2008_Watershed_Study.pdf

TABLE OF CONTENTS Tighe&Bond i

LETTER OF TRANSMITTAL

SECTION 1 PROJECT DESCRIPTION

SECTION 2 EXISTING WATERSHED AND STORM DRAIN SYSTEM

DESCRIPTION

SECTION 3 HYDROLOGIC AND HYDRAULIC COMPUTER MODELING

SECTION 4 EXISTING PROBLEM AREAS

SECTION 5 PROPOSED IMPROVEMENT ALTERNATIVES

APPENDIX A TABLES

A-1 SUMMARY OF EXISTING CONDITIONS PEAK FLOWS

A-2 SUMMARY OF PROBLEM AREAS AND PRIMARY CAUSES

A-3 SUMMARY OF PROPOSED IMPROVEMENT ALTERNATIVES

A-4 CONCEPTUAL LEVEL CONSTRUCTION COSTS

(PROJECTED TO JUNE 2009)

APPENDIX B PLAN SHEETS

B-1 WATERSHED PLAN

B-2 EXISTING PROBLEM AREAS PLAN

B-3 PROPOSED IMPROVEMENT ALTERNATIVES PLAN

J:\M\M0432\REPORT\Manchester Report Rev 2-20-08.doc

SECTION 1 PROJECT DESCRIPTION Tighe&Bond

Dry Brook Culvert Hydrologic and Hydraulic Study, Manchester, CT 1-1

Background

Dry Brook is a watercourse that drains a 612-acre urbanized watershed including the central business district in Manchester, Connecticut. The entire brook and most of its tributaries are enclosed in a piped collection system that includes large diameter pipes and box culverts. The system’s main trunk line is known as the Dry Brook Culvert and principally runs southerly, to the east of Main Street, before it discharges to Hop Brook which flows westerly along the north side of Interstate 384.

Many areas within the Dry Brook watershed experience flooding problems which result in street hazards, property damage, and neighborhood nuisances. The Town desires to implement improvement alternatives that will alleviate these problems.

Purpose

The purpose of this study is to identify the frequency and magnitude of the flooding throughout the watershed and to evaluate improvement alternatives to alleviate the problems. The evaluation for each improvement alternative will consider the magnitude of the flooding problem to be addressed, the extent that the improvements will correct the flooding problem, permitting requirements, property acquisition requirements, and the approximate cost to construct the improvements.

Approach

The approach to this study involved the following tasks:

• Assemble Available Information - Available information including the Town’s GIS mapping and storm system survey data was reviewed for consistency and completeness. GIS data within the project area that was identified as missing or incomplete was subsequently addressed by limited field checks and a review of existing data performed by the Town of Manchester Engineering Division.

Critical data in flooding problem areas was also spot checked by the Town.

Other information reviewed included soil surveys, Federal Emergency Management Agency (FEMA) flood studies and maps, and previous drainage studies for Dry Brook and its watershed.

• Field Investigation of Hydrologic Conditions – The Dry Brook Culvert Watershed was visited to compare the actual existing conditions with the assembled information and to verify land use and area delineations to catch basins within the watershed.

• Evaluate Existing Conditions / Identify Problem Areas - Using the field-verified information, a hydrologic / hydraulic computer model of the Dry Brook Culvert Watershed was developed to simulate storm runoff flows throughout the

SECTION 1 PROJECT DESCRIPTION Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 1-2 collection system. Storms of various frequency and magnitude were simulated to identify locations within the collection system that have hydraulic deficiencies and result in flooding. The model results were compared to historic observations from previous studies and the Department of Public Works staff’s knowledge. Areas with serious and prevalent flooding problems were identified.

• Identify and Evaluate Improvement Alternatives – Using the computer model, improvement alternatives were evaluated to include: replacing existing pipes/conduits with those of larger size, installing new pipes along streets that currently have none, adding additional catch basins along streets, and modifying the existing stormwater detention facility. The evaluation also included an estimate of order of magnitude cost of construction and prioritization based on input provided by the Department of Public Works.

• Preparation of a Summary Report – A summary report was prepared to include a narrative, tabular summaries, and plan exhibits to summarize the project description, the study approach, and findings.

This report is the culmination of these activities. Upon approval of the final report by the Town, the hydrologic / hydraulic computer model depicting both the existing conditions and the proposed alternatives will be provided to the Town.

SECTION 2 EXISTING WATERSHED AND

STORM DRAIN SYSTEM DESCRIPTION Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 2-1

The 612-acre Dry Brook watershed in Manchester, Connecticut is generally bound by Main Street to the west, East Center Street to the north, Pitkin Street and Autumn Street to the east, and Wells Street and Hartford Road to the south with some outlying areas. The existing closed piped collection system extends to most streets and areas within the watershed and includes approximately 12 miles of pipes/culverts, 740 structures (catch basins, manholes, junction chambers, and drop inlets), and one regional detention basin within East Cemetery.

Land use within the watershed is predominately comprised of ¼ to ½-acre residential lots. Other notable land uses include the densely developed central business district area along Main Street, and East Cemetery which is located in the north and northeast part of the watershed.

According to the United States Department of Agriculture Natural Resources Conservation Service (USDA-NRCS) Web Soil Survey, underlying soils within the watershed are predominately hydrologic soil group “A” soils (90%) which consist of highly permeable sands and gravels, and therefore have very low runoff rates unless covered by an impervious surface. The remaining 10% of the underlying soils are hydrologic soil group “B” soils which consist of somewhat permeable materials, and have relatively low runoff rates unless covered by an impervious surface.

For descriptive purposes, the overall watershed has been divided into 10 subwatersheds. The subwatersheds were delineated based on the tributary area corresponding to the trunk line pipes within the collection system. Refer to Appendix B-1 for the Watershed Plan. A description of each subwatershed is as follows:

Main Street Subwatershed (123 Acres) – This subwatershed includes the Dry Brook Culvert itself which flows in a southerly direction from Bissell Street to the system outfall located south of Hartford Road near the on-ramp to I-384 West. Most of the central business district is located within this subwatershed and generates relatively high runoff rates due to a high percentage of impervious cover.

The upstream end of the Dry Brook Culvert is considered to be located at a point approximately 400 feet east of the Main Street / Bissell Street intersection that receives storm flows from the Northwest Subwatershed. From this point, the Dry Brook Culvert conveys flows beneath parking lots to the rear of Main Street through a series of box culverts with various dimensions (2.5’x5’ and 1.7’x7’ box culvert). At Birch Street, the Dry Brook Culvert changes to a 2.5’x6’ box culvert and continues to a junction chamber approximately 500 feet south of Birch Street. The Dry Brook Culvert along this section is at a relatively flat slope (less than 0.2%) and has very shallow cover (less than 2 feet)

SECTION 2 EXISTING WATERSHED AND STORM DRAIN SYSTEM DESCRIPTION Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 2-2

From the junction chamber, the Dry Brook Culvert increases to a 3.2’x9’ box culvert continuing southward toward Oak Street for approximately 300 feet. From Oak Street, it increases to a 3.2’x10’ box culvert to Maple Street. The Dry Brook Culvert along this section is slightly less flat (ranging between 0.5% and 0.7%) but still has very shallow cover (less than 2 feet).

At Maple Street, the Dry Brook Culvert drops approximately 6 feet and increases to a 6’x7’ box culvert that flows west down Maple Street to Main Street, and then south down Main Street to eventually discharge at the outfall. This section of the culvert was installed in the early 1990’s and replaced the older box culvert downstream as the trunk line carrying flows from upstream.

Except where it was disconnected at Maple Street, the older box culvert, which varies in size between 3.2’x10’ and 4’x11’, still remains to convey runoff flows from portions of the southern Main Street Subwatershed. The older box culvert crosses Eldridge Street, School Street, and Wells Street, and runs along Main Street where the flow splits at a junction chamber. A portion of the flows are conveyed by a 48” diameter pipe to the Hop Brook immediately to the east of South Main Street, and the other portion flows westerly through a 4’x6’ box culvert to a 48” diameter pipe where it discharges at Hop Brook approximately 1,500’ west of the 6’x7’ box culvert. Please refer to the attached Sheets 1-3 in Appendix B for additional information.

Northwest Subwatershed (47 Acres) - This subwatershed includes the Main Street area between Park Street and Center Street, and side streets including Madison Street, Myrtle Street and Locust Street. Land use in this area includes some open space, ¼-acre to ½-acre residential lots, and the central business district. Storm flows are collected from side streets by the trunk line within Main Street and are discharged to the Dry Brook Culvert trunk line as a 24” diameter pipe at a point approximately 400 feet east of the Main Street / Bissell Street intersection.

North Subwatershed (94 Acres) - This subwatershed includes Spruce Street north of Florence Street, as well as Hamlin Street, Holl Street, Harrison Street, and the streets between. Land use in this area is predominately ¼-acre to ½-acre residential lots with the East Cemetery covering the east portion of the subwatershed. Storm flows are collected by the trunk line which zigzags southwesterly along Harrison Street, Pearl Street, and then cross-country to Spruce Street. The trunk line, which continues southerly along Spruce Street, consists of two pipes in parallel (24” and 27” diameter pipes). The North Subwatershed trunk line combines with the Central Subwatershed trunk line at the Spruce Street / Florence Street intersection.

Northeast Subwatershed (142 Acres) - This subwatershed includes the neighborhood around Pitkin Street, Kensington Street, Glenwood Street, and the northern portions of Autumn Street and Grandview Street as well as portions of East Cemetery. Land use in

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 2-3 this area is predominately ½-acre or greater residential lots. Storm flows are collected from side streets by the trunk line that starts at Pitkin Street and zigzags southwesterly along Academy Street, cross-country to Kensington Street and Autumn Street. The trunk line, which conveys flow southwesterly through the East Cemetery, discharges as a 36” diameter pipe to a regional stormwater detention facility. The stormwater facility has approximately 10 feet of available storage between the top of the embankment and the permanent pool. The permanent pool has a surface area of approximately 0.25 acres. The discharge pipe from the stormwater detention facility’s control structure is a smaller, 24-inch diameter pipe which is the upper end of Central Subwatershed trunk line.

Central Subwatershed (43 Acres) - This subwatershed includes Florence Street and Clinton Street, and the residential block bounded by Cottage Street, Birch Street, Spruce Street and Oak Street. Land use in this area is predominately ¼-acre residential lots with some multi-family residential uses. Storm flows are collected by the 24” diameter trunk line from at the stormwater detention facility in the East Cemetery and directed westerly on Florence Street. The North Subwatershed trunk line combines with the Central Subwatershed trunk line at the Spruce Street / Florence Street intersection. The Central Subwatershed trunk line, which continues southerly along Spruce Street, consists of two pipes in parallel (24” and 27” in diameter) until it connects to the Oak Street Subwatershed trunk line at a point approximately 200 feet south of the Spruce Street / Florence Street intersection. The Central Subwatershed trunk line then continues cross-country in a westward direction as a 3’x5’ box culvert until it connects to a junction chamber on the Dry Brook Culvert in the municipal parking lot to the east of Main Street.

Oak Street Subwatershed (44 Acres) - This subwatershed includes Oak Street east of Spruce Street, Ashworth Street and portions of connecting side streets. Land use in this area is predominately ¼-acre residential lots. The trunk line starts on Ashworth Street, turns south on Autumn Street, and then turns westerly again on Oak Street to Spruce Street. At Spruce Street, the trunk line turns northerly and then connects to the Central Subwatershed trunk line at a point approximately 300 feet north of the Spruce Street/Oak Street intersection as a 24” diameter pipe.

Maple Street Subwatershed (21 Acres) - This subwatershed includes Maple Street east of Cottage Street. Land use in this area is predominately ¼-acre residential lots. The trunk line starts at the Maple Street / Spruce Street intersection and flows westerly. At a point approximately 200 feet west of the Maple Street / Cottage Street intersection, the trunk line connects to the Dry Brook Culvert as an 18” diameter pipe.

Eldridge Street / School Street Subwatershed (43 Acres) - This subwatershed includes Eldridge Street and School Street east of Spruce Street. Land use in this area is predominately ¼-acre residential lots. The trunk line starts at the School Street /

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 2-4

Norman Street intersection and flows westerly. At the School Street / Spruce Street intersection, the trunk line turns northerly on Spruce Street and then turns westerly on Eldridge Street. At a point approximately 200 feet east of the Main Street / Eldridge Street intersection, the trunk line connects to the Dry Brook Culvert as an 18” diameter pipe.

Wells Street Subwatershed (37 Acres) - This subwatershed includes Wells Street and Battista Road. Land use in this area is generally a combination of ¼-acre and ½-acre residential lots. The trunk line starts at the Wells Street / Ansaldi Road intersection and flows westerly on Wells Street. West of Vine Street, the trunk line consists of two pipes in parallel (18” and 36” diameter pipes) to the point of connection with the Dry Brook Culvert at the Main Street / Wells Street intersection.

Lindman Street Subwatershed (18 Acres) – This subwatershed generally includes the residential block bound by Glenwood Street, Oak Street, Autumn Street and School Street and includes Lindman Street, a dead end road. Most of the runoff from this watershed is directed to a large depression at the end of Lindman Street by sheet flow.

The only formal drainage system appears to be a small piped system at the Autumn Street / School Street that discharges to the depression as a 15” diameter pipe. There are no known outfalls from this depression. Therefore, there is no contribution of discharge from this subwatershed to the Dry Brook Culvert.

SECTION 3 HYDROLOGIC AND HYDRAULIC

COMPUTER MODELING Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 3-1

The SewerGEMS computer modeling program by Bentley Systems Inc. was used for this study to analyze the hydrology (storm runoff) of the watershed and the hydraulics (flow routing) of the collection system. The SewerGEMS program implements Technical Releases Nos. 20 & 55 stormwater modeling methodologies (TR-20 & TR-

55) which were developed by the United States Department of Agriculture Natural Resources Conservation Service (USDA-NRCS).

For any given storm event, the SewerGEMS program can generate runoff values from the multiple subwatersheds that comprise the entire watershed and route the flows through the collection system accordingly. The SewerGEMS program can identify flooding by simply running queries for surcharged pipes, manholes, catch basins, and drop inlets. The modeling program can also illustrate the results by showing the hydraulic grade line (i.e. the flowing water surface within the collection system) on the pipe profile. When the model depicts a location on the pipe profile where the hydraulic grade line exceeds the ground surface, it is understood that flooding occurs at that location.

By running queries, SewerGEMS can also readily identify catch basins and drop inlets that have incoming surface flows which exceed the grate capacity and result in street flooding or bypassing to the downgradient catch basin or drop inlet.

The hydrologic data used to develop the SewerGEMS model included:

• Aerial survey/topography from the Town of Manchester’s Geographic Information System (GIS) database. This information was used for determining watershed delineation areas, land uses and impervious cover, slopes and drainage travel paths.

• Soil data from the USDA-NRCS Web Soil Survey website. This information was used for identifying hydrologic soil groups, which combined with land use information, is used to determine storm runoff rates.

• Precipitation data based on intensity/duration/frequency (IDF) curves for Hartford County available from the Connecticut Department of Transportation (ConnDOT) Drainage Manual, 2000 Edition. This information was used to identify the various storm frequencies and magnitudes.

The hydraulic data used to develop the SewerGEMS model included:

• Catch basin and drain manhole geospatial locations provided by the Town of Manchester Engineering Department in a GIS format. This information was used to generate the horizontal and vertical pipe alignments in the model.

SECTION 3 HYDROLOGIC AND HYDRAULIC COMPUTER MODELING Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 3-2

• Pipe / conduit sizes, alignments, lengths, invert elevations, and materials also provided by the Town of Manchester Engineering Department in a GIS format.

This information was used to identify slopes and roughness coefficients for determining pipe / conduit capacities.

• Record drawings of major sections of the collection system including the Dry Brook Culvert and the East Cemetery stormwater detention facility. This information was used to verify some of the GIS pipe and structure data.

• Federal Emergency Management Agency (FEMA) flood studies and maps, and drainage reports entitled “Stormwater Management Program for the Dry Brook Drainage Area”, prepared by Metcalf & Eddy dated December 17, 1976, and “Stormwater Management Report, East Cemetery Expansion” prepared by the Department of Public Works Engineering Division dated October 19, 2001.

The information within these previous studies was used only to ascertain the degree of flooding and flow rates at certain locations within the watershed.

However, when given in these previous studies, the flow rates and flood elevation data did not necessarily correlate with the model results developed by this study since each of the previous studies used different hydrologic / hydraulic methodologies and modeling techniques.

If any pipes or other conduits are partially blocked by accumulated sediments or debris, the computer model can be adjusted to account for the reduced capacity. Since there has been no specific knowledge whether any such blockages exist, all computer models were simulated based on the assumption that the pipes and conduits throughout the collection system are clean and free flowing.

The rainfall data used for all storm simulations are based on a Type III 24-hour duration storm as defined in TR-55. Table 1 provides the precipitation amounts for various storms anticipated in Manchester, Connecticut (Hartford County) based on the ConnDOT Drainage Manual.

Frequency refers to the probable recurrence interval that a storm of a given magnitude or greater is anticipated to occur based on historical precipitation data. For example, a storm that produces 3.2 inches of precipitation or greater over a 24-hour duration will typically occur once every 2 years, or a 50% chance of occurrence annually.

Since these values are based on probability, it is possible that multiple 2-year frequency storm events could occur in the same year, or possibly in the same month. Conversely, five years may pass without a single storm that exceeds 3.2 inches of precipitation over a 24-hour duration. However over a 50-year period, it is likely that a 3.2 inch precipitation event or greater would occur approximately 25 times.

SECTION 3 HYDROLOGIC AND HYDRAULIC COMPUTER MODELING Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 3-3

TABLE 1

24-Hour Precipitation Amounts for Various Storms Manchester, Connecticut (Hartford County)

Storm Frequency Precipitation in Inches

1-year 2.5

2-year 3.2

5-year 4.1

10-year 4.7

25-year 5.5

50-year 6.2

100-year 6.9

Source : Chapter 6 /Appendix B - ConnDOT Drainage Manual, last revised May 2002

SECTION 4 EXISTING PROBLEM AREAS Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 4-1

Using the available information described in Section 3, the SewerGEMS program was used to prepare an Existing Conditions Model. Storms of various frequency and magnitude were simulated to identify locations within the collection system that have hydraulic deficiencies and result in flooding. Appendix A-1 - Summary of Existing Conditions Peak Flows presents peak flow values at several key locations for various storm events in tabular format.

Typically, storm drain collection systems are designed to accommodate the 10-year or 25-year storm. However, the Existing Conditions Model results indicated that a majority of the Dry Brook Watershed collection system would be surcharged during the 10-year storm event causing widespread flooding.

With the understanding that the majority of the system cannot be reasonably replaced for financial reasons, this evaluation focused on the smaller, more-frequent storms (e.g.

1-year and 2-year frequency storms) to identify discrete areas that have prevalent or severe problems.

A description of the location, the storm frequency at which the problem is evident (as determined by the model), and the primary cause of the problem at 12 discrete areas is summarized below. Appendix A-2 - Summary of Problem Areas and Primary Causes presents this information in tabular format. Appendix B-2 is the Existing Problem Areas Plan.

Problem Area A - The area above the upper section of the Dry Brook Culvert, especially the parking lot between Birch Street and Oak Street, starts to flood at approximately between the 1-year and 2-year storm event. Based on Existing Conditions Model results and anecdotal information from the Town and the public, this area has significant flooding issues beyond street and parking lot flooding. Adjacent houses, businesses and a church have experienced flooding during several storms in recent years. The primary cause of the problem is the limited available capacity of the Dry Brook Culvert (north of Maple Street) due to its small cross-sectional area and very flat slope.

Problem Area B – The area around Main Street at Bissell Street starts to flood at approximately the 2-year storm event. The problem is primarily caused by the single 24” diameter pipe on Bissell Street which discharges from this area, whereas there are four 24” diameter pipes and one 15” diameter pipe flowing into it at the junction chamber on Main Street. The other issue is related to Problem Area A, when the Dry Brook Culvert backs up and creates a tailwater, the pipe on Bissell Street also begins to back up.

Problem Area C – The area around Academy Street and Putnam Street starts to flood at approximately the 2-year storm event. This problem is caused by undersized sections

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 4-2 of the trunk line between Pitkin Street and Autumn Street. Another issue is the lack of catch basins on Pitkin Street between Academy Street and Boulder Road.

Problem Area D - The Maple Street area near Spruce Street starts to flood at approximately the 2-year storm event. The main reason is that the trunk line pipe is undersized to handle the additional flow from the adjacent Eldridge Street / School Street Subwatershed through a 15” cross connection overflow pipe. Another issue is the lack of catch basins on the east end of Maple Street between Spruce Street and Clinton Street. As a result, the sheet runoff along Maple Street exceeds the inlet capacity of the catch basins at the Spruce Street / Maple Street intersection and causes recurrent street flooding.

Problem Area E – The Eldridge Street and School Street area near Spruce Street starts to flood at approximately the 2-year storm event. The trunk line pipe is undersized.

The lack of catch basins on the east end of Eldridge Street results in sheet runoff along Eldridge Street that exceeds the inlet capacity of the catch basins at the Spruce Street / Eldridge Street intersection. The trunk line on Eldridge Street connects to the older Dry Brook Culvert lower section which has plenty of available capacity. Therefore, there are no tailwater effects from the Dry Brook Culvert.

Problem Area F - The area around East Center Street / Hamlin Street intersection starts to flood at approximately the 2-year storm event. This problem is caused by the undersized 15” diameter pipe along Hamlin Street.

Problem Area G - The area around the Wells Street / Spruce Street intersection starts to flood at approximately the 2-year storm event. This problem is caused by undersized sections of the 15” diameter pipe.

Problem Area H - The area around the Oak Street / Glenwood Street / Ashworth Street intersections starts to flood at approximately the 5-year storm event. This problem is caused by the undersized 12” diameter pipes along Ashworth Street and Glenwood Street.

Problem Area I - The area along Florence Street east of Holl Street starts to flood at approximately the 5-year storm event. The trunk line along Florence Street is undersized to convey the discharge from the East Cemetery Stormwater Detention Facility. Based on the model results, it has been concluded that the detention facility provides some flow attenuation for very large events (e.g. 100-year storm), but provides little flow attenuation for smaller storm events (e.g. 1-year storm to 10-year storm). This is generally due to the large weir (3’x3’) openings in the control structure that serve as a trash/debris grate rather than a flow control structure during the smaller storms. The cross-sectional area of the 24” diameter pipe discharging to Florence Street from the control structure actually acts as the flow restriction since it has significantly

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 4-3 less cross-sectional area than the weir openings. This conclusion is consistent with the findings in the “Stormwater Management Report, East Cemetery Expansion” prepared by the Department of Public Works Engineering Division dated October 19, 2001.

Problem Area J - The area in the Pearl Street / Harrison Street intersection and a portion of the adjacent East Cemetery starts to flood at approximately the 5-year storm event. This problem is caused by this section of the trunk line being undersized.

Problem Area K - The area around the Madison Street / Hawley Street intersection starts to flood at approximately the 2-year storm event due to the downstream trunk line being undersized. Additionally, field observations indicate that there are no catch basins located at the low spot along the Madison Street.

Problem Area L - The area around the Spruce Street / Pearl Street intersection experiences regular flooding, but this flooding is pronounced at approximately the 2-year storm event. An inadequate number of catch basins on the Spruce Street north of Pearl Street results in sheet runoff along Spruce Street that exceeds the inlet capacity of the catch basins at the Spruce Street / Pearl Street intersection.

SECTION 5 PROPOSED IMPROVEMENT ALTERNATIVESTighe&Bond

Dry Brook Culvert Hydrologic Model & Study Study, Manchester, CT 5-1

Potential improvement projects were developed by modeling various alternatives in SewerGEMS that increase the system capacity to alleviate the magnitude and frequency of flooding at the problem areas identified in Section 4.

Each of the improvement alternatives were conceptualized to convey a minimum 10-year storm event which would significantly reduce the magnitude of building and street flooding during larger storms. For example, flooding during the 25-year storm would be significantly reduced, but some flooding would still likely occur.

A description of the location, the general scope of work, and the phasing (if deemed appropriate) is provided for eight improvement alternatives summarized below.

Appendix A-3 - Summary of Proposed Improvement Alternatives presents this information in tabular format. Appendix A-4 presents the Conceptual Level Construction Costs for each improvement alternative and phase including a breakdown of trunk line pipe/conduit replacement sizes and other major work items projected as June 2009 dollars. Appendix B-3 is the Proposed Improvement Alternatives Plan.

Of note, the sizes, lengths and configurations of pipe sections described below are preliminary and would be further refined as the project advances to the design and construction phases.

Improvement Alternative 1 – This improvement alternative involves replacing the upper section of the Dry Brook Culvert from Bissell Street to Maple Street, and replacing the 24” diameter pipe on Bissell Street from Main Street. When 6’x7’ box culvert between Maple Street and the outfall was installed in the 1990s, the invert of the box culvert was lowered approximately 6 feet at Maple Street. The proposed improvements would use this available change in grade to increase the slope of the proposed box culvert upstream. By replacing the upstream conduit to larger 4’x10’ and 3’x10’ box culverts at a steeper slope than the existing conduit, Dry Brook Culvert will be able to convey the 50-year storm from Bissell Street to its outfall at Hop Brook without experiencing backup problems. This improvement would alleviate Problem Area A flooding. This alternative also includes the replacement of the existing 24” diameter pipe on Bissell Street with a new 3’x 5’ box culvert which will also provide hydraulic relief for the 50- Year storm event in Problem Area B (similar to Problem Area A).

Improvement Alternative 2 - This improvement alternative involves replacing the trunk line from Pitkin Street to Autumn Street with 24” and 36” diameter pipes. The trunk line will also be extended further north on Pitkin Street to Elwood Road. This alternative also includes modifications to the outlet structure at the East Cemetery Detention Facility to utilize the available storage by detaining more flow for frequent storms, but still release peak flows from the 50-year and 100-year storm without overtopping the facility embankment. The proposed outlet structure configuration

SECTION 5 PROPOSED IMPROVEMENT ALTERNATIVES Tighe&Bond

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 5-2 consists of a 10” diameter orifice at elevation 255.25’, a 12” diameter orifice at elevation 257’, and a 3’x3’ grate opening at elevation 265’. It should be noted that the orifices and weirs can be designed in various configurations, numbers, sizes and shape and still provide flow detention. These improvements would alleviate Problem Areas I and C flooding and can be feasibly divided into three discrete phases, but the pipe replacement must be sequenced starting downstream and working upstream.

Improvement Alternative 3 - This improvement alternative involves adding catch basins at the low spot on Madison Street, enlarging the immediate trunk line pipes downstream, and extending collection systems on Hawley Street and Foster Street.

These improvements would alleviate Problem Area K flooding and can be feasibly divided into two discrete phases, but must be sequenced starting downstream and working upstream.

Improvement Alternative 4 - This improvement alternative involves replacing the trunk line on School Street / Eldridge Street and western portion of Maple Street, and extending the collection system to the eastern portions of Maple and Eldridge Street.

These improvements would alleviate Problem Areas D and E flooding and can be feasibly divided into three discrete phases. The Eldridge Street / School Street trunk line should be phased first such that the 15” diameter pipe overflow cross connection to the Maple Street trunk line can be abandoned, which will alleviate some of the flooding on the western portions of Maple Street. After that, either of the two remaining phases could proceed.

Improvement Alternative 5 - This improvement alternative involves replacing the trunk line between Spruce Street (at the Central Subwatershed connection) and Harrison Street. Along a majority of this work, the existing double pipes would be replaced with a single larger pipe. The improvements also include pipe replacement on Hamlin Street between Pearl Street and East Center Street. These improvements would alleviate Problem Areas F and J flooding and can be feasibly divided into two discrete phases, but must be sequenced starting downstream and working upstream.

Improvement Alternative 6 - This alternative involves the installation of six additional catch basins and connecting manholes onto the existing drain pipe along Spruce Street between Pearl Street and East Center Street. These improvements would alleviate Problem Area L flooding.

Improvement Alternative 7 - This improvement alternative involves replacing the trunk line along a portion of Wells Street. These improvements would alleviate Problem Area G flooding.

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 5-3

Improvement Alternative 8 - This improvement alternative involves replacing the trunk line along a portion of Glenwood Street and Ashworth Street. These improvements would alleviate Problem Area H flooding

Recommendation / Overall Phasing

Improvement Alternative 1 is considered to be the highest priority for several reasons.

First of all, the flooding that occurs along the upper section of the Dry Brook Culvert (Problem Areas 1 and 2) tends to be more frequent and of a greater magnitude than the other problem areas. Also, its location being within the central business district has a significant affect not only to the immediate property owners, but also the general public that patronizes the businesses and uses the streets. Of most importance, if Improvement Alternative 1 is not completed first, the implementation of other improvement alternatives upstream would result in higher peak flows to the Dry Brook Culvert and cause even more frequent and widespread flooding in Problem Areas A and B.

Based on input received from the Manchester DPW – Engineering Division, Improvement Alternative 2 is also considered to be a high priority since the problem is relatively frequent and widespread in the Academy Street and Pitkin Street area (Problem Area C).

Beyond Improvement Alternatives 1 and 2, there is no specific prioritization for the other improvement alternatives.

Wetlands Permitting

Since most of the work involves replacement of existing pipes within existing streets, parking areas and lawns, significant wetlands permitting for these improvement alternatives is not anticipated. However, work within the stormwater detention facility and along some cross-country sections that have existing wetland areas may require permits from the Manchester Planning and Zoning Commission which handles Inland Wetlands issues.

Improvements that impact wetlands also require authorization from the U.S. Army Corps of Engineers (ACOE) and the Connecticut Department of Environmental Protection - Inland Wetlands Division. However, the ACOE’s Programmatic General Permit (PGP) allows for certain impacts to occur which are under predetermined thresholds. For impacts under 5,000 square feet, utility line projects (such as storm drains) may qualify under Category 1 of the Connecticut PGP. Also, if any in-stream work is undertaken (e.g. streams, river, ponds), it must be conducted during the low flow season, which is between July 1 and September 30.

Dry Brook Culvert Hydrologic Model & Study, Manchester, CT 5-4

The permitting requirements would be further defined for individual improvement alternatives that proceed to the design phase. In this case, the alignment of the improvements would be surveyed to identify the extent of wetlands and adjacent buffer areas. Identification would be performed by a licensed wetlands/soils scientist.

Property Acquisition

Most of the work involves replacing existing pipes within public streets and therefore would not require property acquisition. If the existing right-of-way is not wide enough or is already too crowded with other utilities, additional right-of-way width may need to be acquired from private properties along the road frontage.

For cross-country pipe alignments, the proposed improvements are required to be within permanent easements if located on private property. Some of the pipe alignments may already be within permanent easements, which may be of adequate width to accommodate the proposed improvements. The construction of the improvements will also likely require temporary construction easements.

The property acquisition requirements would also be further defined for individual improvement alternatives that proceed to the design phase. In this case, the alignment of the improvements would be researched and surveyed by a registered land surveyor.

J:\M\M0432\REPORT\MANCHESTER REPORT REV 2-20-08.DOC

Tighe&Bond

APPENDIX A‐1

Summary of Existing Conditions Peak Flows

2-Year 5-Year 10-Year 25-Year 50-Year Location Peak Flow Peak Flow Peak Flow Peak Flow Peak Flow

(cfs) (cfs) (cfs) (cfs) (cfs)

48" Outfall East 10 17 21 27 33

48" Outfall West 35 57 64 78 90

7'x6' Dry Brook Outfall 135 190 210 235 260

Junction Chamber at Main Street and Bissell Street 30* 30* 30* 30* 30*

Maple Street Junction Chamber 129 167 173 177 184

3'x5' Box Culvert between Spruce Street and Cottage Street 74* 120* 130* 138* 142*

East Cemetery Detention Facility** 13 / 13 28 / 25 45 / 32 65 / 38 84 / 42

* Flow is restricted by the hydraulic grade line within the downstream pipe, which results in flooding at this location ** The two flow values represent incoming and outgoing flow at the detention facility

J:\M\M0432\REPORT\Problem Area Summary T&B format.xls 2/14/08

Area ID Street Location Subwatershed Primary Cause of Problem

*Storm Frequency at which the problem is evident

A Dry Brook Culvert Trunkline Main The Dry Brook Culvert between Maple Street and Bissell Street has insufficient capacity being undersized and constructed with a near flat slope.

1 to 2-Year

B Main Street at Bissell Street Main The main cause of the backup is due to the pipe in Bissell Street having insufficient capacity. When the Dry Brook Culvert backs up, the resulting tailwater contributes to flooding at Main Street.

2-Year

C Trunkline from Autumn Street to Pitkin Street Northeast The trunkline between Autumn Street and Pitkin Street has insufficient capacity. There is a lack of catch basins on Pitkin Street. 2-Year

D Maple Street Maple The pipes on the west end of Maple Street have insufficient capacity and there is a lack of catch basins / storm drains on the east end of Maple Street.

2-Year

E Eldridge Street and School Street Eldridge / School

The pipes on the west end of Eldridge Street have insufficient capacity and there is a lack of catch basins / storm drains on the east end of Eldridge Street. The pipes have insufficient capacity on School Street East of Spruce Street.

2-Year

F 135 North The pipes along Hamlin Street have insufficient capacity. 2-Year

G Wells Street and Spruce Street Wells The pipes on Wells Street just east of Spruce Street and downstream to Vine Street have insufficient capacity. 2-Year

H Ashworth Street Oak The pipes in Ashworth Street and Glenwood Street have insufficient capacity. 5-Year

I Florence Street Central The pipes from East Cemetery Detention Facility to Spruce Street have insufficient capacity. 5-Year

J Harrison Street North The pipes downstream of Harrison Street to the junction chamber on Spruce Street, between Florence Street and Oak Street have insufficient capacity.

5-Year

K Foster Street, Madison Street, and Hawley Street Northwest

The pipes downstream from Madison Street have insufficient capacity and there is a lack of catch basins / storm drains on Foster Street and Hawley Street.

2-Year

L North section of Spruce Street North There is a lack of catch basins / storm drains along north Spruce Street. 2-Year

*Assumes pipes are clean and free flowing

APPENDIX A-2

Summary of Problem Areas and Primary Causes

Proposed Improvement

Alternative

Affected Problem

Area Location Project Phase Proposed Improvements

1 A,B Dry Brook Culvert Trunkline and Bissell Street / Main Street 1-1 Replace upper portions of the existing Dry Brook Culvert with 1,270 l.f. - 4'x10' box culvert, 300 l.f. -

3'x10' box culvert, 390 l.f. of 3'x5' box culvert. Relocate existing sanitary sewers as necessary.

2-1 Modify the East Cemetery Detention Facility Outlet Structure to detain more flow within the pond.

2-2 Replace pipes from the Autumn Street entrance of East Cemetery to Pitkin Street with 1,700 l.f. of 36" pipe, 330 l.f. - 30" pipe, and 180 l.f. -24" pipe.

2-3 Construct a new collection system on Pitkin Street with, 100 l.f. - 18" pipe and 460 l.f. - 15" pipe and appropriately placed catch basins.

3-1 Replace pipes on Hazel Street to Madison Street with 300 l.f. of 18" pipe.

3-2 Construct a new collection system on Hawley Street and Foster Street with, 920 l.f. - 15" pipe and appropriately placed catch basins.

4-1 Replace pipes on Eldridge Street , Maple Street, Spruce Street, and School Street with 1,740 l.f. of 30" pipe, 1,990 l.f. of 24" pipe, and 490 l.f. of 18" pipe. Abandon existing 15" cross connection pipe to Maple Street.

4-2 Construct a new collection system on the east end of Eldridge Street with 600 l.f. of 18" pipe, 600 l.f.

of 15" pipe and appropriately placed catch basins.

4-3 Construct a new collection system on the east end of Maple Street with 600 l.f. of 18" pipe, 600 l.f.

of 15" pipe and appropriately placed catch basins.

5-1 Replace double pipes on Spruce Street and x-country east to Hamlin Street with 1,300 l.f. of a single 42" pipe and replace the existing pipe on Pearl Street with 700 l.f. of 36" pipe and 400 l.f. of 24" pipe.

5-2 Replace pipes on Hamlin Street with 900 l.f. of 24" pipe.

6 L North section of Spruce Street 6 Construct an additional 6 catch basins on the existing storm drain pipe on the north section of Spruce Street.

Harrison Street, Hamlin Street, and Spruce StreetF,J5

Madison Street, Hawley Street, and Foster StreetK3

Maple Street, Eldridge Street, and School StreetD,E4

APPENDIX A-3

Summary of Proposed Improvement Alternatives

From Florence Street below the East Cemetery detention basin

North-East along the trunkline to Pitkin Street

I,C2

Proposed Improvement

Alternative

Affected Problem

Area Location Project Phase Proposed Improvements

APPENDIX A-3

Summary of Proposed Improvement Alternatives

7 G Wells Street 7 Replace pipes on Wells Street with 690 l.f. of 30" pipe and 430 l.f. of 18" pipe.

8 H Ashworth Street 8 Replace pipes on Ashworth Street and Glenwood Street with 670 l.f. of 18" pipe.

1Linear Foot costs include structures (manholes, and catch basins), pipe/ conduit (mainline and CB connections), trenching & backfill, restoration, and utility / service adjustments.

2Survey, design, permitting, construction administration and easement acquisition costs are not included.

15-Inch 18-Inch 24-Inch 30-Inch 36-Inch 42-Inch 3'x5' Box 3'x10' Box 4'x10' Box $230 $290 $330 $370 $420 $470 $630 $950 $1,050

1 1-1 390 300 1270 1960 $1,864,000 Abandoned Bridge Abutments Removal ($20 K), Sewer Line Relocation Allowance ($100 K) $120,000 $1,984,000 $1,990,000

2-1 0 $0 Detention Facility Outlet Control Structure Modifications $20,000 $20,000

2-2 180 330 1700 2210 $896,000 $896,000

135 460 100 560 $135,000 $135,000

3-1 300 300 $87,000 $87,000

3-2 920 920 $212,000 $212,000

4-1 490 1990 1740 4220 $1,443,000 $1,443,000

4-2 600 600 1200 $312,000 $312,000

4-3 600 600 1200 $312,000 $312,000

5-1 400 700 1300 2400 $1,037,000 $1,037,000

5-2 900 900 $297,000 $297,000

6 6 0 $0 6 catch basins and connecting pipes and manholes. No main pipe required. $40,000 $30,000 $40,000

7 7 430 690 1120 $380,000 $380,000 $380,000

8 8 670 670 $194,000 $194,000 $200,000

Total $7,380,000 1 Linear Foot costs include structures (manholes, and catch basins), pipe/ conduit , trenching & backfill, restoration, traffic control, replacing CB laterals with a minimum pipe size of 15", and utility / service adjustments.

2 Survey, design, permitting, construction administration, and easement acquisition costs are not included.

Cost

Phase No.

Alt.

No.

$1,340,000

$1,060,000

$300,000

Phase Cost

Total Alternative Cost (Rounded Up)

$2,070,000

APPENDIX A-4

Conceptual Level Construction Costs (Projected to June 2009)

Pipe / Conduit Replacement Costs Cost per Linear Foot

Other Improvement Costs

Total Length

Total Pipe / Conduit Costs Description