Trash Screen
Content Table
Introduction
Culverts are primarily structures which provide a route for watercourses to pass under obstacles such as roads, railways and urban developments. As such, they must be engineered and maintained effectively to minimize flood risk to property, members of the public and infrastructure. In-channel debris can have a significant impact upon conveyance through culverts if it becomes lodged within the structure, especially during storm events, as this can lead to flooding and sometimes damage to the culvert itself. This problem has had real consequences in recent flooding events and is alluded to in the Pitt Review (Pitt, 2008) where it is noted that (pg xix), ‘Many of the responses received by the Review have blamed the extent of flooding last year on ....... debris being allowed to build up’. The problem was also highlighted in the Foresight (2004) report in which it was recognised that (pg 309), ‘…..debris within the urban system may also increase the potential for blockage and impair performance of outlets to the urban drainage system’. This blockage hazard is often combated by fitting a trash screen at the structures inlet but these can be a hazard in themselves as they are, by their very nature, inherently designed to block and may therefore cause flooding if an effective maintenance regime is not implemented. The problems associated with effective screen operation practices are widely recognised amongst asset managers who are well aware that undertaking capital works through fitting a screen does not ensure a reduction in flood risk unless the maintenance regime and necessary associated budget are adequate to ensure that the structure operates effectively over its design life. Unfortunately, it can be argued that often too much emphasis is placed upon the construction of new defence works and maintenance budgets are sometimes an afterthought which, in the case of trash screens, can lead to an actual reduction in flood resilience. This problem has been highlighted in the Environment Agency’s Strategy for Flood Risk Management (Environment Agency, 2003) (pg 9), ‘Flood defence assets and their management are extremely important. .......The replacement value of our flood defence asset base is more than £15 billion, with only £100m (0.7% of asset value) spent on maintaining and operating the assets. The operation and maintenance of this major asset infrastructure is key to the minimisation of damage from flooding and this level of expenditure is believed to be inadequate’.
There have been few systematic investigations of blockage by debris at hydraulic structures such as culverts in the UK. However, an ongoing research project within the Flood Risk Management Research Consortium Phase 2 (FRMRC 2) and SAWA (Wallerstain et al, 2009-10) is currently tackling this issue. Within this project primary research is being conducted to produce methods by which the maintenance of culverts that have trash screens in place can be optimised with respect to balancing the competing requirements of maintaining and servicing these structures on the one-hand to ensure they perform as designed while keeping the consequent operational costs to a minimum.
It should be noted that the term ‘trash’ is used synonymously with the term ‘debris’ here. These two terms refer to both organic material such as leaves, branches, trees, grass, etc. which enter the channel through the processes of wind transport, bank erosion and wash-in from the catchment surface and anthropogenic material such as litter, general household waste, and large non-domestic waste such as shopping trolleys and pallets, etc. which have been dumped, or blown into the channel.
 |
Figure 1: Example of organic debris (top), and anthropogenic trash (bottom) deposited at trash screens.
References
Pitt M. (2008) Learning the lessons from the 2007 floods. An independent review by Sir Michael Pitt. London, UK: Cabinet Office. June 2008. Available at: www.cabinetoffice.gov.uk/thepittreview.
Foresight (2004) Future Flooding. London, UK: Office of Science and Technology. Available at:
www.foresight.gov.uk.
Wallerstein N. P. and Arthur S. (2010a) Improved methods for predicting trash loading at culverts and trash screens. In Proceedings of the Flood and Coastal Risk Management Conference 2010. Telford, UK: Environment Agency and defra. 29th June-1st July 2010.
Wallerstein N. P. and Arthur S. (2010b) Development of equations for prediction of blockage at trash screens. In Proceedings of the First European Congress of the IAHR. Edinburgh, UK. 4th-6th May 2010.
Wallerstein N. P., Arthur S. and Sisinngghi D. (2010) Towards predicting flood risk associated with debris at structures. In Proceedings of the IAHR APD Conference 2010. Auckland, NZ. 21st-24th March 2010.
Wallerstein N. P., Arthur S. and Sisinngghi D. (2009) Relationship between river discharge and debris blockage at culverts with trash screens. In Pasche E., Evelpidou N., Zevenbergen C., Ashley R. and Garvin S. Proceedings of the Final Conference of COST Action C22 Urban Flood Management in cooperation with UNESCO IHP/PARIS: Road map towards a flood resilient urban environment. Paris, France. 25th-27th November 2009.
