Resource efficient drainage?

Working on a Sustainable Drainage Strategy for a new industrial building recently, “resource efficiency” in drainage design snuck into my mind. The details of  how and why aren’t important, but at the end of the day the site ended up with a far reduced drainage scheme – permeable block paving replacing both the concrete service yard and tarmac car parking, leaving only the roof as impermeable. And all the roof drainage collected at high level by a syphonic system before discharging immediately adjacent to the attenuation tank.

Which left a very “lean” below-ground surface water drainage system – just an attenuation tank (less than half the size proposed in the Flood Risk Assessment) with its inlet & outlet manholes, and a small diameter drain connecting this to the outfall manhole. And no other underground surface water drainage on the site at all – hence my “resource efficiency” thoughts.

Obviously, replacing the concrete yard with permeable paving eliminated its associated drainage system, petrol interceptor, attenuation tank etc, so there’s an obvious “efficiency” there, and at the end of the day the block paving could be taken up and reused, so a “plus” there too. But it was our preference for putting in underground drainage systems around buildings for roof drainage that really made me think – why do we insist in getting rainwater below ground at the earliest opportunity? Have you ever seen components of a below-ground drainage system recovered for reuse? Neither have I.

The obvious way to minimize the underground drainage system is to collect the rainwater together at high level and bring it to a suitable discharge point before dropping it down to ground level. (Syphonic drainage systems are well established now, so this doesn’t mean huge gravity drainpipes hanging from your roof!)  So what are the resource efficiency benefits of this approach?

  • Material use – Because syphonic systems can carry far more water than a gravity system in the same size pipe, overall the pipework is smaller.  And no manholes either, or pipe bedding & surround.
  • Adaptability – Above ground systems are assembled from components which can be dismantled and reassembled into different arrangements if needs change. Below ground systems are fixed rather than flexible – they can only be extended or re-routed (and any unused bits abandoned).
  • Maintainance – Above ground systems are visible and readily accessible for maintenance, repair or replacement. Below-ground systems by their nature are difficult & disruptive to get at if they fail.
  • Recovery / reuse – Above ground systems are generally made of metal or high-grade plastic, which has significant value and a ready market. At the end of the life of the building an above ground system WILL be dismantled and recovered for reuse or recycling. Below ground systems are rarely reused when a site is redeveloped, or even minor components recovered for reuse

And, not directly “resource efficiency” issues, but worth also taking into consideration:

  •  Waste – Fewer trenches = less excavation = less arisings to dispose of.
  • Groundwater – Fewer trenches mean less need for dewatering operations on sites with high groundwater tables
  • Soil impact – Fewer trenches mean less disturbance of the ground, so less risk of “soft spots”.
  • Site management – Fewer drainage trenches mean safer access and easier traffic management around the site

So, remind me – Why exactly do we prefer underground drainage systems?