28 ENVIRONMENTAL CONNECTION / THIRD QUARTER 2025 / IECA.ORG these stream diversion works in 2023, we have managed to fill the area to a sufficient height to allow for a large sediment retention pond with a capacity of 12 acres (5 ha) of catchment, to be constructed at the most downstream extent of the site (Figure 5). Site access and other limitations have led to the development of unconventional ESCs, which require third-party technical review before installation. One example of this unconventional ESC is a sediment pond outlet structure that sought to disperse discharge flows to prevent erosion on a 30-degree slope (Figure 6). Using a concrete manhole riser and six perforated outlet pipes, the authors were successfully able to prevent scour and erosion. The device performs as well as those with traditional outlet structures in terms of meeting discharge requirements (Figure 7). This innovative approach received the Excellence in Innovation award from the International Erosion Control Association (Australasia). Given the ecological and cultural significance of the landscape, effective ESCs are essential. A comprehensive monitoring programme informs the construction team about the effectiveness of controls on-site. This includes baseline monitoring of water quality, ecological markers, routine monitoring of devices and controls, rainfall-based water quality, ecological monitoring, continuous in-stream nephelometric turbidity unit (NTU) monitoring and in-stream static sampling. Telemetered in-stream NTU monitors at six locations across the alignment provide realtime insight into stream conditions (Figure 8). The Project Regulator (Taranaki Regional Council) receives automatic notifications when downstream NTU exceeds an agreed “trigger level.” This encourages open communication between the Alliance and Regulator regarding on-site activities. Additionally, we have been able to use the NTU monitors to develop a “risk-based” approach when completing in-stream works. Escalating NTU trigger alerts are sent to key staff members, and construction work will cease before the downstream agreed trigger level is reached, which eliminates sediment deposition and downstream ecological impacts. >> Figure 7. The temporary sediment pond in operation with a dispersal outlet system. INTERNATIONAL PEER REVIEW Figure 6. The temporary outlet system of the sediment pond during construction. Figure 5. Temporary sediment retention pond constructed in Fill 12 with a catchment capacity of 5 ha (12.3 acres). Figure 4. The scaffold staircase features 700 stairs and is used daily for worker access to the site.
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