Inflitration practices are a key element of runoff reduction, but they require careful design and testing to work properly. One of the big upgrades with this Bay-wide design specification is dealing with the scale and amount of infiltration that is done at a site. The specification outlines three scales for infiltration: micro-infiltration, small-scale infitration and conventional infiltration. The spec also outlines a process for evaluating whether the proposed site will be a designated stormwater hotspot where infiltration is restricted or prohibited.
Most of us in the stormwater business are quite conflicted when it comes to infiltration. On one hand, infiltration has the highest runoff reduction capability of any stormwater practice, and probably comes closest to replicating predevelopment hydrology. On the other hand, infiltration practices have experienced consistent problems and failures over the years. These anecdotal reports, along with groundwater concerns, have greatly limited the use of infiltration practices in most communities.
The basic infiltration design specification has not changed much over the last 20 years, with three exceptions. While these design changes have merit, they have also had the unintended consequence of reducing the number of sites where infiltration can be used. The first design change was increasing the minimum infiltration rate from a half inch to one inch per hour (which meant that most C and D soils don’t qualify).
The second change involved more extensive on-site soil infiltration testing to establish infiltration rates. As a result, many designers eschewed costly soil infiltration testing when other stormwater practices could be designed with less subterranean analysis. Third, the number of site constraints that limit or prohibit infiltration has multiplied in recent years. Slope, soil, flow, water table, hotspot, aquifer protection, underground injection and karst terrain constraints have cropped up in many manuals, and have become so extensive that many designers automatically rule out infiltration.
The challenge is to craft a new specification that promotes more widespread use of infiltration while minimizing the risk of failure. Toward this end, CSN has prepared a new design specification for infiltration practices for review and comment from the Network. A visual summary of key design choices involved in the new infiltration design spec are described below.
- Small is Beautiful. Perhaps the most important lesson learned over the years is to not force a lot of runoff into a single infiltration practice. It makes more sense to spread out the infiltration investment into a series of small infiltration practices rather than a single large facility. With this in mind, the new design spec designates three categories of infiltration practice, based on the relative size of their contributing drainage area or CDA.The smallest category is micro-infiltration that receives runoff from 250 to 2500 square feet (sf) of CDA, most of which is presumed to be impervious. The second category is small-scale infiltration that accepts runoff from 2500 to 20,000 sf of CDA, and the third is conventional infiltration which takes runoff from 20,000 sf to 100,000 sf of CDA (note that this implies a maximum upper limit of CDA for infiltration of about two acres). The design spec outlines more engineering “bells and whistles” as CDA increases. Also, micro and small scale infiltration practices can be sized to accept a fraction of the desired water quality volume, as long as a downstream practice treats the remainder.
- Moving Back and Forward at the Same Time. The new spec proposes a lower minimum soil infiltration rate of 0.5 inches/hour, but then reduces the field rate by half during the actual design computations. This change increases the feasibility of infiltration, but also accounts for the post construction decline in infiltration rates that have been noted by several investigators. Does this new approach make sense to the Network?
- Pretreatment and Combo Systems. The new spec also tries to outline more clearly what kinds and how much pretreatment are needed to protect infiltration practices from clogging. Experience has shown that it makes more sense to design combo filtering/infiltration systems when CDA gets above two acres. The basic idea of the combo system is to split the water quality volume equally between a filtering practice cell and an infiltration cell. The filtering cell provides more reliable pretreatment to the infiltration cell.
- Elevated Backup Underdrains? Another idea advanced in the spec is to utilize backup underdrains that are elevated a foot or two above the bottom elevation of a conventional infiltration practice. The basic idea is that the elevated underdrains can still provide some measure of runoff reduction through extended filtration in the event of temporary or permanent clogging (particularly at sites with soils that are close to the minimum allowable infiltration rate). This seems like a fairly low cost design feature, but it would be great if other designers in the Network could provide feedback on the feasibility of this concept.
- Working Observation Wells and Caps. While it’s a good idea to install observation wells for infiltration practices, it also evident that shattered PVC pipes is a diagnostic indicator of infiltration in the field. It seems that that anything extending above the ground surface is a fair target for mutilation by mowers, vehicles and disgruntled teenagers. PVC pipes are cracked, broken off, pulled out, or un-capped. Even if they do survive, most observation well caps have “lockjaw” and are so frozen they are only good for one round of inspection.Even when wells/caps are installed flush with the ground surface, they mysteriously seem to disappear beneath it after a few years, as any inspector well knows. Also, do we really need observation wells for micro-infiltration or even small scale infiltration practices? If anybody on the Network knows of a better way to make observation wells survive, I’d love to hear about it.
- And What about Dispensing with Cleanout Pipes? Not only do cleanout pipes suffer from the same carnage as observation wells, it’s questionable whether they actually serve their purpose. Can anyone recall a single occasion where cleanout pipes were successfully used to back flush an infiltration practice to relieve clogging? If not, does it make sense to dispense with this unnecessary design feature?
- Future Land Use Matters. Concerns have always been expressed about the risk of groundwater contamination for infiltration practices that serve stormwater hotspots. These hotspots possess an increased risk of spills, leaks or other discharges that could allow pollutants to migrate into groundwater. Until now, there has not been a definitive list of land uses or operations qualify as hotspots, and are therefore subject to restrictions or prohibitions on the use of infiltration.The new spec contains my first crack at such a hotspot list. While compiling a list is extremely useful for plan reviewers and designers, it is fraught with danger. If the list is too narrow, it increases the risk of potential groundwater contamination in the future. If the list is too broad, it may mean that infiltration is eliminated as an option at sites where it could be used safely. So, I would appreciate your sense of risk and reward on an appropriate hotspot list.
- The Specter of Underground Injection Permits. Although most stormwater engineers are dimly aware that certain infiltration practices might require an underground injection (UIC) permit, we must admit that we don’t go overboard to learn much about this important groundwater protection requirement. The notion of getting yet another permit is daunting, particularly if it is accompanied by possible long term monitoring requirements and a perpetual groundwater liability for the owner.Thankfully, the groundwater and stormwater folks at EPA headquarters have come together and issued joint guidanceto clarify the UIC permit issue. The basic rule is that underground injection permits are only required if the depth of the infiltration practice is greater than its average surface width, under most conditions. The proposed design specs for infiltration have all been tailored to avoid this outcome.
- Infiltration in Karst Terrain? Designers have always been afraid of infiltration in karst due to concerns about sinkhole formation, despite the fact that karst is one of the finest naturally infiltrating terrains out there. It seems that micro-scale and small-scale infiltration can work fine in karst, although larger scale infiltration should be avoided. The karst working group is teasing out these details as we speak, so expect more on this topic later in the winter.
- What about Permeable Pavers? Although infiltration practices and permeable pavers are quite similar in many respects, they are different enough to warrant a separate design spec.