Hou Zhijun  Yao Wenyi and Chang Wenhua

 Yellow River Institute of Hydraulic Research, YRCC Zhengzhou  450003

Abstract: In the paper, actualization of combine with the narrow channel excavation dredging and dykes reinforcing startup work in the lower narrow reaches of the Yellow River, Process experimental research be handed in excavation dredging mechanism on excavation dredging by using physical model. Research result shown: river dredging deposition reduction mechanism is opened out in certainly degree and the sediment deposition reduction effects have be approved by prototype observation datum of excavation dredging and dykes reinforcing startup projects in 1998. The hydraulics problems of river dredging and channel dredging up may be calculated through commonly hydraulics knowledge. The calculation content include: the water level of the dredging research reaches, water surface slope as well as flow resistance highness of the dredging reach end. The results are basic sameness between the calculation and the physics model experiment, and it is proved by prototype reaches dredging datum. It is shown that the reaches dredging geometry parameter can be calculated through hydraulic formula in case of the changed reaches boundary condition after simplified . This paper would offer convenient design calculation method for river excavation dredging up project in the future. 

Key words: channel excavation dredging and dykes reinforcing; effect of sediment deposition reduction; dredging reaches sedimentation reduction mechanism, startup project

1.  Excavation dredging deportment physical model’ design and Verification

There are many measures to maintain the healthy life of the Yellow River. Harnessing sedimentation is the key problems. “Essentials of Planning for Yellow River Harnessing and Development” worked out in 1990s. Based on the former practice experience of harnessing sediment in the Yellow River, the comprehensive harnessing of “retaining, draining, discharging, regulating and excavating ” and measures of sediment utilization have been pointed out. In which the “excavating” is an important measure of them to treat sediment. In the paper, actualization of combine with the narrow channel excavation dredging and dykes reinforcing startup work in the lower narrow reaches of the Yellow River, Process experimental research be handed in excavation dredging mechanism on excavation dredging by using physical model.

1.1  Predigest physical model design

Excavation dredging deportment physical model’ test is a prophase prepare test for concert startup project actualize ongoing. This model select 20.64Km length reach between Yihaoba in Xihekou. Deportment model is designed according to gravitation resistance conform.  Moreover, it suffice flow state conform and least water deepen request. This model’ plane scale is 1:500, vertical scale is 1:55. Channel’ landform select flood frond landform in 1992. Model’ offal water level is controlled according to Xihekou water level in1992.

1.2 Verification of predigest physical model

     This model have been done according to flood landform in 1992.Firstly, in condition to this landform. We validated water surface profile and roughness. Result of verification as table 1. This model’ precision essential achieve design requirement. After the channel dredged, reaches cross sectional sharp and roughness will not more big change. Experimental result of reaches dredging up express basically real case.

Excavation dredging deportment physical model test is a prophase prepare test for concert startup project. This model 20.64km long between Yihaoba and Xihekou. The model is designed according to gravitation resistance similarity principles.  Moreover, it can also meet flow state similarity and least water deep request. The plane scale is 1:500, vertical scale is 1:55. Channel elevation use the flood frond landform in 1992. Theexitwater level is controlled according to Xihekou water level in1992.

Tab.1   Model and Archetypal slope and roughness of “92.8” flood of all level discharge

2.Experiment results of excavation dredging model

   Base on verification experiment, three dredged channel plan compare have been done. The dredged channel place was selected in the middle of Qianzuo—Yuwa reach. The reach was 9.34km long, the dredged channel size (width×Depth)were 50 m×3 m and 200 m×2.5 m .In the model , water levels and velocities of all levels discharge along channel are measured. It must be point that the experiment done in fixed the channel bed. The experimental results is equivalent to the after excavation dredging.  For example, water level, slope and velocity of the initial flow.

2.1 The adjustment of water level and the change of slope and velocity along the up and down reach of the dredged channel

In the model, the water levels are measured for the three dredged schemes and six different discharges, the change trend of water level is in accordance with the three dredged schemes. After river channel is dredged, the cross section area increases the velocity and friction resistance loss decreases, river channel water level drops slope become slow, water level falls. The head level of the dredged channel decreases most, the more of the dredged channel area, and the slow of the slope, and the more of the fallen water level. Because of the effect of the abrupt water level drops in the front reach of the dredged channel, the slop of upper reach become steep, velocity increases. When the discharge is equal to 1000 m³/s, the affected slopes of dropped water level in Qianzuo station surpass that of Yihaoba station, the lower reach have little been affected by the dredged channel relatively, the velocity change along the reach have the below laws: (1) Before dredging, the original channel, the flow is even. (2) After dredging, the area of water flow increase, velocity decreases, the more of the dredged area, the more of the deceased velocity; the same dredged area, with the increase of dredged area to flow area cross section decrease, the decreased ratio to the velocity is relatively little. (3) Dredging is to dig a same deep slot, After dredging, the slow, the channel flow is uneven, (4) Because of the effect of the Qianzuo water level, slop greatly, the river bed elevation decrease, the section velocity increase.

2.2 Discussion on the mechanism of the dredged channel

The measured data of the water level, slop, and velocity change uncovers the dredged and sediment reduction mechanism. (1)After dredging, velocity decrease, channel deposition, again, (2) Because of front channel water level drops greatly,  river channel slope increases, velocity increases, in the upper reach, channel become eroded from down to up, with the increases of dredged area, the eroded from down to up, with the increase of dredged area, the erosion strength and affected scope increase. (3) Dredging have little effect on the lower reach, the erosion and deposition of the lower reach is determined to the sediment quantity and capacity of the sediment transport, this is belongs to the common flow the sediment erosion and deposition.

When studying the dredged mechanism, the reach should include the upper, the dredged and the lower reach. When studying the effected of reduction deposition, one should calculate not only the quantity of sediment deposition and erosion the three reach, but also consider the sediment excavation quantity, and that of old channel in the same condition. For the lower reach of the Yellow River, from the year 1965 to 1996, the sediment deposition and erosion quantity is respectively7.91×10⁴m³/ year﹑km, and 7.06×10⁴m³/ year﹑km for the Jinan reach and Estuary reach.   

3. Channel hydraulics calculation

   Behind excavation reach near symmetrical flow surface line calculation excavation dredging, dredged channel flow through primary channel symmetrical flow changes unsymmetrical gradual change flow. Now unsymmetrical flow still absence theory calculation formulary. can but subsection calculation according to symmetrical flow. For predigest calculation, dredged reach flow shall be calculated by way of almost symmetrical flow, and regard as follows assume: (1) Dredged channel bottom Yuwa cross section all energy main decided lower reach transportation  flow ability, dredged channel influence it small. (2) Behind dredged channel, reach conformation and channel bottom riverbed sediment diameter changes small, so we think reach roughness same in dredged channel before and after. (3) When hydraulics factors was calculated, it adopt that of all cross section average hydraulics factor. Dredged reach average water surface slope may be calculated according to the following formula:

                                          (1)

   In formula n_o is old roughness, the of v_c and h_c are respectively of all cross section average velocity and depth.

3.1 The calculation of the Yuwa section local raise water level in the end of dredged channel

   The whole energy of Yuwa section, the end of dredged channel, is determined to the capacity of river channel, and it is consider that the section whole energy of after dredging is the same as that of dredging before. According to energy and ignoring the local loss, the raised water level of Yuwa section can be calculated by the following formula:

                                                (2)

   In formula v_co is the average velocity of Yuwa section that of before dredged old channel cross section, v_c1 is the average velocity of Yuwa section that of after dredged channel. Water stage was calculated approximately according to that of before dredged channel. According to formula (2), the raise flood height are calculated dredged of the channel two scheme 50×3m²,  200×2.5m², and all level discharge. Reckon in raise flood later Yuwa water stage of considering the raised water level are listed in Tab.2. In Tab.2 list also the model measured water stage commonly. Calculated water stage and model measure water stage is very close.

   Tab.2    The model measured and calculated water levels after dredged channel

3.2 The calculation of local drop level height in Qianzuo section of the Dredged channel  head

Dredged reach slope become slow, the Qianzuo water level drops rapidly of the head dredged channel. When discharge is 1000m³/s to 3000m³/s, water level drop 0.69m to 0.52m. Upward Qianzuo reach becomes obvious drop level curve reach. When the flood flow into dredged channel, as across flood the water area of cross section change rapidly, the level link up with dredged channel water surface according to raise water curve. As Qianzuo cross section local drop water and raise water level calculation are general difficult, it can be calculated according to the following function

                                   (3)

   In formula v_bo is average velocity of Qianzou section before dredged channel old channel, v_b1 is average velocity that of after dredging in Qian Zou section. It can be calculated according to water stage of before dredged. Base on experimental datum, k is 0.2, considering the decreased water level of Qianzuo section, The water level is listed in Tab.2. It is shown that calculated result is closer with measure datum.

4. Conclusion and Discussion

The physical model experiment research results shown: (1) When the reaches have been dredged, it’s increase for area of over flow, discharge velocity of cross section decreased,  flow hydraulicratiowill drop down greatly, and the sediments of dredging channel occur back-deposition. (2) In the up reach of dredged channel, because it is influenced for dredging channel ,There are bigger water level fall down in head of dredging channel, lead to water level slop ratio increasing, velocity of channel flow increasing, the reaches of the up reach occur trace to the source erode. The trace to the source eroded intensity and influence scope become more bigger with the dredged channel area under the dredging channel length. (3)The lower reaches of the dredged channel is influenced small. The reaches deposition-erosion changes main is decided by amount of incoming sedimentation and ability transport sediment. Because the reaches boundary condition general not change, the lower reaches basic belong to general deposition-erosion reaches and is influenced small. Dredging reaches experimental results can response the prototype beginning flow movement of dredged channel. Experimental results open out basically river-dredging sediment-deposition reduction mechanism, the dredging reaches occur sediment back-deposition , upwards reaches occur sediment trace to the source erode, downward reaches change small, it belongs to general reaches. dredging reaches hydraulics questions can be calculated according to general hydraulics method, moreover it rather satisfactory results can be obtained. The dredging channel sedimentation deposition reduction mechanism are also approved farther by the channel excavation dredging and dykes reinforcing work archetype measured datum in 1998. Excavation dredging sediment deposition reduction ratio is 0.44～0.73 in the reach of Lijin section to Qing6 section, in other words, reduction 1m³ sediment, the dredging sediment is only 0.44～0.73m³，so the effects sediment deposition reduction is obvious. the effects of deposition reduction