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The rapid growth and industrialization of Yonkers, N.Y., in the second half of the 19th century took its toll on the Saw Mill River, which served sawmills, gristmills, and carpet mills as both a power source and a receptacle for waste materials. By the 1920s, the heavily polluted waterway was regarded more as a public burden than an asset. In response, the U.S. Army Corps of Engineers constructed a massive flume that carries the river beneath the city at Larkin Plaza.
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Recently, the city decided to restore this natural asset to public use by daylighting the river and making it the centerpiece of a public park. In turn, the park will serve as a catalyst for subsequent urban renewal projects involving both public- and private-sector investments. The project was the brainchild of former Governor George Pataki and has been championed by Yonkers Mayor Phil Amicone.
From a design and engineering perspective, this project created a variety of challenges related to both park aesthetics and a complex, functioning hydrological system. At the same time, with the challenges came opportunities to take advantage of the varied natural characteristics of the river to provide solutions.
Challenge 1: Range of river flows
The Saw Mill River experiences a wide range of flow conditions. While confined to the flume, the river flood threat is substantially mitigated; uncovering the watercourse removes, to some extent, the protection of the flume. The low end of the flow range offers a second challenge: To fulfill its aesthetic role as the focal point of the public space, the river must have sufficient volume to create sensory interest.
Solution – The design addresses all river flow scenarios. The most important decision was to retain the existing flume in order to preserve its flood control role. This is accomplished by creating a chamber within which a diversion channel directs the river's base flows to a varied new watercourse. In low-flow conditions, all flow is directed through the park; during precipitation events, excess stormwater runoff remains in the flume and travels onward to the Hudson River. A confluence chamber at the foot of the park allows the flow within the open channel to rejoin the water in the flume.
Additional flood prevention measures include raising the level of walkways and park amenities above the 100-year flood level. The sides of the park thus form an unobtrusive but effective containment wall. Architectural wall systems, curbing, and transitions in grade all serve as integral flood prevention elements in the plan. The slopes along the waterway are protected from storm flow velocities by a slope armoring system that integrates plants into the rock to soften the appearance of the slope protection while providing erosion control.
The low-flow scenario was also of concern. At minimum flow, the stepped design of the channel takes advantage of the sloping topography and employs a section of riffles – a short stretch with a large grade differential that increases water speed – to maintain the flow and audibility necessary to draw the attention of park visitors. The pools serve as a visible "still" feature at either end of the riffles.
Challenge 2: Hudson River tidal influence
Within the corridor to be daylighted, the Saw Mill River makes the transition from a freshwater system to the brackish water of the Hudson River, thus containing two distinct riparian environments. The design must control the tidal influence without hampering marine life migration. As the tidal influence would also affect the flow regime within the daylighted waterway, hydraulic design of the daylighted portion would be controlled by the flow characteristics of the Hudson River.
Solution – The redesigned river corridor incorporates a series of pools. The lowest of these, located at the foot of the park where the water rejoins the flume, is subject to tidal advance and retreat. A stepped weir at the upstream end of the lowest pool accommodates the ebb and flow of the tide without creating a barrier for wildlife migration. The rise in elevation built into the weir confines the brackish water to the lowest pool, effectively separating the tidal and freshwater environments. A secondary fish ladder provides an additional route upstream for American eel and other aquatic species.
Challenge 3: Water quality degradation
The 26.5 square mile watershed of the Saw Mill River occupies a highly urbanized setting, where debris introduced in runoff finds its way downstream. This, in combination with the absence of light within the confines of the flume, has created a degraded environment for riverine flora and fauna. Therefore, the design must mitigate that water quality degradation and create a restored habitat where a variety of marine species could survive and thrive.
Solution – Multiple aspects of the daylighting scheme work to restore a viable wildlife habitat. First is the daylighting itself – the reintroduction of light to facilitate vegetation growth. The riverine vegetation exerts a cleansing effect while providing a food source for some wildlife and a habitat for others. The beneficial effects of exposure to the open air are augmented by design elements, such as the riffles, that aerate the water in the course of its passage. The "plunge pool," immediately below the inlet from the flume, allows sediment to drop out of the stream. Further, a debris netting system at the inlet culvert screens out the waterborne debris that passes through the river.
Challenge 4: Surrounding landmarks and utility infrastructure
The project creates potential issues relating to the preservation of historic resources and aging underground utilities. For example, Philipse Manor, formerly the home of the family who owned most of Yonkers from the 1670s up to the American Revolution, is one of the most important historic structures in the county. The Manor stands immediately adjacent to Larkin Plaza. Other historically important elements include the Gold Star Mothers Memorial, the Larkin Flag Pole, the Spanish-American War Memorial, and a fountain. The potential for damaging utility infrastructure also needed to be considered.
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Solution – The project design calls for daylighting the west end of the plaza rather than the easternmost end. There are several reasons for this decision. First, opening the river at the narrow, eastern end of the plaza would have created a deep crevasse with limited visual and physical access. Equally important is the proximity of Philipse Manor. To avoid any adverse impact on the historic structure, nearby construction activity was intentionally kept to a minimum. The park design also improves the appearance of the small monument park at the head of the plaza, enhances its visual prominence, and builds on its complementary relationship with the Manor and grounds.
The fountain and three monuments are preserved by various means. The fountain will be moved offsite to another city park. The Larkin Flag Pole, a meaningful local landmark, will join the Spanish-American War Monument in the monument plaza. The war monument will maintain its position of eminence as the visual anchor of the park, but will be moved slightly to better complement the rest of the plaza. Finally, the Gold Star Mothers Memorial, an emotionally moving commemoration of mothers whose sons did not return from war, will be moved to a prominent location in the park.
The design includes the improvement to surrounding utility infrastructure and the roadway system. The storm sewer system, in particular, will be improved by the installation of tidal valves which allow flow in only one direction, preventing tidal flow from overwhelming the system. Sanitary sewers and water distribution systems in the roadways surrounding the park also will be replaced.
Challenge 5: Vehicular and pedestrian patterns
Larkin Plaza in its existing configuration is easily circumnavigated and can be crossed at a number of points. Pedestrians and motorists have access to the public library, train station, post office, Philipse Manor, and monuments. The plaza design needed to maintain or improve access to these resources, to adjoining neighborhoods, and to local businesses.
Solution – To replace the previous north-south route across Larkin Plaza (a walk-through parking lot), a pedestrian bridge will be constructed across the river at its midpoint, allowing pedestrian flow to various destinations. The bridge will serve the added function of drawing people into the park. Within the park, pedestrians will circulate along an esplanade and a series of walkways that lead through a variety of spaces and sensory experiences including the sequence of pools, riffles, wooden amphitheatre, and diverse landscape design elements. Vehicular circulation around the plaza will remain essentially unchanged.
Challenge 6: Public access and safety
How do people safely access the water from the street? Over its length, the Saw Mill River corridor within Larkin Plaza undergoes a substantial change in elevation as it slopes toward its confluence with the Hudson River. Additionally, the river elevation is well below street level, making it a challenge to allow easy access from the street and to provide safe passage along the walkways. Safety must be ensured without creating a barrier between the pubic and the water.
Solution – Safe access, including an ADA accessible path, to the water's edge is provided by walkways that gradually lead visitors to the water level at the riffles, guiding them in the process along the series of interest points described above. Where grade separations are present, pedestrians are protected from fall hazards by an open railing system that does not create a visual barrier.
Challenge 7: Potential presence of environmental contaminants
The historic fill within the banks of the Saw Mill River potentially can contain an unknown quantity of contaminants. Reducing or eliminating the need to work within these conditions would also be a crucial consideration.
Solution – Disruption of these potentially contaminated soils alongside the flume is minimized by leaving the flume in place. A further barrier to potential contaminants is a new PVC lining, which separates the new construction from the historic fill while preventing water loss from the pools.
Challenge 8: Sustainability practices
The city of Yonkers is committed to incorporating sound sustainability practices in design and construction. This commitment adds a new set of parameters to any project, affecting choices in energy use, construction materials, and waste disposal, to name a few.
Solution – The daylighting plan upholds the sustainability principles of maintaining community connectivity, restoring habitat, maximizing open space, and controlling stormwater. Among the primary sustainability-related benefits is the elimination of the heat island effect. Removing the expanse of black asphalt parking contributes greatly to this end. Less reflective gray concrete and stone are used throughout the park's construction. Park vegetation will have a shading effect and the water itself will be a cooling factor, creating the effect of a small urban oasis in the summertime.
The project employs environmentally friendly low-wattage LED lighting that provides nearly 50 percent energy savings, optimizing energy performance while also reducing light pollution. The water-efficient landscape plan provided by co-consultant Saratoga Associates utilizes indigenous plant species that can survive in an urban environment with minimal irrigation.
Expected results
Groundbreaking on the Saw Mill River Daylighting project took place in December 2010, and utility work is underway. Substantial completion is expected by year's end; final completion is expected midsummer 2012.
Completion of the Saw Mill River Daylighting project will deliver significant benefits to Yonkers in the realms of environment, economics, aesthetics, community interaction, and quality of life. The daylighting will create a point of interest – a destination rather than a pass-through – that will attract people to the neighborhood and bring business to surrounding establishments. It will provide a point of connectivity between surrounding neighborhoods, complement other civic improvements being made nearby, and contribute to a larger goal of urban renewal. In bringing people to the water, the city will again harness the power of the river that once ran the mills.
Joseph Fleming P.E., PP, executive vice president, PS&S, serves as principal-in-charge of the firm's Land Group and is directly involved in a wide range of industrial, commercial, retail, research and development, and infrastructure projects. He can be contacted at jfleming@psands.com.
