Central Wharf Plaza

Challenge

The design team began working from the belief that the site, a fully paved traffic island only 4,000 sf in size, would succeed only if the spatial result was strong and positive. Furthermore, summer shade was deemed the primary characteristic that would appeal to visitors, offering a contrast to the more open parks of the neighboring Rose Fitzgerald Kennedy Greenway. Set among busy streets along the waterfront of downtown Boston, the site’s location posed difficult conditions above and below grade. Close proximity to the harbor meant that plants would have to be carefully selected and arranged to withstand high winds and salt. Because the site was once part of the bay, the sub-soils and urban fill below the surface could not support normal spread footings without considerable settlement. In addition, the tangled network of underground utilities further contributed to the complexity of the site.

Solution

Working with city agencies, the team expanded the site to over 13,000 square feet - just enough scale to provide identity and spatial coherence. Many large caliper trees were installed to create a complete canopy with instant shade, while still allowing unhindered pedestrian circulation through the site. Red and Pin oaks, both native species, were selected primarily for their form, wind and salt tolerance, and relative resistance to health problems. Below the surface, a specific soil and infrastructure matrix was designed to accommodate settling sub-soils and complex utilities, while optimizing tree growth. Mini-piles and spanning grade beams were utilized to support the granite benches and catenary posts, while shallow spread footings and sand-based structural soil provide support for seat walls and stairs. These allow for a continuous layer of soil and unobstructed root zone for the trees and offer flexibility for managing utility configurations. To support the long-term function and appearance of the site, design team members sustain a continued relationship with the client and are an ongoing part of maintenance decisions. Should any trees need to be replaced in the future, the dry-laid paving allows for a quick turnaround and minimal impact to the site’s ground plane.

• 25 closely spaced, 13-in DBH trees provide a pervasive canopy and extensive shade for the plaza. They are a mix of Red and Pin oaks, two native species that are wind and salt tolerant and relatively resistant to health problems.
• Pervious surfaces cover almost 10,000 sf, or approximately 75% of the site, in the form of dry-laid granite pavers with permeable joints, stonedust and planting soils. The pavers and stonedust allow stormwater to infiltrate while supporting heavy public use.
• 12 slot drains collect all excess runoff from the site and carry the stormwater directly to 13 trees for passive irrigation, with any water not taken up by tree roots continuing to percolate down through the soil. In addition, the drains convey air to the root zone through a perforated lattice below the surface.
• Flood bubblers provide an efficient deep root mechanical watering system for the trees. A rain sensor automatically adjusts irrigation based on the level of precipitation, while moisture sensors installed on select trees allow for periodic manual adjustment depending on soil moisture.
• The continuous layer of sand-based structural soil below the surface allows for an unobstructed root zone by providing additional support for seat walls and stairs with shallow spread footings, filters stormwater as it percolates down, and enables better distribution of moisture across the site.
• Mini-piles and spanning grade beams support the granite benches and catenary posts, reducing physical barriers throughout the root zone and allowing flexibility for utility configurations below grade.
• The pitched plane of the plaza, sloped at a 2% gradient, provides elevated views toward the water and a stepped gathering place at the active harbor edge where over 1.3 million Aquarium visitors and 335,000 ferry riders pass by annually.
• 290 linear feet of granite seat walls with wood insets and freestanding wood benches, provide a place of respite for tourists, neighbors and commuters.
• A system of LED lights hung from catenary wires threaded through the tree canopy provides efficient and subtle lighting, extending the useful hours of the plaza.
• All streets surrounding the plaza were narrowed and a separate vehicle drop-off area was added on the wharf/aquarium side. The new drop-off area allows vehicles to pull through when leaving instead of having to back up into the flow of traffic.

• The integrated planting system on the site contains several elements in its design beyond what is typically found in an urban plaza. These added features improve the conditions for root growth, increasing tree health, size and longevity. Compared to the 4.3% average annual mortality rate of a typical urban street tree, the trees on the site have a current annual mortality rate of only 1.6%. Based on these rates, in a typical urban plaza, all 25 trees would have to be replaced in 23 years, while in this plaza only 9 would have to be replaced over the same time frame, a savings of nearly $50,000.

• The key role that soil biology plays in maintaining the high performance of the plaza’s urban tree canopy became evident when the condition of several trees started declining immediately after installation due to the stresses of transplantation. An arborist with a specialty in soil biology was brought on to the project to help stabilize the trees. He suggested that the best way to accomplish this was to activate the latent biology of the manufactured soil on the site through compost tea and mycorrhizal applications. The applications would quickly recreate the trees’ native soil biology, helping to support good root growth and nutrient uptake, leading to robust, healthy trees. This has become part of the routine maintenance of the site with a single annual application used to adjust soil biology based on yearly testing results. As the site’s soil biology changes over time, an annual decision is made with the owner on whether or not to continue this process.

Granite Pavers: Cold Spring Granite                  

Granite Walls: A Lacroix et Fils Granite

Lighting: BK Lighting

Soils: Dexter and Harpell Inc.

Trees: Halka Nurseries Inc

Project Team

Client: Frog Pond Foundation
Architect: Chan Krieger Sieniewicz
Landscape Architect: Reed Hilderbrand, LLC
Structural/MEP Engineer: ARUP
Civil Engineer: Vanasse Hangen Brustlin
Soil Scientist: Pine Swallow Environmental
Lighting Designer: LAM Partners
Specification Writer: Rico Associates
General Contractor, Construction Management: Turner Construction
Landscape Contractor: ValleyCrest

Role of the Landscape Architect

The landscape architect worked with the client, a private philanthropy dedicated to supporting urban open space initiatives, to develop and apply exemplary, high-performance urban design and planting practices. On behalf of the client, the landscape architect negotiated with city agencies to expand the project site from the original 4,000 sf traffic island to create a 13,100 sf plaza. Through collaboration with the design team of an urban designer/architect, soils engineer, soils biologist, arborist and structural, civil and geotechnical engineers, the landscape architect created an integrated and optimized infrastructure to support healthy trees and heavy public use.