Industry guidance

There are a number of technical guides that provide up-to-date information about methods used for urban drainage modelling and also set out recommendations about

Fig. 18.14 Attenuation of flood hydrograph by sustainable urban drainage systems (SUDS) flood storage.

Table 18.8 Key guidance documents for urban drainage hydrology

Flow :-Modelled 'Current' —- Modelled 'Scenario'

Flow :-Modelled 'Current' —- Modelled 'Scenario'

Fig. 18.14 Attenuation of flood hydrograph by sustainable urban drainage systems (SUDS) flood storage.

The SUDS manual (CIRIA report C697), 2GG7

Designing for exceedance in urban drainage: good practice (CIRIA report C635), 2006

Code of Practice for the Hydraulic Modelling of Sewers, 3rd edn (WaPUG; the Wastewater Planning Users Group, part of the Chartered Institute of Water and Environmental Management), 2GG2

Guidance on the planning, design, construction, operation and maintenance of sustainable urban drainage systems. Includes landscaping, biodiversity, public perception and community integration, water quality treatment and sustainable flood risk management

Advice for design and management of urban sewerage and drainage systems to reduce the impacts that arise when flows occur that exceed their capacity. Includes both underground systems and overland flood conveyance. Advice on risk assessment procedures and planning for extreme events

Covers all aspects of model building for hydraulic analysis and testing, flow surveys and verification, and documentation the choice and application of methods as recommended by relevant technical working groups within the UK water industry. Many of the guidance notes relevant to urban drainage have been produced by CIRIA. Three key documents of direct relevance for hydrologists are given in Table 18.8.

In some cases the specific guidance contained within these documents has been based on a combination of scientific, practical and historical considerations. There can therefore be variations in the advice given to practitioners in different situations. Ultimately, the hydrologist should form a clear understanding of the assumptions and data that are used in the various methods so as to be able to judge their suitability in different circumstances.

For example, some guidance notes recommend that for estimating design peak flow rates for small rural ('greenfield') catchments of less than 200ha (2km2) the methods for small catchment flood estimation published in Institute of Hydrology Report No. 124 (IH124; Marshall and Bayliss, 1994) should be used, whereas for larger catchments, the FEH is recommended. The IH124 formulae have the advantage of drawing on a sample of gauged data specifically from smaller catchments. However, despite this empirical base, analysis of the results presented in IH124 shows that the peak flow relationships obtained for small catchments were not unequivocally superior to those of similar models available at the time based on the Flood Studies Report (Natural Environment Research Council, 1995; e.g., when compared with the existing FSR (Supplementary Report 16) empirical equation for time-to-peak, the IH124 results were better in about half of the 24 smallest catchments and worse in others).

The newer FEH methods are designed to allow a hydrologist to incorporate the most up-to-date and more representative available data for a particular catchment. The required catchment descriptors can be defined with due care down to a scale of approximately 50 ha, and, given a reasoned judgement about the representativeness of the data, an FEH peak flow estimate could be scaled by area to provide runoff values for smaller areas that are consistent with those for the wider catchment. Statistical analysis of the residuals about the FEH equations for estimation of median annual flood show little sign of bias or strongly increasing uncertainty towards the smaller catchment areas and so the empirical evidence does not make it obvious that the model is unsuitable for use in areas smaller than 200 ha. This is not to say that one method is universally or automatically better than another but that, with a good understanding of the methods and the catchment area, a hydrologist may consider a range of applicable methods and choose appropriately between them.

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