Stormwater design like any other engineering design requires that the rules and standards of various local authorities are followed. Specifically stormwater design pipe sizing, spacing and material choices are dictated by standards for a good reason.
We have observed a worrying trend towards designs that rely solely on local authority parameters and inaccurate dimensions. One report that recently crossed our desks stated that Google Earth had been used to determine catchment boundaries. On another project no potholing or service location was undertaken prior to the design drawings becoming construction drawings, the contract being let and the contractor arriving on-site.
The contractor commenced service location and potholing, and found 4 fibre ducts, a 200mm AC watermain, a 150 PVC watermain, telecom cables, and an unspecified earthenware pipe. These services were obstructing the path of a 375mm pipeline proposed to be laid at 0.12%.
It was at this point that we decided to make a hat, from the construction drawings because that is all they were good for.
This experience led me to ask the question: how critical is detailed design on a basic stormwater project? Charts exist to assist with pipe capacity calculations, and if it is a matter of getting water from A to B, on-site design may be the best option. We compiled a portfolio of stormwater design projects (hats) and looked for common faults and errors; compared the outcomes and ruminated upon ways to do things better.
We concluded that once those actual problems were accurately identified, packaged together and let as ‘design and build’ contracts with defined outcomes, life become simpler. Whilst this “we don’t care how you get there, just get there and meet the standards” approach is not necessarily limited to stormwater, we find poor outcomes following detail design to be most prevalent in this area.