04 - Geology, Soils, and TopographyDEIS 4-1 9/12/2018
Chapter 4: Geology, Soils, and Topography
4.1. INTRODUCTION
This Chapter describes the Project Site’s existing geology, soils, and topography, and addresses potential impacts to on-Site surface and subsurface geologic resources. As such, these analyses
address the potential for the Proposed Action to have one or more significant adverse environmental impacts that were identified in the Lead Agency’s Positive Declaration (see Appendix A-5). Bedrock geology, surface soils, and steep slopes are described based on Site-specific topographic surveys data from the Natural Resources Conservation Service (NRCS) and the New York State Museum.1 Potential impacts to these resources are based on the potential for the Proposed Project to cause soil erosion or to impact geologic resources or groundwater resources as a result of cut-and-fill activities during construction.
The majority of the Project Site and associated Project disturbance is to the “Urban Land” soil type. The subsurface geology of the Site has been investigated and documented in a Preliminary Geotechnical Engineering Report (see Appendix I). Existing soil and rock conditions of the Site are documented in that report and construction of the Proposed Project would be in accordance
with the recommendations of the Preliminary Geotechnical Engineering Report. Elevated groundwater levels were found in one area of the Site, potentially indicating perched water. However, as described below, construction measures of the Proposed Project would
accommodate this condition and would not adversely affect groundwater conditions. With the implementation of a Village of Rye Brook (the “Village”) approved Erosion and Sediment Control Plan (ESCP), adverse impacts from the disturbance of steep slopes and on-Site soil
would be avoided and additional mitigation measures would not be required.
4.2. EXISTING CONDITIONS
4.2.1. SOILS
AKRF, Inc. consulted the United States Department of Agriculture’s NRCS Soil Survey to identify the soil mapping units within the Project Site. Different soil units exhibit variable water storage, erosion potential, and other characteristics that could impact
development. Table 4-1 contains a list of the soil mapping units located within the Project Site and their primary characteristics. The spatial arrangement of these soil types, as mapped by the NRCS Soils Survey of Putnam and Westchester Counties (1994), is shown in Figure 4-1.
1 The New York State Museum publishes surveys of bedrock and surficial geology.
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Table 4-1 On-Site Soils
Symbol Soil Series Name Percent of Site Depth to Restrictive Layer Drainage Characteristics
ChB
Charlton Fine Sandy loam, 3 to 8 percent slopes
<1% More than 80 Inches
Drainage class: Well drained Depth to water table: More than 80 inches Permeability: Moderately slow to rapid (0.14 to 14.17 in/hr) Surface runoff: Low Water capacity: Moderate Hydrologic group: B
PnB
Paxton fine sandy loam, 3 to 8 percent slopes
6% 18 to 39 inches to bedrock
Drainage class: Well drained Depth to water table: about 18 to 37 inches Permeability: Very slow to moderately slow (0.00 to 0.14 in/hr) Surface runoff: Medium Water capacity: Low Hydrologic group: C
PnC
Paxton fine sandy loam, 8 to15 percent slopes
<1% 20 to 39 inches to bedrock
Drainage class: Well drained Depth to water table: about 18 to 37 inches Permeability: Very slow to moderately slow (0.00 to 0.14 in/hr) Surface runoff: Medium Water capacity: Low Hydrologic group: C
Uf Urban land 78% N/A* N/A*
UhB
Urban land-Charlton complex, 2 to 8 percent slopes
2% More than 80 inches
Drainage class: Well drained Depth to water table: More than 80 inches Permeability: Moderate or moderately rapid (0.57 to 5.95 in/hr) Water capacity: Moderate Hydrologic group: B
UhC
Urban land-Charlton complex, 2 to 8 percent slopes
6% More than 80 inches
Drainage class: Well drained Depth to water table: More than 80 inches Permeability: Moderate or moderately rapid (0.57 to 5.95 in/hr) Water capacity: Moderate Hydrologic group: C
WdB Woodbridge loam, 3 to 8 percent slopes 6% 20 to 39 inches to bedrock
Drainage class: Moderately well drained. Depth to water table: More than 80 inches Permeability: Slow to moderately slow (0.00 to 0.14 in/hr) Surface runoff: Very rapid Water capacity: Low Hydrologic group: C
Note: *The variability of “Uf: Urban land” soil mapping units prevents the NRCS from publishing most soil parameters, including drainage class, hydrologic soil group, etc. Source: NRCS Soil Survey of Putnam and Westchester Counties, New York.
4.2.2. TOPOGRAPHY
The Project Site is dominated by the gently sloping surface parking lot in the eastern and northern portion of the Site. The Project Site ranges in elevation (el.) from a low point of
approximately 220 feet at the southeastern corner (Arbor Drive and King Street) to approximately 276 feet at the northwestern corner of the property (see Figure 4-2). The center of the Site contains an abrupt, likely human-made, rise in elevation from the
relatively flat parking area to the east (~el. 246), to a higher plateau (~el. 260) to the west. This rise contains areas of steep slopes (see Figure 4-3). Areas of steep slopes also occur on the eastern portion of the Site around the existing stormwater basin (i.e.,
Wetland D) and Stream S to its south, the southern portion of the wooded area between the existing Site building and The Arbors, the northern boundary of the Site, and the Site’s frontage along King Street.
The Village regulates steep slopes in Chapter 213 of the Village Code. Steep slopes are defined as a topographical gradient of 15 percent or greater with a minimum area of 100 square feet (sf). Steep slopes are further categorized as a moderately steep slope (slope
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equal to or greater than 15 percent, but less than 25 percent), very steep slope (slope
equal to or greater than 25 percent, but less than 35 percent), and extremely steep slopes (slope equal to or greater than 35 percent) (§213-2 of the Village Code). As shown in Table 4-2, 85.9 percent of the slopes on-Site are not considered to be steep slopes as
defined by Chapter 213 of the Village Code.
Table 4-2 Steep Slopes Analysis
Slopes Category Total Area Percent of Project Site1 Area of Disturbance
<15% 668,842 sf (15.35 acres) 85.9% 530,125.20 sf (12.17 acres) Moderately Steep Slope (15–25%) 58,554 sf (1.34 acres) 7.7% 19,166.40 sf (0.44 acres)
Very Steep Slope (25–35%) 29,885 sf (0.69 acres) 4.0% 18,295.20 sf (0.42 acres)
Extremely Steep Slope (>35%) 18,304 sf (0.42 acres) 2.4% 6,098.40 sf (0.14 acres)
Note: 1 Numbers may not add due to rounding. Sources: JMC Engineering; Village Code Chapter 213.
The existing topography of the Site is the result of several periods of development that shaped the land. As shown in Figures 4-4a, 4-4b, 4-4c, 4-4d, and 4-4e in 1934 the
Project Site was largely covered in trees and foliage with a relatively small building on its eastern edge. Over time, as shown in Figures 4-4a, 4-4b, 4-4c, 4-4d, and 4-4e farmland surrounding the Project Site was graded and developed with roadways and single-family housing (see aerial imagery from 1963). By 1974, the property adjacent and to the southeast of the Project Site was cleared of vegetation and graded to allow for the construction of the Blind Brook Middle School and associated parking lot. By 1985, the land to the southwest of the Project Site was cleared of existing vegetation, graded, and developed into The Arbors townhome community. Also in 1985, the Blind Brook Middle School property to the southeast of the project site continued to be cleared of vegetation and developed with sports fields. The 1985 aerial also shows the complete redevelopment of the Project Site, including removal of most of the remaining
vegetation within the center of the Site and grading the area in the center and east of the Site for the purpose of developing a parking lot. By 2006, Blind Brook Middle School had expanded its playing fields to the east by clearing vegetation and grading the fields.
4.2.3. SUBSURFACE SOIL AND GEOLOGY
This section is a summary of the Preliminary Geotechnical Engineering Report that presents the results of the geotechnical field exploration of the Project Site completed by AKRF, Inc. in November 2017 (see Appendix I). A total of 13 geotechnical borings (i.e., PB-1, PB-2, etc.) were drilled on-Site (see Figure 4-1) and two temporary groundwater observation wells (i.e., OW-1 and OW-2) were installed in completed boreholes PB-8 and PB-4.
An approximately 4- to 5-inch thick layer of topsoil consisting of sand and silt was encountered in borings PB-1 through PB-3, PB-6, and PB-11 and is characterized by the presence of roots. Asphalt measuring approximately 4 inches thick was encountered in borings PB-4, PB-5, and PB-7 through PB-13. Uncontrolled fill was encountered beneath the topsoil at discrete locations and generally consists of brown to grey sand with varying amounts of gravel, silt, and other miscellaneous fill such as brick. The fill
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is characterized as very loose to very dense, and ranged in thickness between
approximately 4 and 9 feet. Brown sand was encountered in all borings below the topsoil, asphalt, and fill layers, and ranged in thickness from approximately 9 to 31 feet.
Soft rock2 was encountered in all borings, except boring PB-1, and consisted of
brownish, orange and black, fine- to medium-grained sand with varying amounts of silt and mica. The top of the soft rock was encountered at depths ranging from 9 feet to 35 feet below existing grade. The soft rock extended to the boring termination depths,
except in borings PB-1, PB-4, PB-6, PB-8, and PB-13.
Foliated rock was encountered in five of the borings to boring termination depths. The depths to the top of the foliated rock ranged from approximately 11 feet below existing grade in PB-13 and 44 feet below existing grade in PB-6, corresponding to an elevation of approximately el. +232 and +211, respectively. The foliated rock generally consisted of gray to black schistic gneiss of the Harrison Gneiss Formation and is intensely to moderately fractured and fresh to slightly weathered.3
Groundwater levels were observed at three wells (OW-1, OW-2, and OW-2x). OW-1 and OW-2 are identified on Figure 4-1, and the third is approximately 10 feet southwest of OW-2.4 The water level at OW-1 was observed to stabilize at a depth of approximately 6 feet below existing ground surface corresponding to an elevation of approximately el.
+239. The water level at OW-2 was observed to stabilize at a depth of approximately 2 feet below ground surface, corresponding to an elevation of approximately el. +245. Considering the high water level in OW-2 with respect to OW-1, it is likely that there is a
possibility of perched water flowing beneath the asphalt into OW-2, causing water level readings to be erroneous in OW-2. As such, monitoring well OW-2x was installed to provide supplemental geotechnical data. Similar readings were found in OW-2x that were
found in OW-2. Considering the high water level in OW-2 with respect to OW-1, there is a possibility of perched water flowing beneath the asphalt into OW-2.
4.2.4. OTHER GEOLOGIC FEATURES
As noted by the EAF Mapper results, the Project Site contains no known unique geologic resources (see Appendix D-1). Therefore, the Proposed Project would not have an adverse impact on unique geologic resources.
4.3. POTENTIAL IMPACTS OF THE PROPOSED PROJECT
The analyses below address the potential for the Proposed Action to have one or more significant adverse environmental impacts that were identified in the Lead Agency’s Positive Declaration (see Appendix A-5).
As previously discussed, the Proposed Project was designed to focus disturbance within the Site to areas previously disturbed by development to the maximum extent practicable; that is, the
2 “Soft rock” is a general term used to describe sedimentary rocks, which are fairly soft and can break apart or crumble easily.
3 As shown in Figure 3 of the Preliminary Geotechnical Report, the Geologic Map of New York shows that the bedrock underlying the site is comprised of the Harrison Gneiss Formation.
4 OW-2x was installed during the Phase II Environmental Site Assessment, as described in Chapter 15, “Hazardous Materials.”
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Proposed Project seeks to redevelop that portion of the Site that was previously graded and on
which a building and parking lot were constructed. As such, the Proposed Project seeks to avoid, or minimize, impacts to mature vegetation, native soils, and native topography to the maximum extent practicable. For example, the soils proposed to be disturbed are classified as “Urban Fill,”
based on the fact that they had previously been disturbed through mass grading and building construction, and the vast majority of the steep slopes proposed to be disturbed are human-made steep slopes, including those around the existing stormwater basin on the site’s eastern edge and
the vegetated slope towards the Site’s western edge. As demonstrated in this Chapter, as well as in Chapter 5, “Waters and Wetlands,” Chapter 6, “Stormwater,” and Chapter 7, “Vegetation and Wildlife,” the impacts of the disturbance to the Site’s soils and topography are not, in the Applicant’s opinion, anticipated to have a significant adverse environmental impact to other environmental features, including to wetlands, waterbodies, stormwater, or mature vegetation.
4.3.1. SOILS
The Proposed Project would result in the disturbance of the Urban land (Uf) soil type, with a small amount of disturbance to the Paxton fine sandy loam (PnB) soil type in the northeast corner of the Site as shown in Figure 4-1.
The grading for the Proposed Project was designed to create a relatively level building pad, which is necessary for the proper functioning of an age-restricted residential
community. As such, the western portion of the Site, with the exception of the area within the footprint of the existing building, is proposed to be “lowered” in elevation through the removal, or cut, of approximately 42,600 cubic yards of earthen material and
the central portion of the Site, with the exception of the area where the underground parking is proposed, is proposed to be “raised” in elevation through the fill of approximately 51,600 cubic yards of earthen material (see Table 4-3). The grading plan allows for the creation of underground parking, which will be screened from view and which allows for the amount of impervious surface coverage on the Site to be reduced from its current condition. Material excavated from the Site would be used as fill material, where appropriate. Unsuitable or excess earthen material would be removed from the Site by truck to an appropriate receiving facility. Similarly, additional material required for “fill” would be obtained from an appropriate facility.
Table 4-3 Cut-and-Fill Analysis
Total Cut (cubic yards) Total Fill (cubic yards) Net Cut-and-Fill (cubic yards)
±42,600 ±51,600 ±9,000 net import
Source: JMC Engineering.
As described in Chapter 7, “Vegetation and Wildlife,” the grading of the Project Site would result in a temporary loss of habitat for species that use highly fertilized, mowed lawn as the dominant habitat. Immediately adjacent to the Project Site is a similar
habitat that will be available to wildlife. No species of special concern were found on-Site. After construction, the Proposed Project would result in a net increase in the amount of lawn habitat for existing wildlife.
To reduce the potential for erosion of soils during construction and protect from loss of mature vegetation, a Stormwater Pollution Prevention Plan (SWPPP) and ESCP have been prepared (see Appendix C-1). The preliminary SWPPP is described in Chapter 6,
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“Stormwater Management,” and the preliminary ESCP is described in Chapter 16,
“Construction.” These plans include measures to prevent untreated stormwater runoff or sediments from leaving the Project Site during construction. These measures include the installation of stabilized truck entrances, silt fencing, inlet protection, and a temporary
sediment basin. Prior to final Site Plan approval, the Village will review and approve the final SWPPP and ESCP to ensure compliance with state and local regulations. With the implementation of the Village-approved SWPPP and ESCP, the Proposed Project would
not be expected to have a significant adverse impact to on-Site soils.
4.3.2. TOPOGRAPHY
The Proposed Project would require approximately 13.17 acres of disturbance to the Site, much if not all of which has been previously disturbed. Of that disturbance, approximately 12.20 acres, or 93 percent, would occur within areas having a slope of less than 15 percent. Areas of steep slope impacted are primarily associated with the human-made slope in the center of the Site. Areas of steep slopes along the northern, western, and southeastern portion of the Project Site have been avoided.
The grading of the Proposed Project was designed to create a relatively level building pad, which is necessary for the proper functioning of an age-restricted residential community. As such, the western portion of the Site, with the exception of the area within the footprint
of the existing building, is proposed to be “lowered” in elevation and the central portion of the Site, with the exception of the area where the underground parking is proposed, is proposed to be “raised” in elevation. As shown in Figure 4-5, areas of steep slope in the
proposed condition that would be similar to the existing condition would occur on the eastern portion of the Site around the existing stormwater basin (i.e., Wetland D) and Stream S to its south, the southern portion of the wooded area between the existing Site building and The Arbors, the northern boundary of the Site, and the Site’s frontage along King Street. New steep slope areas in the proposed condition would be on the western side of the proposed IL building on either side of the entrance drive to the sub-surface parking garage, on the western portion of the site to the rear of the townhomes, and on the eastern side of the proposed IL building between the building and the proposed interior drive. Table 4-4 summarizes the steep slopes in the proposed condition.
Table 4-4 Proposed Steep Slopes
Slopes Category Total Area Percent of Project Site1 Net Change from Existing Condition
<15% 637,736 sf (14.64 acres) 82.4 -31,106
Moderately Steep Slope (15–25%) 69,518 sf (1.60 acres) 9.0 +10,964
Very Steep Slope (25–35%) 45,641 sf (1.05 acres) 5.9 +15,756
Extremely Steep Slope (>35%) 22,690 sf (0.52 acres) 2.9 +4,386
Note: 1 Numbers may not add due to rounding. Sources: JMC Engineering; Village Code Chapter 213.
As described above, the Proposed Project includes a SWPPP and ESCP. The preliminary SWPPP is described in Chapter 6, “Stormwater Management,” and the preliminary
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ESCP is described in Chapter 16, “Construction.” These plans include measures to
minimize the erosion potential resulting from the disturbance of steep slopes. These measures include minimizing steep slope impacts to the greatest extent possible and stabilizing soil on slopes with slope stabilization erosion control measures. With the
implementation of these measures, the Proposed Project would not be expected to have a significant adverse impact to steep slopes.
Because construction of the Proposed Project would require disturbance to 0.97 acres of
slopes in excess of 15 percent, the Proposed Project would require a Steep Slope Work Permit pursuant to Chapter 213 of the Village Code. The Planning Board would be the approval authority for the permit (§213-7B) and a public hearing would be required (§213-10D) prior to final Site Plan approval. The Proposed Project would meet the requirements for granting a Steep Slope Work Permit, as defined in §213-10C. Specifically:
• The steep slope disturbance can be completed without increasing the possibility of creep or sudden slope failure and would minimize the potential for erosion to the maximum extent practicable through the implementation of the previously described
ESCP (§213-10C(2));
• The steep slope disturbance would not adversely affect existing wetlands, waterbodies,
or watercourses as the potential for increased sedimentation would be avoided through proper implementation of the Village-approved ESCP (§213-10C(3));
• The steep slope disturbance would not affect any wells or sewage disposal systems, nor would it adversely impact any threatened or endangered species as none are known to occur on or adjacent to the Project Site (§213-10C(4));
• The steep slope disturbance, and the Proposed Project generally, is compatible with public health and welfare (§213-10C(5)), as demonstrated throughout this Draft Environmental Impact Statement (DEIS); and,
• The steep slope disturbance has been minimized to the maximum extent practical while balancing the need to avoid impacts to the Site’s wetlands (§213-10C(6)).
4.3.3. CONSTRUCTION OF SUBSURFACE STRUCTURE
Two subsurface infiltration practices would be installed to improve the water quality and the water quantity of the runoff coming from the Project Site. As described in Chapter 6, “Stormwater Management,” the bottom of infiltration basin 1A-2 and infiltration basin 1A-3 are at elevations 238.75 and 239.70, respectively. The practices proposed require a
3 foot buffer between the bottom of the practice and rock or groundwater. According to geotechnical borings described earlier in this chapter, the highest rock elevation is at 238.50 while the lowest rock elevation is at 220.00. Most rock elevations observed are below the 235.00 and 235.95 elevations needed for infiltration basin 1A-2 and 1A-3, respectively. As such, it is not anticipated that rock blasting would be required to accommodate the construction of the subsurface infiltration systems. Accordingly, it is
not anticipated that construction of the Proposed Project would require materials processing (i.e., rock crushing) on-Site.
If materials processing is required during construction, a permit would be obtained from the Westchester County Department of Health (DOH). According to the Westchester County DOH Air Quality Program, the minimum permit filing requirements needed in order to operate an approved portable rock crusher and screener on-Site are as follows:
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• a licensed and New York State-registered Professional Engineer must prepare the applications;
• an application fee of $4,000 consisting of a one-time $1,000 processing fee and $1,000 operation permit fee for each year of the 3-year operation cycle;
• an authorization letter signed by the owner or owner’s representative authorizing the
Professional Engineer to file applications and plans on the owner’s behalf;
• a notarized Certificate of Resolution form;
• an Environmental Assessment Form;
• calculations of air emissions generated from power generator, rock crusher(s), and
screener(s);
• proof of Workers’ Compensation and Disability Benefits Insurance; and,
• one completed copy of the Westchester County DOH Application for Certificate to Construct and Operate (Form R).
• Air quality monitoring is not required to receive this permit from the County.
The subsurface parking garage would be constructed to an elevation of 242.50, which is approximately 4 feet below the finished floor of the existing office building. If groundwater is encountered during construction of the parking garage, an underdrain system will be designed and implemented to divert groundwater away from the structure. The underdrain system would daylight into the existing stream at the southeastern portion of the Site.
With respect to the potential impacts of the Proposed Project on groundwater, as
described in Chapter 5, “Waters and Wetlands,” it is important to note that neither the Project Site, nor nearby parcels, utilize groundwater for their operations. Instead, the Project Site and adjacent properties utilize a municipal water system. The Proposed
Project would reduce the amount of impervious coverage on the Project Site, which would enhance groundwater infiltration that helps to sustain hydrology and improve water quality to downstream wetlands and streams during dry weather.
Based on these data and the foregoing analyses, it is the Applicant’s opinion that the Project Site is suitable for the Proposed Project.
4.4. MITIGATION MEASURES
In the Applicant’s opinion, and based on the foregoing analyses, the Proposed Project is not anticipated to have a significant adverse impact on geology, soils, or topography. As described above, the Proposed Project includes an ESCP to avoid and/or mitigate impacts associated with
the disturbance of on-Site soils and steep slopes during construction. Construction of subsurface structures, in the manner described above, would not have an adverse impact on the Site’s geology or groundwater. As such, no additional mitigation measures are required.
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Figure 4-1
Existing Soil Conditions
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Aerial Images of the Project Site Over Time
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Figure 4-4b900 KING STREET
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Figure 4-4c900 KING STREET
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Figure 4-4d900 KING STREET
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