A cross-section of a tree box, the star of today’s Photography Friday! Read on to learn what this Best Management Practice means to your favorite ocean. Image from http://ciceet.unh.edu/unh_stormwater_report_2007/treatments/tree_box/design.php.

When you’re in the middle of, say, Indiana, it’s easy to feel disconnected from the ocean. The coastline is hundreds of miles away, and you probably have plenty of environmental problems to worry about closer to home. But marine conservation is not solely the purview of coastal communities: since all rivers ultimately lead to the ocean, the entire mainland acts as one big funnel to the nearest ocean. The Mississippi River watershed, for example, includes about 41% of the continental United States, meaning that rainwater rushing down the streets of Lincoln, Nebraska, will ultimately empty into the Gulf of Mexico. Before it gets there, though, it will accumulate dissolved nutrients and sediment as it passes through agricultural and disturbed land. Those additions can promote eutrophication and hypoxia in the ocean, as embodied by the dead zone in the Gulf of Mexico.

The focus of today’s Photography Friday is known as a tree box, and although it might be found in neighborhoods hundreds of miles from the shore, its presence can have meaningful implications for marine water quality. Tree boxes are a type of best management practice (BMP), which refers to a device or system recommended for stormwater or wastewater treatment. Technically, BMPs are endorsed by a federal or state agency as an appropriate means to address water quality concerns, but the term is often used more broadly . BMPs include retention and detention basins, riparian buffers, permeable pavement, rain gardens, and bioswales. Most BMPs are intended to slow the flow of stormwater so it can filter into the ground rather than flow quickly offsite.

Tree boxes are located along the curb (/kerb, for our UK audience), and at first glance appear to be simply ornamental.  However, a closer look will reveal a gap in the curb along the length of the planter, similar to the opening of a storm sewer grate.  The tree box itself is a concrete box located entirely underground, with the tree planted so the mulch at its base is flush with the street.  After a rain event, stormwater will flow through the gap, covering the mulch and eventually infiltrating a thick layer of soil below.  The soil contains an engineered media that traps sediment and facilitates pollutant removal.  This remediation is augmented by the tree roots, which immobilize nutrients like nitrogen and phosphorous for growth.  After water passes through the soil media, it either enters a perforated pipe that discharges it offsite, or trickles into the groundwater through crushed stone at the bottom of the box.  During large storms, high volumes of water will bypass the tree box through a pipe along the curb, since the BMP is not intended to handle excessive water.  This design highlights the fact that BMPs should be selected for the needs of the site in question; the mere inclusion of a BMP in a stormwater management plan does not guarantee results.  Tree boxes, for example, might be ideal for small parking lots, but will not benefit expansive streets that accumulate a lot of runoff.

If used wisely, tree boxes can be an important means of reducing pollutant loads that would otherwise be shunted directly to local waterways.  They might make your neighborhood street look a little snazzier, but they can also help beautify the ocean, however far away.

A bonus photo: two adjacent tree boxes. Look for the gap in the curb that allows stormwater to enter the system. Image from http://www.filterra.com.

For more info:

EPA’s guidance for BMPs: http://www.epa.gov/oaintrnt/stormwater/best_practices.htm

Rutgers’ explanation of tree boxes: http://njaes.rutgers.edu/pubs/fs1209/