Mechanism of Efficient Sediment Transport by Hyperconcentrated Flow in the Lower Yellow River (Part VI)

5. Powerful Sediment Transport Capacity of Hyperconcentrated Floods Passing through the Lower Yellow River

Under the long term natural evolution as well as the human’s activity, different river regimes and channel types have been developed along the 800 km long lower Yellow River from Tiexie to the river mouth (Qi et al. 2002). Consequently, each river reach develops its own channel evolution and sediment transport characteristics. The Yellow River in the 300 km long reach from Tiexie to Gaocun with a bed slope of 0.0027 to 0.0017 has a typical wandering, “wide and shallow” channel. The Yellow River in the 165 km reach from Gaocun to Taochengpu has a transition type channel with a bed slope of 0.0017 to 0.0011. The Yellow River in the 460 km reach from Taochengpu to the river mouth with a bed slope of 0.001 has a meandering, “deep and narrow” channel. Figure 6 shows the longitudinal sediment concentration variations at different stations along the lower Yellow River for five flood events with highest sediment concentrations in the past years. The sediment concentration decreased sharply from 220～320 kg/m³ to 80～150 kg/m³ along the “wide and shallow” channel upstream of Gaocun. However, the average sediment concentration even increased a bit, in the 300 km “deep and narrow” channel downstream of Aishan with a mild slope. Such phenomenon shows the powerful sediment transport capacity of the hyperconcentrated flow.

As shown in Table 7, sediment transport ratio from Aishan to Lijin (0.0001 bed slope) is 0.97 to 1.04 for three flood events with high sediment concentration in year 1973 and 1977, and there was no sediment deposition for the flood with discharge of 3,000 m³/s and maximum sediment concentration of 200 kg/m³. Although as sediment concentration increases, sediment particles are easier to be suspended; in order to convey hyperconcentrated flow smoothly, it is critical to keep the flow in turbulent region and avoid unsteady transient flow or intermittent flow. Thus, under a given river channel condition, there is an upper limitation of concentration for efficient sediment transport.

Figure 4 Sediment Concentration of Hyperconcentrated Floods Varies along the Lower Reach for Different Events

Since in most cases the hyperconcentrated flow in natural channel is fully turbulent, and possesses the same friction characteristics as clear water, its roughness can be calculated by using Manning’s formula. The water depth and velocity of lower Yellow River were calculated by using water depth and velocity relationship established for low sediment-laden flow, and the result is listed in Table 8. It can be seen in Table 8, under the same discharge, the water depth and velocity at Lijin Station are smallest. Thus, the flow condition of Lijin Station is chosen as control factors for hyperconcentrated flow.

Supposing that the minimum discharge to transport sediment Q_min = 3,000 m³/s, the maximum sediment concentration S_max = 800 m³/kg, d₅₀ = 0.036 mm, the percentage of sediment with d < 0.01 mm is 20 %. By using the formula of Xiangjun Fei (Fei, 1993), the shear stress t_B = 0.33 kg/m²，viscosity m = 0.00093 kg×s/m²，the density of the sediment flow ρ = 153 kg·s²/m³, the Reynold’s number = 3.03 x 10⁶, which is much higher than the critical Reynold’s number Re = 2,000 for fully turbulent flow. From the calculation above, it is proved that the flow with discharge of 3,000 m³/s and sediment concentration of 800 kg/m³ is fully turbulent, and the sediment-laden flow with concentration as high as 800 kg/m³ can be transported properly in to the sea as long as the discharge is kept at 3,000 m³/s or more.

In conclusion, under the current channel condition of the lower reach,  if the flow discharge is kept from 2,000 to 3,000 m³/s, not only floods with sediment concentration lower than 200 kg/m³, but also floods with concentration 400 to 500 kg/m³ can be transported. The flood with maximum sediment concentration of 800 kg/m³ can be properly conveyed to the sea.

 Table 7 Sediment Transport Statistics of Hyperconcentrated Floods from Aishan Station to Lijin Station of the Lower Yellow River

River	Date	Sediment Flow Condition					Morphological Change and Sediment Transport Ratio
		Peak Discharge (m3/s)	Maximum Daily-average   Discharge (m3/s)	Maximum Sediment Concentration (kg/m3)	Duration for S > 400 kg/m3 (hr)	d50 (mm)	Percentage for d < 0.01 mm    (%)	Depth of Main Channel Erosion (m)	Length of Channel Erosion (km)	Sediment Transport Ratio (%)
Weihe	07/05/77~07/13/77	5,550	4,120	690	43	0.04~0.06	15~20	-2.5	165	97
	08/12/64~08/17/64	3,970	1,999	670	120	0.03~0.04	14~22	-0.4	165	108
	07/16/64~07/21/64	3,120	1,870	600	30	0.05~0.06	12~22	-0.5	165	120
	08/02/70~08/10/70	2,930	2,250	800	24	0.03~0.04	17~25	-0.32	165	104
	07/24/75~08/01/75	2,290	1,350	645	30	0.03~0.05	10~40	-0.25	165	100
Beiluohe	07/28/75~07/31/75	2,190	1,120	725	32 (53)	0.04~0.05	15~19	-131	87	90
	08/17/71~08/20/71	1,100	504	885	79 (79)	0.04~0.06	10~20	-1.13	87	96
	07/06/77~07/08/77	3,070	1,080	850	60 (72)	0.04~0.06	10~16	-3.16	87	112
	07/30/69~08/02/69	1,290	504	880	81 (89)	0.04	8~16	-0.51	87	120
	08/25/73~09/03/73	765	380	860	130 (176)	0.04~0.05	10~17	-1.63	87	123
	08/06/77~08/09/77	800	298	1,010	84 (84)	0.04	10~16	-0.64		100

Table 8 Variations of River Bed Roughness at Different Discharge Levels

Discharge (m3/s)		500	1,000	1,500	2,000	3,000	4,000	5,000	6,000
	H (m)	1.5	1.8	2.0	2.5	3.2	4.0	4.5	5.0
Aishan	V (m/s)	1.0	1.5	1.8	2.0	2.5	2.8	3.0	3.2
	H (m)	2.5	2.5	2.6	3.4	4.6	6.0	7.0	8.0
Luokou	V (m/s)	1.0	1.5	1.9	2.1	2.35	2.5	2.7	2.8
	H (m)	1.5	1.8	2.0	2.3	2.8	3.4	3.8	4.2
Linjin	V (m/s)	0.9	1.5	1.8	2.0	2.2	2.4	2.5	2.6
	n	0.014	0.01	0.009	0.009	0.009	0.009	0.010	0.010

Because the river channel of lower Yellow River is relatively wide in the upstream part and narrow in the downstream part as shown in Figure 7, the channel presents the following sediment transporting characteristics during the flood with high sediment concentration. Although the main channel can discharge more sediment as the flood discharge increases, when the flood spills out of the channel onto the floodplain, huge amount of sediment will deposit there by overbank flow. Thus, in order to make full use of the channel to transport sediment by hyperconcentrated flow, the critical issue is to transform the wide and shallow meandering channel into deep, narrow and stable channel. 

6．Conclusion

In most cases, the hyperconcentrated flow in a natural river is fully turbulent. It possesses the same resistance characteristics as the low sediment-laden flow, and can be calculated by using Manning’s formula. The main reason for high sediment transport efficiency of hyperconcentrated flow is that the flow viscosity increases with the sediment concentration increasing, which makes particle fall velocity decrease dramatically, while the roughness of the river bed remains unchanged.  　

As the sediment particles of the Yellow River are rather fine, e.g. d₅₀ = 0.03 - 0.10　mm, sediment concentration distribution becomes more uniform in vertical direction as the sediment concentration increases. When the sediment concentration is higher than 200 kg/m³, the S - v³/ghω₀ relationship appears a reverse tendency.

From the analysis of the influence of sediment concentration on flow structures, and the relationship between sediment concentration and flow velocity profile in vertical direction, it is concluded that the sediment is the most difficult to be transported by the channel when concentration is around 200 kg/m³. This has also been proved by various observed flow data. The hyperconcentrated flood has high efficient sediment transporting capacity.

The recorded maximum sediment concentration downstream of Aishan in the lower Yellow River is 200 kg/m³. However, due to high sediment transport capacity of the hyperconcentrated flow, a flow with concentration as high as 800 kg/m³ can be conveyed through the channel into the sea when the flow discharge is 2,000 m³/s to 3,000 m³/s. The key point is to transform the existing wide and shallow channel into deep, narrow and stable channel.