ASSESSMENT OF SUSPENDED SEDIMENT CONCENTRATION IN THE
SURFACE WATER USING REMOTE SENSING
Asif Mumtaz Bhatti 1, Seigo Nasu 2 and Masataka Takagi 3
1

PhD Student, Department of Infrastructure Systems Engineering, Kochi University of Technology,
Tosayamada-cho, Kami-City, Kochi, 782-8502, Japan. asif_engr@yahoo.com
2, 3Professor, Department of Infrastructure Systems Engineering, Kochi University of Technology, Kochi,
Japan.

ABSTRACT
Remote sensing is an effective tool for the assessment of suspended sediment in surface water. Experiments
were conducted in the laboratory to investigate the spectral response of suspended sediments concentration,distribution and settling with time. A hyperspectral Field SpecPro FR Spectroradiometer was used to measurereflectance from the water in the electromagnetic spectrum region of 400 nm to 900 nm. Reflectance of waterwith varying concentration of clayey red soil ranging from 50 to 1000 mg/l was measured. Significant change
in reflectance was observed for the same suspended sediment concentration (SSC) with time. However, thechange in reflectance was more in visible domain. This paper analyses the relation between suspendedsediment concentration and the reflectance. The near- infrared (NIR) wavelength was found to be optimum
range for assessment of suspended sediment concentration in surface water.
Keywords: Remote sensing, Suspended sediment assessment, Optimum range

1 INTRODUCTION
High concentration of suspended sediment in water is a critical element in the economic feasibility of a projectand could shorten the useful life of reservoirs & dams. It is estimated that approximately 1,100 km3 ofsediments have been accumulated in the world’s reservoirs, taking up almost one fifth of the global reservoir
storage capacity [1]. The reservoir resulting from the construction of a dam in a river is a site for thesedimentation of solid particles transported by the river, due to the decrease in the flow transport capacity. On
the one hand, this sedimentation process has engineering consequences because it leads to a reduction of thestorage capacity of the reservoir [2] and, hence, of its efficiency. It is important to incorporate sedimentassessment as an integral part of water resources and environmental planning and management. The reason forsuccess of remote sensing in such surveys is the strong positive relationship that exists between SSC andremotely sensed spectral reflectance.
Variations of sediment type (grain size and refractive index) and changing illumination conditions affect thereflectance signal of coastal waters and limit the accuracy of sediment-concentration estimations from remotesensing measurements [3]. It is believed that the main differences between the two types of waters are locatedin the 0.4–0.7 m spectral range [4], where the turbid water has significantly larger reflectance than the clearwater. This formed the basis for the detection of turbid water and SSC estimation algorithms. If the range ofsuspended sediments is between 0 and 50 mg/l, reflectance from almost any wavelength will be significantly
related to suspended sediment concentrations [5]. As the range of suspended sediments increases to 200 mg/l
or higher, curvilinear relationships have to be developed with reflectance in the longer wavelength. Significant
relationships have been shown between suspended sediments and radiance or reflectance from spectral wave
bands or combinations of wave bands on satellite sensors. Ritchie et al. [6] using in situ studies concluded thatwavelengths between 700 and 800 nm were most useful for determining suspended sediments in surface
waters.
Suspended sediments are the most common pollutant both in terms of weight and volume in surface waters offreshwater systems. Suspended sediments may serve as a surrogate contaminant in agricultural watersheds
since phosphorus, insecticides, and metals adhere to fine sediment particles. Remote sensing is an effective
tool for the assessment of suspended sediment in surface water. Numerous researchers have developed
algorithms for the relationship between the concentration of suspended sediments and radiance or reflectance.

A few studies have taken the next step and used these algorithms to estimate suspended sediments for anothertime or place [7]. The aim of the study was simply to investigate the relationship between suspended sedimentconcentration (SSC) and reflectance in a column of water. The objective was to understand the change inreflectance with SSC and change in reflectance for the same SSC with time.

2 METHODS
The experiments were performed in Laboratory at Kochi University of Technology. The experiments werecarried out in a black painted room to avoid extraneous reflectance. Field SpecPro FR Spectroradiometer(Analytical Devices, Inc., Boulder, CO) was used to collect the upwelling radiance from the water surface.
This portable spectrometer combines three spectrometers to cover the wavelength range from 350 to 2500 nm.
A photodiode array spectrometer is used to cover the 350 to 1000 nm spectral range (UV/VNIR), while two
fast-scanning (0.1 sec) spectrometers provide coverage for the two wavelength ranges from 1001 to 1800 nm
and 1801 to 2500 nm (SWIR 1 and SWIR 2). The sampling interval is 1.4 nm. Radiation input to the ASD
FieldSpec FR spectrometers is through a fiber optic bundle, 5 meters in length. The fiber optic cable provides
the ability to quickly and easily point the spectrometer field of view at different targets, especially when using
the pistol grip. The spectroradiometer was calibrated in the laboratory before measuring water surface radiance.
The warm up time, sensor height, cable flexure test and illumination angle was measured by making radiancemeasurements of reference panel in a black room. The reference plaque was a Labsphere white Spectralon ofdimensions 12.5cm x 12.5cm.
The clayey red soil samples served as ingredient for sediment loading were collected from the central area of
Okinawa Island (coordinates: 26º23´N and 127º73´E). Soil properties are closely related to geological andgeomorphological components [8]. The soil sample was air dried and manually sieved to assure uniform grain
size. For this study, data from 400 to 900 nm was used because noise was pronounced at wavelength shorterthat 400 nm and longer than 900 nm. The sensor was attached to the mechanical arm and positioned over thewater at the height of 1 m. Radiance from the water surface with suspended sediments concentration (clayey
red soil) ranging from 50 to 1000 mg/l was measured in a black coated water tank. The depth of water column
was kept constant at 1 meter. The sediments were kept in suspension by manually stirring at regular intervals.
In addition, radiance was measured for the same SSC without stirring after 1,2 and 3 minutes to investigate theeffect of SSC distribution on reflectance value.
Reflectance was calculated as a simple ratio between target and reference panel using following equation.

% R (λ ) =L (λ )× Cal ( λ )S (λ )

Where L(λ) is the radiance measured from the water surface, S(λ) is the radiance from the reference panel andCal (λ) is the calibration factor for the reference panel. Using this method, all parameters that are multiplicativein nature, such as the spectral irradiance of the illumination source and the optical throughput of the field
spectrometer, and present in both the spectral response of a reference sample and the target material, aremathematically eliminated.

3. RESULTS AND DISCUSSION
3.1 EQUIPMENT CALIBRATIONThe ASD FieldSpec FR spectroradimeter is calibrated in the laboratory to achieve maximum accuracy ofradiance measurements. The calibration room was completely darkened, the only source of illumination was
the halogen lamp. The instrument’s fiber optic probe is placed in a stationary position at a 45 degree angle tothe Spectralon panel. The collection of accurate spectra requires an awareness of the influences of the various
sources owing to sensitivity of the instrument. Instrument warm up time, field of view, illumination geometry,sensor height above the water surface and target characteristics must be considered for accurate measurements.
The experimental design must be modified to account for the characteristics of the available instrumentation.
Experiment was performed by making radiance measurement of reference panel with different warm up time.

Sensitivity of spectroradiometer changes with temperature, and this is particularly significant when it iswarming up. The measurements were made during instrument warm-up, after the spectrometer was turned on.
11.0000

Fig. 2

Fig. 1R e fl e c ta n c e

0.9800a0.99bc

0.98

Reflectance

0.99000.9700d0.9600

abc

0.97

0.96

0.95000.95

0.9400
400

500

600

700

800

0.94
400

900

500

600

Wavelength (nm)

700

800

900

Wavelength (nm)Figure 1. Reflectance measurement of reference panel with different warm up time after the spectroradiometer
was turned on, a: After 90 minute b: After 120 minute c: After 150 minute d: After 10 minute
Figure 2. Reflectance measurement of reference Panel with different sensor height above the water surface, a:
1.2 meter above the water surface b: 0.5 meter above the water surface c: 1 meter above the water surface

11Fig. 3
0.99

abc0.97

a0.99

b0.98
R e fle c ta n c e

0.98

R e f le c t a n c e

Fig. 4

dc

0.97

0.96

0.96
0.95

0.95
0.94

0.93
400

0.94
500

600

700

Wavelength (nm)

800

900

400

500

600

700

800

900

Wavelength (nm)Figure 3. Reflectance measurement of reference panel with different cable positions, a: one twist in cable b:zigzag cable c: Straight cable d: many twist
Figure 4. Reflectance measurement of reference panel with different illumination angle; a: 60 degree b: 30
degree c: 45 degree

The warm up time of 150 minutes was selected and kept for all experiments (fig.1). Also, radiancemeasurement was taken with different sensor height above the water surface, different illumination angle withconstant distance between the reference panel and the light source and different cable placements. Flexure of
the cable effect the radiance measurement of the target by changing the signal level. Sensor height of 1 meter
above the water surface was selected for all experiments (fig. 2), with straight fiber cable position (fig. 3), andillumination angle of 30 degree (fig. 4) was kept during radiance measurement of water surface with varying
suspended sediment concentration (SSC).
3.2 SUSPENDED SEDIMENT REFLECTANCE
The reflectance spectra were computed for suspended sediment concentration ranging from 50mg/l to 1000
mg/l. The spectral shape is almost similar, however, the magnitude of reflectance increased with increase in
SSC as depicted in figure 5. It is noted that reflectance value increased at all wavelengths above 500 nm with
the increase in SSC. However from 700 to 900 the reflectance increased systematically with increase in SSC.
A strong correlation exists between SSC and reflectance in the red and NIR region. In the present research
work the bottom effect was not considered owing to already done research work. At a depth of 80 cm, thebright bottom signal is virtually non-existent when suspended sediment concentrations were above 100mg/l.
At wavelengths between 740 and 900nm, bottom brightness has no impact due to the absorption of nearinfrared energy by water [9].

0.2

R e f le c t a n c e

0.161000 mg/l
900 mg/l

0.127000 mg/l
400 mg/l

0.08

300 mg/l
200 mg/l

0.04

100 mg/l
50 mg/lClean water

0400

500

600
700
Wavelength (nm)

800

900

Figure 5: Spectral reflectance of water with varying concentration of clayey red soil

The low reflectance value is due to the black coated inner walls of the tank, which absorb much of the incident
light. Maximum reflectance is observed in the visible domain from about 550 to 700 nm followed by thesecond peak at 750 to 850 nm. High absorption is observed beyond 750nm in the near infra-red region.
Increases in suspended sediment load lead to a shift in the reflectance spectral profile upwards. The results
agreed with work done by the numerous researchers.
3.3 DISTRIBUTION OF SUSPENDED SEDIMENTS WITH DEPTH
Assessment of suspended sediment in water bodies is based mainly on assumption of uniform distribution ofsuspended sediments along the depth. Most of the sediment which is eroded from the land surface is in theform of fine particles which are transported in water courses as a suspended load. These fine sediments
account for the turbidity often observed in rivers and their natural fall velocity in water is as low that thenatural turbulence maintains them in suspension. A small proportion of the sediment, perhaps around 10%, iscourser material which is transported as bed load. These sediments roll along the bed of the river or saltate(hop) close to the bed. Reservoirs act as settling basins for both types of sediment [10]. However, thedistribution of sediments is not uniform and varies with depth depending on sediment characteristics,
precipitation, run-off, temperature, water flow, and wind speed. Specular reflection from a wind-roughened
water surface constitutes a serious problem in the interpretation of spectral data [11].
The objective to perform the experiment was to investigate the effect of non-uniform distribution of suspended
sediment on remotely sensed data. Sediments were mixed in water uniformly by manually stirring the water
sample for a few minutes. Spectral reflectance of water is measured for the constant amount of sediments
immediately after the mixing of sediments. Also for the same concentration of suspended sediments thereflectance signals was measured with time without stirring the water sample.
0.14

Fig. 6
0.12

Reflectance

0.1abc
d0.08
0.06
0.04
0.02
0400

500

600

700

800

900

Wavelength (nm)Figure 6. Spectral reflectance of water with constant concentration of clayey red soil (500 mg/l) a: Spectral reflectanceafter manual mixing 500mg/l clayey red soil b: spectral reflectance after 1 minute c: Spectral reflectance after 2 minutes
d: Spectral reflectance after 3 minutes
0.18

Fig. 70.160.14aReflectance

0.12bcd0.1
0.080.06
0.04
0.020400500600700800900

Wavelength (nm)Figure 7. Spectral reflectance of water with constant concentration of clayey red soil (1000 mg/l) a: Spectral reflectanceafter manual mixing 1000mg/l clayey red soil b: spectral reflectance after 1 minute c: Spectral reflectance after 2 minutes
d: Spectral reflectance after 3 minutes

Suspended sediments settled with time in the water and reflectance value changed with the change in sediment
distribution and caused error in accurate estimation of suspended sediments. With the time the reflectancevalue decreases for the same amount of sediments owing to settling of sediment with time. Sediment grain size
is one of the major factors for the distribution of sediments. These results indicate that the sediment
distribution with depth affects the reflectance values significantly.

4 BAND RATIO MODEL DEVELOPMENT

Rrs 825 / Rrs 560

Band ratio model was developed to investigate the relationship between suspended sediment and reflectance.
This relationship is based on a simple reflectance ratio between near-infrared (825nm) and visible (560 nm)
wavelengths. It was observed that strong correlation exists between SSC and the reflectance ratio of nearinfrared & visible wavelength [Rrs (825)/Rrs (560)] as depicted in the figure. The polynomial function plottedon the graph (R2=0.98) allows an accurate estimation of suspended sediments from Rrs measurements. It wasalso observed that in case of multispectral remotely sensed data (Landsat ETM), the ratios of band 4 & band 2plotted against SSC will provide solution for the assessment of suspended sediments.

10.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
02

y = -0.0032x + 0.1022x + 0.1896
2R = 0.9893

0100 200 300 400 500 600 700 800 900 1000
Suspended Sediment (mg/l)

Figure 8. Ratio of reflectance [Rrs (825nm)/Rrs (560 nm)] plotted against suspended sediment (mg/l)

5 CONCLUSIONS
Soil characteristics (color, grain size, texture) are important parameters that influence the spectral reflectanceof suspended sediment. With the increase in SSC, the reflectance signal increase uniformly with scattering in
the visible domain and absorption in the near-infrared domain. Hence, it was concluded that the visible domain
is useful from qualitative point of view and near-infrared wavelength is useful for quantitative assessment. The
optimum wavelength for measuring SSC is 750 nm to 900 nm. However, visible wavelength 525nm to 650nm
is found to be optimum for characterizing and differentiating suspended sediments. Assessment of suspended
sediment concentration is important for basic and applied field application. Effect of suspended sediments
distribution on the spectral signatures in water bodies needs to be explored for future research.

ACKNOWLEDGMENTS
The research work was financially supported by the Kochi University of Technology, Kochi Japan. The
authors wish to acknowledge the financial assistance provided by the University. The assistance of all
members of Nasu Lab is gratefully acknowledged.

REFERENCES
1 Mahmood K., 1987: Reservoir sedimentation: impact, extent and mitigation. World Bank Technical Paper
71.
2 Graf H. W., 1983: The hydraulics of reservoir sedimentation. Water Power Dam Construct. 35, 45–52.
3 D. Doxaran, Jean-M Froidefond & P. Castaing, 2003: Applied Optics-LP. Volume 42, Issue 15, pp. 26232634.
4 Rong-Rong Li, Yoram J. Kaufman, Bo-Cai Gao, & Curtiss O. Davis, 2003: Remote Sensing of Suspended
Sediments and Shallow Coastal Waters. IEEE Transaction on Geoscience and Remote sensing. Vol. 41, No.
3 PP. 559.
5 Ritchie, J.C. & F.R. Schiebe, 2000: Remote Sensing in Hydrology and Water Management. Water Quality
In: G.A. Schultz and E.T. Engman (eds.) Springer-Verlag, Berlin, Germany, pp. 287-303, 351-352.
6 Ritchie, J.C., Schiebe f. R., & Mchenary J. R., 1976: Remote sensing of suspended sediments in surfacewaters. Photogrammetric Engineering and Remote sensing, 42, 1539-1545.
7 Ritchie J.C. and C. M. Cooper, 1988: Comparison of Measured Suspended Sediment Concentration withSuspended Sediment Concentrations Estimated from Landsat MSS data. Int. Journal of Remote Sensing,Vol. 9, No. 3, pg. 379-387.
8 Buondonno,C., Leone, A. P., Ortolani, F., Pagliuca, S., & Tedeschi, P., 1989a: Rapportitra evoluzione
geomorfologica e processi pedogenetici in aree significative della Campania. Bollettino della Societa,
Geologica Italiana, 44, 41–51.
9 B. L. Tolk, L. Han & D. C. Rundquist, 2000: The impact of bottom brightness on spectral reflectance ofsuspended sediments. Int. j. remote sensing, 2000, vol. 21, No. 11, 2259–2268.
10 White W. R., 2005: World water storage in man-made reservoirs. Foundation for Water Research,
FR/R0012, April 2005.
11 L. Han, D. C. Rundquist, 1998: The impact of a wind-roughened water surface on remote measurements ofturbidity. Int. j. of remote sensing, 1998, vol. 19, no. 1, 195± 201.