Flooding & Drainage

Stomor specialise in civil engineering design and assessments for housing, commercial developments and educational projects.

Stomor Ltd are able to provide Flood Risk Assessments in accordance with The National Planning Policy Framework, Flood Risk Standing Advice. Studies include obtaining historical or modelled flood level data from the Environment Agency and identifying the risk of flooding from fluvial sources. Flood compensation can be designed where development encroaches upon the flood plain, providing sufficient land is available.

Drainage strategies are prepared as part of the FRA which identify options for foul and surface water drainage, identifying the risk of flooding from local drainage infrastructure inadequacies and ensuring that the proposed development does not present a flood risk to downstream properties.

Preparation of drainage strategies include design for flow control, storage and Sustainable Drainage Systems (SuDS) for storms up to 1 in 100 year plus allowance for Climate Change. Flood routes are identified for extreme storm events above the 1 in 100 event, and strategies are forwarded to the Environment Agency and Drainage Authorities for approval as necessary.

Assessments of existing drainage networks can be carried out to establish existing drainage routes and network capacity. Jetting and CCTV surveys can be arranged where necessary.

Flood Risk Assessments

What is a Flood Risk Assessment?
A Flood Risk Assessment is a document which identifies whether the site in question could be susceptible to flooding at present or as a result of future development. It will also determine if any proposed development could cause a risk of flooding to neighbouring properties or areas downstream of the site. A Flood Risk Assessment is a vital part of most Planning Applications.

Preparation of Flood Risk Assessments in accordance with National Planning Policy Framework, Flood Risk Standing Advice includes the following processes:

  • Obtaining historical or modelled flood level data from the Environment Agency.
  • Identifying risk of flooding from fluvial sources
  • Identifying the risk of flooding from local drainage infrastructure inadequacies.
  • Preparing Drainage Strategies to reduce or maintain flows to downstream watercourses or existing sewer network, including design for flow control, storage and Sustainable Drainage Systems (SuDS) for storms up to 1 in 100 year plus allowance for Climate Change.
  • Indentifying flood routes for extreme storm events above the 1 in 100 event.
  • Negotiations with Environmental, Planning and Drainage Authorities.

Do I need a Flood Risk Assessment?
The local planning authority will generally advise whether or not a Flood Risk Assessment is required, either in consultation with the Environment Agency or in accordance with their own policies.

However, as a general rule; if your site is within Flood Zone 2 or 3 you will need a flood risk assessment. If it is outside these zones you may still need one, particularly if the development is over 1ha or in close proximity to a river or other watercourse.

Costs and time scales?

  • Up to 1-3 weeks to receive data
  • Up to a further 1-4 weeks to complete the report
  • Each project is assessed on an individual basis, please contact us with site details for a budget cost

Drainage Strategies

A drainage strategy is preliminary drainage design that is often used to form the basis of a detailed design in the future. The majority of planning applications require the submission of a drainage strategy to demonstrate that a proposed development is able to discharge both surface and foul water. The strategy should include the use of Sustainable Drainage Systems SuDS where possible and be designed in accordance with the Local Water Authority and Environment Agency regulations.

Existing Drainage Assessments

Stomor provide onsite inspections and assessments of existing site drainage networks to assess/establish current capacity and connection routes. Often jetting and/or a CCTV survey is required to establish the condition of the sewers and the exact number and route of any underground connections. Should you require a CCTV survey please contact us for further details.

Stomor can provide site specific solutions to existing problems, compliant with the current building regulations.

Attenuation Specification and Sizing

It is standard practice for surface water flows from new developments to be restricted to match existing flows, or equivalent greenfield runoff flow rates, in order to minimise the impact of development upon downstream watercourses. This practice helps to safeguard areas downstream of the proposed development from flooding as a result of urbanisation of upstream sites.

As a result, water is stored at source, on the site. There are many options for provision of attenuation which include ponds, swales, cellular storage, granular storage or tanks, among others. We prepare designs to suit site characteristics, space, operation and maintenance, with sustainability and cost effectiveness as key factors in our specification.

Design of Flow Control Measures

In order to restrict runoff from development sites, in order to match existing flows or equivalent greenfield runoff rates, the choice of flow control device is fundamental to ensuring sustainable and effective operation of the entire drainage system.

Options for flow control can include pumping, orifices, Hydrobrakes or similar, each with different flow characteristics and suitable in different situations.

Our designs are carried out to identify the most suitable and efficient method for flow control, with associated attenuation specification and sizing.

Sustainable Drainage Systems SuDS

SuDS are physical source and water flow control systems which aim to mimic greenfield run-off conditions on new developments.
The greenfield run-off conditions are dependent upon soil type and topography, as are SuDS solutions.

Our drainage designs aim to utilise the most appropriate and effective SuDS, based upon a sustainibility hierarchy as follows;

  • Basins and Ponds; including balancing or retention ponds, detention basins.
  • Filter Strips and Swales; for attenuation and improved water quality.
  • Infiltration Devices; including soakaways or infiltration trenches.
  • Permeable Surfaces; including porous paving, gravelled areas.
  • Tanked Systems; including oversized pipework or cellular storage systems

Designs are carried out in accordance with CIRIA good practice guidelines.

Drainage Modelling in Micro Drainage®

MicroDrainage is the Industry Standard software for modelling and simulation of drainage systems. Our designs for foul and surface water sewer systems, including flow control, attenuation, and infiltration are modelled and simulated using MicroDrainage in order to check the effective hydraulic operation of the system.

MicroDrainage enables us to identify where cost savings can be made in the design, by refining the size of pipes and storage facilities.

We are then able to forward full design calculations or even the computer model to the Local Water Authority for them to review.

LLFA compliant Drainage Strategies

The Lead Local Flood Authority (LLFA) for Hertfordshire became a statutory consultee on 15th April 2015 in relation to the management of surface water drainage. Discussions with our Clients have identified that the LLFA technical requirements for Flood Risk Assessments and Drainage Strategies has been a major stumbling block for planning applications. This is primarily down to the increase in the amount of detail required at the earlier stages of the planning process and justification of Sustainable Drainage System (SuDS) selection. To help planning consultants understand the requirements of the LLFA, Stomor has produced a briefing note that sets out what the LLFA will need in order to satisfy their requirements. It also sets out what information would be required and the role of the Client and Planning Architect in the process. We have held detailed meetings with the LLFA and, as such, have a thorough understanding of their requirements and the information they are looking for. Stomor has recently produced a Flood Risk Assessment and Drainage Strategy for a proposed new 2FE primary school to be located in Bishop’s Stortford. These proposals have been agreed with the Hertfordshire County Council (HCC) LLFA, who provided the following response (dated January 2016). “The applicant has provided sufficient detail to demonstrate that there is a feasible drainage scheme for the site, including rain gardens, swales, ponds and permeable pavements. The proposed layout for the development makes the provision of sustainable drainage features which they have demonstrated is a feasible option to provide the required attenuation volume.”

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