A flow split is created when two or more pipes leave a manhole node. When this occurs, InfoSewer will automatically calculate the flow being routed to each downstream pipe. The user also has the option of over-riding the automatic flow split calculation and replacing it with a fixed percentage or assigning a flow split curve that represents a routing condition.
A flow split must first be digitized by the user. A split consists of having two or more pipes exit a single manhole structure. Once digitized, the user can elect to do nothing or change the flow split type. If nothing is done, InfoSewer will automatically calculate the flows for all downstream pipes. If the user wishes to enter a flow split percentage or assign a flow split curve, merely highlight a downstream pipe and click on the flow split icon located on the Attribute Browser.
Once this is done, the following dialog box will be displayed.
There are four types of flow splits:
Automatic Calculation - The default option. InfoSewer automatically determines the flows entering each downstream pipe based on their invert elevations and downstream slopes.
Fixed Split Percentage - When this option is selected, the user will need to specify the amount of flow entering the selected pipe as a fixed percentage of the combined flows entering the upstream manhole.
Variable Flow Percentage - When this option is selected, the user must provide a Flow Split Curve ID that represents the percentage of flow entering the pipe as a function of the total flow entering the upstream manhole.
Inflow-Outflow Curve - Similar to the variable flow percentage approach, this method requires specification of flow split curve. Under this option, however, the curve represents the actual outflow (not percentage) as a function of the inflow. Click here to learn more about flow split curves.
In an automatic calculation, flow splits are determined via the invert elevation of the downstream pipes. As shown in the figure below, those pipes with the lowest invert are assumed to fill first. As the depth of water increases in the lowest pipe, flow will be routed until the next highest invert is achieved. The change in flow is then equally distributed to each downstream pipe via a cross sectional evaluation. The slope of each downstream pipe is also factored into the flow routing regime. While the automatic flow calculation is not 100% accurate, it does take away a degree of uncertainty for the engineer who may not have the time or resources to flow meter both downstream pipes in order to create an accurate flow split curve.
The fixed percentage is the least accurate method due to the fact that flow entering a split pipe is not directly proportionate to the flow entering the upstream manhole. A fixed percentage need only be assigned to one of the two downstream pipes as InfoSewer will automatically determine the difference for the second pipe. Likewise, if three pipes are found to be leaving a single manhole, InfoSewer will require that fixed flow percentages be assigned to at least two of the pipes. Again, the remaining pipe percentage will be automatically determined by InfoSewer.
If the user is able to ascertain the flow regime for the flow split, a curve can be assigned to represent the amount of flow entering a downstream pipe in relation to the flow entering the upstream manhole. As with the fixed percentage option, only one curve needs to be supplied as InfoSewer will automatically determine the difference for the second pipe. The same holds true for the three pipe scenario in that two curves will need to be assigned while InfoSewer will compute the third.
This method is similar to the variable flow percentage method described above, except that under this option the curve represents the actual amount of the incoming flow to be diverted to a downstream pipe. This flow split method is designed to accommodate flow conditions where hydraulic structures such as weirs and dams are used to regulate downstream flows Click here to learn more about flow split curves.