I once asked why the religious service I attend regularly was so long. The explanation: the sages over the years wanted to add new prayers, but didn’t feel authorized to discard any of the old prayers. Thus, the service just got longer and longer.
I think something similar has happened with stormwater BMP specifications. Blending religion and stormwater management is probably untoward, but my real point is about BMP specifications and cost inflators. We have added new standards to the specifications, but not felt authorized to dispense with much of the old stuff. The upshot is that BMPs have likely become more expensive than they need to be.
In this context, a cost inflator is a design feature, analysis, or material that is required as part of a specification that doesn’t contribute to, or may even detract from, the practice’s performance, but increases the cost of design, installation, or even long-term maintenance.
I have been thinking lately about BMP cost inflators in this age of wider TMDL-driven implementation across our watersheds. This is real money that is intended to have real benefits. Cost inflators serve to increase budgets, potentially reducing the number of BMPs and cumulative water quality benefits. Cost inflators also have opportunity costs, as the “extra” funding could be used for other public or private purposes.
I am interested to hear what you think some of the key cost inflators are in our ever-evolving stormwater universe. Here are some that I have encountered.
Wrap It Up With Fabric
For bioretention in particular, we used to put filter fabric between the soil media and underdrain. This led to some clogging issues, so this layer was removed. However, it is still common to put filter fabric on the sides of the excavation, presumably to keep the surrounding soil from contaminating the valuable engineered soil media. There are also many different flavors of geotextile: needle-punched, non-woven, woven, webs, mats, etc. Fabric or geotextile doesn’t just come in chocolate and vanilla; we can also choose butter pecan or even rocky road (not recommended for bioretention).
Geotextile is great stuff, and each product has intended and specific purposes (e.g., separation barrier, structural element, replaces another more expensive material). I have seen some bioretention plans that wrap everything in some type of geotextile: the underdrain stone, the underdrain pipe, a strip above the underdrain, even between mulch and soil. What is the specific and intended purpose and is it really necessary for performance?
Speaking of performance, research has indicated that a good deal of exfiltration takes place out of the sides of the practice, and not just the bottom. Why are we putting filter fabric on the sides when we are trying to promote runoff reduction? (Note that we have to mindful of building foundations, fill material, etc., so that is a warranted specific purpose for use of some fabrics, liners, or geotextile in some cases).
Some professionals will claim, perhaps rightly, that limiting the ponding depth to 12” maximum is the largest cost inflator for bioretention, as that storage must be made up with the much more expensive soil media, stone, and overall depth or footprint of the practice. It is a notion worth entertaining. My own experience with deeper ponding depths is that they haven’t performed well and can lead to nuisance issues.
Permeable Pavement: Dig ‘er Deep & Wide
Anybody that has looked at relative BMP costs (or costs per pound of pollutant removed) has concluded that permeable pavement lives the life of a king. It is an expensive practice in relative terms, but also has multiple benefits and can promote efficient land use where land values are high. It is an important BMP in the portfolio, but are there elements that make it more expensive than it needs to be?
Recently, I have encountered some sidewalk or patio applications intended only for pedestrian loads. I have seen all kinds of subsurface layer cakes comprised of different size stone and sand, often extending to 18” or even 24” below ground. Shallower profiles are available (e.g. Interlocking Concrete Pavement Institute pedestrian applications with 6” of stone reservoir), but there seems to be an industry standard that is much deeper. Only 3-4” is needed for typical water quality storage requirements; the rest is to support the intended structural load. This load is well understood and documented for traffic-bearing surfaces, but maybe the standard can be reduced for lighter duty projects?
The contractors that warranty their work need support from the engineering community to justify a shallow profile, and, ultimately, it could lead to less excavation, fewer materials, and more permeable pavement on the landscape.
A Little This n’ That in the Plant List
The planting list in a stormwater design will dictate not only initial installation costs, but also long-term maintenance procedures and costs. This initial plant stock is generally not a big ticket item compared to other components (e.g., soil media), but the long-term O&M is certainly an important BMP cost factor.
Typical designs include a little of this and that: trees, shrubs, herbaceous layer, and generous helpings of mulch. Aside from the false expectation that these practices should require annual re-mulching, the “this and that” approach can be very hard to maintain. Is it done with mowers, whackers, clippers, loppers, edgers, bushhogs, lot of people bending down, or some as-of-yet unimagined landscape tool? Oddly, we don’t generally ask this question at the intitial design and installation stage or match the initial design with the maintenance regime (as calibrated to the crew who theoretically will be handling the task).
Here’s one take on a potentially more cost-effective process that can also result in more ecologically-relevant designs. Use low-cost native plant plugs and maybe seed mix, plant very densely with initial installation to get quick surface coverage, and cut-back annually in early spring as part of the operations plan. If done successfully, there will be no need for re-mulching once the “green mulch” takes over (but informed management during the first couple of growing seasons is key). There is an increasing amount of guidance on this approach, and it make take some trial and error, but is an attractice option for some practices. Here’s one upcoming workshop that may interest you:
These are only three examples of BMP inflators, but they may be minor in the grand scheme of things. The required sizing for BMPs bears the greatest influence on cost. Current sizing seems to be supported by research, especially if runoff reduction is a primary objective. My previous post about Bioretention Soil Media addresses an issue that could be another potential cost inflator. I feel confident that some of you have encountered other vexing cost inflators in your work. Let me know what they are and I will report back in a later blog.