This topic outlines the approach taken by the network sizing eng= ine and how it can be controlled.

The auto size process is completed based on the ** Network Design ** specif=
ied for the phase. Some aspects, such as Minimum Cover Depth, Velocities an=
d Backdrops are optional and will only be used if turned on. However the au=
to sizing engine will always look to ensure that a pipe/channel has suffici=
ent capacity to take the calculated flow. Equally the Design Level will be =
applied to align pipe/box culverts to one another.

Network design will begin at the head of the Flow path selected, which a= cts 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 th= e network sizing the pipe/channel in turn. Note: Notional connections (No D= elay, Lagged Flow and Attenuated Flow) cannot be sized so will be ignored. = This sizing is completed as follows:

**E****stablish Total Area & Base Fl= ow**- The Inflows contributing to each connection will be summed (f= or the Rational Method and UK Modified Rational Method) and combined in par= allel (for the Methode de Caquot) as the calculation moves down the network= to establish the totals used for sizing:**Area/Roof Area**- Depending on which Runoff Method the = contributing area will be established as:**Area Inflow**- Area * Preliminary CV**Roof Area:****Time of Concentration****Time Area Diagram****Green Roof**

**Hydrograph**- The peak flow of the Hydrograph wil= l be considered as a base flow.

*Note: Where there are multiple upstream branches the totals of all= branches will be combined for the downstream pipe/channel totals.*

**Set Initial dimensions for Pipes/Channels**- To ensure = that the smallest possible size is considered the dimensions of the pipe/ch= annel will be set to a suitable minimum before starting:- Pipe - The diameter is set to the Smallest pipe size in the Pipe Size L= ibrary.
- Rectangular/Trapezoidal Channel - The Base Width is set to 600mm.
- Triangular Channel - The Height is set to 0.2m as this is the dimension= that is adjusted

**Determine Travel Time****-****The Travel Time is calculated as the largest upstream time of concent= ration taken from:**- The Preliminary Time of Concentration on the incoming Area Inflows
- The Travel Time through the Upstream connections
- Min. Time of Concentration from the
**Network Design**.

The travel time is then restricted to the= Max. Travel Time from the Network Design Criteria which helps to prevent i= ntensities dropping too low in a large network.**Rainfall Intensity**- The Rainfall Intensity is determi= ned from the IDF data, specified on the**Network Design**, based on the Tra= vel Time for the given connection.**Determine Flow****-****=**The selected Calculation Method on the Network Design C= riteria will then be used to establish the flow that the pipe/channel must = have capacity to convey.**Rational Method****:**Flow =3D Rainfall I= ntensity x Total Area + Total Base Flow**(UK) Modified Rational Method:**Flow =3D Rainfall Inten= sity x Total Area * Cr(1.3) + Total Base Flow**Methode de Caquot:**Flow is calculated via the&nbs= p;Methode de Caquot equations which are considerably more involved that the= Rational Method or (UK) Modified Rational Method equations. The metho= d's inputs are the Caquot (Base and Advanced) parameters, selected Desborde= s rainfall parameters and the Runoff Path Length and Catchment Slope of the= Inflows upstream of the pipes belonging to the auto-sized Flow Path. Furth= er information can be found on the**Methode de Caquot**page.

Note

The calculated design flow is not allowed to reduce below the largest up= stream value.

**Sizing routine**- The auto size calculation will the= n 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 dete= rmine what the minimum depth at each end of the pipe is. This will ensure t= hat the pipes have sufficient cover and align to the lowest incoming soffit= /invert level based on the Design Level.Notes

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.<= /p>

If there are differing cover levels at the downstream end of the incomin= g connection(s) and upstream end of the outgoing connection(s) the lowest v= alue for all of them will be used to ensure the correct alignment.

If the connection is the first in a run (first in either the flow path o= r a branch line), and minimum cover depth is not checked in the network des= ign criteria, then the initial upstream level will remain at the connection= s upstream invert level, so long as it is non-zero and does not break cover= .

If Reduce Channel Depths is checked on in the network design criteria, a= ny rectangular or trapezoidal channels now have their depths set to match t= he largest, non-zero, down stream depth of incoming connections. If no non-= zero depths exist, or if no incoming connections exist, the depth of the ch= annel remains the same.

**Determine the range of slope**- The min and max slop= e entered in the sizing criteria will be compared to the initial slope calc= ulated from the initial levels as well as the range of Min/Max slopes avail= able for the pipe sizes.**Perform sizing**- The auto size calculation will then s= tart from the flattest slope and attempt to find a diameter that gives suff= icient capacity to take the calculated flow as well as providing a valid ve= locity (if selected)**Upsizing a connection**- the Pipe/Channel does not have= capacity it will be up-sized:*Pipe*- The next pipe size from the entered Pipe Size library w= ill be selected. if the largest pipe in the file is not big enough for the = flow then the program will continue based on the Additional Increment speci= fied (75mm by default) until sufficient capacity is reached.*Rectangular/Trapezoidal Channel*- the base width is increased = by 100mm.*Triangular Channel -*The Height is increased by 20mm.

**Adjust slope**- When a solution has been found for a gi= ven slope it is recorded and the calculation will make the pipe slightly st= eeper and repeat the check.**Selecting the solution**- The solutions for different s= lopes are considered and the one providing the minimum cover depth at the d= ownstream end, as well as meeting the velocity checks if applicable), will = be selected and assigned to the pipe.

**Branch lines**- These will be sized in the same way as = the main line with the upstream area and base flows summed and used to size= each connection as above. However when the branch line connects into the m= ain line its downstream level needs to be considered as it may be above, or= below, the pipes in the main line (or other incoming branch lines). When t= hey are not aligned the calculation will attempt to align them all based on= the Design Level. It is however possible to allow a backdrop on the Sizing= Criteria, which will leave any backdrop above the minimum, but below the m= aximum, and then remove any below and align any above to the maximum.

** **

it is possible to lock a Pipe/Channel to prevent it being sized, by togg= ling the Auto Size setting (hidden by default) on the Connections form for = a given connection(s). When the Auto Size calculation encounters this, it w= ill not adjust the dimensions of that connection. The Area and Base flow to= tals will still be determined and passed downstream, and the downstream con= nections will still be aligned to the locked connection.

Where a locked slope option is selected on the Sizing Criteria the engin= e will bypass the standard sizing routine. Instead the software attempts to= size the connections as follows:

* Slope *

This option will stop the Network Design = from adjusting the slopes of the connections in the current path. Only the = diameter/height of section will be adjusted until the connection has capaci= ty and then the connections will be aligned based on the cover depths and d= esign level options. This is restrictive and is likely to lead to pipes bei= ng deeper than necessary.

** Slopes and Invert Levels **

Only the connection diameter/height can b= e adjusted by the engine with the soffit level being adjusted to accommodat= e any change in the height. This option prevents triangular sections from b= eing 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 b= e adjusted by the engine with the invert levels being adjusted to accommoda= te any change in the height.