|Click to download the METRIC model files.|
You can carry on from the previous Chapter or switch to the Sizing of Treatment Phase by selecting from the drop-down list on the left Tree View panel if you have not already done so.
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. As well as this we are getting flooding in our pipe networks which we do not want, therefore we will now carry out a sizing of the treatment system 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. As we have already discovered, we have broken this rule for both cases so we will need to redesign our network. 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. With this IDF curve a time of concentration and rainfall intensity is generated for individual pipes, this 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.
Note: There is surcharging in the lower pipe for the 1:2 year critical storm, is due to the water depth in the pond being greater than the soffit of the pipe (as can be seen in profile view).
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.
You can carry on from the previous Chapter or open XP_Tutorial6.xpdx and switch to the Completed Design phase by selecting from the drop-down list on the left Tree View panel if you have not already done so.
More information on the Export Pipe Function can be on Export Pipe Network.