The winter of 2019 has been one of the wettest on record…or you’d be excused for thinking so.
In October and November 2019, spells of high intensity rain fell across central parts of the country, with the Environment Agency reporting that from the 20th to 27th November, Central England received 183% (nearly double) of the monthly long-term average rainfall.
Figure 1. EA Weekly Rainfall Summary for October to December 2019
Our perception of this winter’s weather has been one of excessive rainfall, however, outside this period, we have experienced typical or even below average rainfall. For example, the Environment Agency reports that from 1st to the 21st of January, the rainfall across all parts of England was between 63 to 81% of the norm.
So why did we experience so much flooding?
Our winter weather is changing, storms are increasing in size, with high intensity events occurring more frequently. These overwhelm the ability of the drainage network to contain the peak flows, increasing the flood risk. If the same amount of rain fell over a longer period of time, the river network would be able to contain the flows, reducing the risk of flooding.
While our winter storms are becoming more intense, our summer storms are becoming less so, with some parts of the country experiencing noticeably drier summers, for example, a recent BBC article (https://www.bbc.co.uk/news/uk-scotland-51347881) warned that dry hot summers may become the norm for Scotland.
While we get approximately the same amount of rainfall annually as previously, the rain is falling on fewer number of days; therefore, the intensity (mm/hr) is increasing, raising the risk of flood events substantially.
Prolonged periods of wet weather saturate the soils, limiting their ability to absorb water. This increases the amount and rate at which the surface water run-off will enter into the drainage network, further raising the flood risk.
The flow of water across the grounds surface, escalate the rates of soil erosion, transporting large quantities of sediment (silt) into our waterways.
Construction sites have the potential to be a big source of silt pollution.
Figure 2. Erosion rates from different land-based activities (Dunne & Leopold, 1978)
Soil, containing silt and sediment, which inundate a watercourse at a higher level than that which occurs through natural processes, causes physical harm to benthic creatures, fish and plants. Soil and silt, in the wrong place, equal pollution.
In addition to silt pollutions effect on the environment, it may also damage the economy, therefore the financial benefits generated by ensuring the environment is protected are significant, particularly to rural communities.
Silt: The World’s Biggest Pollutant
In a survey of English Rivers, the Environment Agency (EA) reported that:
• Over 23% of rivers are at risk from high levels of silt and sediment
• In 2004 50% of rivers with salmon action plans were at risk of missing their egg deposition targets
• In 2008 90% of trout spawning beds studies contained enough fine sediment from soil to kill 50% of the eggs and larvae
• Silt and sediment pollution accounts for 40% of the industry’s damaging water pollution incidents
• Is often very visual, damages reputation and can lead to prosecution
• Can result in criminal prosecution, prison sentences and fines of up to £3 million
How can silt reach receptors?
Sediments can be mobilised from a number of areas on site:
• roads and drainage ditches
• excavations and dewatering areas
• wheel washing facilities
• surface soil stripping
• river crossings
• material storage areas and stockpiles
Figure 3. Poor surface water and sediment control on a construction site Figure
4. North Sea discoloured due to do silt mobilisation (Credit Julia Fleming)
Once sediment is mobilised it will travel the path of least resistance, often resulting in silty water escaping from site and entering our waterways. Key pathways include:
• land drains
• ditches and streams
• overland flow
• surface water and foul drains
Construction Surface Water Management Plans
Preventing a pollution event in the first place is always less expensive than having to manage the problem once it’s happened. A company who fails to plan for excess surface water and groundwater discharged can suffer harm to their business:
• Project Risks and Program Delays
• Fines and Criminal Sanctions
• Brand Damage
A successful company will manage its operations to ensure that the risks associated with water pollution are minimised, allowing the company to operate in a cost-effective way.
A surface water management plan is central to understanding how a construction site ensures its operation will protect the aquatic environment from the ingress of pollutants. It will document the control measures to minimise the risk of pollution, such as how to:
• Minimise the level of contaminants being generated such as silt
• Minimise water entering an excavation site, such as rainfall or runoff
• Prevent contaminated water moving to a river or stream
• Dispose of water that enters the excavation, including any silt control interventions
• Maintenance plan for silt control interventions and water quality monitoring plan
Control of contaminated run-off
There are three simple principles which should be followed to minimise the volume of
contaminated run-off water being generated:
• DIVERT clean water away from exposed soils and working areas in a controlled manner
• MINIMISE erosion of exposed soils – retain vegetation cover, minimise soil stripping and establish new vegetation on bare ground at the earliest opportunity
• PREVENT contaminated water from entering watercourses untreated – the last resort; this may require an engineered treatment solution or removal from site, both potentially expensive options
Consider these three principles carefully and place particular emphasis on minimising the problem up front. If the potential for run-off is unavoidable seek the help of an expert to define the most effective solutions to manage the issue for the specific site conditions. For instance, if settlement ponds are installed flocculants are often required; these must be used with great care to prevent another source of pollution being created.
Prior to the use of any flocculant, approval from the environmental regulator (EA, NRW, SEPA) will first need to be obtained. Acquiring approval will take time, meaning that discussions with the Regulator need to commence well in advance of the site works.
Prior to commencement, ensure the magnitude of the water management issues have been assessed correctly and the potential for the site to impact on the local environment, fully understood.
The key questions which need to be answered during the planning stage are:
1. How much water will be generated?
2. Where will excess water be discharged to?
3. What Permits, Consents, Licences will be required?
4. How clean does the discharge need to be?
5. How is the required treatment standard going to be achieved?
6. How much land is required for treatment?
7. How are we going to monitor compliance?
The vast majority of engineering works carried out next to rivers will result in some physical changes to the bed and banks of the worked area. When the permanent works are complete, these areas should be appropriately reinstated to prevent silt and sediment pollution occurring post-construction.
Doing this correctly is complex, there is “no one size fits all” solution and a full understanding of the pre-existing site condition and the changes that have occurred as a result of the work is required.
Where possible undertake early and phased reinstatement of the site as individual areas are completed.
Figure 5. Reinstatement of the river banks, post works at Silverton Mill
(John F Hunt Regeneration)
Get help if needed
Responsible construction companies always take surface water management seriously; however, this is a complex subject. Keeping control under changing circumstances adds significant challenges to a construction project.
Planning well, engaging with regulators and making the right decisions on site prevents costly fines, project delays and bad press.
Engaging with a trusted advisor as early as possible to assist you with these duties will save time and prevent avoidable mistakes being made.
Jonathan Goldsmith and Simon Skentelbery
John F Hunt Regeneration, Water Tech Division