- Email: [email protected]
Discussion by Geneviève A. Marquis and André G. Roy
Pressure- and velocity-measurements
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Vollmer, S., Francisco, d.l.S.R., Daebel, H., Ku¨hn, G., 2002. Micro scale exchange processes between surface and subsurface water. J. Hydrol. (269). Wilcock, P., Barta, A., Shea, C., et al., 1996. Observations of flow and sediment entrainment on a large gravel bed river 32, 2897–2909. Willemetz, J., 1997. DOP1000. Signal Processing User’s Manual. WSA, 2004. Geschiebezugabe Iffezheim. Flyer. Zanke, U., 2001. Zum EinfluX der Turbulenz auf den Beginn der Sedimentbewegung. Mitteilungen des Instituts fu¨r Wasserbau und Wasserwirtschaft, Universita¨t Darmstadt (120).
Discussion by Genevie`ve A. Marquis and Andre´ G. Roy We compliment the authors for their very innovative approach to the problem. The experimental setup is meticulously designed and surely has the potential to answer some of the questions on the interactions, effects or feedbacks between flows above and within a porous gravel bed. It addresses the very difficult technical problems of measuring interstitial flow. The data presented be the authors raise several interesting issues. Here, we would like to discuss the importance of large-scale turbulent flow structures and the interaction between interstitial and above the surface flows. Firstly, the significance of the 1D-ADCP results may have been underestimated. The slower and faster packets of fluid detected and shown in Fig. 4.6 are similar to largescale turbulent structures found in flows above rough boundaries. Such structures scale with flow depth (Roy et al., 2004). The origin of these large-scale flow structures is not well known and interstitial flow may play an active role in their generation and maintenance. Looking at Fig. 4.10, it appears that there is a peak in the spectra of all pressure sensors (except the one above the bed) between 0.4 and 0.7 Hz. This range may indicate a direct relationship with large-scale turbulent structures. Second, the data showing the mechanisms of interactions between interstitial and surface flows raise interesting questions. In Figs. 4.8 and 4.12, amplitude and frequency of pressure fluctuations are diminishing with depth, the higher values being associated with the surface flow. This observation raises the bottom-up or top-down question. Are the pressure fluctuations in the interstitial flow inducted by the surface flow and is it the other way around? What is the nature of the feedback effects between the interstitial flow structure and the surface flow structure and at what scale do they operate? The shape of the pressure distributions may contain part of the answer. In Fig. 4.9, we see that the pressure distributions change from a symmetric shape within the bed to a positively skewed one above the bed. Is the mechanism responsible for this change in the shape of the distributions related to dampening of the impacts of largescale downward sweeping motions on the interstitial flow? Could the authors speculate on the role, generation or maintenance of large-scale turbulent flow structures of the surface flow in relation to the interstitial flow? References Roy, A.G., Buffin-Be´langer, T., Lamarre, H., Kirkbridge, A.D., 2004. Size, shape and dynamics of large scale turbulent flow structures in a gravel bed river. J. Fluid Mech. 500, 1–27.
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Vollmer, S., Francisco, d.l.S.R., Daebel, H., Ku¨hn, G., 2002. Micro scale exchange processes between surface and subsurface water. J. Hydrol. (269). Wilcock, P., Barta, A., Shea, C., et al., 1996. Observations of flow and sediment entrainment on a large gravel bed river 32, 2897–2909. Willemetz, J., 1997. DOP1000. Signal Processing User’s Manual. WSA, 2004. Geschiebezugabe Iffezheim. Flyer. Zanke, U., 2001. Zum EinfluX der Turbulenz auf den Beginn der Sedimentbewegung. Mitteilungen des Instituts fu¨r Wasserbau und Wasserwirtschaft, Universita¨t Darmstadt (120).
Discussion by Genevie`ve A. Marquis and Andre´ G. Roy We compliment the authors for their very innovative approach to the problem. The experimental setup is meticulously designed and surely has the potential to answer some of the questions on the interactions, effects or feedbacks between flows above and within a porous gravel bed. It addresses the very difficult technical problems of measuring interstitial flow. The data presented be the authors raise several interesting issues. Here, we would like to discuss the importance of large-scale turbulent flow structures and the interaction between interstitial and above the surface flows. Firstly, the significance of the 1D-ADCP results may have been underestimated. The slower and faster packets of fluid detected and shown in Fig. 4.6 are similar to largescale turbulent structures found in flows above rough boundaries. Such structures scale with flow depth (Roy et al., 2004). The origin of these large-scale flow structures is not well known and interstitial flow may play an active role in their generation and maintenance. Looking at Fig. 4.10, it appears that there is a peak in the spectra of all pressure sensors (except the one above the bed) between 0.4 and 0.7 Hz. This range may indicate a direct relationship with large-scale turbulent structures. Second, the data showing the mechanisms of interactions between interstitial and surface flows raise interesting questions. In Figs. 4.8 and 4.12, amplitude and frequency of pressure fluctuations are diminishing with depth, the higher values being associated with the surface flow. This observation raises the bottom-up or top-down question. Are the pressure fluctuations in the interstitial flow inducted by the surface flow and is it the other way around? What is the nature of the feedback effects between the interstitial flow structure and the surface flow structure and at what scale do they operate? The shape of the pressure distributions may contain part of the answer. In Fig. 4.9, we see that the pressure distributions change from a symmetric shape within the bed to a positively skewed one above the bed. Is the mechanism responsible for this change in the shape of the distributions related to dampening of the impacts of largescale downward sweeping motions on the interstitial flow? Could the authors speculate on the role, generation or maintenance of large-scale turbulent flow structures of the surface flow in relation to the interstitial flow? References Roy, A.G., Buffin-Be´langer, T., Lamarre, H., Kirkbridge, A.D., 2004. Size, shape and dynamics of large scale turbulent flow structures in a gravel bed river. J. Fluid Mech. 500, 1–27.