This topic outlines the approach taken by the pipe sizing engine and how its can be controlled.
The auto size process is completed based on the Pipe Sizing specified for the phase. Some aspects, such as Minimum Cover Depth, Velocities and Backdrops are optional and will only be used if turned on. However the auto sizing engine will always look to ensure that a pipe/channel has sufficient capacity to take the calculated flow. Equally the Design Level will be applied to align pipe/box culverts to one another.
Pipe sizing will begin at the head of the Flow path selected, which acts as the mainline. Typically the head of a Flow Path will normally be an Inflow (Area or Hydrograph). It will then move down each Pipe/Channel in the network sizing the pipe/channel in turn. Note: Notional connections (No Delay, Lagged Flow and Attenuated Flow) cannot be sized so will be ignored. This sizing is completed as follows:
The calculated design flow is not allowed to reduce below the largest upstream value.
Sizing routine - The auto size calculation will then attempt to size each connection to achieve both the required capacity for the calculated flow as well as the sizing options selected.
This works as follows:
Determine initial upstream and downstream levels - Considering the min cover depth (if applicable) and the incoming pipes determine what the minimum depth at each end of the pipe is. This will ensure that the pipes have sufficient cover and align to the lowest incoming soffit/invert level based on the Design Level.
Channel section types will always be aligned as level inverts regardless of the Design Level setting.
Where a Design Flow leads to a reduction in pipe size between two pipes they will also be laid as level inverts to prevent permanent water levels.
If there are differing cover levels at the downstream end of the incoming connection(s) and upstream end of the outgoing connection(s) the lowest value for all of them will be used to ensure the correct alignment.
Determine the range of slope - The min and max slope entered in the sizing criteria will be compared to the initial slope calculated from the initial levels as well as the range of Min/Max slopes available for the pipe sizes.
it is possible to lock a Pipe/Channel to prevent it being sized, by toggling the Auto Size setting (hidden by default) on the Connections form for a given connection(s). When the Auto Size calculation encounters this, it will not adjust the dimensions of that connection. The Area and Base flow totals will still be determined and passed downstream, and the downstream connections will still be aligned to the locked connection.
Where a locked slope option is selected on the Sizing Criteria the engine will bypass the standard sizing routine. Instead the software attempts to size the connections as follows:
This option will stop the Pipe Sizing from adjusting the slopes of the connections in the current path. Only the diameter/height of section will be adjusted until the connection has capacity and then the connections will be aligned based on the cover depths and design level options. This is restrictive and is likely to lead to pipes being deeper than necessary.
Slopes and Invert Levels
Only the connection diameter/height can be adjusted by the engine with the soffit level being adjusted to accommodate any change in the height. This option prevents triangular sections from being sized as the calculation is unable to adjust the height given this is simply the distance between the cover level and the invert level.
Slopes and Soffit Levels
Only the connection diameter/height can be adjusted by the engine with the invert levels being adjusted to accommodate any change in the height.