Working In-Stream?
With the current acceleration of civil construction extending through the historical QLD wet season, we’ve been asked by a number of clients in the past few months for guidance on in-stream works where, for various reasons, there is limited opportunity for the diversion of inflow. Demonstration of General Environmental Duty during instream works can be tricky, especially where the work area cannot be isolated from inflow. So we’ve put together a list of points to consider when planning and undertaking the works.
First and foremost, consider re-scheduling works for a drier period of weather. Or re-assess the construction methodology to minimise the risk of disturbance (such a directional drilling rather than open trenching through waterways). In some instances a more expensive construction technique can result in a significantly cheaper project outcome through reduced timeframes, potential for issues/failure and reduced risk of fines or clean-up actions, should something go wrong..
Assess the contributing catchment and anticipated flow conditions. This will provide an understanding of potential flow rates over time (hydrograph) and inform decisions in regard to damming, bypass requirements, scour protection, and rainfall forecast triggers.
Block inflow on the upstream extent of works. The type of dam will depend on the anticipated flow rates, bed conditions and available materials. Always consider the potential for high flows overwhelming installed dams, and minimising disturbance during installation. Prior to commencing any works within the waterway, ensure you have the necessary approvals and means to comply with any conditions of such approvals.
Block outflow to prevent backflow entering the work area, and also trap dirty site water.
Establish bypass piping (gravity) or pumping. For all but the smallest streams/catchments it is unlikely that the bypass will achieve a similar or even significant proportion of the flows from the most likely rain events, but will reduce headwater levels during periods of low-flow. It’s always worth checking fish-passage requirements at this point as well.
Develop contingency protocols for the likely event that flow conditions will exceed the dam and pipe/pump capacity. These contingencies should be based on rainfall forecast triggers adopted for the catchment that are known to produce runoff that exceeds the low-flow conditions for which the dam and bypass were designed. Contingencies usually involve site stabilisation, removal of plant/equipment and loose material, and re-establishing normal flow conditions. Stabilisation may consist of carefully pinned or weighted geofabric, secured plastic sheeting, rock, concrete blinding or permanent/long-term surface treatments.
Rainfall forecast triggers for re-establishing normal flow and stabilising any disturbance should based on catchment conditions and rainfall/runoff experience. It will be important to regularly review these triggers at a site level to suit actual conditions and the pump/pipe flow rates that can be achieved on site. Triggers should consider the timeframes, materials and machinery required to secure the site prior to flow.
Dewater the work area in accordance with the water quality objectives for the project. If the upstream and downstream dams can be constructed with minimal disturbance often the water within site can be dewatered immediately downstream. Otherwise treatment will be required, or pumping to truck. During construction, ingress will need to be managed in a similar manner.
Manage runoff from surrounding areas to prevent lateral inflow entering the work ar
Monitor, review and report. Continually re-assess new or developing risks.