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When designing a cellular soakaway, infodrainage automatically assumes a 1m2 plan area chamber between tank soffit and exceedance level (which would have been specified as ground level). In other structures you can specify freeboard and exceedance level as two seperate quantities but this is not an option for geocellular storage.
The additional volume makes the presentation of results to Councils confusing as it appears more storage is available than there is in reality.
Also had queries on this with extra capacity being allowed for in the model which doesn't exist and therefore hiding potential flood waters.
thanks for your feedback and comments. We reviewed this in the past and unfortunately we did not move forward with its implementation due to the limited feedback received and the fact that using the tank SWC type would sort this issue related to the additional storage volume provided by the access shaft.
That said, if we see growing interest in this change, we will be happy to revisit this
This issue caused problems in getting a drainage strategy accepted by a LLFA, as they correctly identified that the total volume quoted on the results printout was greater than the tank volume as stated in the strategy. The smaller the tank, the greater the discrepancy - in our case the fictitious 1x1m shaft gave the tank around 40% more capacity than was available in reality (for a large tank, the discrepancy might be negligeable).
To satisfy the LLFA that there would be no flooding if this additional capacity wasn't available, we had to increase the size of the tank to ensure the peak water level was within the tank depth, which resulted in an inefficient design.
We would welcome this issue being resolved in a future update.