Residence Times in Vegetated Stormwater Ponds

Bringing together lab work, field work, and computational modelling to present a complete picture of mixing processes in vegetated ponds.

Ponds are often placed besides roads or in developments in order to capture rainfall run-off. Their purpose is two fold. To help reduce urban flooding and to improve run-off quality. This EPSRC funded project—"Residence Times in Vegetated Stormwater Ponds"—is focused on the quantifying the latter. The project is aimed at understanding and modelling how vegetation impacts on water movement in ponds. This will allow for better predictions of sedimentation and the ability of ponds to improve water quality. The work being undertaken is a collaboration between the Universities of Sheffield and Warwick.
Updated July 25th, 2016
The project is wrapping up! Our Final Workshop was a success and the presentations are available online here. We hope to have several journal publications shortly submitted describing our work in more detail. We also hope to soon make available an open access archive of our research data. The photo on the left shows visitor Sergio Saul Solis (far left) with the project team, from left to right, Mahshid Golzar, Fred Sonnenwald, Ian Guymer, Patrick West, James Hart, Nathan Wilson, and Virginia Stovin performing field work at our validation site, Longbridge Pond.
Project Overview
Storm water run-off typically contains and transports a wide range of pollutants, resulting in negative environmental effects with potential threats to ecosystems and health. Hundreds of run-off treatment ponds intended to moderate these impacts are likely to be delivering sub-optimal (and perhaps actually below legally required) levels of improvement in water quality due to poor understanding of flow patterns and the effects of vegetation. This project will generate a unique dataset to describe the influence of different types and configurations of vegetation on the pond's fundamental flow—and treatment—characteristics. We will also deliver a validated set of vegetative resistance and mixing parameters that are essential if 3D computational modelling tools are to be used with confidence. These tools will ensure that future pond designs meet all their water quality and ecosystem services objectives for current legislation and the increasingly stringent EU regulatory framework anticipated over the next decade.

Stormwater ponds take run-off from urban areas, highways and agricultural land, providing detention and attenuation of peak storm discharges and improving water quality. Stormwater ponds are able to provide protection to downstream drainage components and receiving waters by holding or treating run-off at or near the source and provide additional nature conservation and amenity benefits. Within the Highways Agency Asset Inventory System alone there are currently over 800 stormwater ponds. Pond performance (pollutant treatment efficiency) is directly related to hydraulic residence time, a function of the internal flow field, which in turn is controlled by the pond geometry and the distribution and type of vegetation present.

The prediction of water quality improvements within drainage features is gaining importance with stormwater professionals. However, performance prediction is complex since water quality processes are functions of the pond hydraulic residence time. Current evaluations employ the nominal residence time which assumes plug flow through the pond, as the design consideration. It is accepted that the nominal residence time (pond volume/discharge) provides a poor estimate of the actual mean (or median) residence time, with overestimates of treatment times of 100% or more not being uncommon. However, it is still in use, even 'the norm'.

In wastewater treatment wetlands, treatment is good since a high degree of engineering is adopted in creating an efficient, often linear, shape with uniform, dense, vegetation. In contrast, stormwater ponds must fit into existing water courses or urban environments. Together with the additional requirements for biodiversity and ecological function, this leads to pond layouts that may be less than ideal from a hydraulic perspective.

Vegetation can have either a positive or negative role in water quality treatment within stormwater ponds. It provides the appropriate environment for the support of biofilms and the colonisation by algae, enhancing treatment, yet variable spatial distribution influences the spread of the hydraulic residence time. This proposal seeks to better understand and quantify the physical, vegetation-driven, flow mechanisms occurring within a stormwater pond and to develop a robust physically based modelling tool. The research proposed here will deliver improved understanding of the effects of vegetation (type: emergent, floating and submerged; physical characteristics: porosity and spatial distribution) on flow patterns and residence time distributions (RTDs) within stormwater ponds. The validated computational modelling approach will permit the assessment of short circuiting, a measure of poor performance, and provide estimates for vegetation contact times, sediment deposition regions and rates. This will provide a tool for predicting the treatment efficiency of vegetated stormwater ponds.
Project Objectives
The aims of the project are to derive an understanding of how the hydraulic residence time distribution (RTD) within a stormwater pond is affected by the type and spatial distribution of vegetation and to identify appropriate modelling tools for the representation of the water quality performance. The following objectives apply:
  1. To collect comprehensive new laboratory data on the bulk porosity and flow resistance of different pond vegetation types (emergent, floating, and submerged) as a function of seasonal growth cycles over a range of typical pond flow velocities.
  2. To conduct laboratory solute tracer studies to quantify the mixing (transverse, longitudinal, and RTD) within, and the exchange coefficient between, clear and vegetated flow zones.
  3. To utilise the laboratory data to validate computational fluid dynamics (CFD) modelling procedures and quantify parameters appropriate for describing the influence of vegetation on velocities and mixing.
  4. To perform field measurement of RTDs in mature vegetated ponds over seasonal growth cycles to validate the 3D CFD methodology.
  5. To conduct scenario modelling to explore the sensitivity of RTDs to vegetation mosaics, discharge and pond shape.
  6. To facilitate the dissemination of the validated modelling methodology, together with appropriate parameter values for the flow resistance and mixing processes.
Project Grant
Professor Ian Guymer and Dr. Virginia Stovin were successfully awarded EPSRC grants EP/K025589/1 and EP/K024442/1 to complete this research. The project started September 1st, 2013 and will end August 31st, 2016.
Ian GuymerProfessor Ian Guymer, the University of Warwick
Principal Investigator

Prof. Guymer is a long time researcher of mixing processes in both urban and river contexts. He is currently at the University of Warwick, where he leads the Warwick Water research group. Along with his research projects, he also teaches students about mixing processes. With this project he hopes to increase the practical understanding of mixing at the vegetation-water interface. Learn more about Prof. Guymer.
Virginia StovinDr. Virginia Stovin, the University of Sheffield

Dr. Stovin has spent the last several years investigating the potential of urban drainage systems, like ponds. At the University of Sheffield she is a Reader in Urban Drainage, teaching students about hydrology and SuDS design. She believes this project will lead to innovative new methods of evaluating both pond designs and existing ponds. Learn more about Dr. Stovin.
James hartDr. James Hart, the University of Warwick
Research Fellow

Dr. Hart is a recent graduate of the University of Warwick, where he did his PhD on longitudinal dispersion in transitional pipe flow. Dr. Hart continues at Warwick as a post-doctoral researcher for the project. He is responsible for the design and construction of experimental facilities, carrying out lab and field work to measure vegetated flows, and the analysis of associated results. Learn more about Dr. Hart.
Fred SonnenwaldDr. Fred Sonnenwald, the University of Sheffield
Research Associate

Dr. Sonnenwald has recently graduated with a PhD from the University of Sheffield, where he developed a deconvolution methodology for identifying residence time distributions (RTDs) from experimental solute transport data. He has remained at Sheffield to carry out post-doctoral research as part of the project. His work focuses on developing practical computational fluid dynamics (CFD) techniques to model vegetated ponds. Learn more about Dr. Sonnenwald.
Patrick WestPatrick West, the University of Warwick
Research Student

Patrick West is a current PhD student at the university of Warwick. His research topic is to quantify the exchange of mass between vegetation and mass at low flow velocities. He has been working with Dr. Hart to develop the new experimental facilities for the project.
Mahshid GolzarMahshid Golzar, the University of Sheffield
Research Student

Mahshid Golzar is a current PhD student at the university of Sheffield. Her research topic is still to be decided, but will involve CFD modelling of ponds.
Nathan WilsonNathan Wilson, the University of Warwick
Research Student

Nathan Wilson is a new PhD student at the university of Warwick who, while still developing his final research topic, has been performing project field work. Nathan has been working at the Longbridge Pond field site with Patrick West, Sergio Saul Solis and Ian Baylis to collect solute transport data for CFD model validation.

Experimental Facilities
New experimental facilities have been developed at the University of Warwick for this project. An existing wide flume has been equipped with a laser induced florescence (LIF) system. This new system allows for the detection of solute at better than millimetre resolution across a line in 2D space across the channel width. Furthermore, a new deep flume has been constructed, allowing for flow depths of over 1 meter. It has similarly been equipped with the new LIF system. Combined these flumes allow for investigations into full channel width and floating or submerged vegetation. They also allow for investigation of the vertical and horizontal shear layers that form at the vegetation-water interface to be studied in great detail.
The Deep Flume
This new flume is 24 meters long by 1.5 meters deep, but still under construction.

Before side panelling
Looking into the flume
The LIF Measurement System
In the laser induced florescence system, a laser cuts across the channel from the side, and any dye passing it fluoresces, making it visible to the camera looking up from under the flume.

The laser window in the channel bed
The image acquisition system
The lasers turned on
Real Vegetation
One of the main project objectives is to obtain a set of measurements around real vegetation. Some of the first work using real vegetation has been carried out.

Vegetation as it arrived in the lab
Vegetation in the wide flume
Mid-project Workshop
A mid-project workshop was held on Wednesday March 18th, 2015 at the University of Warwick. There were over 40 attendees, including our guest presenters Dr. Jean Lacoursière, Dr. Mick Whelan, Dr. James Shucksmith, Dr Scott Arthur, and Dr. Ramesh Ponnambalam. Unfortunately, Dr. Andrea Bottacin-Busolin was unable to attend. The day was very successful, and several of the presentations are available below.
Mid-project Workshop Presentations
Residence Times in Vegetated Stormwater Ponds
Presented by Professor Ian Guymer, University of Warwick and Dr. Virginia Stovin, University of Sheffield
This presentation is a project overview.
Feasibility of the porous zone approach to modelling vegetation in CFD
Presented by Dr. Fred Sonnenwald, University of Sheffield
Real Vegetation — Laboratory and Field Studies
Presented by Dr. James Hart, University of Warwick
Measuring Residence Time Distributions in a Vegetated Pond
Presented by Dr. Jean Lacoursière, Kristanstad University, Sweden
Vegetation Enhanced Mixing in Open Channel Flows
Presented by Dr. James Shucksmith, University of Sheffield
Monitoring SuDS Pond sediment transport using the lanthanide group of elements
Presented by Dr. Scott Arthur, Heriot-Watt University
Final Workshop
An end of project workshop was held on July 6th, 2016 to present research findings at the University of Warwick. The workshop was well attended and our guest presenters, Matthew O'Hare, Anders Wörman, Thorsten Stoesser, Andy Bailey, and Ellen Bedford stimulated interesting discussion. Unfortunately David Naismith was unable to present as planned. The day was very successful and several of the presentations and posters are available below.
Final Workshop Presentations
Residence Times in Vegetated Stormwater Ponds
Presented by Professor Ian Guymer, University of Warwick
This presentation is a brief project outline.
Pond Plants: assemblages, traits and attributes
Presented by Matthew O'Hare, Centre for Ecology & Hydrology
Remediation measures of agricultural streams
Presented by Anders Wörman, KTH Royal Institute of Technology
Overview of Laboratory Studies
Presented by Dr. James Hart, University of Warwick
Benefits and Operation of Stormwater Ponds for Highway Drainage
Presented by Andy Bailey and Ellen Bedford, Highways England
Field Study – Longbridge
Presented by Nathan Wilson, University of Warwick
CFD models of real pond scenarios
Presented by Virginia Stovin, University of Sheffield
This presentation includes project next steps.
Supplementary video files: Animation of convergence fluctuation in steady-state vegetated pond model
Final Workshop Posters
Effect of Basin Shape and Vegetation Patterns on Wetland Performance
Andrea Bottacin-Busolin, Nima Sabokrouhiyeh, and Andrea Marion
Feasibility of the porous zone approach to modelling vegetation in CFD
The presentation of a conference paper presented at the 34th International School of Hydraulics, the paper will soon be available
Residence Times in Vegetated Stormwater Ponds
A poster presented at the 2014 IAHR UK Chapter Annual Technical Meeting
Pollution in the Environment ~ Aquatic Vegetation
A poster presented at the 2014 IAHR UK Chapter Annual Technical Meeting
The University of Sheffield
The University of Warwick