Design Ideas Portal
We have recently been challenged by Wiltshire LLFA regarding half drain times.
As I understand it, Flow calculates the half drain time to CIRIA 156 which gives a significantly longer half drain time than the BRE365 method that Micro Drainage uses. As the infiltration tests are typically carried out using BRE365, the LLFA are getting confused with their requests. Therefore, it would make sense to have the option of which method Flow uses to calculate the half drain time to better reflect the original testing methodology. It is also essential to have the half drain time to be a printable option for the results.
This is a crucial issue for us now and going forward on other schemes with infiltration features.
Calculating the time to half empty is done automatically by Flow and Flow is aware of the Flood Risk level in ‘Other’ so it will base the Time to half empty calculation based on that freeboard (Flood risk) specified. Regarding the time to half empty calc, for structures that do not discharge, such as soakaways, Flow follows the recommendation from BRE 365. This guideline applies to structures that only release water through infiltration (i.e., no outflow). The time to half-empty is calculated based on the structure being at full capacity, regardless of the water level. According to BRE 365, we assume the structure is full when calculating the half drain time, so changing any variables, including the return period, will not affect the calculation other than the infiltration rate.
If the system accounts for outflow in addition to infiltration losses, the time to half-empty can be determined based on either the peak volume during the critical storm or the total available storage volume when filled to the flood risk level. This depends on your preference settings in Flow. The time to half-empty is recalculated each time the analysis is run. Different guidelines exist for calculating the half drain down time, assuming the structure is at either full or peak volume. Therefore, we provide a drop-down menu for users to select their preferred method. If you choose "Structure volume," the results will be similar for varying return periods as it assumes the structure if filled up to the flood risk level. However, if you choose "Peak volume," the results will vary depending on the return period and storm events. These two methods apply when there is additional discharge/outflow besides infiltration from the system. The time to half-empty results can be found in the Storage form in the design tree view or in the approval results if the Approval setting is used.
The overall run time can be set using the Drain Down option in Simulation Settings. The analysis will run for the duration of the storm plus the specified drain down time. It is important to run the analysis long enough to capture the storm's peak in most cases. To accurately calculate the time to half-empty, it is crucial to increase the Drain Down Time by at least 24 hours. This ensures that the structure will be half empty after 24 hours, so the analysis drain down time should be adjusted accordingly.