TABLE OF CONTENTS

PAGE

SECTION	NUMBER

I. INTRODUCTION	1

Goals	3

II. SCOPE OF STUDY	4

III. BACKGROUND REASEARCH AND TECHNICAL OVERVIEW	8

A.	Previous Surveys	8

IV. HYDROLOGIC AND HYDRAULIC ANALYSIS	14

V. FLOOD HAZARD EVALUATION	19

V. FLOOD AUDIT PROGRAM	23

VI. CONCLUSIONS & RECOMMENDATIONS	25

REFERENCES	27

I.	INTRODUCTION

The Town of Peterborough is located in the west central portion of Hillsborough County, in southwestern New Hampshire. The area of the town is 38.6 square miles. It is approximately 50 miles southwest of Concord and halfway between the cities of Keene and Manchester. Peterborough is bordered by the towns of Hancock and Greenfield on the north; Greenfield and Temple on the east; Temple and Sharon on the south; and Jaffrey, Dublin, and Harrisville on the west.

In 1977 the population of Peterborough was 4480. By 1990 the population had grown to 5239 and the 1998 state estimate set the population at 5686.

Elevations range from approximately 680 feet North American Vertical Datum of 1988 (NAVD88) along the Contoocook River to approximately 2300 feet NAVD88 at the peak of Pack Monadnock Mountain, the highest point in the town.

The climate of Peterborough is typical of Southern New Hampshire, with warm summers and cool winters. Prolonged periods of severe cold are rare. Average annual precipitation is 44 inches, of which approximately 34 percent is in the form of snow.

Geologically, the Town of Peterborough lies in the Merrimack Valley section of the western highlands of New England. The area is characterized by numerous land forms resulting from continental glaciation. These include symmetrical, elongated drumlins and ice- shaped bedrock hills, gentle north-west slopes thickly mantled with till and steep southeast slopes thinly veneered with till. The valleys are composed of stratified and unstratified material deposited by the retreating glacial ice sheet. Along the streams, alluvial silt covers the glacio-fluvial deposits to form the flood plains, a condition which creates a significant potential for erosion from floods. The bedrock consists largely of mica schist.

The principal watercourse in the Peterborough is the Contoocook River, which drains Contoocook Lake in Jaffrey and the eastern slopes of Mount Monadnock. The river enters Peterborough at the southwest corner of the town and flows north through the center of town and into Hancock and Greenfield at the northern town boundary.

The river continues to flow in a generally northeasterly direction, and is tributary to the Merrimack River approximately 6 miles north of Concord.

Land use in the flood plains in Peterborough varies from agriculture to commercial and residential. The heaviest concentration of structures are located from the State Route 101 by pass to North Village Dam, where the Contoocook River flows through downtown Peterborough.

The major tributary to the Contoocook River in Peterborough is Nubanusit Brook. The source of Nubanusit Brook is in the hills to the northwest of Peterborough. It drains a series of lakes of which Nubanusit Lake is the largest. The brook flows in an easterly, then southerly direction through Hancock and into Peterborough. The Edward MacDowell Dam is located approximately 3.5 miles above the confluence with the Contoocook River. This dam, which was constructed in 1950 and is operated by the U.S. Army Corps of Engineers, New England District, provides flood control for a majority of the 47 square mile drainage area of Nubanusit Brook.

Below the dam, the stream passes through West Peterborough, where it turns eastward, passing over a mill dam. The brook then passes over several more mill dams before it flows into the Contoocook River, near the center of Peterborough.

Otter Brook, the other significant tributary to the Contoocook River in Peterborough, has its headwaters in Greenfield and flows in a generally westerly direction through low, swampy terrain. After entering Peterborough, the stream steepens, passing through the village of Happy Valley. It is tributary to the Contoocook River approximately one mile above the Peterborough-Greenfield-Hancock Corporate limits.

According to historical records, flood damage to structures in Peterborough occurred in 1836, 1927, 1936, 1938 and 1950. The 1938 flood was the largest of record. The 1959 flood had an estimated frequency slightly greater than a 10 year event. The estimated frequency of the 1938 flood was equal to a 100 year event.

The potential for severe flooding exists in early spring as a result of snowmelt and heavy rains in conjunction with ice jams. Autumn is also a critical season for flood danger because of heavy rainfall associated with tropical storms. Minor flooding can occur in Peterborough at any time of the year, as heavy rain storms can result in rapid runoff and flooding of steep streams.

Goals

There were several goals established for this study at the time of its initiation, some of which evolved during the course of the study. The primary goal of the study has been to develop consistent and accurate data regarding flood hazards along all of the major watercourses in Peterborough.

Another goal of the study has been to precisely define the degree of flood hazard at many specific locations in the town. The flood potential at many structures and sections of roadway has been evaluated. The data that has been generated has been utilized to consider the potential for Hazard Mitigation Projects at high hazard locations

A third major goal of this study has been to develop flood hazard data and recommendations for individual property owners in the form of flood audits. The audits have evaluated potential flood heights and impacts to structures at locations throughout the town.

II.	SCOPE OF STUDY

A significant level of study of the major watercourses in Peterborough has previously been completed and many of the issues addressed in this study have been identified. A focused analysis of the flood hazards was undertaken in this study in an effort to better understand the dynamics that impact flood hazards and exposure. This study has utilized the available data generated in previous studies to the greatest extent practicable and has gathered a substantial amount of detailed new data in areas where additional data was considered to be beneficial.

Collection of Data

All available technical data, maps, studies and reports regarding the conditions in the flood plains and watersheds were collected and reviewed. This data included the following:

•	Previous studies completed for the Town of Peterborough, and other regional, state and local authorities in the watershed

•	File documents and correspondence from the Town of Peterborough the State of New Hampshire and the Federal government

•	Existing topographic mapping

Site Reconnaissance

•	Detailed site reconnaissance was undertaken by the members of the study team. The site reconnaissance involved observations, field studies and measurements to document and verify existing conditions. The study team also confirmed available data regarding the location and the condition of watercourses and structures within the study area.

Field Verification of Flood Hazards

•	DELTA developed additional detailed data to precisely define the extent and severity of flood hazards at critical locations throughout the town. This work included field surveys and hydraulic analyses of flood prone locations.

Flood Study - Otter Brook

•	DELTA obtained the back up data from the currently effective Flood Insurance Study of Peterborough. We utilized the original FIS data and Peterborough's topographic mapping to develop a revised flood study of the brook. Development of a revised study for Otter Brook assures consistency in the vertical datum and the quality of flood data on all the major watercourses throughout the town.

Flood Audits

•	DELTA developed flood audits for the residential and commercial properties that participated in a town wide flood audit program. The flood audits identify flood hazards at properties and provide information regarding flood response actions that can be undertaken by property owners to mitigate potential flood damage.

Bridge Replacement Hydrologic and Hydraulic Analysis

•	DELTA evaluated the impact of bridge replacement and repairs both on upstream flood hazards and on downstream flood hazards resulting from the elimination of backwater valley flood storage.

Route 202 (North) Flooding

•	DELTA verified the accuracy of flood elevations and performed an analysis of flood hazards along the reach of the Contoocook River from the northern Town boundary to the North Village Dam.

•	We verified flood data in the vicinity of the access road to Southfield Village.

•	We reviewed mitigation options to keep Route 202 open.

•	We identified potential flood impacted structures and facilities.

Route 202 (South) Flooding

•	DELTA verified the accuracy of flood data in the following areas:

A&P Plaza / Basket Factory area

Noone Falls to Route 202 / Sharon Road

The entrance to the New Hampshire Ball Bearing facility

•	We identified mitigation options to keep Route 202 open during severe flood events.

•	We identified potential flood impacted structures and facilities.

•	DELTA developed flood hazard mitigation strategies for the area.

Summer Street Area

•	DELTA verified existing available flood data for the Summer Street area.

•	We investigated potential mitigation measures in the Summer Street area.

Town / Community Facilities

•	DELTA verified the accuracy of flood data at the following locations:

Landfill

Wastewater Treatment Plant

Summer Street Well

Downtown Flooding

•	Utilizing updated topographic data which reflects current conditions for the downtown area, DELTA provided further analysis to verify the flood data in the area and identify the most effective options for flood hazard mitigation.

•	The options that were evaluated included potential modifications to the Transcript Dam such as spillway lowering, gate construction and spillway augmentation.

•	We also evaluated the potential for other structural improvements such as construction of berms or floodwalls to prevent flood waters from overflowing the riverbanks and flooding the downtown area.

Flood Hazard Analysis / Automated Flood Warning System

•	DELTA worked with the Office of Community Development in the evaluation of the potential for an automated flood warning system for Peterborough.

Mapping

•	A series of AUTOCAD maps of the study area were developed using the Town of Peterborough topographic mapping as a base. The detailed maps indicate the 100 year and 500 year flood hazard areas, the floodway, flood elevations and river cross sections along the entire reach of the Contoocook River, Nubanusit Brook and Otter Brook.

III.	BACKGROUND RESEARCH AND TECHNICAL OVERVIEW

Previous Studies

Numerous studies that have evaluated the flood hazard areas in the Town of Peterborough have been completed over the past several years. A brief discussion of the studies which had the most significance to this study is provided below:

The earliest study that provided extensive data regarding flooding in Peterborough was the US Army Corps of Engineers Flood Plain Information Study which was completed in 1974. A description of the FPI study is provided later.

The most extensive study that has been completed to date is the 1979 Flood Insurance Study (FIS) of Peterborough which is currently in effect and is the basis for floodplain management regulations and flood insurance purchase requirements in the Town.

Flooding caused by the overflow of the Contoocook River, Nubanusit Brook, and Otter Brook was studied in detail in the FIS. Specifically, the Contoocook River was studied from the Peterborough-Hancock-Greenfield corporate limits to the Jaffrey corporate limits; Nubanusit Brook, from its confluence with the Contoocook River to a breached dam just downstream of Edward MacDowell Dam; and Otter Brook, from its confluence with the Contoocook River to the Peterborough-Greenfield corporate limits.

Approximate study methods were used in flooding analyses of Nubanusit Brook, from the tailwaters of Edward MacDowell Reservoir to the Dublin corporate limits; and Bogle Brook, from its confluence with the Contoocook River to a point approximately 0.5-mile south of Sand Hill Road. In addition, approximate study methods were used for MacDowell Reservoir, Topside Pond, Cranberry Meadow Pond, Cunningham Pond, and several unnamed streams. These areas were studied by approximate methods because of their lack of development or because they were areas of minimum flood potential.

The following discharges for the Contoocook River, Otter Brook, and Nubanusit Brook were utilized in the in the 1979 FIS:

Drainage Area	Peak Discharges (cfs)

Location	(sq. miles)	10-year	50-year	100-year	500-year

CONTOOCOOK RIVER

At downstream corporate limits	107.0	3,235	6,065	9,150	13,900

Transcript Dam	79.9	2,890	5,420	7,500	12,415

Upstream of Nubanusit Brook	74.9	2,660	4,990	7,150	11,430

At USGS Gage	68.1	2,300	4,310	5,700	9,890

At downstream confluence	54.0	1,850	3,470	4,300	7,950

with Gridley River

OTTER BROOK

At confluence with	15.4	440	1,070	1,300	2,820

Contoocook River

NUBANUSIT BROOK

At confluence with	49.0	650	700	800	900

Contoocook River

Two documents prepared by the U.S. Army Corps of Engineers relating to flooding in Peterborough were consulted extensively during the development of the FIS.

The first document contained Contoocook River profiles developed from high-water marks of the 1936 and 1938 floods. The second document was the Flood Plain Information report for the Contoocook River and Nubanusit Brook.

Peak discharges and flood elevations at structures on Nubanusit Brook in the FIS agreed exactly with the FPI report. Peak discharges used in FIS study for the 100 and 500 year floods on the Contoocook River also agreed exactly with the 100 year and Standard Project Flood discharges in the FPI report.

Profiles in the Flood Plain Information report were based on high-water marks and limited hydraulic analysis at structures. The 100 year elevations developed for the FIS study agreed closely with elevations in the report at all structures, except State Route 101, Morrison Road, and Drury Road.

At those structures, high-water marks were the primary source of information used to develop elevations in the Flood Plain Information report and hydraulic conditions at State Route 101 and Morrison Road had changed significantly since 1938. At Drury Road, the profiles in the FIS are higher than in the U.S. Army Corps of Engineers reports.

On September 10, 1997, the Army Corps of Engineers performed a Community Assistance Visit to the Town of Peterborough and the State of New Hampshire Office of Emergency Management. The community assistance visit was authorized by the Federal Emergency Management Agency (FEMA) through its Community Assistance Program.

Based on a review of the 1979 FIS mapping it was determined that during a 100 year flood, the shopping area in downtown Peterborough would be inundated by shallow flooding. That determination was based upon a comparison of the 723 foot National Geodetic Vertical Datum (NGVD) base flood elevation to ground elevations determined at points at the shopping area.

Inspection of the 100 year flood profile shown in the FIS for the Contoocook River indicated that flood levels in the downtown area are controlled by the 6 foot high Transcript Dam, located 100 feet upstream (south) of Main Street. The FIS indicates that flood waters back up behind the dam into the left overbank (looking downstream) where the shopping plaza is located. The FIS flood profile indicates flood levels in the area to be approximately 723.3 feet (NGVD).

The Corps compared flood elevations from the FIS to those in the 1974 Corps FPI report. By superimposing the 100 year flood profiles from the FPI and the FIS upon one another, it was determined that elevations in the FPI flood profile indicated lower 100 year peak water surface elevations for several hundred feet upstream of the Transcript Dam. The FPI flood profile indicated that, immediately upstream of the dam, the peak flood elevation was approximately 2 feet lower than that of the FIS.

Although both the FIS and the FPI used the same peak flood flows and a HEC-2 computer model to establish flood profile elevations, cross-sections were obtained for the FIS from 5 foot contour maps with the below-water portions from field-surveys. FIS bridge and structure data were obtained from the Corps, but all bridges for which data were unavailable or out-of-date were surveyed. The FPI used several sources including field surveys, bridge and dam plans from various agencies, and the USGS topographic map dated 1953 with 20 foot contours. Since the flood boundaries in the FPI were delineated on a less accurate map than the maps used in the FIS, the Corps of Engineers concluded that the FPI flood boundary was less accurate than the FIS boundary.

Next, the Corps obtained the 1979 FIS HEC-2 input for the Contoocook River. The FIS HEC-2 input was coded into HEC-RAS and the computer model was run. The nearest cross-section to the Centertown Shopping Area is cross-section 8.513 (FIS cross-section J), located roughly 250 feet downstream of the shopping area. The HEC-RAS output indicates that the 100 year flood in the left overbank of this cross-section to be approximately one foot in depth.

Transcript Dam serves as the hydraulic control for the vicinity of Centertown Shopping Area. The Corps performed multiple HEC-RAS runs using the original FIS input data, but changing the Transcript Dam crest elevation in the model input. At cross-section 8.513 (FIS cross-section J) it was determined that the existing peak 100 year water surface elevation would be lowered from its current 723.3 feet (NGVD) to 722.5 feet (NGVD) if the dam crest were lowered one foot, and to 721.6 feet (NGVD) if the dam crest were lowered two feet.

The Corps drew the following conclusions at the completion of their evaluation. Because the FIS was based upon more detailed modeling, and delineated the floodplain on more detailed maps, the flood boundaries shown in the FIS, with the Centertown Shopping Area shown in the floodplain, are likely more accurate than the boundary shown in the FPI. No information was found indicating the FIS to be incorrectly modeled. Because of the criticality of flood levels on the Centertown Shopping Area, and the age of the existing FIS, however it was recommended that, after new detailed topographic maps were developed, the Contoocook River floodplain be reanalysed and the FIS accordingly revised.

Pursuant to that recommendation, in October 1999 after the Town of Peterborough had developed detailed topographic mapping, the engineering firm SFC Engineering Partnership, Inc. (SFC) was retained to conduct a detailed review of the flood hazards along the Contoocook River and Nubanusit Brook. The resulting hydraulic analysis of the is contained in a January 2000 hydraulic study prepared for the Town of Peterborough by SFC. That study utilized the U.S. Army Corps of Engineers HEC-RAS hydraulic model and the detailed Town of Peterborough topographic mapping to update the 1979 Flood Insurance hydraulic model of the Contoocook River and Nubanusit Brook.

The hydraulic analysis that was completed by SFC is summarized as follows:

•	The existing effective FEMA multiple flood model was manually input into HEC2 format and run to reproduce current existing FIS results.

•	As the next step in the process the duplicate HEC2 model was imported into HEC-
RAS format and run to reproduce the duplicate HEC2 model output. Only changes made to duplicate model import were converting dams modeled as bridges in HEC2 to Inline Weirs for HEC-RAS.

The HEC-RAS models developed using the FIS data were then imported into BOSS RMS for conversion of the overbank data to match the town's 2000 basemap data. The following is a summary of the conversion process:

•	SFC obtained the town's digital base mapping. Base mapping included planimetrics and topography from 1997 aerial photography.

•	Existing FIS HEC2 cross-sections (147) were located and delineated on the basemap.

•	HEC-RAS models were imported into the basemap under the BOSS RMS program operating within an AutoCAD platform. Imported cross-sections were linked to the basemap.

•	Cross-sections were re-stationed to center the channel at station 5000, midway between the left and right bank points, to accommodate the new expanded cross- sections.

•	Each cross-section was "re-cut" through RMS to automatically re-write the existing overbank data with the new basemap topography, up to and including the channel banks.

•	New channel banks were defined per the new basemap planimetrics, but all existing channel data within the bank stations was preserved.

•	SFC then ran a Type 1 encroachment of the FEMA floodway using HEC-RAS multiple flood cross-sections.

•	The final models for both the Contoocook River and the Nubanusit Brook were re-
imported into RMS for automated delineation of the floodplains and floodway.

SFC also completed a hydrologic evaluation of the water courses, which included the collection and review of flood data from the US Geological Service (USGS), the ACOE, and the Federal Emergency Management Agency (FEMA). SFC analyzed the data using the ACOE – HEC statistical computer model "FFA – Flood Frequency Analysis".

Per the FEMA FIS Guidelines and Specifications, the 1979 Peterborough FIS gauge discharges were compared to the 95% and 5% confidence limits of the new statistical analysis to determine if new updated flood estimates were statistically different from the values used in the 1979 FIS.

For the Contoocook River, the 1979 FIS flood flows at the gauge were within the acceptable confidence limit range of the updated statistical calculation.

For the Nubanusit Brook, the 1979 FIS flood flows at the gauge varied somewhat in statistical significance to the revised gauge analysis. The disparities were attributed to ACOE's operation of the MacDowell Reservoir.

IV.	HYDROLOGIC AND HYDRAULIC ANALYSIS

DELTA's efforts on Peterborough's Project Impact Study were initiated in January 2001, shortly after the completion of SFC's hydrologic and hydraulic analysis. Our initial work included a detailed evaluation of the impact of potential modifications to the Transcript Dam on Flood Hazards in the commercial area in the center of the town.

In DELTA's opinion, SFC's recalculated 100 year flood elevations was the best available flood hazard information and therefore DELTA utilized the updated analysis in our initial evaluation of the hydraulics of the Transcript Dam.

DELTA reviewed SFC's hydraulic analysis and concluded that the procedures that were utilized were consistent with standard hydraulic model development using the best available technical data and analytical techniques.

The final SFC hydraulic model for the Contoocook River in the vicinity of the Transcript Dam indicated that the 100 year flood elevation at cross section J, (river station 8.513), approximately 15 feet upstream of the Transcript Dam, is 722.12 feet (NAVD88). The currently effective FIS 100 year flood elevation at river station 8.513 is 723.3 feet (NGVD).

DELTA recoded SFC's 2000 HEC-RAS model and reran it utilizing modified geometry for several Transcript Dam lowering options. The analysis utilized the cross section coding developed by SFC. The results of DELTA's initial hydraulic analysis indicated that lowering the dam crest elevation by 1 foot would result in a base flood elevation of 721.23 feet (NAVD88) at river station 8.513 and a 1.5 foot lowering would result in a base flood elevation of 720.76 feet (NAVD88) at the same location.

Based on a review of the topographic mapping of the town center area that was available in the Spring of 2000, it appeared that with modifications to the dam spillway, there would be some limited residual flooding of the riverside parking area but essentially all buildings in the Town Center would be outside the 100 year flood hazard area.

A number of variations of hydraulic configurations were evaluated including a partial lowering of a limited section of the structure. To document the impact of one such modification, DELTA ran a model with 58 feet of the spillway (approximately ½) lowered by 1.5 feet. The resulting 100 year flood elevation at river station 8.513 was 721.09 ft (NAVD88). This analysis indicated that the upstream flood elevation would be more significantly impacted by lowering the crest elevation than by extending the spillway length.

Subsequent to the completion of DELTA's initial project assignment, our Scope of Services was expanded as described in Section II.

DELTA completed a comprehensive review of the previously completed hydrologic and hydraulic analyses that had been completed as part of the flood insurance study and the work completed for the town of Peterborough by SFC Engineering Partnership in January 2000.

DELTA reviewed and confirmed the accuracy of both the hydraulic coding and performed sensitivity analysis of the in channel cross sections used by SFC. Based on DELTA's initial review there were concerns regarding the precision of the data generated by the SFC model at several critical locations along the river that were identified as areas that required additional data to provide the most accurate flood hazard analysis.

In addition, the sensitivity analysis included adding channel cross sections at critical locations along the Contoocook River.

DELTA focused on the reach of the river from a point approximately 5,000 feet downstream of the Transcript Dam to a point approximately 1,000 feet upstream of the dam.

Of critical importance to the entire project was the determination that the vertical datum and control used throughout the town was consistent for all of the flood studies and evaluations that were completed.

The Town of Peterborough's newly completed topographic mapping was developed utilizing the North American Vertical Datum of 1988 as vertical control.

New horizontal and vertical controls have been adopted as standards throughout the United States over the past several years. The original Flood Insurance Study and much of the historical survey information data that is available in Peterborough, utilized National Geodetic Vertical Datum of 1929. Based on information provided by the U.S. Geodetic Survey the difference in these two datums in Peterborough is approximately .67 feet with NGVD being lower than NAVD. As a result, an elevation of 100 ft (NAVD88) recorded in NGVD would be 100.67 ft (NGVD).

The effort to assure that proper vertical control was utilized throughout the study was of critical importance and the results of using a consistent vertical datum have a significant impact on the results of this study.

Since the SFC hydraulic analysis utilized the town of Peterborough's topographic mapping for the overbank cross sections coding the final SFC model was significantly more accurate than the data contained in the original flood insurance model.

DELTA completed field surveys of three channel cross sections within the detailed study reach in addition to completing an elevation survey of the Transcript Dam. The data from DELTA's channel surveys were input into the HEC-RAS models of the Contoocook River and the sensitivity analysis was completed to determine the impact of revising channel sections on water surface profiles.

The result of the analysis indicated that there are random variations in the channel geometry on the order of the difference between the datums used in the two studies. As a result, it was determined that no significant improvement in the accuracy of the hydraulic analysis would be gained by revising the channel cross sections utilized by SFC. The one significant and critical revision that did result in the resurvey work was the recoding of the Transcript Dam weir profile. Based on DELTA's field survey the crest of the Transcript Dam is at an elevation of approximately 714.65 ft (NAVD88). The crest elevation used in the original FIS analysis and SFC modeling was 715.26 ft (NGVD).

DELTA's hydraulic analysis of the Contoocook River resulted in a 100 year flood elevation upstream of the dam of 721.46 ft (NAVD88). This elevation is shown on the revised flood maps of the Town.

The results of DELTA's models were then used to evaluate the flood hazards at many of the structures identified as being located within the 100 year flood hazard area on the City of Peterborough detailed topographic mapping.

The final flood elevations were used to evaluate the results of the flood audits completed at structures in the floodplain.

In addition to completion of the revisions to the hydraulic models of the Contoocook River, DELTA also reviewed the analysis completed by SFC for the Nubanusit Brook and developed a revised hydraulic analysis for Otter Brook.

DELTA's review of the Nubanusit Brook indicated that the flood boundaries produced by SFC's modeling were accurate. As a result DELTA produced maps for the Nubanusit Brook in a format consistent with the detailed maps produced for the Contoocook River, but did not revise the SFC models.

DELTA's effort on Otter Brook was substantially more extensive. DELTA obtained the microfiche backup data for Peterborough from FEMA Region I. Copies of the HEC-Z models for Otter Brook were made and coded into DELTA's computer system. The HEC-Z models were rerun to develop a duplicate effective Flood Insurance model. A HEC-RAS model was then developed reestablish the FIS hydraulic conditions.

In the final step in DELTA's hydraulic analysis of Otter Brook the cross section from the FIS model were recoded using the revised geometry from the town of Peterborough detailed topographic mapping.

The resulting hydraulic model of Otter Brook was then utilized to produce detailed flood mapping in a format and at a level of detail consistent with the mapping produced for the Contoocook River and Nubanusit Brook.

Bridge Hydraulic Analysis

Delta reviewed the engineering reports produced for two recent bridge replacement projects in Peterborough. The review was undertaken to evaluate the potential impact that bridge replacement hydraulic sizing has on upstream and downstream hydraulics and resulting flood hazards. The two bridges that were reviewed were the Spring Road Bridge on Nubanusit Brook and the Steele Road Bridge also on Nubanusit Brook. The results of DELTA's review indicated that the unique locations and geometry of these two structures are such that the their design does not have a significant impact on hydraulics. The hydraulics at each location considered for a bridge replacement or construction must be reviewed carefully to assure that excessive backwater or loss of important valley flood storage does not result from a structural modification.

Alternatives Analysis-Transcript Dam

DELTA received input from CLD Inc. regarding their structural analysis of the Transcript Dam and utilized the alternative spillway modifications that were evaluated by CLD to prepare hydraulic analyses of the impact of each alternative.

The results of DELTA's analyses are included in the technical appendix of this report.

Essentially all of the proposal modifications to the Transcript Dam that have been considered by CLD will provide additional spillway capacity at the dam. Any increased spillway capacity will result in reduced flood elevations in the reach upstream of the dam.

The alternative of adding approximately 11 feet of additional spillway length at the western end of the dam results in a 100 year flood elevation upstream of 721.45ft (NAVD88). Adding additional spillway length at the eastern end of the spillway in addition to the repairs at the western end would result in a 100 year flood elevation of 721.35 ft (NAVD88). The alternative of adding one foot of breakaway flash boards to the dam crest would result in lowering the 100 year flood elevation to 720.29 ft (NAVD88).

V.	FLOOD HAZARD EVALUATION

The areas reviewed and studied in detail by DELTA included the locations of essentially all structures identified as being impacted by the 100 year flood hazard along the Contoocook River.

DELTA identified each structure that appeared to be potentially impacted by flooding and produced a set of detailed topographic maps that highlighted each impacted structure. These maps served as a graphic representation that highlighted the areas along the river that required the most focused attention.

Review of the initial mapping effort indicated areas of concern in the northern section of town in the vicinity of the Peterborough Waste Water disposal facility, in the reach of the river upstream of the North Village Dam to the Transcript Dam; upstream of the Transcript Dam in the Town Center; in the vicinity of Peterborough Basket Company; in the Noone Falls Area and in the vicinity of the Sharon Road Bridge in the southern section of town.

DELTA conducted a detailed field inspection of all of these areas and conducted flood audits of structures in each reach of concern.

Additional detailed elevation surveys were completed to provide more accurate information regarding the depth and extent of the flood hazard area at critical locations along the river.

The ground surveys were supplemented with additional river cross section surveys which were completed to confirm the accuracy of channel cross sections that were utilized in the SFC analysis.

DELTA's detailed review of flood hazards in Peterborough indicated that a significant number of structures are located in the floodplain but that relitively few of the structures are subject to significant depths of flooding during a 100 year flood or regular flooding by common floods. The majority of the structures DELTA inspected are located along the sloping floodplain valley walls at elevations significantly above the river channel. The majority of the flooding problems identified during detailed flood audits are basement or lower level flooding.

There are sections of major roadways which are subject to inundation during a 100 year flood. During a 100 year flood, inundation of sections of Route 202 (Concord Street) north of the town center will occur. A significant section of Route 202 (Jaffrey Road) south of the town center in the vicinity of Sharon Road, south of Noone Falls Dam will be inundated by a significant depth of flooding making the roadway potentially impassable.

The detailed analysis of the flood hazards in the vicinity of the Transcript Dam and in the Town Center that has been completed as part of this study indicates that the majority of the businesses in the town center are not subject to inundation during a 100 year storm. The exception to this condition are those structures that are constructed on top of the retaining walls along the downstream section of Nubanusit Brook and the business located immediately west of the Transcript Dam. While these buildings have floors elevated above the 100 year base flood elevation the fact that the channel walls are part of their structure causes them to be subject to flood hazard designation.

The reach of the Contoocook River downstream of the North Village Dam has a flat gradient and wide floodplains that include the floodplain wetlands along the river.

This downstream reach is the location of the Town's municipal wastewater disposal facility as well as an apartment complex located immediately downstream of the wastewater disposal facility.

DELTA's analysis indicated that the closed landfill in the northern section of the town was constructed within the Contoocook River floodway as designated in the 1979 Flood Insurance Study. DELTA's remodeling the floodway and adjusting the floodway limits in the area for this addressed the impact of the landfill on the upstream flood hazards.

Based on DELTA's analysis the wastewater lagoons are protected from inundation from the floodwaters during a 100 year flood by the earthen dikes that surround the lagoons. In addition, the detailed elevation surveys in the vicinity of the wastewater treatment building completed by DELTA indicated that the structure is located outside of the flood hazard area and not subject to inundation during a 100 year flood.

Similarly, DELTA's elevation surveys indicated that the apartment buildings immediately downstream of the wastewater treatment building are elevated above the 100 year flood elevation and will not be subject to inundation during a 100 year flood. Floodwaters will inundate the parking lot but will not reach the buildings.

The reach of the Contoocook River upstream of the North Village Dam and downstream of the Main Street Bridge was the location of intensive review during this study.

Route 202 (Concord Street) runs along the east side of the floodplain with commercial and residential development along both sides of the road and Summer Street is located along the west side of the river with primarily residential development along both sides of the road.

Detailed elevation surveys along both Concord Street and Summer Street were completed along with detailed river channel cross sections surveys and spot elevations surveys in the vicinity of many of the structures in this area. In addition, flood audits were completed at several residences and businesses throughout this area.

In general, it is DELTA's conclusion that while several structures in this reach and a section of Concord Street will be subjected to flooding during a 100 year flood, flood depths will not be significant and flood hazards can potentially be mitigated by increased preparedness and some modifications to floodplain uses by businesses and residences.

DELTA's review of the reach of the Contoocook River upstream of the town center, and the confluence of Nubanusit Brook, did not indicate significant flood hazards through the reach downstream of the Route 101 bypass bridge. Upstream of the bypass bridge, DELTA's review and detailed elevation survey indicated that the shopping plaza building is located outside of the 100 year flood hazard area.

Upstream of the shopping plaza the Peterborough Basket Company is located along the northern edge of the 100 year flood hazard area of the Contoocook River as it meanders through a broad floodplain area with significant flood plain wetlands along its banks.

Flooding of the Basket Company property will result in inundation of the southern portion of the property and potential flooding of the southeastern portion of the building and a section of the yard area in the vicinity of the furnace building.

A flood audit was completed at the Peterborough Basket Company that provides recommendations for flood mitigation at that facility.

Upstream of the Basket Company the river flows under Morrison Road through an undeveloped area of floodplain and then through a steep reach of river up to the Noone Falls Dam.

Upstream of Noone Falls Dam the mill pond created by the dam backwaters upstream through Old Powers Bridge Road which causes an increase in flood elevations and then through the Sharon Road Bridge which also causes significant backwater. The impact on flooding caused by the combination of the dam and the two bridges results in a wide area of inundation during a 100 year flood including flooding of two significant lengths of Route 202 (Jaffrey Road).

The flood hazards along Jaffrey Road are considered to be significant because while few structures are potentially impacted, flood depths and the extent of flood hazards are great.

Based on DELTA's hydraulic model and mapping, sections of Jaffrey Road will potentially be inundated by floodwaters with depths in excess of two feet during a 100 year flood for a length of approximately 1,000 feet south of the intersection of Jaffrey Road and Sharon Road. A major cause for this flood hazard is the backwater and significant head loss resulting from floodwater first flowing through the restricted opening of the Sharon Road Bridge and then overtopping the roadway both north and south of the structure.

Options for hazard mitigation for this area may include acquisition of flood prone properties, modifications to the waterway opening or replacement of the Sharon Road Bridge to reduce or eliminate the significant backwater effect caused by the bridge.

Upstream of the area of flood hazard the roadway elevation increases significantly and Jaffrey Road is flood free to the south as far as the Jaffrey town boundary.

VI.	FLOOD AUDIT PROGRAM

The flood audit program was developed in the early 1980's by the USDA Natural Resource Conservation Service (NRCS) and the Connecticut Department of Environmental Protection to help reduce flood damage to contents and nonstructural building components for buildings within the 100 year floodplain of selected rivers. In Connecticut the program is performed in conjunction with the installation of municipal ALERT flood warning and response systems.

A flood audit provides homeowners and businesses with information on flood levels and the relationship of the flood levels to their structures. When a flood is forecast for the area, the individual takes the actions identified in the flood audit for the corresponding flood level. The audit includes an individual action plan which helps owners react quickly and effectively to flood warnings. In anticipation of flooding, an individual can move furniture, and appliances out of basements and other low areas and secure closures to prevent floodwaters from entering buildings. As part of the program in Connecticut, flood audit data is also loaded into local community's flood warning system computer database. The computer display provided includes an elevation graph for each audited structure. With the computer display, municipal and state officials can quickly spot the lowest structures in flood-prone areas and notify audited homeowners and small businesses to begin taking actions to reduce flood damages. Audits result in a package of information that property owners maintain and review annually.

The elevation at which water from the river will spill into the building through an opening, such as a door or window, is identified as a critical component of all audits.

When a flooding event is imminent, home owners and businesses take the actions prescribed in the audits, including evacuation when flood heights are at a level that threatens lives and roads are flooded.

DELTA developed a modified flood audit program for the Peterborough Project Impact Study. The Peterborough flood audit program uses the same basic principles utilized in the Connecticut program but does not currently incorporate the early flood warning and notification aspects of the program.

The Peterborough flood audits included detailed inspections of eighteen structures either located in the floodplain or near the floodplain of the Contoocook River. Detailed elevation surveys were completed at the majority of the structures to give precise elevation data for the lowest point of entry and to identify the ground elevations in the vicinity of the lowest levels of the audited structures.

The elevations at the structures were then compared to the recalculated flood elevations determined as another component of the Peterborough Project Impact Study to enable property owners to make decisions regarding the severity of flood hazards at their location.

In some cases the information may be used to complete a site specific hazard mitigation project that will protect the structure or its contents from flood damage.

In other cases, for properties near the limit of the flood hazard area, the flood audit data can potentially be used to with an elevation certificate to better define the true flood risk or as the basis for a request for a letter of map amendment from FEMA to more accurately define the flood risk of a structure.

In the future, the flood audit program can potentially be expanded to include other structures not included in this study and the program may be expanded to incorporate the early flood warning components if development of such a system is completed in Peterborough.

The completed flood audits for each of the eighteen structures that were performed as part of this project are included in the technical appendix of this report. A brief description of the program and a copy of the individual flood audits have also been made available to property owners that participated in the program.

Table 1

Peterborough, New Hampshire

Flood Audit Addresses

Property Address
Comments
Mr. Richard Fernald Esq.

14-20 Grove Street	Property is located outside of the 100 year floodplain based on revised flood elevations resulting from this study.
Mr. Brian Ewing

Active Media Robotics

44 Concord Street	The lower floors of this commercial property are located significantly below the 100 year flood level. Recommendations to protect valuable equipment and relocate certain critical activities to upper level.
Mr. Brian Ewing

Active Media Robotics

46 Concord Street	The basement floor of this commercial property are located below the 100 year flood level. Recommendations to protect valuable equipment and flood proof certain non-critical items.
Mr. Bruce Hunter

Community Theater

9 School Street	Property is located outside of the 100 year floodplain based on revised flood elevations resulting from this study.
John & Lynne Graham

73 Summer Street	Residential structure near the edge of the 100 year floodplain
Mr. George Sterling

Sterling Business Print & Mail

206 Concord Street	Commercial structure located near limit of 100 year floodplain. Building floor subject to inundation by flood water. Recommendations to protect damageable equipment provided.
Mr. Robert Martin

61 Summer Street	This residence is located outside of the 100 year flood hazard area. The property owner was concerned regarding the development of an adjacent undeveloped lot that is located in the floodplain. Guidance regarding floodplain development regulations and constraints was provided.

Ms. Nancy French

US Post Office

23 Grove Street	The Post Office is not located in the flood hazard area as defined by the 1979 FIS or this study. The 100 year and 500 year flood for Nubanusit Brook is confined by the masonry walls constructed along both sides of the brook. The only concern identified in the event that flood water did escape from the channel was at a lower level window along the south side of the building
Mr. Robert Kipka

40 Concord Street	The rear lower level of this multi-family rental property is located near the flood hazard area. The sewage pump station and the rear yard and vehicle parking area are in the flood area.
Campbell Kipka

66 Concord Street	The residence is located entirely within the 100-year floodplain. The family room at the rear of the structure is vulnerable to limited flooding during a 100-year storm. The basement is subject to flooding.

Table 1 (cont.)

Property Address
Comments
Mr. Norman Makechnie, Esq.

26A Concord Street	The commercial property is located entirely within the 100-year floodplain. The elevation of the lowest floor is above the 100-year flood elevation. Flooding would be limited to the garage, parking areas and the lawn area.
Peterborough Basket Factory

130 Grove Street	The commercial property is located entirely within the 100-year floodplain. The elevation of the rear door is approximately 9-inches lower than the 100-year storm. Recommendations to protect any critical equipment provided.
Ms. Michelle Stahl

Historical Society

Grove Street	The commercial property is located outside of the 100-year floodplain.
Jennifer Allen

32 Concord Street	The residential property is located near limit of the 100-year floodplain. The lowest entry to the basement is approximately 6- inches below the 100-year flood elevation. Recommendations to flood proof the entryway provided.
Loraine Strang

Old Sharon Road	The residential property is located entirely within the 100-year floodplain (elevation of 778 feet NAVD.) Many flood proofing modifications have been made on the structure. However, the first floor is at elevation 774 feet making it one of the most flood prone locations in town.
Mary Guilmette

76 Concord Street	The residential property is located near limit of the 100-year floodplain. The section of Concord Street in front of the residence would be inundated with approximately 1-foot of water during a 100- year flood.
Roy

Roy's Market

20 Main Street	Property is located outside of the 100-year floodplain based on revised flood elevations resulting from this study.
Francis Carr

28-30 Main Street	The commercial property is located near the limit of 100-year floodplain.

VII.	CONCLUSIONS & RECOMMENDATIONS

This study has gathered a wide range of data related to the flood hazards in Peterborough, New Hampshire.

Detailed hydraulic analyses have provided precise information on the extent and the severity of flood hazards on the Contoocook River, Nubanusit Brook and Otter Brook.

Flood Audits were completed at eighteen separate properties along the Contoocook River to provide property owners with site specific information regarding flood hazards and techniques that can be utilized to mitigate flood damage potential.

Many of DELTA's recommendations will require further effort to develop detailed plans to achieve the positive results that are anticipated by effective implementation of the recommended programs. The recommendations contained in this report may be used as the basis for development of a detailed flood hazard management plan for the Town of Peterborough.

The development along Otter Brook is primarily rural in nature with few structures located in the flood hazard area. Careful floodplain management should protect the Otter Brook floodplain from significant flood damage in the future.

Nubanusit Brook was once a significant contributing tributary to the Contoocook River. Flood hazards along the brook downstream of the Edward MacDowell Dam have been essentially eliminated by construction of the dam and management of the upstream flood control pool by the U.S. Army Corps of Engineers.

The Contoocook River remains the watercourse with the most significant potential for severe flood damage in the town. DELTA inspected and evaluated the flood hazards along the entire length of the Contoocook River through Peterborough in detail.

Based on our analysis flood hazards downstream of the town center are considered significant but manageable and increases in future damage can potentially be mitigated by careful floodplain management and by the implementation of flood damage reduction recommendations contained in the flood audits prepared by DELTA as part of this study. In some locations minor structural projects such as stream bank stabilization may also prove to be effective.

The detailed analysis that was conducted at the Transcript Dam and in the town center area indicates that based on revised topographic data and survey of the dam the majority of the structures in the town center will not be subject to flooding during a 100 year flood. Implementation of recommendations for structural repairs to the dam that were evaluated by CLD Inc. including increasing spillway capacity will further mitigate flood hazards in the town center area and are recommended. Any increase in spillway capacity provided will provide additional flood hazard mitigation benefits.

Upstream of the town center implementation of Flood Audit recommendations contained in DELTA's flood audit and careful planning and construction of any future expansions or modifications to the facility, should help to mitigate the potential for increased flood damage at the Peterborough Basket Company.

The remaining area of greatest concern is the section of Route 202 in the vicinity of Old Sharon Road Bridge. An extremely flood prone residence is located in this area. The bridge causes significant backwater during all but the most common flood events. DELTA recommends further detailed evaluation of this area and consideration of a bridge replacement and property acquisition project.

02011501/113

REFERENCES

Town of Peterborogh NH Aerial Planimetric and Contour Digital Files, dated 3/30/00, prepared by Fuss & O'Neill, Inc. of Manchester CT;

Town of Peterborough NH Flood Insurance Study, FEMA, November 1979.

Town of Peterborough NH Flood Insurance Rate and Boundary Maps, FEMA, May 1980.

Town of Peterborough NH 1979 Flood Insurance Study backup data and model runs from FEMA fiche.

Floodplain Information for the Contoocook River and Nubanusit Brook, Peterborough NH, ACOE New England Division, April 1974.

USGS 15-minute Quad Mapping, Peterborough North and Peterborough South, 1987.

Transcript Dam sensitivity analysis, ACOE, 9/97.

SFC Engineering Partnership Inc., FIS Remapping Study, January 2001.

DELTA #D99-188-10

Dear Mr. Ryner:

DELTA Environmental Services, Inc. (DELTA) is pleased to submit the Project Impact study of the Town of Peterborough.

It has been a pleasure working with the Town of Peterborough on this study. We look forward to the Town's review comments. We will revise the report as appropriate subsequent to the review of the sponsoring agencies.

Very truly yours,

Robert E. Sonnichsen, P.E.

President

02011501/113

PROJECT IMPACT STUDY

PETERBOROUGH, NEW HAMPSHIRE

PREPARED FOR:

Town of Peterborough

Office of Community Development

Town House, 1 Grove Street

Peterborough, NH 03458

DELTA NUMBER: D99-188-10

DATE: December 31, 2001

PROJECT IMPACT STUDY

PETERBOROUGH, NEW HAMPSHIRE

TECHNICAL APPENDIX

PREPARED FOR:

Town of Peterborough

Office of Community Development

Town House, 1 Grove Street

Peterborough, NH 03458

DELTA NUMBER: D99-188-10

DATE: December 31, 2001

APPENDIX A

HYDRAULIC MODELS

CONTOOCOOK RIVER

FINAL EXISTING CONDITIONS 2001 MULTIPLE FLOOD MODEL

CONTOOCOOK RIVER

FINAL EXISTING CONDITIONS 2001 FLOODWAY MODEL

CONTOOCOOK RIVER

ALTERNATIVE MODEL 1 - 121 FT TRANSCRIPT DAM SPILLWAY

CONTOOCOOK RIVER

ALTERNATIVE MODEL 2 - 138.2 FT TRANSCRIPT DAM SPILLWAY

CONTOOCOOK RIVER

ALTERNATIVE MODEL 4 - 138.2 FT TRANSCRIPT DAM SPILLWAY

12 INCH FLASHBOARDS SPILLWAY CREST 713.65 FT NAVD88

CONTOOCOOK RIVER

SENSITIVITY ANALYSIS MODEL 1 - DELTA TRANSCRIPT DAM SURVEY

CONTOOCOOK RIVER

SENSITIVITY ANALYSIS MODEL 2 - DELTA TRANSCRIPT DAM SURVEY

DELTA CHANNEL SECTIONS RECODED OVERBANK SECTIONS

CONTOOCOOK RIVER

ALTERNATIVE MODEL 3 - 138.2 FT TRANSCRIPT DAM SPILLWAY

4 FT x 11 FT FLASHBOARDS

CONTOOCOOK RIVER

APPENDIX C

Macroinvertebrate Survey Report