12 16 20 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 0 0.2 0.4 0.6 0.8 1 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 0 20 40 60 80 100 Water discharge (km 3 ) Sediment load (10 9 t) SSC (kg/m 3 ) (A) (B) (C) Year Monthly water discharge (A), sediment load (B) and suspended sediment concentration (C) at Station Lijin from 1950-2009 Before Xiaolangdi Reservoir TNH LZH TDG LM TG SMX XLD HYK GC ASH LJ 0 200 400 600 800 1000 1200 Annual sediment load (Mt/yr) Sediment source A TNH LZH TDG LM TG SMX XLD HYK GC ASH LJ 0 5 10 15 20 25 30 35 Median grain size (μm) B TNH LZH TDG LM TG SMX XLD HYK GC ASH LJ 0 5 10 15 20 25 30 35 Median grain size (μm) B TNH LZH TDG LM TG SMX XLD HYK GC ASH LJ 0 200 400 600 800 1000 1200 Annual sediment load (Mt/yr) Sediment source A 0 20 40 60 80 100 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 20 40 60 80 100 Monthly SSC (kg/m 3 ) Cumulative ratio of sediment load at station Lijin (%) 1950-1999 35 kg/m 3 C 0 20 40 60 80 100 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 20 40 60 80 100 Monthly SSC (kg/m 3 ) Cumulative ratio of sediment load at station Lijin (%) 2000-2006 Month C 35 kg/m 3 After Xiaolangdi Reservoir Operation of Xiaolangdi Reservoir 11:00 16:00 21:00 02:00 07:00 12:00 17:00 22:00 03:00 08:00 13:00 Jun. 21, 2005 Jun. 22 Jun. 23 Time Water depth (m) -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Interrupted by storm D SSC (kg/m 3 ) 10:00 15:00 20:00 01:00 06:00 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0 20 40 60 80 100 120 140 160 180 Sept. 18, 1995 Sept. 19 Water depth (m) Hyperpycnal event Hyperpycnal event Hyperpycnal event <3 PSU <3 PSU >24 PSU SSC (kg/m 3 ) D Time Figure caption: Annual sediment load at stations along the main stream 1950-1999 (A); Annual median grain size at stations along the mainstreas 1950-1999 (B); Monthly SSC at station Lijin 1950-1999 (C) and hyperpycnal plumes observed at the river mouth in September 1995 (D) Figure caption: Annual sediment load at stations along the main stream 2000-2006 (A); Annual median grain size at stations along the mainstreas 2000-2006 (B); Monthly SSC at station Lijin 2000-2006 (C) and hypopycnal plumes observed at the river mouth in July 2005 (D) Before high intervention from human activities on the river system, nearly 90% of the Huanghe-laden sediment is delivered to the sea during the period when the hyperpycnal flows are prominent. Since the middle Holocene, high-SSC drevied hyperpycnal flow at the river mouth has been a dominant pattern for the terrestrial sediment dispersal in the coastal ocean, and has acted as a driving force for the rapid accretion of the mega-delta. Intensive human activities during the past several decades including dam-orientated flow regulation, sediment entrappment and soil-conservation works have altered the river system. The SSC decreased drastically and sediment became much coarser than ever. Those are unfavorable for the formation of hyperpycnal flows at the river mouth. Significant changes have happened for the delta morphology. Sediment transfer within the framework of Source to Sink has been controlled by the different behaviors of human activities, such as the high sediment yeild due to agriculture since the middle Holocene and rapidly decreasing sediment load to the sea due to dam operation during the past several decades. In the future, delta erosion will be a dominant pattern instead of the rapid accretion due to insufficient sediment supply. The delta will be more vulnerable than ever to the sea-level rise and attack from storm surges, as well as to the natural and accelerated subsidence. Average of 1950s Huanghe (Yellow River) catchment Figure caption: Correspondance to H.Wang: [email protected] (A) (B)
