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Right Parameters for Drainage Modelling

March 22, 2024

We have recently experience more and more Lead Local Flood Authorities (LLFAs) querying rainfall data and parameters used for the hydraulic modelling.

In this article we talk about FSR and FEH rainfall Data, Cv values and more. We find out HOW to set up your hydraulic model to avoid post-planning surprises.

FEH and FSR Data

The first parameter to consider when hydraulically modelling drainage networks is whether to use FEH or FSR rainfall data.

Some LLFA’s will identify whether a specific dataset should be used in their SuDS requirements, whilst others do not mention SuDS requirements at all.

FEH is generally considered to be more accurate, due to the data being based on more recent and more detailed rainfall information, however, this methodology does not include sub-hourly events and although extrapolation is possible, flood estimation guidance from the Environment Agency recommends the use of FSR for events under one hour.

To this end, FEH data is now widely accepted as being the preference when designing attenuation system where critical attenuation times are above 60 minutes in duration.

When designing pipe networks where time of concentration / conveyance is typically below 60 minutes (with the 15-minute duration event often being the critical event for pipe sizing), FSR rainfall data is preferred due to sub-hourly rainfall data more heavily included in its research.

In light of the above, we would suggest modelling hydraulic networks (encompassing pipes and attenuation structures) with both FSR rainfall data (for the sub-hourly events i.e. 15, 30 and 45mins) and FEH rainfall data for all events 60+ mins and provide all results to the LLFA.

Whilst it depends on catchment and location of a given site, from our experience FEH-13 and FEH-22 data can easily provide attenuation volumes of 10-15% higher than with FSR data and hence using this data in the wrong context can be costly.

Cv values

The volumetric runoff coefficient – Cv, is the proportion of the rainfall volume materialising within the positive drainage system.

Cv varies depending on the permeability of the catchment, the soil type and the wetness among other factors.

Historically a value of 0.75 and 0.84 was used for summer and winter events respectively (as adopted from the Wallingford Procedure). We however are seeing the accuracy of this approach now scrutinised by LLFAs who request a Cv closer or equal to 1.

We have noticed some LLFA SuDS guidance documentation explicitly request Cv to be set at 1, or to provide evidence of why it should be set lower.

This clearly results in additional attenuation volume when compared to the default values recommended by the Wallingford Procedure and other hydraulic drainage design guidance.

Indeed, whilst designers will explicitly need to follow the Cv values set by LLFAs to gain their approval, it is our view the use of Cv’s approaching 1.00 should be questioned and evidenced.

 What are the consequences of using the wrong data sets and parameters?

The used of FEH or FSR and different Cv values, will have a significant impact on the attenuation volumes to be provided.

A significant risk to any project is when a Drainage strategy is submitted for planning using a given size for an attenuation tank or basin. This informs the size of the landscape and architectural design. Subsequently, if planning feedback states that different parameters should be used increasing tanks or basins by as much as 20%, this can have a serious detrimental effect on the masterplan.

How can we design accurately without oversizing our SuDS features? 

  • Read the SuDS guidance for each specific council to determine if their requirements include a specific data set or Cv value.
  • Do not assume the default parameters in the modelling software are correct from approval. Justify the use of a lower or higher Cv value and ask questions like – Are all the areas fully impermeable? Do my catchment areas include swales, raingardens, basins, etc?
  • Talk to the LLFA in advance. Most councils offer a first consultation over the phone or email for free or for a small fee. Evidence of their response will support the strategy submitted for planning.
  • The FSR is still valid for events under 1 hour. Even when the FEH data is required, it should be acceptable to determine the attenuation volumes for rainfall durations below one hour with the FSR methodology and avoid unnecessary attenuation volume.

In summary make sure you have checked parameters with your local council and LLFA and do not assume that the modelling software is correct. CHECK!

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