Persistent rainfall, flooding and how civil engineering is rising to the challenge

The unpredictable nature of rainfall events is something that has been an issue for our civil infrastructure throughout history. In the UK, many of our cities were established by the Romans and experienced a significant amount of growth during the industrial revolution. A lot of the drainage infrastructure during these times were developed by trial and error, with most of the rainfall channelled via the road network to the nearest watercourse without any real consideration to managing rainfall volumes.

Fast forward to 2024, our civil engineering team now consider, on all projects, the consequences that flooding has on property, infrastructure, human life, travel, and the environment.

The major difference in how we design for rainfall events compared to previous decades is that now we consider extreme weather events and the impact that climate change has on every scheme. The Flood and Water Management Act 2010 included the Lead Local Flood Authority as a statutory consultee as part of the planning process. This put drainage infrastructure design at the forefront of granting the permission on new developments for both new and refurbishment projects. We now design all our drainage schemes for rainfall events up to and including the 1 in 100-year return period with an additional climate change factor, which is up to 50% extra in some parts of the UK. This ensures schemes are future proofed for their lifetime.

Our experience of the planning process has identified a few key items that must be provided to gain a planning permission for your development, most of which reference the CIRIA Sustainable Urban Drainage Schemes (SuDS) design manual. Although simple in concept, these items can cause great issues when coordinating with other on multi-disciplined large scale projects.

Flow restriction

The Lead Local Flood Authorities (LLFA) require that all proposed developments must have a restricted stormwater discharge from the development equivalent to Greenfield Run-off Rate (GRR). GRR is the flow from the proposed development as if there were only vegetation on the site and is calculated using actual rainfall data, region-specific soil, and hydrological characteristics. When a development increases the percentage of impermeable area, the run-off rate also increases. The impact of this on downstream elements is flooding of property, infrastructure, and damage to the environment.

The restriction can be created in several ways but the most common is to use a flow control device with a small outlet orifice that limits the flow to GRR from the development.

Attenuation

When we increase impermeable areas and restrict flow to GRR, we create a volume of surface water that must be contained within the planning redline boundary for slow release to the downstream network. The preferred way to do this is via a series of natural features that mimic the pre-developed site, such as permeable surfaces, green roofs, swales, ponds, and infiltration basins. In extreme cases where plan area is limited, these attenuation features may become below ground structures, such as low carbon lightweight crate system, oversized recycled plastic pipework, or porous recycled stone sub-base.

Water quality

Restricting flows and attenuating storm water is not enough to gain planning approval. The proposed drainage scheme must provide an improvement on water quality and demonstrate that the future use of the site is adequately considered. Any potential pollution within the development, such as vehicle and operational use, must be treated by the SuDS onsite before final discharge to the downstream network or watercourse.

Amenity and biodiversity

It is also increasing common to see requests from the LLFA on how drainage schemes contribute to the improvements of amenity and biodiversity of proposed schemes. This is a difficult item to measure as there is limited information available if this can be achieved with current experience, considering it is a cross discipline element that needs landscape, ecology and a planning specialist to collate the information.