With Low Impact Development (LID), also known as Environmental Site Design (ESD), not all solutions are created equal. There is a hierarchy in the regulations for what is the preferred technology. So it’s not so much is it approved or not approved… there are certain preferred technologies that must be considered first, and if that doesn’t work, you consider the next option in preference.
This is a huge difference in conventional stormwater regulations, which says technologies must be approved by state of local jurisdictions – and then everything is on a level playing field. See the solution staircase to help you better navigate this regulatory hierarchy based on an approximation of Low Impact Development regulations around the country.
Step One: Surface Infiltration
Let’s start at the top with surface infiltration. Where possible, LID regulations in general are promoting surface infiltration (including limiting impervious surface). So, the preferred approach of just about any LID regulation is don’t create runoff, just infiltrate it back in the ground. In a lot of cases though, that won’t work. You may have a highly urban site or a site that is already paved out. Or perhaps you’re facing space constraints – surface infiltration can take 5% to 15% (or more) of the development area, so it can be pretty space intensive.
Step Two: Subsurface Infiltration
So if surface infiltration won’t work, the next alternative is to go underground with infiltration. There are many underground infiltration systems available, including perforated pipe, chambers and concrete structures. But… there are many times when subsurface infiltration just won’t work. Your site may have high groundwater, bad soils, bed rock, etc. that prohibit this type of technology.
Step Three: Rainwater Harvesting
So if you go back to the basic principle of LID – to keep the water onsite – if these items don’t work what are you going to do to keep the water onsite? The answer, is our next step… rainwater harvesting. This is the step where you can see where stormwater can provide a benefit – really be that resource – that is one of the core principles of LID.
Step Four: BioFiltration
The first 3 steps have really focused on runoff reduction (a basic principle of LID) which is why they are at the top level. The next technology, biofiltration, kind of straddles the line between runoff reduction and treatment. It can offer some runoff reduction – it holds a lot of water in the soil – but it’s not a huge amount in the terms of 80% of the annual runoff. So it’s really a compromise between the older conventional methods and the new LID methods.
Steps Five - Seven: Filtration, Hydrodynamic Seperation, or Detention
And if that doesn’t work – space, cost, whatever the reason may be – then you can move into the more conventional approaches – like filtration, hydrodynamic separation and detention. Another option is a treatment train combination of the three.