CTMP Revision 0 thereby allowing the rain which falls on the plywood to run to waste. Floc boxes include a sliding lid that adjusts the catchment tray size in a similar way. The required tray size will be calculated and submitted with as-builts associated with each SSESCP and included in Appendix D. Figure 6: Roof tray design. 4.1.2 Header Tank Outlet Spacing Rainfall from the catchment tray is drained into a header tank which has two outlets offset vertically. This provides a storage capacity that avoids dosing during initial rainfall following a dry period and to attenuate dosing at the beginning and end of a rainstorm event. The volume between the drain (lowest) header tank outlet and the first dosing outlet is equal to the volume of 12mm of rain on the catchment tray and the volume between the first and second dosing outlets is the same. The required header tank outlet spacing details will be calculated and submitted with as-builts associated with each SSESCPs and included in Appendix D. Header tank management in summer months will be as per the GD05 guideline, which requires: • After 3 days without rain – reduce volume by 50%. • After 6 days without rain – empty completely. 4.1.3 Sediment Laden Runoff Channel and Dosing Point for Rainfall Activated System The chosen chemical needs to be added to the sediment laden runoff channel to provide mixing with the sediment laden runoff before it reaches the area of ponded water in the forebay or the sediment control device itself. All sediment laden runoff from the catchment should be combined into a single channel if possible before it reaches the chemical dosing point. To maximise mixing, the dosing point should be located at least 10 metres prior to the point where the runoff reaches the inlet of the device (in the case of a SRP, the inlet of the forebay).
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