Accurately calculate the base and bank-top surface areas of flow through ponds for all shapes

The flow through pond storage option allows the user to select a side slope, depth and base area. It can then predict the bank-top area of that structure. The user can also adapt the shape of the pond/basin in Flow and Live Design and these updates are reflected in the areas shown in Flow, however Flow is always assuming a circular pond despite the shape the user makes. This isn't made clear to the user and it matters for all shapes that aren't circular - which is commonly the case for a basin/pond. The situation is exacerbated with particularly elongate structures. The base area is important for ensuring the correct interception requirement has been met for SAB applications, and if Flow always assumes a circle then a manual workaround is required to get the correct bank-top and base areas for non-circular ponds/basins.