3.4 Sediment rating curves
Strong correlation is often observed between suspended sediment
concentration and discharge in most streams. Compared to large river
systems that usually contain an abundance of materials, the suspended
load of small mountain streams usually depends on episodic events that
transport fine materials from banks and upland areas Thus, suspended
sediment concentration depends on both supply of sediments and discharge
(Yaksich & Verhoff, 1983; Zhang et al., 2021). Nonetheless, suspended
sediment concentration and discharge can be plotted to create what is
known as a sediment rating curve. One benefit of developing a sediment
rating curve is that it can be applied to interpolate missing data
during the observation period (Asselman, 2000). Here, the sediment
rating curve is a power function expressed as:
SSC = a×Qb
where SSC is in mg/L, Q is the water discharge (in
m3/s), and a and b are fitting coefficients.
When b > 1, the increase in sediment volume per 1 unit of
flow volume can be nonlinear. Such situations imply that more than half
of the sediment load is carried by high flows that account for less than
15% of the water volume or less than 5% of the period of measurement.
Another characteristic of suspended sediment concentration during high
flow is their increased variability. Both factors indicate that more
sampling should take place during periods of high discharge. However,
one common limitation of suspended sediment data from small mountainous
catchments is the lack of measurements conducted during high flows.
Storms make access difficult and measurements hazardous. Nevertheless,
high-resolution data collected during high flows are essential for the
development of good sediment rating curves. Due to the infrequency of
high flows and logistical problems, collecting adequate high flow
measurements cannot be achieved through hand sampling alone.
Suspended sediment rating curves were determined for Ming Yong glacial
catchment using discharge and suspended sediment concentration data
between August 2013 and July 2017. The rating curves were fit by
nonlinear least-squares curve fitting using the Levenberg-Marquardt
(L-M) algorithm produced in Origin 2021b. One of the major limitations
of the sediment rating curve is the assumption that the rating
coefficients will remain constant (Zhang et al., 2021). As mentioned by
Yaksich and Verhoff (1983), small mountainous catchment often results in
“event response” streams, which causes large scatter between sediment
concentration and discharge and hence poorer relationships between
suspended sediment concentration and discharge. The low accuracy of the
sediment rating curves may be attributed partly to hysteretic effects,
where at a given discharge, the sediment concentration on the rising and
falling stage of the hydrograph differs (Khanchoul & Jansson, 2008).
Separate rating curves are needed for the rising and falling limbs to
account for seasonal variations so as to enhance the accuracy of the
estimated suspended sediment concentration (Khanchoul & Jansson, 2008).
In this study, the hydrological year is defined as August to July, with
the rising stage (February-July) and falling stage (August-January in
the next year).