Introduction to sediment transport in open channels
6.2 SIGNIFICANCE OF SEDIMENT TRANSPORT .1 Sediment transport in large alluvial streams
The transport of sediment is often more visible in mountain streams, torrents and creeks (e.g.
Fig. 6.1). However, larger rivers are also famous for their capacity to carry sediment load: e.g.
the Nile River, the Mississippi River and the Yellow River.
The Nile River is about 6700 km long and the mean annual flow is about 97.8 X 10^m^ (i.e.
3100m^/s). Large floods take place each year from July to October with maximum flow rates over 8100m^/s, during which the river carries a large sediment load. In Ancient Egypt, peasants and farmers expect the Nile floods to deposit fertile sediments in the 20 km wide flood plains surrounding the main channel.
The Mississippi River is about 3800 km long. Its mean annual discharge is about 536 X 10^m^ (or 17000m^/s). On average, the river transports about 7000 kg/s of sediment in the Gulf of Mexico. During flood periods much larger sediment-transport rates are observed.
Mcmath (1883) reported maximum daily sediment-transport rates of 4.69 X lO^kg (i.e.
54300kg/s) in the Mississippi River at Fulton (on 10 July 1880) and over 5.97 X lO^kg (or 69 110 kg/s) in the Missouri River^ at St Charles (on 3 July 1879). During the 1993 flood, the river at the Nebraska Station carried 11.9 X lO^kg (4580kg/s) of sediment load during the month of July. Between 12 and 28 July 1993, the Missouri River at Kansas City scoured the bed by 4.5 m and the sediment load reached 8700 kg/s between 26 June and 14 September 1993 at the Hermann gauging site (Bhowmik, 1996).
The Yellow River (or Huang Ho River) flows 5460 km across China and its catchment area is 745000km^. The annual mean (water) flow is 48.3 X 10^m^ (i.e. 1530mVs) and the average
^ The Missouri River is the longest tributary (3726 km long) of the Mississippi River and it is sometimes nicknamed 'Big Muddy' because of the amount of sediment load.
Fig. 6.1 Aerial view of Camas Creek flowing into the North Fork of the Flathead River, Montana, USA (12 July 1981) (by P. Carrara, with permission of US Geological Survey, Ref. 356ct).
annual sediment load is 1.6 billion tons which comes primarily from the Middle reach regions.
In the lower reach the river bed has been subjected to numerous changes, and the location of the river estuary has varied by as much as 600 km over the past 3000 years.
6.2.2 Failures caused by sediment-transport processes
Moore Creek dam, Tamworth, Australia
The Moore Creek dam was completed in 1898 to supply water to the town of Tamworth, NSW (325 km North of Sydney) (Fig. 6.2). At the time of construction, the 18.6 m high dam was designed with advanced structural features: i.e. thin single-arch wall (7.7 m thick at base and 0.9 m thick at crest), vertical downstream wall and battered upstream face made of Portland cement concrete. The volume of the reservoir was 220 000 m^ and the catchment area is 51 km^.
Between 1898 and 1911, the reservoir was filled with 85 000 m^ of sediment.^ In 1924 the dam was out of service because the reservoir was fully silted (25 years after its construction).
The dam is still standing today, although Moore Creek dam must be considered as an engineer- ing failure. It failed because the designers did not understand the basic concepts of sediment transport nor the catchment erosion mechanisms.
^For the period 1898-1911 observations suggested that most of the siltation took place during the floods of February 1908 and January 1910.
.*, . - * ^ -
6.2 Significance of sediment transport 145
' ^ - • • * •" . . . ^
Fig. 6.2 Photographs of the Moore Creek dam: (a) old photograph (shortly after completion), (b) recent photograph (14 June 1997).
Old Quipolly dam, Werris Creek, Australia
Completed in 1932, the Old Quipolly dam^ is a concrete single-arch dam: it is 19 m high with a 184 m long crest (1 m thickness at crest) (Fig. 6.3). The reservoir was built to supply water to the
Outlet
0 10 20 30 40 50m
^ J I
2.02 m
13.33 m
1.83m
Fig. 6.3 Sketch of the Old Quipolly dam.
^ Also known as Coeypolly Creek dam No. 1 (International Commission on Large Dams, 1984). A second dam was built in 1955, 3 km downstream of the old dam.
6.2 Significance of sediment transport 147 town of Werris Creek, NSW. The catchment area is 70 km^ and the original storage capacity was 860000 m l
In 1941, 130 000 m^ of sediment had accumulated. In 1943 the siltation volume amounted to 290 000 m^. By 1952, more than half of the initial storage had disappeared.
The reservoir was closed in 1955. Nowadays the reservoir is fiilly silted. The dam is still standing and the wall is in excellent condition (author's inspection in 1997). The reservoir presently acts as a sediment trap for the new Quipolly dam built in 1955 and located 3 km downstream.
The Old Quipolly dam is nevertheless another engineering failure caused by reservoir sedi- mentation. It is probably the most extreme siltation record documented in Australia. Indeed, several dams have also suffered siltation in South-East Australia (Table 6.1).
Mount Isa railway bridges, Queensland, Australia
In the early 1960s, the reconstruction of the railway line to Mount Isa (mining town in the Australian desert) required the construction of several bridges across creeks. Several bridges failed because of erosion of the pier foundations: e.g. bridge 235 across Julia Creek, bridge across Eastern creek (Fig. 6.4) and bridge across Corella Creek (Nonda).
The bridges collapsed because of the inability of (overseas) engineers to understand the local hydrology and associated sediment motion.
Shihmen dam, Taiwan
The Shihmen dam (Taoyuan County, Taiwan ROC) is a 133 m high dam built between 1958 and 1964. The maximum reservoir capacity was more than 60 000 000 m^ and the catchment area is 763 km^.
Although the dam was inaugurated in 1964, the reservoir began filling in May 1963. In September 1963, 20 000 000 m^ of silt accumulated during cyclonic floods (typhoon Gloria).
Between 1985 and 1995, the reservoir was dredged and over 10000 000 m^ of sediment was removed. But, during that period, the volume of sediment flowing into the reservoir amounted to about 15 000 000 m^ (over 10 years). More than 100 sediment-trap dams were built upstream of Shihmen reservoir to trap incoming sediment. In 1996 only one sediment-trap dam was still fimctioning, all the others having filled up. It is believed that the maximum depth of water in the Shihmen reservoir was less than 40 m in 1997.
The Shihmen reservoir (Fig. 6.5) was designed to operate for at least 70 years. Thirty years after completion, it has, in fact, become a vast sediment trap with an inappropriate storage capacity to act as a flood control or water supply reservoir (Chang, 1996).
Discussion
A spectacular accident was the Kaoping river bridge failure on 27 September 2002 in Taiwan.
Located between Kaohsiung and PingTung city, the 1 km long bridge failed because of scour at one pier abutment. The bridge had been in operation for about 22 years. Illegal gravel dredging upstream was suspected to be one of the causes of failure. A 100 m long bridge section dropped taking 18 vehicles with it, but there were fortunately no fatalities. Witnesses described a rumbling sound as the four-lane bridge broke and fell into the river. Recent references in bridge failures and scour include Hamill (1999) and Melville and Coleman (2000).
Table 6.1 Examples of reservoir siltation in South-East Australia Dam
(1)
Corona dam, Broken Hill NSW, Australia
Stephens Creek dam. Broken Hill NSW, Australia
Junction Reefs dam, Lyndhurst NSW, Australia Moore Creek dam, Tamworth NSW, Australia
Koorawatha dam,
Koorawatha NSW, Australia Gap weir, Werris Creek NSW, Australia
Cunningham Creek dam.
Harden NSW, Australia
Illalong Creek dam, Binalong NSW, Australia Umberumberka dam.
Broken Hill NSW, Australia
Korrumbyn Creek dam, Murwillumbah NSW, Australia Borenore Creek dam. Orange NSW, Australia
Tenterfield Creek dam, Tenterfield NSW, Australia Old QuipoUy dam, Werris Creek NSW, Australia
Inverell dam, Inverell NSW, Australia
Illawambra dam, Bega Valley NSW, Australia
Year of completion (2) 1890?
1892^
1896 1898
1901 1902 1912
1914 1915
1919 1928^
1930^^
1932
1939 1959
Catchment area (km^) (3)
15 510
- 51
-
160 820
130 420
3 22 38 70
600 19
Original reservoir capacity (m^) (4)
120 000^
24325 000
200000^
220000^
40500 t _t
260000^
13197000
27300^
230000 1500000 860000"^
153 000000^
4500^
Cumulative siltation volume (m^) (5) 120000 Fully silted 1820000 2070000 4500000 De-silting*
Fully silted 85 000 - 2 0 0 0 0 0 Fully silted Fully silted Fully silted 216000 258600 379000 522 600 758000 75% siltation 3 600000 4500000 5013000 5 700000 Fully silted
150000 110000 130000 290000 - 4 3 0 0 0 0 Fully silted Fully silted 3000 Fully silted
Date of record
(6) 1910?
1907 1912 1958 1971 1997 1911 1924 1985 1997 1924 1916 1917 1920 1922 1929 1997 1941 1961 1964 1982 1985 1981 1951 1941 1943 1952 1985 1982 1976 1985
Notes: ^Spillway crest raised in 1909; ''dam raised in 1943; '^dam raised in 1974;'''fully silted nowadays; * extensive de-silting in 1971.