Accumulation of Pollutants in Highway Detention Ponds

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This PhD study deals with issues related to water and pollutant transport from highway surfaces caused by rain. It is essential in the study to apply methods and models in which improvements in relation to removal of pollutants can be identified and to be able to predict the yearly discharges of heavy metals and polycyclic aromatic hydrocarbons from an arbitrary detention pond to the natural environment. The present thesis is a part of a co-operation between the Danish Road Directorate (Vejdirektoratet) and Aalborg University and is founded in the Danish construction act for new highways and expressways by Herning, between Odense and Svendborg and between Holbæk and Vig. Special problems occur, when it comes to prediction of pollutant load from road runoff. One of the main problems is that the temporal varying intensity of the rain causes high variation of the runoff. Hence it is impossible from few measurements to predict annual pollutant loads from the runoff. The method that has been shown to be the most effective for coping with the time variation in the rain is a so-called hindcast where several years of measured rain are used for simulating the exact variation in runoff from every single rain event. From the hindcast results it is possible to calculate mean water and pollutant loads. This method is commonly used in urban drainage systems for capacity analysis or for prediction of CSO's.

The challenge is to develop a simplified and still accurate description of flow and transport of pollutant adequate for the long-term simulation of the pollutant transport from highways caused by rain. Because of the strong non linearity in the processes involved it is obvious that methods based on simple average concentrations cannot be applied when it comes to removal of particles in ponds. Measurements of water and pollutant transport are carried out in different highway systems. A geometrically well-defined test pond is established, wherein the deposition of particulate matter can be measured. The result from the test pond is transferred to real detention ponds in which the three-dimensional flow is described with a numerical CFD model. The particulate matter is analysed for grain size distributions, settling velocity distributions and corresponding heavy metal and PAH concentration. Erosion/resuspension experiments for detention pond sediments are carried out in the laboratory with currents and waves.

In general the study shows that the bulk of hydrocarbons, PAH's and heavy metals accumulate in detention pond sediments and the removal efficiency for particulate matter in the detention ponds is around 80 %. An important parameter for retention of particulate matter in Denmark is the wind - in that way, that calm water expedites the settling process contrary turbulent water. The impact from the wind can reduce the pollutant removal efficiency significant and even result in negative efficiencies due to resuspension of already settled particulate matter.

In the study a well calibrated one-dimensional transport model has been set up for describing the:

1. The build up of particulate pollution on highway surfaces.
2. The removal of particulate pollution on highway surfaces due to rain.
3. The transport of water and particulate pollution through the drainage system to the connected detention ponds.

This model can be used for prediction of event loads on detention ponds for several years.

Likewise a well calibrated three-dimensional transport model has been set up for describing the:

1. The transport of water, dissolved and particulate pollutants in wet detention ponds during arbitrary runoff events including the impact from wind and waves on the transport mechanisms.

The pond model is an essential tool for evaluating the ability of removal of particulate matter in existing and planned pond.

Original languageEnglish
Place of PublicationAalborg
PublisherDepartment of Civil Engineering, Aalborg University
Number of pages150
Publication statusPublished - 2008
SeriesDCE Thesis


  • CFD
  • Highways
  • Heavy metals
  • PAH
  • Rain
  • Resuspension
  • Runoff
  • Wind induced currents and waves


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