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CHAPTER 11: SAMPLE CALCULATION FOR BEDLOAD, SUSPENDED LOAD AND TOTAL BED MATERIAL LOAD . Confluence of the Fly River (upper) and the Ok Tedi (lower), Papua New Guinea. The Ok Tedi is laden with sediment from a copper mine. The flow is from left to right. SAMPLE CALCULATION.
SAMPLE CALCULATION FOR BEDLOAD, SUSPENDED LOAD AND TOTAL BED MATERIAL LOAD
Confluence of the Fly River (upper) and the Ok Tedi (lower), Papua New Guinea. The Ok Tedi is laden with sediment from a copper mine.
The flow is from left to right.
The Fly River, Papua New Guinea has been subject to a heavy loading of sediment from the Ok Tedi copper mine. The waste sediment flows 140 km down the Ok Tedi (Ok means “River”) and enters the Fly River at D’Albertis Junction. Mining commenced in 1985. Data for the Fly River at the Kuambit Gaging Station, just downstream of D’Albertis Junction, has been collected since about 1980. Before the commencement of the mine, the total bed material load of the Fly River at Kuambit was estimated (rather crudely) to be in the neighborhood of 4.45 Mt/year (million metric tons per year).
Here a full calculation is performed using actual data, pre-mine for the most part.
Confluence of the Ok Tedi (lower) and Fly River (upper), Papua New Guinea. The lighter color of the Fly River is due to the disposal of sediment from a mine upstream. Kuambit Gaging Station is about 1 km downstream of the confluence.
River slope S = 5.14 x 10-5 near Kuambit.
Bankfull depth Hbf there is 9.45 m, as determined from the cross-section below.
The river cross-section is plotted in undistorted form. It is only when the section is viewed in an undistorted plot that it becomes viscerally apparent how wide most natural alluvial streams are.
The relation between B and H was computed from the cross-section of the previous slide.
The pre-mine grain size distribution of the bed of the Fly River at Kuambit is given below; D50 = Dg = 0.211 mm, D90 = 0.425 mm and g = 1.63
The flow duration curve at Kuambit for 1994 is given below. The choice is because a) detailed pre-mine discharge measurements are lacking, and b) 1994 was a fairly typical year over the available record.
RESULTS FROM CALCULATION OF BEDLOAD AND SUSPENDED BED MATERIAL LOAD
As the flow goes overbank, the channel depth still rises with increasing discharge, albeit much more slowly. This implies a sediment load that increases slowly as stage rises above bankfull. In the case of the Fly River near D’Albertis Junction, the floodplain is over 10 km wide, i.e. so wide that little increase in sediment load is likely realized.
On the other hand, as flow goes overbank in a meandering river, the thread of high velocity can leave the channel and cut across the vegetated floodplain, causing the load to decrease as it loses its source from the river bed. In the case of the Fly, the wide floodplain should suppress this as well.
So as a first approximation, in this case
overbank load = bankfull load
Bankfull discharge Qbf = 3018 m3/s
Mean annual discharge Qm = 2355 m3/s
Bankfull discharge is exceeded 29% of the time
Note that a) the bankfull discharge is less than double the mean annual discharge, and b) the river is overbank for a significant amount of time. Such numbers are common for large, low-slope tropical streams. In most temperate streams, however, a) Qbf is much larger than Qm, and b) bankfull discharge is exceeded a few percent of the time at best.
Mean annual bedload transport rate Qbavan = 0.34 Mt/a
Mean annual suspended bed material load Qsavan = 2.14 Mt/a
Mean annual total bed material load Qtavan = 2.48 Mt/a
Percentage of annual bed material load that is bedload = 13.6%
Ashida, K. and M. Michiue, 1972, Study on hydraulic resistance and bedload transport rate in alluvial streams, Transactions, Japan Society of Civil Engineering, 206: 59-69 (in Japanese).
Wright, S. and G. Parker, 2004, Flow resistance and suspended load in sand-bed
rivers: simplified stratification model, Journal of Hydraulic Engineering, 130(8), 796-805.