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The Integration of Best Management Practice (BMP, or SUDS) and Traditional Sewerage Schemes in a Dynamic Simulation Model

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Courtesy of Innovyze

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BMP/SUDS structures such as swales and permeable car parks are being increasingly used to provide solutions to urban drainage problems including flooding and pollution of local water courses. In most cases these structures are retro fitted into an existing urban area. There is therefore the need not only to design the structures themselves, but to assess their impact on the existing sewerage system. Previously addressed by manual calculations, these tasks are best accomplished using simulation software such as InfoWorks CS, which can represent both the BMP/SUDS structures and the existing sewerage system.

InfoWorks CS simulation software addresses these tasks as follows:

·         Fully dynamic representation of traditional sewerage systems using the full solution St Vennant equations.

·         Representation of the flows through granular material using the Darcy flow equation.

·         Infiltration loss to the surrounding soil from the BMP/SUDS structures.

·         Representation of the infiltration and inflow (I/I) contribution from the catchment surface into the BMP/SUDS structures.

One aspect of many BMP/SUDS structures is that they can retain flow for a considerable time. Their operation is therefore significantly influenced by antecedent conditions. There is therefore the need not only to assess their performance during discrete design events, but also during long term rainfall data series. A simulation approach is ideally suited to meeting the challenges associated with running long time series, and to drawing useful conclusions from the large amounts of data created.

With any new software implementation there is clearly the need to validate the model results through comparison with traditional manual methods of calculation and also to confirm that the model works in real life situations. This has been completed in three case studies that are outlined below.

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Case study 1 - Salford Sports Village

Salford, a city in the north west of England, planned to construct a sports village, incorporating BMP/SUDS concepts in its management of storm water. The site conditions were favorable for infiltration with clayey sands and gravels providing the potential for high infiltration loss rates from the structures into the surrounding soils.

The modeling study involved replicating an infiltration trench, which had been designed using hand calculation. The model was simulated with a range of rainfall durations which supplemented the constant intensity testing which had been used duing the original design.  The results confirmed that the original design was valid as the two approaches gave very similar results.

Having established the validity of the model, it was then used to:

  • optimize the storage volume
  • examine the break up storage into discrete packages across site
  • produce sensitivity analyses of changes to the porosity and infiltration rate and on the impact of the Darcy coefficient (for flow rate through permeable soils).

Case study 2 - Hoylake Park & Ride

The second case study addressed a park and ride scheme in Hoylake, in the north east of England where a permeable pavement was required to provide the necessary storage and attenuation of flow from the site.  The pavement size was 400 yards by 5 yards and 0.6 inches thickness, and the granular material was of 52% porosity.

The original design was again tested, examining the conduit with infiltration loss from base, permeable flow equation through blocks, application of area laterally along unit, and any consequent impacts using design profiles.  The modeling software and the model again produced similar results to the traditional design calculations.

Case Study 3 - Ilfracombe School

A school in England plans to use BMP/SUDS to address drainage from buildings and a car park.  A variety of systems are currently under consideration including:

  • A bio retention basin
  • Swales
  • Treatment infiltration trenches
  • A pond and detention basin

Not yet designed, this project will use BMP/SUDS modeling, which offers many additional benefits over traditional approaches for this type of complex multistage project.

Conclusions

Traditional BMP/SUDS design is often based on very simple equations and approaches.  These case studies confirm that software models can replicate the results of traditional approaches.  However, modeling is more than simply a replication of traditional methods; it can extend far beyond.

In terms of calculation, modeling can handle more detailed rainfall profiles, produce detailed overflow analysis, produce better runoff representation, and allow sensitivity analysis on all key parameters including infiltration rates, porosity, and hydraulic conductivity.  In its presentation of results, modeling allows the viewing of images of failure thresholds in geoplan (surcharge and flooding) and a detailed set of results for every simulation run. Software simulation allows various ‘what if' scenarios and structure configurations to be quickly tested. Importantly it also allows the effect of BMP/SUDS schemes on the whole catchment to be understood.

In essence, software modeling has now developed to the stage where it can provide the same invaluable support for BMP/SUDS design and operation as it has long been providing for collection systems.

This is a summary of a presentation given at the Wallingford Software International User Conference, 2005 by Jamie Margetts of Clear Ltd. 

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