The murky world of sediment pollution and its impacts on fish.

Naturally Compliants Peter Matthews delves in to the impacts of sediment laden water from construction sites finding its way in to fish supporting water courses. He also provides advice on how to get the message across.

For an Environmental Clerk of Works (ECoW) working on a construction site, one of the greatest challenges is preventing the escape of silty water into the aquatic environment such as streams, rivers and lakes. The status of dirty water as a pollutant and the ecological damage it can cause is not always fully understood on construction sites.

A common misconception is that because silt and silty water is non-toxic, non-corrosive and non-flammable it will do little harm to the ecology of a water body. This is not the case, however, and examples of the harm it can cause include the following:

  • Causing suffocation of fish by blocking gills,
  • Blinding of gravel beds causing disruption to spawning fish or failure to spawn,
  • Visual impairment of fish causing a reduction in predation success,
  • Smothering of aquatic invertebrates and loss of habitat, and
  • Reduction in dissolved oxygen if the silt contains significant organic material (the biological oxygen demand).

Legally, pollution, in relation to the water environment, means the direct or indirect introduction, as a result of human activity, of substances (including bacteria and other pathogens) or heat into the water environment, or any part of it, which may give rise to any harm. Therefore, in addition to the physical impacts, sediment entering the water environment as a result of construction activities would therefore meet the definition of pollution and should be treated as such.

A conscientious site operative who has been trained in emergency response is likely to react quickly and appropriately to a serious pollution incident such as an oil spill. But that same operative may not respond in the same way to a silt pollution incident, through a lack of knowledge of its potential to cause damage.

Courses and tool box talks (TBTs) can be used to raise awareness of silty water but these need to be made engaging. Making the connection between silty water and the negative effect it has on fish populations, and therefore fishing, is one way of presenting the problem in the context of a widely practiced hobby. Whatever approach is taken, more needs to be done to communicate the message that silt pollution is a serious environmental issue on a construction site.

How to manage surface water on sites with Amphibian/Reptile Fencing

The amphibian and reptile species found in the UK are subject to one of three different levels of legal protection, each with varying status within national policies. Protection on construction sites is often implemented using physical barriers to movement, known as amphibian, newt or reptile fencing. This can be found on a wide array of sites, often spotted as green, black, white or even metal fencing, unbroken and low to the ground.

Fencing may be used on small projects, such as the installation of a minor drainage in rough grassland, to larger projects such as landfill, quarries or power stations. It is also commonly found on linear projects such as pipeline corridors and road building projects due to their nature of intersecting habitats. Whether amphibian and reptile fencing is required as a precautionary and good practice measure, or as part of higher level protection under specific development licence requirements, then considerations on site will need to be tailored to site conditions.

One particular site condition that often presents a challenge in terms of management is the presence of water, and often what to do when an excess is present on or adjacent to site. The installation of what can be kilometres of an impermeable barrier to water can exacerbate issues with water management on a site. This can lead to knock on impacts, such as excess dirty water in working areas, resulting in potential delays to work. It is therefore imperative that the design of mitigation and site water management should be considered simultaneously and in the early stages of the project, before works on site commence.

Key Issues

So, what are the key water management issues associated with temporary and permanent amphibian and reptile fencing?

  • Water permeable ditch crossings may be required which must last the duration of the project or have sufficient longevity for permanent projects with minimal maintenance
  • Pooling of water within site with no ability to drain from site
  • Control of sediment and soil degradation
  • Fencing may be required to withstand water pressures during heavy rainfalls periods

What do Naturally Compliant Recommend?

Each site is different and therefore we always recommend that the water management plan needs to be discussed at the design stage of the amphibian and reptile mitigation. Despite the differences, here are some solutions that can be considered:

Water Course crossings;

  • Dependent on the species being excluded and the nature of the watercourse it may be a requirement to restrict movement of the animal but allow the flow of water. The Flow history should be obtained, and crossings designed based on peak flows.
  • Permanent or higher flows may require a more robust fence using welded wire mesh made from stainless steel. High strength posts driven deeper into the ground may be required to counter high water pressures.


  • Investigate soil types and conditions before mitigation design to identify potential problem areas.
  • Drainage channels within site can be used to direct excess water towards openings, provided adequate silt protection measures are installed. These could be temporary V channels cut into the earth or more permanent drainage such as French drains. Where excess silt is present, this may require pumping and treatment using methods such as straw bales, catch pits, silt bags or lamella tanks. This in turn should help to keep a site dry and reduce the chances of soil degradation and sediment runoff.
  • The discharge locations should be made up of small hole stainless steel wire mesh between two upright posts.

Maintenance strategy;

  • Weather, soil and hydrological conditions are likely to impact on a site, and where mitigation is in place across changing seasons, this may bring new challenges to maintain site integrity. A maintenance strategy should include regular audits of fence integrity and will likely require a greater number of visits during the wetter months of the year
  • Other seasonal strategies may include clearing of debris from ditch crossings and openings; likely most prevalent during autumn when leaves are known to clog up watercourses and drainage
  • A response procedure with a fencing contractor should be established to respond to any failures discovered