High flow bypass and overflow
It is good design practice and a requirement that assets and vegetation are protected from high velocities and large storm event flows that may result in scour, erosion, damage to vegetation or biofilms, where practical.
This may be achieved using structures such as weirs, pits and pipes. These structures direct smaller and more frequent flows into the asset for treatment. Higher flows are diverted around the main treatment asset either by diverting flows that exceed a design flow rate or diverting flows once water levels rise above a certain level over an overflow, subject to meeting velocity requirements. This means the main treatment element, which may be a vegetated area or a filter bed, is only exposed to flow rates up to the design flow or that high flows associated with infrequent events are diverted around the treatment once the storage capacity is reached.
An asset may be protected by bypass of flows above a specified rate, by an overflow weir that engages at a specified level at the upstream end of an asset or a combination of bypasses and overflow weirs.
It is common to design treatment assets such as wetlands and bioretention to treat limited flows up to the 4 Exceedances per Year or 4EY [1 in 3 month annual recurrence interval (ARI)] design event.
The configuration and model inputs should aim to represent the likely real-world behaviour of flows through the system to accurately estimate treatment performance and correctly size assets for catchment flows. This may include use of high flow bypass, overflow weir and secondary links. The high-flow bypass rate used should match the relevant design flow rate as calculated using an appropriate method.
Design flows for treatment assets are usually calculated using:
- The Rational Method;
- A hydrologic model; or
- Partial series analysis of flows from a MUSIC model with appropriate design and routing.
The user is referred to the Australian Rainfall and Runoff (AR&R) Guidelines 2019 and Part D of the Constructed Wetlands Design Manual (Melbourne Water, 2016) on the Melbourne Water website for calculation approaches including guidance on the use of RORB for calculating design flows. RORB can now calculate 4 EY flow rates directly and this is preferred over rules of thumb. The Rational Method may be used for small infill developments without significant upstream storage.
High-flow bypass and overflow weir configurations
Three examples of possible designs and high-flow bypass configurations are described below:
- No high-flow bypass with overflow weir; all inflows pass into the asset. High flows exceeding the outlet capacity spill over an overflow weir from the sediment basin (for wetlands) or after treatment (all other treatments). Set HFB to 100 cubic metres per second (default)
- High-flow bypass upstream of a treatment asset represented by a single node. Set HFB to design bypass flow rate for asset. MUSIC directs bypassed flows untreated to the immediate downstream node.
- High-flow bypass occurs at an asset and bypasses other treatment asset(s) downstream. Set HFB to design flow rate for asset and use secondary link to direct bypassed flows around treatment assets as required to represent design. The bypassed flow will be directed to a node downstream of the treatment node via the secondary link. This is useful when flows are bypassed upstream of an asset then continue to be bypassed for downstream assets (e.g. high flow bypass upstream of a sediment basin and wetland or sediment basin and bioretention).
Note: It is particularly important that where an upstream or ‘perched’ sediment pond with a high flow bypass overflow weir is used upstream of a wetland that overflows from the sediment pond are directed to bypass the wetland node using a secondary link and this is a requirement for modelling of these assets.
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