D19

Experimental Investigation of the Influence of Paddy Ridges on Suspended Load Deposition
During Inundation〇 Can DING, Kenji KAWAIKE, Hajime NAKAGAWA, Kazuki YAMANOI, Rocky TALCHABHADEL

Paddy field is important to people’s daily life but vulnerable to flood and sedimentation. In this research, physicalexperiments were conducted for better understanding of the impact of paddy ridges on sediment deposition
during inundation disaster. The results show that the flow near the ridges presents obvious three dimensional
characteristics. Along the main flow direction, deposition happened at the ridge back, and erosion was found at
the front and downstream of the ridge. Two-dimensional (2D) numerical simulation results could generally
reflect the deposition area and thickness but is not sufficient for the detailed results around the ridges.
Introduction
In practical situation, paddy field is often divided
by different parts by paddy ridges or roads, theyplay a role just like small embankment. During theflood inundation, it may not only influence the floodpropagation, but also the sedimentation results. The
purpose of the work is to better understand theinfluence of paddy ridges on sedimentation so that
we can accurately predict the sediment deposition
results and appropriately manage the farmland.

height downstream boundary, case 2 with ridges
and the same downstream boundary with case 1,case 3 with ridges and the downstream was set as
open boundary. A discharge of 2L/s was given as
the inflow boundary, and some relevant parametersof cases 1 and 2 are shown in Tab.1. In case 1 and2, velocity and final deposition thickness weremeasured, but in case 3, as the water level is toosmall, only deposition thickness data was collected.
0.3m
embankment

Experimental setup
The experimental setup was installed in Ujigawa
Open Laboratory, Kyoto University. The setup
consists of a 0.3m wide river and a 3m long and1.6m wide inundated floodplain, the height of theembankment and the ridge are 15cm and 5mm,respectively, and the length of the inflow breach is0.2m. Fig.1 refers to the plan view of theexperimental setup and the distribution ofmeasuring points. 3 experimental cases wereperformed, case 1without ridges and with a 4.5cmembankment
Inlet boundary

ridges
1.6m
downstream boundary

3m

Fig.1 Plan view and the distribution of measuring
points of the experimental setup

Table1. Hydraulic condition near the inlet boundary
Inflow
Discharge

Meanvelocity

Flow
depth

Shear
velocity

2 L/s

19.20
cm/s

5.20
cm

1.23 cm/s

Criticalshear
velocity
1.31cm/s

Shearvelocity
ratio

Fr

Rouse
NumbernSetting
velocity

Sediment
density

0.93

0.27

1.53

0.02

0.75 cm/s

2.65g/cm3

Case 1Case 2

Case 3

Fig.2 Final deposition results of 3 experimental casesFig.5 shows the numerical results of cases 2 and 3.
The numerical results could generally reflect thedeposition area and thickness, but due to the three
dimensional flow characteristics near the ridges, 2D
numerical model is not sufficient for the detailed
results and three dimensional (3D) numerical model
could be recommended.
deposition

erosionFig.3 vertical direction velocity of each measuring
point along the centerline of the setup
Results
Fig.2 shows the final deposition results of 3
experimental cases. Compare case 1 and 2, thedeposition area is almost similar, most of deposition
happened in the vicinity of inlet and along the main
flow direction, but in case 2, it is found that
deposition happened on the back side of the ridge,and erosion happened on the front side and thedownstream of the ridge. Fig.3 illustrates theprocess for this phenomenon. Fig.4 shows thevertical velocity of each point along the platform
centerline. For case 3, because of the open
downstream boundary, high bed shear stress caused
strong erosion near the inlet boundary and most ofsediment was transported to downstream. Besides,
through experimental observation, the flow patternpresented stronger lateral velocity in case 3, whichcaused obvious erosion near the side ridges. (Fig.2c) A 2D numerical model was developed to simulate
the corresponding experimental cases. The
governing equation was expressed as:

Fig.4 schematization of flow pattern around theridges
Case 2

Case 3

Fig.5 Numerical deposition results of cases 2 and 3Conclusion
Physical experiments were conducted toinvestigate the influence of paddy ridges onsuspended load deposition in this research. The
results reveal that deposition and erosion happened
near the ridges due to the three dimensional flow
characteristics. Compared with 2D model, 3D
model would be more sufficient to reproduce thisphenomenon.