Bronx River Forest Floodplain and River Channel Rehabilitation Project
In 2004, construction began on the Bronx River Forest Floodplain and River Channel Rehabilitation Project funded by The New York State Clean Air/Clean Water Bond Act. This project was a pivotal component of on-going efforts by the City of New York/Parks & Recreation’s Natural Resource Group, the Bronx River Alliance, and other community groups to restore the Bronx River and its floodplain in the northern end of Bronx Park. The project was designed to help address the reduced resource and habitat value in the Park resulting from the impacts of urban development and neglect.
One of the consequences of urban disturbance in the Bronx Forest is the prevalence of exotic herbs, trees and vines that out-compete the native vegetation. One of the most aggressive exotic plant species is Japanese knotweed (Polygonum cuspidatum). It dominates the floodplain as well as the banks of the river, hindering regeneration of native woody riparian trees and shrubs that grow lower on the riverbank and provide bank protection and aquatic cover. Japanese knotweed also frequently grows in dense stands over eight feet tall, and accelerates the natural accretion of sediment that forms berms along the river banks, thus limiting access to the river and visibility.
Other results of disturbed urban landscape and conflicting management interests in the Park are flashy flows, high sedimentation, the disconnection of the channel and floodplain, and reduced in-stream habitat. Trails and ball fields were difficult to maintain due to flooding in some locations, and in other locations, high flow waters that could have been stored on the floodplains were trapped in the channel due to fill or excessive sedimentation on the banks. The sand-bedded channel was homogenous and contained few in-stream habitat features, such as pools or structural cover. The low flow channel was excessively wide and shallow.
To combat these problems of diminished resource value, the project goals were to:
- Re-establish more diverse, native species-dominated riparian and river bank vegetation that could improve habitat and provide soil stability
- Reconnect the channel to the floodplain where possible by reclaiming filled lands and creating over-flow opportunities
- Convert a playing field in the floodplain to a floodplain forest
- Enhance in-stream habitat
- Provide more opportunities for access, passive recreation, and education along the river.
Bronx Forest Floodplain Restoration Maps
The Bronx River Forest Bond Act Restoration Project is located in the Bronx Forest floodplain, at the north end of Bronx Park, and is bordered by ball fields and the Metro North railroad to the west, the Bronx River Parkway to east, and Kazimiroff Blvd to the south. The total Bronx Forest Floodplain area is about 30 acres and included a section of the historic Bronx River Parkway and bridge, an under-utilized playing field, an asphalt trail along the east side of the river, a forested island and about 6 acres of upland, some of which was fill. The floodplain area was largely forested, and had a varied topography resulting in some areas being more frequently flooded than others. The capital construction project discussed here occurred on about eight acres of the floodplain, and the NRG forester’s additional invasive plant control work is occurring on approximately six acres.
Click an image to view the PDF.
Floodplain and channel
The environmental analysis and design for the Capital Project was conducted from 2002-2003 and included a characterization of channel sedimentation, vegetation, hydraulic and hydrologic conditions, floodplain soils, and channel geomorphology. A combination of strategies were proposed to achieve these objectives, including:
- clearing and grubbing riparian areas invaded by Japanese knotweed
- excavating high-flow channels to better connect the channel and floodplain
- installing boulder and large wood in-stream habitat structures
- grubbing and grading river banks to establish more appropriate planting slopes
- planting native trees and shrubs in containers and using bioengineering techniques
- installing filter fabric to deter invasive species re-growth
Construction was initiated in August 2004, and began with the installation of erosion and sediment control at the toe of the riverbanks, using coir logs. Riverbanks were then cleared, grubbed, and graded to remove invasive plant rhizomes and establish slopes more suitable for planting, particularly of inundation tolerant woody species. Banks were then seeded and covered with coir webbing for erosion control. This fabric was either cut to allow installation of containerized plants in the early fall. In the winter, the fabric was lifted back to allow installation of bioengineering materials, or cuttings from dormant woody shrubs). These were installed in the form of brush fascines (bundles of cuttings) along the toe of the banks, or as brush layers installed perpendicular to the bank to form a brush mattress.
As the bank reconstruction progressed from downstream to upstream along the channel, habitat features were installed. A four-foot diameter oak tree that fell during a storm in the first week of construction was used to create an overhang at a tight outer channel bend. Boulders were used to bin this, and other pieces of large woody debris, and to deflect flow towards the center of the channel. These boulder obstructions in the channel were intended to promote sedimentation towards the banks and scour or deepening towards the center of the channel. We worked with the Bronx River Alliance and their conservation crew to cable LWD to the toe of the bank to provide additional cover where possible.
The existing asphalt trail along the river was re-graded to better convey flows to swales and depressions in the floodplain. At two locations, elevated vehicular boardwalks were constructed over excavated sections of the highly sedimented channel banks to try to allow more frequent flooding to the back areas of the floodplain while permitting the vehicular access that the Parks Department required. Several pedestrian boardwalk sections and an overlook were also installed to allow greater access on the floodplain. The boardwalks were completed for use in April 2005.
The largest area of floodplain reconnection to the channel was at the former Cricket Pitch. Here, over 500 cubic yards of fill were removed and an overflow channel was excavated in the fall of 2004, and thousands of floodplain forest trees, shrubs, herbs and graminoids were installed in the fall of 2004 and spring 2005. Part of the former parkway was also converted to forest when the concrete was removed and converted to a narrower bikeway through the forest.
Finally, an interpretive sign was installed to help explain the project and floodplain processes in the forest on top of the Burke Ave Bridge. In total:
- Over 6 acres of floodplain forest were cleared, grubbed
- Over 8 acres were planted with native trees and shrubs either in containers or using bioengineering techniques
- Approximately four acres of riverbank were grubbed and graded for planting
- Over 16,000 native trees and shrubs and over 7800 herbs and grasses were installed
- Over 2000 feet of riverbank were planted with native woody plants
- Approximately 1000 square yards of high-flow channels were excavated to better connect the channel and floodplain
- Ten boulder and large wood in-stream habitat structures were installed
- Over 8500 square yards of filter fabric were installed to deter invasive species re-growth
Planting sites were irrigated as needed, particularly at the Cricket Pitch, in the summer 2005. Invasive Japanese knotweed was removed mechanically throughout the planting areas in 2005, and is still being removed throughout the planted area, as it re-grows, by the contractor in 2006. Beginning in 2007, Parks will have responsibility for invasive species control in the planting areas.
The project received a design award by the Connecticut chapter of the American Society of Landscape Architects.
On the upland slopes surrounding the Bronx River floodplain, NRG foresters have worked to reduce exotic invasive trees, shrubs and herbs such as cork tree (Phellodendron amurense), tree-of-heaven (Ailanthus altissima), tartarian honey suckle (Lonicera tartarica), mugwort (Artemisia vulgaris), Japanese honeysuckle (Lonicera japonica), garlic mustard (Alliaria petiolata), and Japanese knotweed (Polygonum cuspidatum) through chemical and mechanical means. They have combated exotic invasive herbaceous understory vegetation through mechanical removal, and restored the slopes by placing seed, mulch, and erosion control fabric. In 2002, Foresters planted hundreds of native herbaceous and graminoid species, such as white snakeroot (Eupatorium rugosum) and Pennsylvania sedge (Carex pensylvanica) in containers through the erosion control fabric. Invasive species control work in these areas continues as needed and as staffing allows. Several monitoring plots were installed in the upland hillslope restoration sites, but were not re-sampled this year.
Vegetation, birds, channel morphology, and in-stream habitat were monitored following the completion of the project. The post project monitoring reports can be downloaded for 2006 (PDF, 493KB) and 2007 (PDF, 613KB).
Vegetation monitoring was conducted to evaluate the success of the plantings (percent survival) and impact of the plantings on plant species diversity, cover, and invasive plant dominance in the forest floodplain. Changes in the breeding bird population over the course of the Bronx Forest restoration were measured using a breeding bird census. Channel morphology monitoring was used to determine if significant bank erosion, bank deposition, pool deepening, or pool in-filling is occurring. In-stream habitat was measured using several parameters impact habitat and could be effected by local in-stream structures and improved management, including large woody debris, instream cover, pools, bank condition and riparian zone/canopy.