DSpace at VNU: Correlating mass physical properties with ALOS reflectance spectra for intertidal sediments from the Ba Lat Estuary (northern Vietnam): An exploratory laboratory study

Geo-Mar Lett DOI 10.1007/s00367-013-0327-1 ORIGINAL Correlating mass physical properties with ALOS reflectance spectra for intertidal sediments from the Ba Lat Estuary (northern Vietnam): an exploratory laboratory study Nguyen Thi Ngoc & Katsuaki Koike & Nguyen Tai Tue Received: 31 October 2012 / Accepted: 27 March 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Characterization of the sediment composition of tidal flats and monitoring of their spatiotemporal changes has become an important part of the sustainable management of coastal environments To accurately classify sediments through remote sensing, a comprehensive understanding of sediment reflectance spectra is indispensable The present laboratory-based study explores the performance of the high spatial resolution (10×10 m) Advanced Land Observing Satellite (ALOS) launched in 2006 Relationships between reflectance spectra (bands to 4) and four typical mass physical properties were investigated under wet and dry experimental conditions for intertidal sediments sampled near the Ba Lat Estuary in northern Vietnam Reflectance in the near-infrared region corresponding to ALOS band (0.76–0.89 μm) was found (1) to have a strong negative correlation with sand content (dry wt%) under both wet and dry conditions (linear correlation coefficient r=–0.7859 and –0.8094, respectively), (2) to increase with decreasing relative water content (%) in a N T Ngoc Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan N T Ngoc Faculty of Geology, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam K Koike (*) Department of Urban Management, Graduate School of Engineering, Kyoto University, Katsura C1-2-215, Kyoto 615-8540, Japan e-mail: koike.katsuaki.5x@kyoto-u.ac.jp N T Tue Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan given sediment type (r=–0.7748 to –0.9367 for mud, sandy mud, muddy sand, and sand), (3) to have a positive correlation with organic matter content (r=0.7610 and 0.6460 under wet and dry conditions for contents >0.20 dry wt%), and (4) to be insignificantly correlated with mineral composition assessed in terms of contents (wt%) of quartz, clay minerals, and mica group minerals Positive relationships between reflectance and water content for the pooled data of all sediment types (r=0.6395) or organic matter content contrast with previous findings, and can be attributed to close interrelationships between these properties and the predominance of sand content as controlling factor of reflectance This study clarifies that ALOS band provides the most useful imagery for intertidal monitoring because its reflectance, as simulated using the laboratory data, shows the strongest correlation with sand content In a next step, these experimental findings should be verified by identifying the reflectance relationships at satellite image scales, and also considering the effects of other tidal flat features on reflectance, such as microtopography and biological surface characteristics Introduction Estuaries commonly act as traps for sediments, nutrients, and chemical contaminants (e.g., Dalrymple et al 1992; Eisma 1997; Roy et al 2001; Swaney et al 2008; Taljaard et al 2009) From an ecological point of view, tidal flats lining estuaries play a crucial role for estuarine ecosystems because they provide nesting, feeding, and spawning grounds for invertebrate animals, fish, and birds (Dyer et al 2000) The sediment composition of tidal flats is particularly important because of its strong correlation with species distribution patterns and the structure of animal communities—e.g., bivalves and shellfish—which have been shown to vary with the Geo-Mar Lett type of bottom sediment (e.g., Ysebaert and Herman 2002) Furthermore, sediment distribution can also be a key factor for understanding hydrodynamic conditions, biogeochemical cycles, and morphological change (e.g., Dalrymple et al 1992; Ryu et al 2004; Taljaard et al 2009) Despite these important functions, tidal flats in many parts of the world have been degraded or destroyed by overexploitation, reclamation, industrialization, pollution, but also by detrimental effects of climate change (Dyer et al 2000) The mass physical sediment characteristics of tidal flats and the monitoring of their spatiotemporal change has therefore become an indispensable part of the sustainable management of such coastal environments Remote sensing is one of the most effective monitoring tools because of its relatively low cost, rapid information updating, synoptic view, and repetitive observation capability In this context, a number of remote sensing studies have been devoted to clarifying the sediment composition of intertidal environments (e.g., Yates et al 1993; Rainey et al 2003; Ryu et al 2004; Sørensen et al 2006; van der Wal and Herman 2007; Choi et al 2011) In addition, geomorphological features such as the surface roughness, and tidal channel density of tidal flats (e.g., Bartholdy and Folving 1986; Doerffer and Murphy 1989; Tyler et al 1996; van der Wal et al 2005; Ryu et al 2008; Choi et al 2011) have been characterized with reasonable accuracy These studies essentially examined the usefulness of image classification techniques for monitoring tidal flats by integrating mainly field spectra and satellite imagery By factor analysis of Landsat TM (Thematic Mapper) data, Doerffer and Murphy (1989) revealed that three factors—namely, topography, water content, and surface temperature (in this order)—have the greatest effect on the reflectance spectra of main sediment types in the Wadden Sea Yates et al (1993) found that three image classification techniques (maximum likelihood classification, regression modeling, and spectral mixture modeling) can distinguish mudflats more precisely than sandy flats using Landsat TM bands 1–3 data The spectral unmixing technique can also be appropriate for classifying sediment types into wet mud, dry mud, wet sand, and dry sand from Airborne Thematic Mapper (ATM) images (Rainey et al 2003), and for classification of heterogeneous tidal flats (van der Wal and Herman 2007) In other approaches, the abundance of microphytobenthos on tidal flats has been assessed by means of the absorption spectra of chlorophyll a at about 0.675 μm using laboratory and/or field data in combination with remotely sensed data (e.g., Doerffer and Murphy 1989; Paterson et al 1998; Deronde et al 2006; Kromkamp et al 2006; Murphy et al 2008; Adam et al 2011) These studies made use of hyperspectral and hyperspatial data such as aerial photography with a ground resolution of 0.4 m (Doerffer and Murphy 1989) and a HyMap TM Scanner at 4×4 m pixel resolution (Deronde et al 2006) In addition to satellite image analyses, fundamental studies on the reflectance spectra of intertidal sediments under laboratory conditions are indispensable as ground-truthing for the accurate classification of sediment composition Intertidal sediment consists of three main components: mineral grains, organic matter, and interstitial water Together with the geometric size of the mineral grains, these components strongly affect the reflectance characteristics of the sediment (Asrar 1989; Rainey et al 2000) As shown by Hunt (1977) and Clark (1995, 1999), the wavelength and depth of absorption of reflectance also varies over the visible to infrared part of the spectrum as a function of mineral class (e.g., silicates, oxides and hydroxides, carbonates, and borates) Furthermore, organic carbon content influences the reflectance spectra in the visible to near-infrared region (Stoner and Baumgardner 1981; Sinha 1987; Korsman et al 1999) Several recent studies have considered the effect of grain size and water content on the reflectance of intertidal sediments: the reflectance was measured in the field and/or laboratory, or calculated from satellite images (e.g., Bryant et al 1996; Rainey et al 2000; Ryu et al 2004; van der Wal and Herman 2007; Small et al 2009) Through in situ and laboratory reflectance experiments, Rainey et al (2000) demonstrated that reflectance in the simulated ATM band is positively correlated with sand content, but negatively with interstitial water content Ryu et al (2004) revealed that the Landsat ETM bands and are effective for detecting grain-size composition and surface water content, respectively From relationships between grain size and water content of sediments and laboratory reflectance, Small et al (2009) suggested that grain size and water content may be mapped by duplicating the laboratory reflectance and hyperspectral imagery Most of these earlier studies, however, focused on individual sediment parameters, the effects of interrelationships among them being rarely considered, especially in the case of remotely sensed satellite data In the past, satellite-derived reflectance spectra were mostly based on Landsat data (Landsat TM and ETM+), which had (and still have) a moderate spatial resolution (30×30 m; e.g., Bartholdy and Folving 1986; Doerffer and Murphy 1989; Yates et al 1993) Due to the relatively low resolution, the mapping accuracy was also low, possibly because of the effect of variable water contents (Ryu et al 2004), and mixtures of reflectance spectra caused by footprint overlaps between adjacent sedimentary facies (Sørensen et al 2006) This shortcoming can today be reduced by using satellite imagery from the ALOS (Advanced Land Observing Satellite) AVNIR-2 (Advanced Visible and Near Infrared Radiometer type 2), launched in 2006 The higher spatial resolution (10×10 m) of this satellite would greatly improve the precision of spatial mapping However, applications for the monitoring of tidal flats are currently still very limited Geo-Mar Lett Within this general context, the Ba Lat Estuary (BLE) in northern Vietnam was selected as a case study site, mostly because the BLE was proclaimed a Ramsar site in 1989 (Ramsar Convention Bureau 1997) and subsequently given the status of a nature reserve in 1995, owing to its rich biodiversity as a wetland ecosystem Nevertheless, the tidal flats of the BLE have in recent decades been negatively affected by both natural processes associated with coastal erosion and deposition (van Maren 2007), and human interventions in the form of coastal development, especially land reclamation, shrimp farming, and the exploitation of intertidal benthic organisms Monitoring the tidal flats of the BLE is therefore indispensable for conserving its natural resources and the environment in general Based on the above background, this article aims to clarify comprehensive characteristics of reflectance spectra of intertidal sediments under laboratory conditions with respect to sediment grain-size and mineral composition, as well as water and organic matter contents, as fundamental parameters in remote sensing studies The results were then used to specify which of the ALOS band(s) were the most suitable for obtaining good correlations between the reflectance data and particular sediment types Such specifications can contribute substantially to the mapping of sediment types by means of satellite imagery Study area The Red River originates in the Yunnan highlands of China where the widespread occurrence of red laterite soils give local river waters its reddish-brown color It has a length of 1,200 km and a catchment area of 160,000 km2 (Milliman et al 1995) The Red River flows southeastward through large cities and industrial centers (e.g., Phu Tho, Ha Noi, Thai Binh, and Nam Dinh) before draining into the Gulf of Tokin The main discharge period coincides with the summer monsoon season The sediment load of the Red River (approx 160×106 a b c d Fig Location of the study area showing the Ba Lat Estuary in northern Vietnam, and the positions of the 101 sampling stations on the tidal flat Photos are examples of a a shoal, b a shrimp pond, c a mangrove forest, and d a clam farm Geo-Mar Lett metric tons per year) is ranked 9th worldwide Siltation in the distributaries of the lower delta region resulting from the high sediment load in combination with tidal pumping, for example, is a constant threat to the waterway leading to the harbor of Haiphong (Lefebvre et al 2012) The BLE, which is the largest estuary in the Red River system of northern Vietnam (Fig 1), is shaped roughly like a broad arrow jutting out into the sea It acts as a gateway for sediment currently transported from the land to the sea at a rate of 31×106 m3 annually This volume is estimated to be equivalent to 38 % of the total sediment transported by the Red River system per year (Duc et al 2007) The water discharge of the Red River Delta is characterized by seasonal variations Daily discharges are low in the dry season, ranging from 1,500 to less than 1,000 m3/s at the hydrological station of Son Tay, which is located on the Red River at the entrance of the delta High discharges occur in the rainy season with an average of 14,000 m3/s, a maximum value of 33,600 m3/s having been recorded in August 1971 The mean concentration of sediment varies from 0.2 kg/m3 in the dry season to 1.4 kg/m3 in the rainy season, reaching kg/m3 during floods (van Maren 2005) The tides in the BLE are diurnal and classified as mesotidal, averaging at about 2.3 m and ranging from 0.1 to 3.7 m (Marine Hydro-meteorological Center 2009) The tidal influence, in particular salinity intrusion, is conspicuous in the Red River Delta in the dry season Seawater intrudes up to 20 km landward from the Ba Lat mouth (Luu et al 2010) For the purpose of this study, a tidal zone within m water depth at high spring tides was selected (Fig 1) There are numerous islets and shoals, the latter composed mainly of well-sorted and relatively homogenous fine sand (Duc et al 2007) Mangroves grow naturally in the northern part of Con Lu and along tidal creeks, new mangroves having been planted in the muddy flats of the southern part of Con Lu since 1997 The mangroves on the Con Vanh and Con Ngan islets were converted to shrimp ponds starting in the 1980s (Fig 1) The tidal flats in the northeastern and southwestern parts of the estuary are occupied by clam farms (Meretrix lyrata) Rapid deposition has occurred along the BLE coastline in the last 50 years, extending the land several kilometers downstream and offshore (van Maren 2007) Materials and methods Field sampling and laboratory treatment Duplicates of about 200 g of intertidal sediment were collected at 101 stations (Fig 1) from the uppermost layer (