Erosion and Sediment Control Assessment Report Rev. 0 This is an uncontrolled copy if photocopied or printed from the Intranet. Copyright © 2016, SouthernSkies Environmental Ltd. All rights reserved. Page 22 of 30 spillways, runoff residence time in the pond reduces. As a result, the sediment retention efficiency reduces with the rising limb of the event, and then increases again through the falling limb as the inflow rate to the pond increases. • Antecedent soil condition i.e. drier soils generally absorb more rain such that less enters the pond, as well as a longer lag period before runoff reaches a pond). • Surface roughness i.e. generally runoff flowing across rough surfaces generates less sediment than that across smooth surfaces. • Period between rainfall events i.e. the extent that the live storage has drained down before the event. Summer conditions also provide evaporative reduction in dead storage levels. • Pond maintenance and available capacity i.e. the level of sediment accumulation within the pond. • Gradient and length of contributing slopes i.e. steeper and longer slopes have greater erosion potential. • Erosion control practices i.e. less sediment entering the pond means less discharging from the pond for a given efficiency. As a result, a compliance trigger such as 100g/m3 is appropriate as a response trigger but is not suitable as compliance limit. This approach is well accepted and adopted within the Auckland region and has also been accepted and adopted by the Environment Court and Horizons Regional Council in the consenting of the Te Ahu a Turanga; Manawatū Tararua Highway Project. 9. ASSESSMENT OF EFFECTS 9.1. ESC Design As stated, the principles of TR2009/02 will be adopted for the management of sediment-laden runoff from the works sites and whenever practicable, ESC measures will be designed and managed in full accordance with that guideline. Through necessity, deviation from specific TR2009/02 design will be required as follows. 9.1.1.Willows Road Site The works associated with the Willows Road Site drain to the Mataura Stream and then to the Ohinemuri River. The surface works of the Willows Road Site will be undertaken as separate elements and treated by separate ESC measures. Sediment laden runoff generated during the earthworks for the main surface facilities (noise bunds, car parking, effluent disposal field, pipe storage/stockpiling area, offices, portacoms, and related facilities) will be treated through the Detention Pond, which will be significantly oversized (10,000m3) when compared to the 2% of contributing catchment minimum required by TR2009/02 (1,000m3). Consequently, a high level of sediment retention efficiency is expected. The Detention Pond will be constructed first and used as the SRP for the site until the site works are completed. The Collection Pond will be built to service the enabling earthworks associated with the access road construction and Willows Rock Stack (WRS). A super silt fence will be installed below the footprint of the Collection Pond to capture any runoff during the construction of the pond. The Collection Pond has a design capacity of 19,000m3. The combined contributing earthworks area will be approximately 6ha. This significantly exceeds a minimum TR2009/02 sized pond, which at 3% would have a capacity of 1,800m3.
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