The current study aims to verify the existing equations for incipient motion for a rigid rectangular channel. Data from experimental work on incipient motion from a rectangular flume with two different widths, namely 0.3 and 0.6 m, were compared with the critical velocity value predicted by the equations of Novak & Nalluri and El-Zaemey. The equation by El-Zaemey performed better with an average discrepancy ratio value of 1.06 compared with the equation by Novak & Nalluri with an average discrepancy ratio value of 0.87. However, as the sediment deposit thickness increased, the equation by El-Zaemey became less accurate. A plot on the Shields Diagram using the experimental data had shown the significant effect of the sediment deposit thickness where, as the deposit becomes thicker, the dimensionless shear stress θ value also increased. A new equation had been proposed by incorporating the sediment deposit thickness. The new equation gave improved prediction with an average discrepancy ratio value of 1.02.

[1] During the past decades, the research into fluid-controlled geodynamic processes in the upper crust of the Earth is put foreward regarding the stress accumulation, deformation, and seismicity. In a large-scale injection experiment at the deep borehole site KTB (Kontinentale Tiefbohrung der Bundesrepublik Deutschland) in Germany, more than 84.000 m 3 fresh water was injected in 4000 m depth over 10 months, and a lot of geoscientific investigations were connected to this injection test. The pore-pressure change of more than 10 MPa yielded an induced deformation, which was detected by a tiltmeter array. The used five borehole tiltmeters of the ASKANIA type with a resolution of better than 1 nrad belong to the most sensitive tiltmeters worldwide. The poro-elastic finite-element modeling used for the interpretation of the observed tilts revealed an uplift of 3.1 mm above the injection point. Furthermore, it was shown that the induced stress and deformation fields depend mainly on the fluids inside the fault zones, particularly the SE2-reflector which is the Franconian fault zone, and the local Nottersdorf fault: While the maximum pore pressure is concentrated close to the injection point at the SE2 zone, the maximum deformation of about 3 cm is located at the intersection line of both faults in 4 km depth. The area of the modeled maximum deformation is highly correlated with the region of the observed induced seismicity, thus linking seismicity and deformation. The connection of tilt observation and modeling provides a useful tool for the investigation of fluid coupled geoprocesses.

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The understanding of how the sediment deposit thickness influences the incipient motion characteristic is still lacking in the literature. Hence, the current study aims to determine the effect of sediment deposition thickness on the critical velocity for incipient motion. An incipient motion experiment was conducted in a rigid boundary rectangular flume of 0.6 m width with varying sediment deposition thickness. Findings from the experiment revealed that the densimetric Froude number has a logarithmic relationship with both the thickness ratios t/d and t/y (t: sediment deposit thickness; d: grain size; y: normal flow depth). Multiple linear regression analysis was performed using the data from the current study to develop a new critical velocity equation by incorporating thickness ratios into the equation. The new equation can be used to predict critical velocity for incipient motion for both loose and rigid boundary conditions. The new critical velocity equation is an attempt toward unifying the equations for both rigid and loose boundary conditions.

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