Earthworks can significantly change the way water moves across a development site. Once vegetation is cleared or soil is exposed, rainfall can loosen and carry sediment into stormwater networks, neighbouring properties, streams and other receiving environments.
Erosion and sediment control is therefore an important part of planning many subdivisions, land developments and infrastructure projects. The objective is not simply to capture muddy water at the bottom of a site. A good approach starts by limiting erosion in the first place, managing clean and sediment-laden water separately, and adjusting controls as construction progresses.
Early erosion and sediment control planning can also make it easier to coordinate earthworks, construction staging and consent requirements before machinery arrives on site.
Although the terms are often used together, they deal with different parts of the problem.
Erosion control aims to prevent soil from being disturbed or washed away. This can involve limiting the area of exposed ground, diverting clean water around active earthworks and stabilising completed areas as soon as practical.
Sediment control deals with soil that has already been mobilised. Measures such as sediment retention ponds, decanting earth bunds and silt fences are used to slow or capture sediment-laden runoff before it leaves the site.
The most effective strategy usually combines both. Relying only on sediment devices after erosion has occurred can place unnecessary pressure on those controls, particularly during prolonged or heavy rainfall.
The level of control required depends on the extent of land disturbance and the sensitivity of the surrounding environment. They are commonly relevant where a project involves:
Smaller sites can still create environmental effects if runoff is concentrated or there is a direct path to a drain or waterway. The appropriate controls therefore need to respond to the actual site rather than simply the overall size of the project.
No two erosion and sediment control plans should be exactly the same.
Slope, soil type, catchment size and existing drainage patterns all influence how quickly water moves through a site and how much sediment it may carry. Steep ground can increase the speed of runoff, while fine-grained soils may remain suspended in water for much longer than coarse material.
The location of existing drains, waterways and low points is also important. Water entering the site from surrounding land may need to be diverted around active earthworks so it does not become unnecessarily contaminated.
This separation of clean and dirty water is a fundamental part of effective sediment control. The less clean runoff that enters disturbed areas, the smaller the volume of sediment-laden water that needs to be treated.
Erosion and sediment controls should reflect how the site will actually be constructed.
Opening an entire development at once can expose significantly more soil than necessary. On larger projects, staging earthworks can reduce the active area and allow completed sections to be stabilised while work continues elsewhere.
The control measures may also need to change as construction advances. A sediment pond positioned for bulk earthworks may need to be modified once roads, drainage systems and building platforms are formed.
For this reason, an erosion and sediment control plan should be considered a practical construction document rather than something prepared only to satisfy a consent condition.
Standard sediment controls rely heavily on soil particles settling out of water. This can be less effective where a site contains very fine soils that remain suspended for long periods.
Chemical treatment can sometimes be used to encourage these fine particles to bind together and settle more readily. It is generally considered where normal physical controls are unlikely to achieve an appropriate level of treatment on their own.
A chemical treatment management plan can establish when treatment should be used, how it will be applied and how the process should be monitored.
Chemical treatment is not automatically required on every earthworks site. Its suitability depends on factors such as soil characteristics, the treatment system being used and the receiving environment.
Installing the controls is only the beginning. Their performance needs to be maintained throughout the earthworks period.
Sediment devices can fill, diversion channels can become blocked and stabilised areas can be damaged by construction traffic. Rainfall events can also reveal runoff paths that were not obvious during dry weather.
Regular inspections are therefore important, particularly after significant rain. Sediment should be removed when required, damaged controls repaired and completed areas stabilised as construction progresses.
This ongoing management can be just as important as the original design. Even a well-designed system will become less effective if it is not maintained as site conditions change.
Construction-phase controls are temporary. Once a development is complete, the site still needs a permanent approach to managing stormwater.
Depending on the development, this may include bioretention design, wetlands, detention systems, soakage or other treatment methods.
The two stages serve different purposes. Erosion and sediment controls manage runoff while the ground is disturbed, whereas permanent stormwater systems manage runoff generated by the finished development.
Considering both early can help avoid situations where temporary controls conflict with the location or construction of the final stormwater infrastructure.
Erosion and sediment control can influence more of a project than the location of a few temporary devices.
Space may be required for sediment retention, clean-water diversions need suitable routes, and earthworks staging can affect the construction programme. On constrained sites, these requirements may influence how the development is approached.
Planning the controls alongside the wider civil and environmental design gives the project team more opportunity to identify workable locations and construction sequences before work starts.
It can also reduce the risk of sediment-control requirements being treated as an afterthought once contractors are already on site.
Civix provides erosion and sediment control planning for subdivision, land development and infrastructure projects.
Where site conditions require additional treatment, this can also be coordinated with chemical treatment management plans and permanent environmental solutions such as bioretention design.
Considering these requirements alongside the wider development allows temporary construction controls and long-term stormwater management to form part of a more coordinated environmental approach.
Earthworks are often one of the first major stages of a subdivision or land development project. They involve reshaping the existing ground so the site can support buildings, accessways and other future construction.
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For private land developments, Civix Town Planners, Chartered Engineers and Licensed Cadastral Surveyors have got you covered from the initial site visit through to council sign off.
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