Doubling Down

Borough Council has voted on July 16th to install stormwater bumpouts on Wynnedale Road - the next step in a plan to build 25 more bumpouts on residential streets across Narberth. Most residents don’t know this is happening.

WHAT IS HAPPENING

The Borough has engaged an engineering firm (Pennoni Associates) to design two stormwater bumpouts on Wynnedale Road at a projected cost of approximately $29,000 each.¹

This is part of a larger Capital Improvement Plan (CIP Item NARB_CIP_030) to construct 25+ more stormwater bumpouts on residential streets Borough-wide, budgeted at up to $730,000, at a rate of roughly five streets per year.²

The streets already identified in Borough planning documents include many of the roads on which you live or drive every day. Eight have already been constructed.³

STREETS IDENTIFIED IN THE BOROUGH’S OWN PLANNING DOCUMENTS

WHY RESIDENTS ARE RAISING CONCERNS

• The Borough’s own analysis identifies at least five alternative locations that deliver significantly greater environmental benefit than Wynnedale Road - yet those sites were passed over.⁴

• Weak rationale has been provided for why 2 bumpouts are necessary on Wynnedale Road and no analysis of the incremental benefit has been shared.

• There is no established process, plan or procedure to measure whether any installed bumpout actually achieves its stated purpose.

• When residents proposed alternative methods for the second bumpout, Borough Council declined to consider them - even those that are accepted by the Pennsylvania Department of Environmental Protection as viable.⁵

A PARKING-COMPATIBLE ALTERNATIVE EXISTS, IT WASN’T EVALUATED ON EQUAL TERMS

A conventional bumpout removes on-street parking permanently: the curb is extended into the parking lane, the extension is planted, and that stretch of curb is gone for good. That is not the only engineered way to capture stormwater at the curb line.

Permeable, or pervious, pavement is a mature, code-recognized alternative that keeps the parking lane intact. In this design, interlocking concrete pavers (or a similar porous surface) are set flush with the roadway grade, with open joints or a porous matrix that lets rainwater pass through the surface itself. Beneath the pavers sits a bed of uniformly graded, open-void crushed stone - functionally the same “capture trough” concept - which temporarily holds the water and lets it infiltrate into the soil below, exactly as a bumpout’s planted soil media is meant to do. The Federal Highway Administration’s own technical guidance describes this as a “stone reservoir” system and states plainly that it is designed to provide “a strong pavement surface for parking, walkways, trails, and roads” while managing and treating runoff at the same time - not a tradeoff between parking and stormwater capture, but both at once.⁶

This is not a theoretical product. The City of Chicago has installed permeable pavement of exactly this kind across its network of public alleys. These surfaces carry daily vehicle traffic and provide direct driveway and parking access to garages. Chicago’s own program materials describe permeable paving as reducing rainwater ever reaching the sewer system while functioning as ordinary vehicle-bearing pavement.⁷ Washington, D.C.’s Department of Energy and Environment funds permeable pavers, including in parking areas, as one of the standard green infrastructure practices under its RiverSmart program.⁸

Nor is this an American or big-city-only practice. The United Kingdom’s national technical body for sustainable drainage design (backed by the Construction Industry Research and Information Association, and widely used by UK local authorities and their engineers) formally recognizes permeable and porous paving. This includes car parks and other parking areas and is a standard component of an urban drainage system, on equal footing with swales, basins, and other more familiar green infrastructure.⁹ Germany has gone further still: Berlin’s public-private rainwater agency actively promotes “de-sealing” (removing impervious surface) and infiltration-capable pavement as core tools of the city’s “sponge city” (Schwammstadt) strategy for managing urban stormwater, applied at the scale of streets, courtyards, and parking surfaces across the city.¹⁰

In fairness, this option comes with real tradeoffs that deserve to be talked about. Permeable paver systems typically cost more per square foot to install than a simple planted curb bumpout, though the gap narrows once a comparable stone reservoir and underdrain are priced into a conventional bumpout design as well. They require the same kind of subgrade soil and infiltration testing that any Best Management Practice(BMP) sited to infiltrate stormwater should already require. Testing that ought to have been done for the any solution regardless of design. Permeable pavement needs periodic vacuum-sweeping or joint maintenance to keep the surface from clogging with sediment over time, an ongoing cost a planted bumpout does not carry in the same way. However, maintenance may be easier to mechanism with this kind of solution as simple road access is already there.

The rationale behind this

Narberth’s own Pollutant Reduction Plan (PRP) - the compliance document Pennoni Associates prepared for the Borough’s MS4 stormwater discharge permit - assigns different sediment removal credit to different types of stormwater practice under Pennsylvania DEP’s "Simplified Method." Stormwater curb bumpouts are credited at 80% sediment removal, the same value given to the Borough’s bioretention swales and facilities. Infiltration basins elsewhere in the Borough are credited at 95%. The one existing permeable pavement installation in Narberth, is a privately owned porous paving system at 100 Forrest Avenue, built by a developer as part of a mixed-use project, not a Borough capital project. This is credited at just 55%, the low end of the range DEP allows for bioretention and rain garden practices generally.¹¹ Judged strictly by that number, a bumpout looks like the more efficient way to hit the Borough’s mandatory 22,645 lb/yr sediment reduction target. That is a real, documented, and quantified reason to prefer bumpouts, and it deserves to be acknowledged as one.

It is also not the end of the analysis. The 55% figure comes from a single small (0.03-acre), privately built retrofit, credited years ago, and there is no evidence it was ever benchmarked against the broader engineering literature on how permeable pavement actually performs when it is properly designed. The U.S. EPA’s National Menu of Best Management Practices technical fact sheet for permeable pavements reports measured pollutant removal rates compiled from five separate peer-reviewed field studies, including a multi-site field survey of permeable pavement installations in eastern North Carolina:¹²

Every one of those ranges, including the low end of PICP performance, exceeds the 55% credit Narberth’s one local example received. Porous asphalt and pervious concrete perform on par with, or better than, the 95% credit the Borough already gives its own infiltration basins. Nothing in the national performance record suggests permeable pavement is an inherently weaker practice than a curb bumpout. It suggests the Borough’s only local data point for permeable pavement was a small, minimally engineered private retrofit that nobody appears to have re-tested against a design built and documented to the same standard as the bumpouts it is effectively being compared against.

A rationale exists, it is quantified, and it rests on one unrepresentative local example rather than on a genuine side-by-side comparison of properly engineered alternatives. The Pennsylvania Department of Environmental Protection BMP Manual that Pennoni’s own project documents cite already lists permeable and porous pavement as a recognized best management practice alongside bioretention and inlet retrofits.⁵ A parking-compatible design was available, in use elsewhere at exactly this scale and performing at least as well by the published numbers. But there is no record anyone modeled it against Wynnedale Road on equal terms before Council’s vote.

WHY THIS MATTERS GOING FORWARD

The bumpouts and those on Wynnedale Road were, in Our Narberth’s assessment, the wrong decision: the lowest-ranked location for environmental benefit among the sites the Borough’s own analysis considered, chosen without a strong rationale weighing it against a properly engineered parking-compatible alternative, without a plan to measure whether it works, and without a demonstrated evaluation of parking-preserving alternatives that other jurisdictions already use at scale and that the national performance data suggest could perform just as well or better. That decision just does not get better because it has momentum behind it.

Voting to proceed with two more bumpouts on Wynnedale Road, and doing so as the first installment of a 25-bumpout, up-to-$730,000 Borough-wide program, does not correct that error. It repeats it, street by street, at increasing cost and increasing loss of on-street parking, before anyone outside the Borough’s engineering contractor has had the chance to ask whether a parking-compatible design, built and documented to the same standard as the bumpouts themselves, could have delivered the same environmental result. Compounding a bad decision is not the same as validating it. It is doubling down on the error.

Thanks for reading.

All numbers and street names referenced above are drawn from Borough planning documents and engineering plans on file with Narberth Borough.