Poster 183 – Click on poster below to view presentation from author.
Tom DeBell
North Carolina State University
Co-Authors: Chadi Sayde, Barbara Doll
Green infrastructure is of growing importance in the face of extreme weather events and environmental degradation. However, the subsurface dynamics of stormwater systems, especially media-based treatment systems such as regenerative stormwater conveyance (RSC), are not yet fully understood. Through technological advancements, the precision and availability of in-situ monitoring tools increased dramatically. However, an accurate method for determining water fluxes through these treatment systems remains uncertain and challenging. Existing methods have limited applications due to their minuscule sphere of influence given the need to understand flux density behavior over large spatial scales. To overcome this limitation, we have implemented fiber-optic distributed temperature sensing (FO-DTS) technology, capable of hundreds of simultaneous measurements and can be configured at various scales. The fiber-optic cable is buried in the treatment system media and water flux is inferred from the FO cable’s thermal response to heat pulses generated by supplying a constant current to the shielding the cable. This methodology enables both continuous heating and temperature measurements along the desired length of FO cable, allowing for fluxes to be measured throughout an RSC cross-section. These measurements are necessary to understand the conditions that facilitate the treatment mechanisms employed to remove pollutants and evaluate the system’s efficacy.
Laboratory tests and field data indicate a potential for FO-DTS applications under various use cases; however, further experimentation is required to address the challenges that these methods present in practice. We will be presenting the theoretical background, preliminary results of laboratory and field experiments, and how our data can evaluate the internal hydraulics of media-based systems to inform better design decisions.
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I think your research focus is extremely important if we want to truly evaluate whether the theoretical treatment processes that we believe are occurring in the media are actually happening. Another dimension that I would be interested in would be the impact of flow regimes on RSCs. At high flow rates (major storm run-off events), is there any treatment happening at all or is the limitation of the system the inherent infiltration rate into and through the media which is independent of the flow above the media?
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Hi Dennis,
Thank you for sharing your shared interest in this work. You bring up an excellent point about how different-sized storms will be treated by RSC’s. I can tell you from the limited number of water quality grab samples that we have collected that the performance can vary not just based on how large of event it is, but what stage you are interested in (first “flush” v.s peak flow). I couldn’t include these initial results in this particular presentation, but I would be glad to share them with you if you are interested.
Inherently this particular system was designed to only fully “capture” the first 1/4″ of run-on from the watershed in pool storage before reaching the conveyance stage. These smaller storms are certainly where RSC’s perform best in terms of hydrologic mitigation and pollution reduction due to the whole storm being treated via infiltration into the media. However, there is still a treatment benefit for larger storm’s initial run-on, which tend to be the most concentrated in pollutants and sediments. Past the point of conveyance caused by larger storms, the RSC structure will also continue to reduce sediment loads due to decreased velocity caused by the cascading structure between pools.
A major point of focus for this research is to see if, up until the point where the system is cascading (conveyance stage), is there a correlation between the stage of ponded water above the media and the infiltration rate? You are correct in saying that for large storms, particularly at peak flow, there is only a maximum amount of driving head that can cause an increase in the rate of infiltration. We hope to identify first, under what conditions do we see the infiltration rate peak, and second if the fluxes through the media are happening uniformly. We hope that these will offer insights into the hydraulic retention time of water in the media to see if those perceived treatment mechanisms can occur. Thank you so much for your comment! I hope I addressed your question, but please don’t hesitate if you have any further thoughts.
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Very nice presentation and interesting (and needed!) project. I was wondering if your field cross-sectional set-up will allow you to gain insight into both vertical and lateral water movement in the subsurface?
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Hi Natasha,
Thank you for the kind comments and question!
The short answer is yes we believe so. Early on in the storm event the flow will primarily be vertical as it infiltrates into the media, but as it becomes fully saturated later in the storm, starting at the bottom of the transect, we expect that horizontal flow will become more dominant. However, the physical model used for calculating fluxes from our fiber optic transect has only been validated thus far for 1-dimensional flow. I am currently working on using a Hydrus model to determine under what conditions it would be appropriate to assume that the temperature responses we see are being caused from vertical v.s horizontal flows.
Thanks again!
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Great presentation and cool use of thermal sensing to infer water movement through RSC systems. I have a couple of questions about your field installation…out of curiosity, where is your fiber optic cable transect located within the overall RSC systems? Also, are you currently collecting supplemental field data (e.g., piezometer levels, other tracers) to aid in interpretation of your heat flux-based estimates? Thanks!
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Hi Trisha,
Thank you for the questions! The transect is located in the middle of the uppermost pool of the system. We selected this location because it seemed to dewater more readily than pools lower down on the structure and allowed for an easier installation. I am glad you asked about supplemental data because I have been monitoring the water table elevation using a Piezometer near our transect and water quality grab samples. Additionally, the estimates of hydraulic conductivity will serve as a reference to see if the values we see from the temperature estimates are reasonable based on the head conditions.
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Hi Natasha,
Thank you for the kind comments and question!
The short answer is yes we believe so. Early on in the storm event the flow will primarily be vertical as it infiltrates into the media, but as it becomes fully saturated later in the storm, starting at the bottom of the transect, we expect that horizontal flow will become more dominant. However, the physical model used for calculating fluxes from our fiber optic transect has only been validated thus far for 1-dimensional flow. I am currently working on using a Hydrus model to determine under what conditions it would be appropriate to assume that the temperature responses we see are being caused from vertical v.s horizontal flows.
Thanks again!
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