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As you saw from reviewing the results, the treatment train is successfully mitigating the volume of water discharging from the site. However, the flow rate is still too great compared to the existing site. Also, we are getting flooding in our pipe network which we do not want. We have not yet used the built in sizing tools, therefore we will now use these to fix these issues.
We are going to size our pipes in line with the Sewers for Adoption (SfA) UK guidance specifications. To meet these requirements, we need to make sure that our network will not surcharge for a 1:2 year event and will not flood for a 1:30 year event. We can either do this by manually altering pipes individually, or we can use the Pipe Sizing Wizard found within the Preliminary Sizing Ribbon.
The Pipe Sizing Wizard works by using an Intensity Duration Frequency (IDF) curve for a particular storm event, for a particular area. We will specify this using FSR data. The Pipe Sizing Wizard reads the IDF curve for the time of concentration of an individual pipe and then a rainfall intensity is generated. This rainfall intensity generates the expected flow the pipe will have to deal with (in the UK, this is done using the Modified Rational Method). The connection is then sized accordingly. This is related to the Wallingford Procedure for the UK, and enables pipes to meet the Sewers for Adoption criteria or similar guidance documents in other countries on pipe surcharging and flooding for various events, as stated above.
The second stage of the Pipe Sizing Wizard allows us to enter the Pipe Sizing Criteria, within here are a variety of conditions that can be specified for our pipes to be designed to. We will be using the (UK) Modified Rational Method (as mentioned above), combined with the FSR 2 Rainfall Source, the two-year return period FSR data from Oxford (UK) as the basis for our pipe design.
Now that we have resized our pipes we want to alter our Dry Pond so that we are meeting the same discharge flow rate as the existing site. As we saw from our phases report for the developed site we are discharging at higher rates than the existing site. To successfully decrease the flow rates we will alter the outflow control and also the pond size, as reducing the flow rate will lead to a higher storage volume required.
Click OK to close and apply the volume. We can now see that the pond has updated its areas and volume based on what was specified in the Sizing Calculator.
The Total Volume shown in the toolbar at the base of the Pond form is calculated up to the Freeboard. In this case the Freeboard is set 150 mm below the Exceedence Level, hence it is different to the total volume measured up to the Exceedence Level as set but the Quick Storage Estimate Calculator.
More information on the Export Pipe Function, refer to Export Pipe Network.