Effective discharge in monsoon controlled rivers
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When does a river really carry sediment? Do lots of small floods carry as much sediment as a single big flood? Although we talk about average annual erosion rates, is sediment transport in rivers episodic rather than continuous? At most discharges, rivers can only transport small amounts of sediment, so there is no reason to expect sediment transport to be continuous at all discharges. Understanding how and when rivers transport sediment is important for both river planning purposes and geomorphology research. River planners need to know when sediment is transported to better understand river systems they are trying to control; geomorphologists benefit from understanding sediment transport regimes when modeling landscape evolution and the effect of rainfall on erosion.
Prior to the 1950s, most people believed that only the largest floods (those occurring less frequently than once every two or three years) did the work of erosion and transportation in rivers. In the 1950s Wolman and Miller (1960) had the great idea to actually collect data to determine what discharges carry the most sediment over time. Essentially they asked whether frequent smaller floods that carry less sediment do more work moving sediment than infrequent large floods because they are more frequent. They showed that in temperate rivers in the eastern US, the bankfull flood, or the flood that occurs approximately once a year, does the majority of the sediment transport work. This discharge is called the effective discharge. Further research showed that as much as 90% of the sediment is moved 10% of the time – much more frequently than people previously believed (Meade, 1982).
Since 1960, the statement that rivers do 90% of the work in 10% of the time has become the common adage for understanding how rivers transport sediment. River engineers around the world have taken the concept further and use the effective discharge to define a specific discharge that is supposed to be equal to all the variations in river discharge over the course of a year. They then set dam discharge to be equal to this idealized discharge instead of worrying about creating a more natural hydrograph (see Crowder and Knapp, 2005 for an overview of effective discharge and controversies surrounding it). This understanding of how rivers work has been extrapolated all over the world based on the work that Wolman and Miller did in the eastern US in the 1950s.
As you can probably guess, there is no good reason to assume that all rivers in the world transport sediment the same way that a handful of rivers in the eastern US do. Unfortunately, the sparseness of most sediment concentration and discharge records, needed to calculate effective discharge, makes it difficult to check these calculations for diverse locations. Luckily for us, the Chinese government maintains an extensive network of sediment and discharge gauging stations all over the country (Figure 1). They have been collecting sediment concentration and discharge daily for over 50 years at some sites. By using data that was collected daily (rather than estimated from rating curves) over a large number of years, the calculations can be done more precisely.
Western China has a climate and hydrology that is strongly affected by the Indian Monsoon. The Indian Monsoon is a summer wind and rainfall pattern that results in nearly all annual rainfall falling between June and September in South Asia and nearby regions. The hydrology of monsoon rivers is so different from temperate rivers that it is reasonable to expect that the effective discharge is not the annual flood. The region also has an extensive network of sediment gaging stations that collect sediment concentration and river discharge daily. In order to calculate effective discharge, we used the sediment concentration and average daily discharge to determine how much sediment was transported each day. We then binned all the sediment transport values by discharge (sort of like making a histogram). We then could calculate how much sediment is moved by a particular range of discharges over your period of record. The peak of the histogram is the effective discharge.
We were able to show that the monsoon discharge, or the average discharge between June and September, is the effective discharge for these monsoon rivers in western China. Unlike the temperate rivers in the eastern US, therefore, the rivers in this part of the world transport 90% of their sediment 33% of the time during the annual monsoon (Figures 2 and 3). Going back to erosion modeling, this means that when we talk about correlations between rainfall and erosion in monsoon regions, it makes more sense to correlate monsoon rainfall to sediment transport rather than annual rainfall.
- Crowder, D. W., and Knapp, H. V. (2005). Effective discharge recurrence intervals of Illinois streams. Geomorphology 64(3-4), 167-184.
- Henck, A., Montgomery, D. R., Huntington, K. W., and Liang, C. (2010). Monsoon control of effective discharge, Yunnan and Tibet. Geology 38(11), 975-978. doi: 10.1130/G31444.1.
- Meade, R. H. (1982). Sources, sinks, and storage of river sediment in the atlantic drainage of the United States. The Journal of Geology 90(3), 235-252.
- Wolman, M. G., and Miller, J. P. (1960). Magnitude and Frequency of Forces in Geomorphic Processes. Journal of Geology 68(1), 54-74.