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NickM

Stream Power Analysis

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I have a relatively simple, but large model and I'm carring out an Manning's sensitivity analysis. I'm looking into the stability of a channel, so I am comparing stream power, bed shear and velocity results. I have carried out two runs using the same files for everything except the materials mapping file and the materials.tmf file.

The results have shown that the stream power and bed shear both increase with increased Manning's roughness. This seems totally incorrect because the velocities should reduce, bringing the other measurement down with it. Incidentally, the velocity does appear to reduce.

I've taken my results along the channel centreline using the dat_to_gis tool to find peak values, followed by the extraction of results along the mapped centreline.

I am wondering whether the peak values have come from a different time to the peak flow, or perhaps stability issues have caused a spike in the results. I don't think stability is the issue because the model seems happy and there are a large number of results along the channel length and I doubt spike would occur everywhere. I guess my questions are; when are peak values of power and shear collected and has anyone else had issues with these values increasing with increased roughness?

Thanks.

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Hi Nick

The BSS is proportional to V^2*n^2/y^.333 where V is velocity, n is Manning's n and y is depth. Therefore, as n increases, BSS increases, but like you say, V will decrease and this will decrease BSS, so the increased n / decreased V somewhat compensate each other. Depth will also change and affect the BSS. Whether BSS should increase or decrease with increased n I can't comment on as this is not an area I've practised in, but it would be good to hear from anyone who is an expert in this field.

Cheers

Bill

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Some more thoughts on this, rather than a clear answer: the true basal shear stress is actually related to near-bed velocities, but TUFLOW gives a depth-averaged velocity. The assumption must be made that the spatial variation in near bed velocities and shear stress would mirror the TUFLOW velocity distribution and that the equation used (see Bill's post) provides an adequate relationship between them. However, as you have noted, the inverse dependence on roughness, n, may be an issue. On the one hand higher shear stresses are generated in rougher areas due to greater turbulence. But on the other hand, a high value of n may be specified on vegetated bars, or those with large boulders, to represent the impediments to flow presented by these obstacles. If this high n were used in the sediment calculations in these areas (which are also often of low depth), then high rates of sediment transport are predicted. This appears counter-intuitive as it is more likely that sediment in transport (which is expected to be of smaller dimensions) becomes trapped in these zones by large boulders, vegetation, debris dams and slack zones behind roughness elements. In other words, the roughness chosen for hydraulic calculations may not always be appropriate for shear stress calculations. One option in such areas could be to keep n for skin friction and use additional form losses for, well, form losses (and vegetation)..... Disaggregation of roughness to remove any vegetation effects is recommended in Mosselman, 2005 (Mosselman, E. “Basic equations for sediment transport in CFD for fluvial morphodynamics”, in Computational Fluid Dynamics: Applications in Environmental Hydrauilcs, Edited by PD Bates, SN Lane and RI Ferguson, 2005, Wiley p71-89).

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