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Photograph - Graph of predicted flow and depth with time

Graph: Flow hydrograph (in red) for the highway drain in the region of the flooding. This was produced by the Micro Drainage model of the drainage system. Note the reverse (negative) flow (at time equals 20 minutes) due to overloading of the drainage system downstream.

Expert witness - flooding from highway drain

Complex flooding problem

One of our senior staff acted as an expert witness in a court case regarding a complex flooding problem in southern England and this involved producing a comprehensive expert witness report and later giving evidence in court. The flooding affected the highway and a residential property, the latter being flooded internally on occasion.

Terms of reference

The terms of reference involved assessing the suitability of an existing Micro Drainage computer model of the drainage system and examining the maintenance regime at the site. Our expert was also asked to construct a separate independent model of the drainage system and to comment on the flooding problem in the light of this.

Micro Drainage modelling

The modelling work carried out was based on data produced by a manhole survey organised as part of the study. Our expert then used Micro Drainage software to firstly construct a model of the drainage system and then to simulate the operation of the real drainage system for various return period storms. The model was constructed in accordance with the Urban Drainage Group's (formerly WAPUG's) Code of Practice for the hydraulic modelling of sewer systems.

Our expert ultimately produced six versions of the original model representing the drainage system at various times. The model was later also used to design notional improvements to the drainage system to alleviate the flooding problem for various levels of protection.

Analysis of results of CCTV sewer survey

The videos and coding sheets produced by a CCTV survey of a proportion of the study drainage system were examined by our expert and this provided further data to refine the models of the drainage systems.

Four causes of flooding

Our expert's modelling work in fact identified four separate causes of the flooding. These included flooding directly from the highway drain both adjacent to the property itself and also from further upstream. In the latter case the slope of the road caused the flood water to arrive at the road outside the property as overland flow.

Further causes of the flooding included siltation of the highway drain and a short length of 100mm diameter pipework within an otherwise 150mm diameter drain. The latter exacerbated the existing flooding problem and was felt to be the result of a poor quality repair. Finally overland flow from an adjacent street, caused by an hydraulically deficient public surface water sewer, also contributed to the flooding problem. Leaf debris causing road gullies to blind was also identified as a contributory cause of the frequent flooding and this was particularly important due to the quantities of overland flow contributing to the problem.


A comprehensive expert witness report was produced which among other things summarised the findings of the audit of the highway authority's model of the drainage system. This identified a significant discrepancy between the performance of the model and the way that the results of the modelling had previously been presented.


Our expert witness was cross-examined at length in court by a barrister specialising in construction matters.

For further details of our services, please see our Expert Witness page.