Bronx River Forest and Floodplain Restoration - Bond Act Project

1-Year Post Project Monitoring and Status Report

Prepared for New York State Department of Protection

by

City of Department of Parks & Recreation, Natural Resources Group

DRAFT July 2006

Authors

Marielle Anzelone, Vegetation Monitoring

Marit Larson, Fluvial Geomorphologist and Project Manager

Ellen Pehek, Bird Monitoring

CONTENTS

1. INTRODUCTION

2. SITE DESCRIPTION

3. PROJECT STATUS

3.1 Floodplain and channel

3.2 Upland Forest

4. MONITORING

4.1. Vegetation

4.2. Birds
4.3. Channel Morphology

4.4. In-stream Habitat

5. RESULTS AND DISCUSSION

TABLES AND MAPS

Table 1. Plants installed and observed mortality

Table 2. Bird Species, Reproductive Phase, Observation

Table 3. Bird Species and Location

Table 4. Channel cross-sections

MAPS

-Project area and schematic design

-Vegetation Monitoring Plots

-Pre- and Post Channel Cross-section locations

-Extent of Knotweed: 1999 and 2005 entitation

-Bronx River Forest Natural Areas Entitation

-Planting Sites

Bronx River Forest and Floodplain Restoration - Bond Act Project

As-Built Monitoring and Status Report

1. INTRODUCTION

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.

The monitoring described in this report is aimed at evaluating the success of the native plantings and invasive species control measures, and the impact of the rehabilitation work on the vegetation community, breeding birds, and the channel configuration and stability.

2. SITE DESCRIPTION

The Bronx River Forest Bond Act Restoration Project is located in the in 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.

3. PROJECT STATUS

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 (approx. 4 acres)

-Over 16,000 native trees and shrubs and over 7800 herbs and grasses were installed

-Over 2000 ft of riverbank were planted with native woody plants and

-Excavating high-flow channels to better connect the channel and floodplain (approx. 1000 square yards)

-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.

Upland Forest

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.

4. MONITORING

4.1. Vegetation

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.

Methods Planting Success
Due to the large numbers of plants installed, a complete post-project count was not conducted. Instead, installed plants were counted in a sub-set of all the planting areas, dead plants were counted, and visual assessments were made of plant loss.
Methods for Cover and Species Richness

Eleven plots were established in 2002, each measured 3m x 3m: one reference wetland planting area plot (number 14); two control plots (numbers 19 and 20); and two reference plots (numbers 24 and 30). Six new floodplain plots were established in 2005 (number 40-43, 50, 51). The location of all plots was marked with wooden or rebar stakes, drawn on a map and GPSed.

During sampling, temporary stakes were used to mark all corners of the 3m x 3m plots and the meter tape was be strung around the plot. A 1m x 1m plot for herbaceous layer sampling was temporarily demarcated in the NE corner of the 3m x 3m plot. Relative percent cover was estimated at three distinct levels: herbaceous, shrub, and tree canopy. Percent cover at each level was visually estimated for each species found in the plot. At each level all species present as well as dominant species were noted.

Methods for Vegetation Communities

An inventory of the vegetation community that has been was conducted using entitation, a process of identifying and describing ecologically distinct plant communities, or entities. Entitation results in a map and database that can be used to identify where certain species are dominant or present, and to help prioritize areas that may be threatened, should be protected, or should be restored. This mapping can serve as a baseline for assessing change. The methods used will be available shortly to download from the City of New York Department of Parks and Recreation’s Natural Resources Group website.

4.2. Breeding birds

To assess changes occurring to the breeding bird population over the course of the Bronx Forest restoration, a breeding bird census was conducted, using spot-mapping techniques based on those employed by the Cornell Laboratory of Ornithology and the National Audubon Society (Robbins, 1970) (see Figure 1). No pre-project monitoring was conducted.

Methods

In 2005 we conducted an abbreviated breeding bird survey due to ongoing construction of boardwalks on the site. We performed three site visits between June 9 and July 6. Each visit began within an half an hour of sunrise, when bird vocalization peaks (Ralph et al. 1993) and lasted 1.5 to 3 hours.

To ensure birds were detectable, we did not conduct visits during moderate to severe precipitation or winds. During each visit, as we walked the census route, we recorded birds seen or heard on a survey map of the site. Species was indicated using the four letter USGS Bird Banding Codes. We also recorded breeding-related behaviors using symbols established by the British Trust for Ornithology (Bibby et al. 1992). We used these behavior registrations to delineate territories and to classify the breeding status of these territories as outlined below. We always walked the same census route, although we varied starting points to avoid surveying the same areas at the same time each morning. We did not actively search for nests during census visits.

We used the territory classification system formulated by the Natural Resources Group Salt Marsh Restoration (Brown & Alderson, 2001), which fuses the National Audubon Society system (Robbins, 1970) with the system developed by the New York Federation of Bird Clubs for the NYS Breeding Bird Atlas. Any “mapped territory” as defined by the Audubon Society constitutes a “Confirmed Breeding” status under the Breeding Bird Atlas system.

We classified each territory as a “Confirmed”, “Probable”, or “Possible” breeding territory according to the following guidelines.

CONFIRMED: To classify a territory as confirmed, we made at least one of the following observations for the species in question:

  • Singing bird within the territory area on at least three consecutive site visits (criterion for a “mapped territory” according to Robbins, 1970)
  • Active nest
  • Bird carrying fecal sac
  • Bird carrying food
  • Unfledged or recently fledged young
  • Distraction display or injury-feigning

PROBABLE: To classify a territory as probable, we made at least one of the following observations for the species in question:

  • Singing bird within the territory on more than one site visit
  • Pair in suitable breeding habitat
  • Chasing of conspecifics, agitated behavior or anxiety calls
  • Bird carrying nesting material or excavation of a nesting hole
  • Courtship and display

POSSIBLE: To classify a territory as possible, we made the following observation for the species in question:

  • Singing bird within suitable habitat on at least one site visit

For the purposes of data analysis, we divided the study site into five sections. See Figure 1 for delineations of these five sections. The thin strip of forest between the Bronx River and the Bronx River Parkway was divided into Parkway 1on the North and Parkway 2 to the South. The other three sections are the swamp forest (Swamp), the strip of mature forest with skunk cabbage, mayapple, and trout lily in the herbaceous layer that lies between the Bronx River and the ballfields (Skunk). and the island. For each section, we tallied the number of species holding possible, probable, and confirmed breeding territories. In addition, for the site as a whole, we calculated the percentage of breeding territories that were held by Red-winged blackbird, Gray catbird, Yellow warbler, and Song sparrow, four of the most common breeding species at disturbed wetland sites. For this calculation, we included only territories and species that qualified as probable or confirmed for breeding. We also calculated breeding species diversity using the Shannon-Weiner index (H’), for all probable and confirmed breeding territories. We calculated H’ for the restoration area and upland areas individually and also for the study site as a whole.

4.3 Channel morphology
Although historically the Bronx River was a naturally meandering channel, it has largely been straightened and confined and anchored in place by bridges, highways, railroads, and rock revetment. Channel morphology monitoring allows us to determine if significant bank erosion, bank deposition, pool deepening, or pool in-filling is occurring.
Methods

In the spring of 2006, one year after project completion, six permanent cross-sections were established in the Bronx Forest at locations where pre-project cross-sections had been surveyed for the purpose of developing a hydraulic model (HECRAS) for the design phase of the restoration project. The pre-project surveys were not permanently marked in the field, so approximate locations were used and the pre-and post-project cross-sections were qualitatively compared to assess whether significant differences in channel geometry could be detected. With the installation of permanent cross-section, we will be able to be accurately re-survey in consequent years to assess the degree of change in channel geometry, whether from erosion or sediment.