Noise Pollution in Bishop City Park

Introduction

Bishop City Park is a small property minimally developed for recreational use. It consists of several parcels owned by the City of Port Townsend, Washington, as well as portions of adjacent public right of way. The use of public right of way as multipurpose trails where they remain unopened and undeveloped is common throughout the City.

My investigation was as to whether this property, which could generously be measured at 5 acres including those portions of the adjacent right of way through which trails run, provides any significant respite from nearby streets and developed parcels. The property’s primary topographic feature is a small ravine that is usually free of running water; this is indistinguishable on topographic maps due to its small size but does offer a small measure of apparent seclusion due to its depth below the surrounding roads and properties. Nearly all of the property is wooded with second-growth timber except where utility lines have been constructed in the various right-of-way parcels.

Methods

I began my investigation by mapping the Bishop City Park property and surrounding parcels in ArcGIS Pro. Most map layers were sourced from Jefferson County GIS via their REST Services. I used (but did not ultimately include on the map due to a lack of data within the small study area) hydrology, utility-line, contour-line and shaded-relief layers obtained from Washington Department of Natural Resources. Structure polygons were obtained from Microsoft Building Footprints data. Creating the base map in advance of field work allowed me to export the map as a georeferenced PDF. I opened that PDF in the Avenza app on my smartphone so I could use it to plot noise measurements.

On the same smartphone, I installed the Sound Meter app. Lacking the equipment necessary to verify the calibration and performance of this app, I tested it indoors and verified that ambient indoor noise approximated the app’s rating in decibels (dB) for “quiet library” and that my speaking approximated the app’s rating for “conversation.”

A Garmin eTrex 20x handheld GPS receiver functioned as my source of coordinates for each noise measurement. While the Avenza app recorded similar information, I judged the handheld device to be a more accurate source of location data than the smartphone’s built-in GPS receiver. The GPS receiver recorded a track log as well as the individual waypoints I created; I later used this track log to add supplemental trails to the base map where the county’s trail data fell short (I did not, however, attempt to reshape the trails included in the county’s data that are quite obviously based on poor sketches).

On foot, I traveled each of the primary trails within the greater park. Using the base map in Avenza to approximate equal spacing and focusing on key points within the small trail network, I paused at 14 locations, creating a waypoint in the GPS receiver and an identically named marker in Avenza. In the Avenza marker, I recorded the average noise rating in decibels from approximately 30 seconds of observation with the Sound Meter app.

Data from the GPS receiver was extracted using DNR GPS. Avenza data were exported from Avenza. The waypoints and track were imported into ArcGIS, the two sets of waypoints joined based on their name fields to form a single table, and the track edited to show supplemental trails as described above. GPS coordinates in WGS84 were projected on the fly by ArcGIS Pro 2.5 onto the base map, which used a NAD 1983 HARN state-plane projection.

Results

The average noise rating recorded at the measurement sites varied from 42dB to 57dB. The highest reading was taken nearest the state highway that borders the park property to the south. This was not unexpected due to the apparently higher traffic volume on that road than on any other surrounding street. The second nearest point to the highway average only 46dB, likely due to its location at the bottom of the small ravine. The third nearest averaged 50dB, as its exposure above the bottom of the ravine provided a more direct line of sight and sound to the highway.

The mapped results show the decibel reading at each measurement point.
The mapped results show the decibel reading at each measurement point.

The remaining measurement points showed generally decreasing noise readings further north and west, away from the highway. It should be noted that no traffic was present on 9th Street at the time of the closest readings to that northern border of the greater park, though the street is used by some through traffic, though at lower volumes and generally lower speeds than the highway on the south end of the park.

No major sources of noise were noted during any one measurement that were not noted during others. Some noise from passing airplanes was present at times, though distant. Birds and other wildlife were noisier lower in the ravine than in most other places, but the measurement app did not seem sensitive enough to include this in the average rating. All measurements were made just before and just after 5 PM on a Thursday evening, timed for the evening commute in an attempt to maximize the amount of noise on surrounding roads and, thus, to highlight any attenuation of the park property.

Conclusion

While this small, little used neighborhood park provides some visual respite from the surrounding streets, its benefit on the ears is limited. The sound of traffic is a constant companion along the trails, especially in areas close to the state highway and approaching it, where the ravine appears to channel noise from the highway through the forested park. However, the short arms of the trail system that reach north and west from the main park parcel are somewhat sheltered from this effect and provide something more peaceful, if not silence.

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