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Evaluation of flow models and pollutant retention isotherms for their application to rain garden bioretention

Evaluation of flow models and pollutant retention isotherms for their application to rain garden bioretention

Quinn, Ruth (2015) Evaluation of flow models and pollutant retention isotherms for their application to rain garden bioretention. PhD thesis, University of Greenwich.

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The primary aims of this research was firstly to develop a computer modelling tool which could predict pollution retention in a rain garden and secondly to use the model and additional experiments to examine various aspects of rain garden design with respect to pollutant retention.
Initially, the behaviour of all contaminants in urban runoff was examined including their retention and possible modelling methods. Heavy metals were then identified as the main focus of this project as this choice was the most beneficial addition to current research. The main factors affecting their retention were found to be macropore flow, pore water velocity, soil moisture content and soil characteristics and the primary method of modelling capture was identified as a sorption isotherm. Thus a dual-permeability heavy metal sorption model was developed; this was based on an intensive literature review of current best practice in both hydrological modelling and pollutant retention fields with respect to rain garden devices. The kinematic wave equation was chosen to model water movement in both the matrix and macropore regions as this provided a simpler alternative to more complex equations while still maintaining good accuracy. With regards to the modelling of heavy metal retention three isotherms were chosen: the linear, Langmuir and Freundlich equations as these were found from previous research to be the most accurate. These isotherms were incorporated into a one dimensional advection-dispersion-adsorption equation in order to model both transport and retention together.
This model was tested against the appropriate literature and accurate comparisons were obtained thus validating it.
Column experiments were designed to both provide a unique contribution to rain garden research and further validate the model. This was achieved by analysing past experiments and identifying an area where research is lacking; this area was the effect of macropore flow on heavy metal retention in rain garden systems under typical English climatic conditions. The findings of these experiments indicated that although macropore flow did not impact the hydraulic performance of the columns, retention of the most mobile of heavy metals, copper, was decreased slightly in one case. The overall retention of the columns was still high however at a value in excess of 99% for copper, lead and zinc. The results of the experiments were also used to further validate the model.
The model was then applied to the development of a rain garden device for a planned roundabout in Kent, U.K. Preliminary design considered an upper root zone layer with organic soil and a sandy storage sublayer each 30 cm thick, for a rain garden area of 5 and 10% the size of the contributing impervious surface. Two scenarios were examined; the accumulation and movement of metals without macropores and the possibility of groundwater contamination due to preferential flow. It was shown that levels of lead can build up in the upper layers of the system, but only constituted a health hazard after 10 years. Simulations showed that copper was successfully retained (no significant concentrations below 50 cm of rain garden soil depth). Finally given concerns of preferential flow bypassing sustainable drainage systems, macropore flow was examined; results indicated that due to site conditions it was not a threat to groundwater at this location for the time frame considered.
These actions successfully completed the objectives of this project and it was deemed successful.

Item Type: Thesis (PhD)
Additional Information:
Uncontrolled Keywords: water modelling; macropore flow; bioretention;
Subjects: T Technology > TD Environmental technology. Sanitary engineering
Faculty / School / Research Centre / Research Group: Faculty of Engineering & Science > School of Engineering (ENG)
Faculty of Engineering & Science
Last Modified: 30 Jan 2017 15:11

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