EGU2020-1875
https://doi.org/10.5194/egusphere-egu2020-1875
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Role of Soil Pipes and Pipeflow in Headcut Migration Processes

Ximeng Xu2, Glenn V. Wilson1, Fenli Zheng3, and Qiuhong Tang4
Ximeng Xu et al.
  • 1USDA National Sed. Lab., Watershed Physical Processes RU, Oxford, United States of America (glenn.wilson@usda.gov)
  • 2Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences,
  • 3Institute of Soil and Water Conservation, Northwest A & F University
  • 4Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences

Headcut formation and migration is sometimes mistaken as the result of overland flow without realizing that the headcut was formed by or significantly influenced by flow through soil pipes into the headcut. To determine the effects of a soil pipe and flow through a soil pipe on headcut migration, laboratory experiments were conducted under free-drainage conditions and conditions of a shallow water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Overland flow and flow into the soil pipe was applied at a constant rate of 68 L/min and 1 L/min, respectively, at the upper end of the flume. The headcut migration rate and sediment concentrations in both surface (channel) and subsurface (soil pipe) flows were measured with time. The typical response without a soil pipe was the formation of a headcut that extended in depth until an equilibrium scour hole was established at which time the headcut migrated upslope. The presence of a soil pipe below the channel, and particularly the phenomena of flow through a soil pipe and into the headcut, whether by seepage from a shallow water table or upslope inflow, significantly impacted the headcut migration. Pipeflow caused erosion inside of the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. This interaction dramatically altered the headcut processes, greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe in addition to pipeflow increased the sediment concentration by 42% and the headcut migration rate by 47% compared with pipeflow under free-drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 minutes compared with free-drainage condition. This study confirmed that pipeflow dramatically accelerates headcut migration especially under conditions of shallow perched water tables and highlights the importance of understanding these processes in headcut migration processes.

How to cite: Xu, X., Wilson, G. V., Zheng, F., and Tang, Q.: The Role of Soil Pipes and Pipeflow in Headcut Migration Processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1875, https://doi.org/10.5194/egusphere-egu2020-1875, 2019

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Presentation version 1 – uploaded on 28 Apr 2020
  • CC1: Comment on EGU2020-1875, Taco Regensburg, 05 May 2020

    Hi Glenn, 

    1. could you please explain the difference between SP&OF&N and SP&OF, what represents N?

    2. how was the pipe constructed and was there any transport possible between the pipe and the surrounding soil, i.e. infiltration and exfiltration to and from the pipe?

    best wishes,

     

    Taco

    • AC1: Reply to CC1, Glenn V. Wilson, 06 May 2020

      Dear Taco

      Thanks for the interest. The details of the experiments are in the ESPL paper which has been accepted but is not published yet.  Here is an exert from the paper that should help. 

      A total of six tests were conducted in this experiment (Table 1) that included three treatments (without a soil pipe, with a soil pipe but no inflow, with a soil pipe having inflow) and two hydrologic conditions (free drainage and seepage), all tests were replicated thus twelve runs were included. Two tests were conducted with no (N) soil pipe present in the soil bed as a baseline treatment to illustrate the headcut migration dynamics in upland surface concentrated flow under free drainage (FD&OF&N) and seepage (SP&OF&N) conditions without soil pipe interactions. The seepage condition involves the establishment of a perched water table at 3 cm below the soil surface which imposes a seepage force on any voids below this depth, whether the presence of a soil pipe or a headcut plunge pool. Note that FD indicates free drainage and SP indicates seepage, whereas OF and PF indicate overland flow and pipeflow, respectively and all tests without an ‘N’ designation included a soil pipe present in the bed.

    • AC2: Reply to CC1, Glenn V. Wilson, 06 May 2020

      I forgot to comment on the second question. The pipe was created as we have done in previous papers, reference below, by packing soil around a rod (pipe) then remove the rod out the lower end of the soil bed. We had a video camer at the back end of the rod to film the pipe as it was removed to ensure the soil pipes integrity. 

      For conditions of pipe inflow under free drainage bed condition, there will be exfiltration from the pipe into the bed. For conditions of a perched water table with no pipe inflow, there is seepage from the bed into the artificial soil pipe that generates pipe flow. For pipe inflow with a perched water table, the re will be limited exfiltration or infiltration from the pipe to bed or bed to pipe, respectively. 

      • CC2: Reply to AC2, Taco Regensburg, 06 May 2020

        Hi Glenn, 

        thank you very much for the explanation. Now I wonder how the compaction of your medium has influenced the likelihood of pipe roof collapse. I would argue that this experiment would only stand because of the compaction rate of the medium, as otherwise the pipe integrity would not have been accomplished. How would you describe the role of the medium's porosity / hydraulic gradient throughout the 15 cm profile in the "success" of the experiment?

        thanks,

         

        Taco 

        • AC3: Reply to CC2, Glenn V. Wilson, 06 May 2020

          Dear Taco

          We have used two different methods of compacting soil beds with soil pipes. In any laboratory experiment of soil beds, the method of compaction is a model of the real world but does not trully represent in situ soil structure.  This is true for laboratory sheet, rill, and gully erosion studies and is also true of soil pipe research. Its a simulation, advantages are controlled conditions but the disadvnatages are you cannot obtain the same structure.  

           

          I will send you the paper and the other papers so you can see the details of the experiments.