Appendix C5. Papermill Creek Watershed Study (2005).pdf

Document Number: D-259

VIRGINIA

Colonial National

Historical Park

Colonial Parkway (PMIS # 57064)

PAPERMILL CREEK

WATERSHED STUDY

VIRGINIA

Colonial National

Historical Park

Colonial Parkway (PMIS # 57064)

PAPERMILL CREEK

WATERSHED STUDY

January 2005 January 2005

Printed on recycled paper.

2300 N Street, NW Washington, DC 20037 Tel 202 912 0200 Fax 202 293 0787

VIRGINIA

Colonial National

Historical Park

Colonial Parkway (PMIS #57064)

PAPER MILL CREEK

WATERSHED STUDY

VIRGINIA

Colonial National

Historical Park

Colonial Parkway (PMIS #57064)

PAPER MILL CREEK

WATERSHED STUDY

VIRGINIA

Colonial National

Historical Park

Colonial Parkway (PMIS #57064)

PAPER MILL CREEK

WATERSHED STUDY

January 2005January 2005

THETHE Louis Berger Group, Louis Berger Group, INC.INC.

Prepared by:

Prepared for:

National Park ServiceNational Park Service Denver Service Center

Document Number: D-259

[This page intentionally left blank.]

EXECUTIVE SUMMARY

Colonial National Historical Park administers two of the most historically significant sites in English North America: Jamestown and Yorktown Battlefield. Representing the beginning and end of English Colonial America, these two sites are connected by the 23-mile scenic Colonial Parkway traversing the Virginia Peninsula.

The Colonial Parkway is a unique concrete roadway dating back to 1937 when the first segment was completed between Yorktown and Williamsburg, Virginia. Rapid regional growth in the area has placed increasing demands on the natural and cultural resources of the park. With this growth, stormwater runoff has increased, specifically in the Paper Mill Creek watershed through which the parkway passes and, as a result, the parkway experiences flooding at certain locations during major storm events. This report addresses the long-term, safeguarding and safety of the parkway.

The National Park Service (NPS), Denver Service Center contracted with the Louis Berger Group, Inc. to conduct a comprehensive watershed study of Paper Mill Creek. The purpose of the study was to identify existing flood causes and outline cost-effective solutions to prevention. This report presents the results.

The watershed analysis also assesses the impact of potential development within the watershed on the existing flooding problem and identifies various solutions to alleviate the flooding.

Specific tasks that were undertaken as a part of this study include:

1. Collecting relevant mapping information pertaining to the Paper Mill Creek watershed and the parkway;

2. Conducting detailed field surveys of the culverts along the parkway;

3. Developing hydrologic and hydraulic models to estimate flow, drainage capacity, and stream erosion potential using the 1-, 2-, 5-, 10-, 25-, and 50-year storm events (these models are used to forecast degradation from future development or to quantify improvements from recommended projects);

4. Developing roadway drainage design computations and stormwater quantity evaluation/ recommendations;

5. Prioritizing problems and developing alternative solutions; and

6. Developing concept plans and estimates for suggested improvements, project objectives, and goals.

The completion of the above listed tasks found that the existing double 36-inch RCP culvert at Location 3 in the Paper Mill Creek watershed is critically undersized. The culvert drains runoff flows from more than 80 percent of the entire watershed, indicating that its poor hydraulic performance is the main cause of flooding of the Colonial Parkway. Culverts in Locations 2 and 4 are also undersized.

While the structural condition of the existing culvert systems appears to be good, each of the seven crossings assessed need cleaning. The main culverts, passing the largest volumes of stormwater, are filled with silt and debris occupying one-third to almost half of their capacity. This clogged condition, in addition to being undersized for storm conditions at some locations, results in the flooding of the parkway.

The following summarizes the findings of the watershed analysis:

Under present (existing) conditions, the culverts at Locations 3 and 4 do not convey the 25-year design storm event.

Under future conditions, the drainage areas for culvert Locations 1, 2, and 3 would be subjected to increased flows.

COLONIAL NATIONAL HISTORICAL PARK

ES-2

Under future conditions, culvert Locations 2, 3, and 4 do not adequately convey the 25-year design storm event.

To address the drainage issues determined above, several improvements to the culverts systems in the Paper Mill Creek watershed were proposed. Two methods of analyses were used, based on the type of culvert system under consideration. A simplified analysis using the standard FHWA design worksheet was used for less complex culvert configurations (see page 10). A more detailed analysis using the FHWA Culvert Analysis program HY-8 is used for the more complex culvert configurations. The HY-8 analysis program is described in FHWA publications HDS-5, “Hydraulic Design of Highway Culverts,” HEC-14, “Hydraulic Design of Energy Dissipators for Culverts and Channels,” and HEC-19, “Hydrology.”

Cost estimates for each option were developed using information on recent bid tabulations for roadway and drainage construction projects in Virginia. Some prices were adjusted due to the context sensitive nature of Colonial Parkway and the need for greater attention to potential construction impacts. Most of the options presented require open cut excavation across the existing Colonial Parkway. One option recommends the jacking of a new culvert under the roadway instead of open cut construction due to the depth of the fill at that location. The cost for property acquisition was estimated based on assessed value per acre and estimated increase for potential loss of future value. Costs for utility relocations were not included.

A summary of the costs and impacts for each option is provided in Table A. In most cases, the environmental impacts would be minor because they require only open cut construction within the existing roadway section with minor impacts at the culvert inlets and outfalls. Cultural resources impacts affecting the landscape might occur during the removal or modification of the existing brick headwalls, or during the construction of new culverts. Two impacts related to operation include the overall performance of the option, specifically the hydraulic performance, and maintenance required after the system is installed.

A ranking system was used to identify the best option at each location. The ranking system assign values to each item and a final rank assessed based on the lowest total. These recommendations can be modified and re-evaluated based on input and review by NPS and FHWA. The historic significance of the existing brick headwalls needs to be determined and may influence whether or not the headwalls can be removed or modified.

TABLE A. COLONIAL PARKWAY STORM SEWER INFRASTRUCTURE COST AND IMPACTS

E C P M Option 1 $20,056 1 1 1 1 1 Modify existing headwalls

Option 2 $20,974 1 2 2 2 2 New culvert and headwalls

Option 1 $218,938 3 3* 2 2 2 New culvert and headwalls

Option 2 $173,438 2 2 1 2 1 Removal of existing headwalls

Option 1 $19,647 1 2 2 2 2 New culvert and headwalls

Option 2 $23,697 1 1 1 1 1 Modify existing headwalls

Grade Scale 1 to 5: 1 - Positive, 5 - Negative E - Environmental C - Cultural Resources P - Performance (Hydraulic) M - Maintenance

*Greater exposure to Paper Mill dam remnants. Additional armoring of base required.

Comments Operations

RankLoc. Option Est. Cost Impacts

TABLE OF CONTENTS

Executive Summary Introduction Technical Approach

Watershed Characterization Paper Mill Creek Hydrology

Identification of Existing Culvert System City of Williamsburg Storm Sewer and Culvert Systems Colonial National Historic Parkway Culvert System

Culvert System Hydraulic Performance Design Storm Concept Culvert Analysis

Estimation of the Peak Flow Discharges The Rational Method TR-55 Model

Analysis of the Culvert System Under Existing Conditions Future Conditions

Future Development Analysis of the Culvert System under Future Conditions

Culvert Hydraulics Performance Stormwater Management Erosion Control

Erosion and Sediment Control Improvements Alternatives Comparison

Location 2 Option 1—Replace Existing 24-inch HDPE with 30-inch RCP Option 2—Add Parallel 24-inch RCP

Location 3 Upstream Ponds and Stage-Storage Analysis Option 1—Parallel Culvert System, double 9 X 5 Option 2 – CON-SPAN Structure, 4' X 28' Additional Options for Location 3

Location 4 ii

Option 1 – Install Parallel 24-inch RCP Option 2 – Replace Existing Culvert with 24-inch RCP

Conclusions References

FIGURES

Figure 1. Location Map Figure 2. Project Study Area Figure 3. Watershed Drainage Area Map—Existing Conditions Figure 4. Analysis Location Map Figure 5. Colonial Williamsburg Planning Area Figure 6. Watershed Drainage Area—Future Condition

TABLES

Table 1. Colonial Parkway Storm Sewer Infrastructure Inventory Table 2. Colonial Parkway Peak Discharge Volumes—Existing Conditions Table 3. Colonial Parkway Peak Discharge Volumes—Culvert Performance for Existing Conditions Table 4. Colonial Parkway Peak Discharge Volumes—Future Conditions Table 5. Colonial Parkway Peak Discharge Volumes—Culvert Performance for Future Conditions Table 6. Colonial Parkway Peak Discharge Volumes—Hydrologic Summary Table 7. Colonial Parkway Storm Sewer Infrastructure Recommendations Table 8. Required Culvert End Treatment Based on Outlet Velocity Table 9. Colonial Parkway Storm Sewer Infrastructure Cost and Impacts

APPENDIXES

Appendix A Hydrologic Data ...................................................................................................................A-1

Appendix B IDF Curve............................................................................................................................. B-1

Appendix C Culvert Analysis ................................................................................................................... C-1 Appendix C-1 Location 1.................................................................................................................. C-3 Appendix C-2 Location 2.................................................................................................................. C-7 Appendix C-3 Location 3................................................................................................................ C-29 Appendix C-4 Location 4................................................................................................................ C-59 Appendix C-5 Location 5................................................................................................................ C-79 Appendix C-6 Location 6................................................................................................................ C-83 Appendix C-7 Location 7................................................................................................................ C-91

Paper Mill Creek Watershed Study iii

Appendix D TR-55 Model (Location 3) ...................................................................................................D-1

Appendix E Conceptual Plans and Cost Estimates ................................................................................... E-1 Appendix E-1 Location 2 .................................................................................................................. E-3 Appendix E-2 Location 3 .................................................................................................................. E-9 Appendix E-3 Location 4 ................................................................................................................ E-15

Appendix F Detail Drawings .....................................................................................................................F-1 Appendix F-1 Outlet Protection Details.............................................................................................F-3 Appendix F-2 CON-SPAN Details ....................................................................................................F-7

Appendix G Location 3 Retention Basin ..................................................................................................G-1 iv

INTRODUCTION

Authorized in 1930, Colonial National Historical Park (NHP) is a 10,221-acre unit of the National Park Service (NPS) located between the James and York Rivers in Virginia (see Figure 1). Originally designated a National Monument (becoming a National Historical Park in 1936), Colonial NHP administers and interprets the sites of Jamestown Island and the Yorktown Battlefield. Central to the original legislation which created Colonial NHP was a plan for a scenic highway to link the sites into a “single coherent reservation.” Free of any “modern” commercial development, the Colonial Parkway was designed to provide continuity to the visitor experience of motoring through nearly 400 years of American colonial history. Traversing a diverse environment, the parkway provides visitors with dramatic open vistas of rivers and tidal estuaries as well as shady passageways through pine and hardwood forests

(NPS 1995).

Colonial Parkway is a unique concrete roadway that was constructed in three ten-foot lanes with an exposed aggregate surface. The first segment of construction was completed in 1937 between Yorktown and Williamsburg. It was not until 1955 that the parkway was extended to Jamestown. Since the completion of the parkway in 1957, the park has been aggressive in its attempts to limit access and fight visual encroachments along the road. Numerous grade crossings have been eliminated by bridges and interchanges and many scenic easements have been acquired to preserve the experience of motoring on a scenic roadway (NPS 1995). To recreate the colonial atmosphere for the visitor experience, all culvert headwalls and parkway underpasses are clad in Virginia-style brick laid in English and Flemish bonds and exposed aggregate in the pavement surface simulates the shell and marl roads of colonial Yorktown.

Rapid regional growth is placing increasing demands on the natural and cultural resources of Colonial National Historical Park. Sections of the parkway between Williamsburg and Yorktown have become popular commuter corridors (NPS 1995). Not only has increased traffic increased stresses on the roadway, increased development in the region has led to increases in stormwater runoff as well. As a result, culverts passing under Colonial Parkway are experiencing frequent flooding. This flooding creates maintenance problems along the roadway with some undermining occurring directly under segments of the road.

The National Park Service wishes to alleviate these identified stresses to the roadway to ensure the Colonial Parkway is properly maintained and ensure visitor safety. To achieve this, the National Park Service used a two-prong approach, focusing on the culvert crossings along the Colonial Parkway associated with the Paper Mill Creek watershed (see Figure 2). First, a watershed analysis was completed.

This included:

• Characterizing the watershed;

• Identifying culvert system within the watershed;

• Assessing the culvert system’s hydraulic capacity;

• Estimating peak discharges;

• Analyzing the culvert system under existing conditions;

• Identifying the watershed future conditions; and

• Analyzing the culvert system under future conditions.

The watershed analysis provided a preliminary assessment of the drainage problems in the Paper Mill Creek watershed study area with a summary of preliminary options identified during the watershed analysis phase to address the drainage problems.

Second, the preliminary options identified during the watershed analysis phase were then studied in greater detail during the hydraulics and conceptualization phase. A review of alternatives was completed with the development of concept plans of the proposed improvements to address the requirements.

The following provides the technical approach, a summary of current conditions, and the alternatives assessed to address the drainage problems along the Colonial Parkway in the Paper Mill Creek watershed.

These alternatives and recommendations are provided in keeping with the goals and objectives of the project to meet or exceed current state and federal regulations for hydraulic performance of drainage structures, erosion and sediment control, and stormwater management.

Lower James Watershed

York Watershed

Great Wicomico- Piankatank Watershed

Lynnhaven- Poquoson Watershed

James River

York River

SR 5SR 5

SR 3

SR 3

1 SR 199SR 199

SR 1

SR 1

SR

SR

0 2 41 Miles

0 10,000 20,000 30,0005,000 Feet

Legend Parkway Study Location

Watershed (HUC-8)

Parkway

Interstate

Primary Road

Secondary Road

Park Boundary

State Boundary

Jamestown

Williamsburg

Yorktown

Study Location

The Louis Berger Group, Inc.

November 2004 Source: NPS GIS Metadata and Data Store.

Figure 1. Location Map

Jamestown and Yorktown, VA

Colonial National Historic Park

259 333/41061

DSC JAN 2005

Jamestown and Yorktown, VA Colonial National Historic Park

0 500250 Meters

0 1,000 2,000500 Feet

Legend Parkway Study Location

Watershed (HUC-8)

Parkway

Secondary Road

Primary Road

Water

Park Boundary

Paper Mill Creek Watershed

SR 199SR 199

S R

S

R 1 32

S R

S

R

SR 5/31

SR 5/31

SR 5/31/162SR 5/31/162

Paper Mill Creek Watershed

York W atershed

Lower James Watershed

The Louis Berger Group, Inc.

November 2004 Source: NPS GIS Metadata and Data Store.

Figure 2. Project Study Area

259 333/41062

DSC JAN 2005

TECHNICAL APPROACH

To complete a watershed analysis of Paper Mill Creek and its drainage impacts on the Colonial Parkway, several tasks were accomplished. The following outlines the seven steps initiated to develop stormwater management improvement options for application in the Paper Mill Creek watershed along the Colonial Parkway:

1. Characterization of the watershed

2. Identification of culvert systems within the watershed

3. Assessment of the culvert system hydraulic capacity

4. Estimation of peak discharges

5. Analysis of the culvert system under existing conditions

6. Identification of the watershed future conditions

7. Analysis of the culvert system under future conditions

WATERSHED CHARACTERIZATION

Located entirely in the City of Williamsburg, the 1,051-acre Paper Mill Creek watershed drains to the James River via College Creek. Flat areas within the Colonial Williamsburg Historic Area and moderately steep ravines leading to three golf course lakes characterize the watershed. Due to limited development within the watershed at this time, impervious surfaces comprise approximately 10 percent of the area resulting in one of the lowest percentages of any watershed in the city. Soils include well-drained and moderately well-drained fine sandy loams.

According to the City of Williamsburg’s Stormwater Management Plan (1996), approximately 64 acres of wetlands occur in the Paper Mill Creek watershed, with 15 acres of tidal wetlands located near the mouth of Paper Mill Creek. Typical for this area, roughly half of the wetland area is classified as Type VII, Arrow Arum-Pickerelweed Community. Other wetlands are classified as Type VI, Cattail Community, which provide habitat for muskrats and birds, and act as a buffer against coastal flooding.

A major element of the Paper Mill Creek watershed includes the Golden Horseshoe Golf Course lakes located in the center of the watershed. The Colonial Williamsburg Foundation (CWF) owns the two golf courses in the watershed and the four connected lakes on the courses collect stormwater runoff that is conveyed to Paper Mill Creek (see Figure 3). Several smaller ponds are located in the stream and drain to the upper and lower lakes. The streams draining to the upper lake are located in a state-designated Resource Protection Area of the watershed.

PAPER MILL CREEK HYDROLOGY

A delineation of the drainage areas was completed for the analysis that includes the identification of sub-drainage areas and the establishment of appropriate pre- and post-development peak discharges (Figure 3). The watershed drainage patterns generally follow existing topographic contours except where the parkway traverses contributing sub-areas. Three smaller sub-areas on the west side of the parkway must pass underneath the roadway and converge with the main flow from the watershed on the east side. The combined flow of more than 800 acres then passes underneath the parkway to the main outfall of the watershed.

259 333/41063

DSC Jan 2005 Figure 3

Detailed information on the drainage areas including travel times for each sub-area is provided in appendix A. The information was used to calculate the peak flows for each point of analysis where a culvert crosses underneath the Colonial Parkway. Both pre- and post-condition peak-flow volumes were developed and were then used for the pre- and post-performance analysis of the culverts.

IDENTIFICATION OF EXISTING CULVERT SYSTEM

The following provides the existing hydraulic conveyance system in the Paper Mill Creek watershed including the location of the culverts, their hydraulic characteristics, and the corresponding drainage areas.

CITY OF WILLIAMSBURG STORM SEWER AND CULVERT SYSTEMS

Located on a ridge, the City of Williamsburg includes 1,333 acres in the York River watershed and 4,459 acres in the James River watershed. The city’s storm drain system serves developed areas in the vicinity of Colonial Williamsburg and along primary roadways throughout the city. The storm drain infrastructure system consists of culverts, inlets, pipes, and stormwater management structures. The overall watershed system includes creeks, streams (both tidal and non-tidal), lakes, ponds, and floodplains, which transmit water flows to the larger waterbodies.

The City of Williamsburg Department of Public Works and Utilities (DPWU) is responsible for the city’s storm drain system, including:

Inspection and maintenance of storm sewer systems, Planning and design of DPWU projects involving new storm sewer systems or improvements to existing systems, Review and approval of new private development (site plan) storm sewer system designs and improvements to existing systems, and

Inspection during construction of storm sewer systems and improvements.

The City of Williamsburg Department of Planning enforces the city’s zoning ordinances and has responsibilities regarding stormwater management including:

Review and approval of best management practices (BMP) design plans to meet state stormwater management requirements, Review and approval of erosion and sediment control plans to meet state erosion and sediment control requirements, and

Inspection of sites for proper BMP construction and erosion and sediment control compliance.

The city maintains the storm sewer systems including the periodic cleaning and replacement or rehabilitation of existing systems. An important element of the maintenance of existing systems is the use of permanent storm sewer easements to ensure that runoff is properly conveyed from public right-of-way.

As the storm sewer systems in the City of Williamsburg typically discharge into channel systems, which then carry flows to larger bodies of water, some streams experience erosion problems due to runoff volumes. Increased runoff can influence the erosion of both man-made and natural conveyance channels and outlets. While current erosion and sediment control laws prohibit increases in runoff resulting from new construction or development activities, many existing storm sewers have led to the further erosion of area streams and channels.

Owned and maintained by the City of Williamsburg, the existing storm sewer systems are located in the northern portion of the Paper Mill Creek watershed. These systems discharge into three drainage paths to the south of Providence Hall and the Williamsburg Inn. A storm sewer system from Newport Avenue and

Nassau Street area discharges into a ditch paralleling the Colonial Parkway near their intersection. The remainder of the storm sewer systems consists of smaller, isolated networks which generally serve as roadway drainage in residential side streets.

The systems contributing to the Paper Mill Creek watershed have experienced deterioration and erosion problems over time; DPWU addresses these. Included in this network are the systems at York Street (between Waller Street and Page Street), Providence Hall, and Quarterpath Park, next to Pocahontas Street. The majority of the systems in this area were installed circa 1967 and 1969 with modifications, improvements, and some additional pipe added in 1983, 1989, and 1991.

Stormwater management improvements have been completed at various locations to mitigate erosion problems and improve water quality. An example of such an improvement is the incorporation of a stormwater management basin into a new parking lot construction project which has helped reduce downstream erosion in a creek which feeds into the Paper Mill watershed. Any new development is closely monitored to ensure full compliance with state and federal stormwater management and water quality regulations.

COLONIAL NATIONAL HISTORIC PARKWAY CULVERT SYSTEM

The Colonial Parkway traverses the western side of the Paper Mill Creek watershed. Topographic mapping of the watershed indicates that this section of the parkway was constructed along the natural contours of the area and the natural primary drainage paths to Paper Mill Creek. This was likely a function of Colonial-era horse and wagon paths through the watershed. As such, the parkway’s ditch and culvert system was not designed to accommodate the existing and future development of the watershed and contributing drainage areas. This includes the contributing portions of the Historic Colonial Williamsburg area of Francis Street, Nassau Street, and Newport Avenue. Major rainfall events, and sustained periods of rainfall have resulted in the flooding of the parkway indicating that the existing culvert systems on the parkway do not have the required capacity for managing upstream stormwater flows.

A detailed survey of the culvert systems associated with the Colonial Parkway was completed for those culverts that convey flows within the Paper Mill Creek watershed (Figure 4), starting near the south end of the parkway and ending at the Newport Avenue ramp onto the parkway. While the structural condition of the culvert system appears to be good, most of the culverts require cleaning and debris removal. The main culverts, which pass the largest volumes of stormwater, are filled with silt and debris occupying one-third to almost half of the culvert capacity. A summary of these culverts is provided in Table 1.

TABLE 1. COLONIAL PARKWAY STORM SEWER INFRASTRUCTURE INVENTORY

Length Invert In Invert Out Drainage Area (feet) (feet) (feet) (acres)

1 33 inch RCP 100 4.65 4.55 34.59 3.7%

24 inch PVC / 30 inch RCP 70 3.65 3.26 68.69 7.4%

3 Double 36 inch RCP 56 4.95 4.37 742.17 79.4% 4 15 inch RCP 61 10.54 8.59 12.49 14.0% 5 15 inch RCP 114 21.70 19.52 6.66 8.7% 6 36 inch RCP 67 18.16 17.38 68.25 7.3% 7 18 inch RCP 102 35.35 28.17 1.53 0.2%

Percentage of Total AcreageLocation Type

259 333/41064

Figure 4

CULVERT SYSTEM HYDRAULIC PERFORMANCE

The assessment of the culvert systems hydraulic performance is in accordance with Federal Highway Administration guidelines and Virginia Department of Transportation Drainage Manual guidelines and addresses the Virginia Stormwater Management Regulations and the Virginia Erosion and Sediment Control Regulations in accordance with the Federal Highway Administration Project Design Development Manual, Chapter 7. The technique used follows the guidance of U.S. Department of Transportation, Federal Highway Administration Publication No: FHWA-DF-88-003, Federal Lands Highway Project Development and Design Manual (June 1996 Metric Revision). Following FHWA guidelines and the project scope, the 25-year design storm event was used for the capacity and performance analysis of the culverts along the parkway.

DESIGN STORM CONCEPT

Hydraulic conveyance structures are designed using a particular storm event associated with a specific quantity of stormwater runoff. This storm, usually termed a design storm, is a rainfall event of specified duration and return frequency (i.e., a storm with the likelihood of occurring once in a given time period).

As stated earlier and following Federal Highway Administration guidelines, the 24-hour 25-year design storm event was used for the capacity and performance analysis of the culverts along the Colonial Parkway.

The rainfall intensity of a specific design storm is usually estimated using Intensity-Duration-Frequency (IDF) curves. These IDF relationships associate the rainfall intensity, the duration of the rainfall, and the return frequency. IDF curves are developed using historic rainfall data for a specific region and allows the engineer to estimate the rainfall intensity given the duration and the return frequency of the storm. IDF curves developed for the city of Richmond were used for this study and are depicted in appendix B.

CULVERT ANALYSIS

The culvert analysis method used for this study is per the standard Federal Highway Administration design worksheet outlined in Publication Number FHWA-NHI-01-020, HDS No. 5, Hydraulic Design of Highway Culverts. The analysis method is based on the location of the controlling flow of stormwater, either inlet or outlet. The worksheets used for the analysis are provided in appendix C. Although control may oscillate from inlet to outlet, the concept of “minimum performance” is applied; meaning that, while the culvert may operate more efficiently at times, it will never operate at a lower performance than calculated.

The analysis procedure begins using the existing culvert information, including pipe size and type, and the headwater elevation for both inlet and outlet control is then calculated. The higher of the two is designated as the controlling headwater elevation. The controlling headwater elevation is compared to the desired design headwater, usually governed by overtopping considerations, to determine if the assumed culvert size is acceptable. The outlet velocity is considered to evaluate the need for outlet protection. If the controlling headwater is based on inlet control, the normal depth and velocity in the culvert barrel is determined. The velocity at normal depth is assumed to be the outlet velocity. If the controlling headwater is based on outlet control, the area of flow at the outlet is determined based on the barrel geometry and tail water depths in relation to critical depth of flow.

Ultimately, the height of the headwater is determined in relation to the culvert configuration and the height of the fill over the culvert; in this case, the height of the roadway section. The allowable headwater is the depth of water that can be ponded at the upstream end of the culvert during the design flood measured from the culvert inlet invert. The headwater depth or elevation may be limited by giving due consideration to inlet and outlet velocities and following upstream water surface elevation controls.

The key design objective of this study was to ensure the safe maintenance of traffic during the 25-year storm event. Consequently, the minimum requirement for the culverts is to ensure the conveyance of the flow associated with a 25-year storm event. Per Federal Highway Administration design criteria, a freeboard of 12 inches must be maintained. Freeboard is the vertical distance between the water surface at the design flood and the top of the road surface. Additionally, the headwater at culvert inlets should not exceed 8 feet.

Other factors to consider include upstream property damage, elevations established to delineate National Flood Insurance Program or other floodplain zoning, and elevation of terrain or ditches that could permit flow to divert around the culvert. Appendix C provides the analysis worksheets for the existing culverts and their performance under peak flow conditions for the different design storm events.

ESTIMATION OF THE PEAK FLOW DISCHARGES

The hydrologic methods used for estimating runoff and determining peak flows included the Rational Method and NRCS WinTR-55. Both methods were selected based on the characteristics of the drainage areas.

THE RATIONAL METHOD

The Rational Method is recommended for estimating the design storm peak runoff for areas smaller than 200 acres. This method is the most commonly used formula for estimating peak discharge from rainfall.

The basis of the Rational Method is that the maximum rate of runoff occurs when the entire watershed area contributes to flow at the outlet. The Rational Method was used to estimate peak flows for watershed locations 1, 2, 4, 5, 6, and 7. The equation for this method is:

Q= C*I*A Equation (1)

Where:

Q = Maximum rate of runoff, cubic feet per second (cfs).

C = Runoff coefficient representing a ratio of runoff to rainfall (dimensionless).

The runoff coefficient is a function of soil type and imperviousness and varies from 0.1, for areas with high infiltration rate, to 1.0, for completely impervious areas such as parking lots.

Cf = Saturation factor.

Less frequent, higher intensity storms require modification of the coefficient because infiltration and other losses have a proportionally smaller effect on runoff. The adjustment of the Rational Method for use with larger storms can be made by multiplying the Rational Formula by a saturation factor, Cf. The product of Cf and C should not be greater than 1.0. Saturation factors for Recurrence Interval of 2, 5, and 10 years is 1.0, for 25 years it is 1.1, 50 years it is 1.2, and for 100 years it is 1.25.

I = Average rainfall intensity for a duration equal to the time of concentration (tc) for a selected return period, inches per hour (in/hr).

A = Drainage area contributing to the design location, acres (ac).

Time of Concentration (tc) - The time of concentration is the time required for water to flow from the hydraulically most remote point in the drainage area to the point under investigation. Use of the rational formula requires the time of concentration (tc) for each design point within the drainage basin. The duration of rainfall is then set equal to the time of concentration and is used to estimate the design average rainfall intensity (I) by using the rainfall IDF curve for the area of study (appendix B).

TR-55 MODEL

The method of analysis used to estimate the runoff and peak flows for the largest drainage area in the study area (location 3) is from the United States Department of Agriculture Natural Resources Conservation Service (NRCS 1986). Natural Resources Conservation Service methods are the most appropriate for calculating peak flows for watersheds smaller than 800 hectares.

The Natural Resources Conservation Service WinTR-55 computer model was used for this analysis.

WinTR-55 is a single-event rainfall-runoff small watershed hydrologic model. The model generates hydrographs from urban and agricultural areas, and at selected points throughout the stream system, and then routes them downstream through channels and/or reservoirs.

The study watershed is composed of sub-areas (land areas) and reaches (major flow paths in the watershed). Each sub-area has a hydrograph generated from the land area based on provided land characteristics. The reaches are designated as channel reaches, where hydrographs are routed based on physical reach characteristics, or storage reaches, where hydrographs are routed through a reservoir based on temporary storage and outlet characteristics. The hydrographs from the sub-areas and reaches are combined as needed to accumulate flow as water moves from the upland areas down through the watershed reach network. The accumulation of all runoff from the watershed is represented at the watershed outlet. This model uses the TR-20 (NRCS 2002) model for all of the hydrograph procedures:

generation, channel routing, storage routing, and addition. Table 2 provides a summary of the peak discharge volumes estimated under the existing condition of the watershed. The results of the model are provided in appendix D.

TABLE 2. COLONIAL PARKWAY PEAK DISCHARGE VOLUMES—EXISTING CONDITIONS

Q2 Q5 Q10 Q25 Q50 Q100

(cfs) (cfs) (cfs) (cfs) (cfs) (cfs) 1 33 inch RCP 17.12 20.93 23.97 30.97 38.35 43.28

24 inch HDPE / 30 inch

HDPE 36.06 44.72 50.49 65.06 79.63 91.96

3 Double 36 inch RCP 200.00 402.60 587.77 767.20 952.31 1118.24 4 15 inch RCP 10.19 12.29 13.79 17.81 21.22 24.54 5 15 inch RCP 4.83 5.91 6.83 8.61 10.49 12.07 6 36 inch RCP 47.30 58.56 67.57 86.71 108.11 122.47 7 18 inch RCP 2.32 2.70 3.03 3.86 4.63 5.27

Location Type g

ANALYSIS OF THE CULVERT SYSTEM UNDER EXISTING CONDITIONS

Using the existing condition peak-discharge volumes, the culvert systems were analyzed accordingly (Table 3). As shown in Table 3, the existing culverts do not convey the 25-year storm event at three locations. The most critical culvert is Location 3, where the double 36-inch reinforced concrete pipe (RCP) system does not convey the 2-year design storm.

TABLE 3. COLONIAL PARKWAY PEAK DISCHARGE VOLUMES—CULVERT PERFORMANCE FOR

EXISTING CONDITIONS

FUTURE CONDITIONS

Colonial Parkway traverses the western portion of the Paper Mill Creek watershed and must pass the majority of the stormwater flow through existing culverts. The watershed also encompasses a portion of the Colonial Williamsburg Historic Area. As described in the existing condition section, the Colonial Williamsburg Historic Area contributes flows to the watershed. The Historic Area lies on the ridge dividing the James River and York River watersheds and, therefore, is not impacted by runoff from upstream development.

FUTURE DEVELOPMENT

This section describes and evaluates in detail the potential effect of future development within the Paper Mill Creek watershed. The information provided was used in the development of the future hydrologic condition factors used in estimating peak flows.

As outlined in the City of Williamsburg Comprehensive Plan, the Paper Mill Creek watershed is encompassed within the Colonial Williamsburg Foundation Planning Area. The plan, completed and adopted by the City Council on July 9, 1998, is currently under revision and expected for completion in 2005. As part of this study, the planning department was consulted with and potential changes that may occur within the Colonial Williamsburg Foundation Planning Area have been incorporated into this study.

The 1,395-acre Colonial Williamsburg Foundation Planning Area is planned as a residential and business community (Figure 5). The plan calls for the majority of the land to be allocated for parks, parkway and recreation, reflecting the three existing golf courses found in the western half of the planning area. Low-density residential uses consume a large component of the land along the developable edges. Commercial and economic development uses are concentrated away from the historic area.

The Colonial Williamsburg Foundation is a primary land owner within the watershed, using some land for golf course construction and leaving some undeveloped. As of this year, the Colonial Williamsburg Foundation has expressed an interest in selling some property including areas zoned single-family residential and redevelopment (mixed-used, office, retail). The Colonial Williamsburg Foundation asked the city government for adjustments in land-use planning to open the existing nine-hole Spotswood Golf Course to a “moderate density” residential development. While it is expected that the Colonial Williamsburg Foundation will maintain and protect some of the “non-core” land outside the historic area, Q2 Q5 Q10 Q25 Q50 Q100

1 33 inch RCP Yes Yes Yes Yes Yes Yes

24 inch HDPE/ 30 inch HDPE Yes Yes Yes Yes No No

3 Double 36 inch RCP No No No No No No 4 15 inch RCP (BB) Yes No No No No No 5 15 inch RCP (AA) Yes Yes Yes Yes Yes Yes 6 36 inch RCP Yes Yes Yes Yes No No 7 18 inch RCP Yes Yes Yes Yes Yes Yes

Conveys Qp (yes/no) selling other non-core real estate to provide capital to help further the foundation’s mission is under review.

Based on the preliminary request, the Colonial Williamsburg Foundation prefers a portion of the sold land be developed at a density of three to five homes per acre, a density similar to that of an existing neighborhood just to the north. The Colonial Williamsburg Foundation has indicated to city planners it may sell an additional 44 acres of hilly woodland south of the Spotswood Golf Course for development of home sites at a higher density than the proposed golf course neighborhood.

FIGURE 5. COLONIAL WILLIAMSBURG PLANNING AREA

Officials believe local government regulations now provide the environmental protection the Colonial Williamsburg Foundation tried to safeguard by buying up all the land. While the Colonial Williamsburg Foundation operates resort and merchandising businesses to support its educational and museum activities, it is not poised to become a large-scale real estate developer.

ANALYSIS OF THE CULVERT SYSTEM UNDER FUTURE CONDITIONS

indicates that future development within the Paper Mill Creek watershed is expected to impact three of the culverts along the southern portion of the Colonial Parkway. The most significant change from the existing conditions occurs in the main culvert Location 3, a double 36-inch RCP carrying the largest quantity of flow from the watershed. Table 4 provides a summary of the future condition peak flow analysis.

TABLE 4. COLONIAL PARKWAY PEAK DISCHARGE VOLUMES—FUTURE CONDITIONS

Using the future condition peak-discharge volumes, the culvert systems analysis (Table 5; Figure 6) revealed the condition of the culvert performance at Location 3 will continue to worsen. In addition, Location 2 (24-inch PVC and 30-inch RCP) will not adequately convey the 25-year design storm.

TABLE 5. COLONIAL PARKWAY PEAK DISCHARGE VOLUMES—CULVERT PERFORMANCE FOR

FUTURE CONDITIONS

Q2 Q5 Q10 Q25 Q50 Q100

1 33 inch RCP Yes Yes Yes Yes Yes Yes 2 24 inch HDPE/30 inch HDPE Yes Yes Yes No No No 3 Double 36 inch RCP No No No No No No 4 15 inch RCP (BB) Yes No No No No No 5 15 inch RCP (AA) Yes Yes Yes Yes Yes Yes 6 36 inch RCP Yes Yes Yes Yes No No 7 18 inch RCP Yes Yes Yes Yes Yes Yes

Conveys Qp (yes/no)

The results from the future peak flow analysis are used in the development of concept plans for the improvements needed on the Colonial Parkway. A comparison of the existing (pre-developed) conditions and future (post-developed) condition is provided in Table 6. The changes to land use as a result of potential future development are reflected in the estimated flows for Locations 1, 2, and 3.

Q2 Q5 Q10 Q25 Q50 Q100

(cfs) (cfs) (cfs) (cfs) (cfs) (cfs) 1 33-inch RCP 25.15 30.92 35.11 45.54 55.97 64.19 2 24-inch HDPE/30-inch 48.06 58.94 68.00 86.77 105.54 121.27 3 Double 36-inch RCP 238.16 458.20 650.64 836.43 1030.23 1202.33 4 15-inch RCP 10.19 12.29 13.79 17.81 21.22 24.54 5 15-inch RCP 4.83 5.91 6.83 8.61 10.49 12.07 6 36-inch RCP 47.30 58.56 67.57 86.71 108.11 122.47 7 18-inch RCP 2.32 2.70 3.03 3.86 4.63 5.27

Indicates no change from existing condition

TABLE 6. COLONIAL PARKWAY PEAK DISCHARGE VOLUMES—HYDROLOGIC SUMMARY

Saturation Factor

Cf Pre Post Pre/Post Pre Post Pre Post Pre Post Pre Post

1 0.22 0.30 1.10 37.1 32.5 3.70 3.95 34.6 34.6 31.0 45.5 2 0.21 0.26 1.10 30.6 27.5 4.10 4.35 68.7 68.7 65.1 86.8 3 742.2 742.2 737.0 803.0 4 0.24 0.24 1.10 18.0 18.0 5.40 5.40 12.5 12.5 17.8 17.8 5 0.25 0.25 1.10 23.5 23.5 4.70 4.70 6.7 6.7 8.6 8.6 6 0.33 0.33 1.10 40.0 40.0 3.50 3.50 68.3 68.3 86.7 86.7 7 0.33 0.33 1.10 10.0 10.0 6.95 6.95 1.5 1.5 3.9 3.9

Note: Discharge Volume Q = CIA(Cf)

TR-55 Model

Drainage Area

A (acres)

Intensity

I (in/hour)

Discharge Volume

Q25 (cfs) Location

Time of Conc.

Tc (min)C

259 333/41066

Figure 6

CULVERT HYDRAULICS PERFORMANCE

The existing double 36-inch RCP culvert at Location 3 in the Paper Mill Creek watershed is critically undersized. The culvert drains runoff flows from more than 80 percent of the entire watershed, indicating that its poor hydraulic performance is the main cause of flooding of the Colonial Parkway. Culverts in Locations 2 and 4 are also undersized.

While the structural condition of the existing culvert systems appears to be good, each of the seven crossings assessed need cleaning. The main culverts, passing the largest volumes of stormwater, are filled with silt and debris occupying one-third to almost half of their capacity. This clogged condition, in addition to being undersized for storm conditions at some locations, results in the flooding of the parkway.

The following summarizes the findings of the watershed analysis:

Under present (existing) conditions, the culverts at Locations 3 and 4 do not convey the 25-year design storm event.

Under future conditions, the drainage areas for culvert Locations 1, 2, and 3 would be subjected to increased flows.

Under future conditions, culvert Locations 2, 3, and 4 do not adequately convey the 25-year design storm event.

The current analysis shows that culvert Location 3 is undersized for the 25-year design storm event and would require the most significant improvements in order to obtain required additional capacity for existing and future flows. The expected capacity for a 36-inch RCP at the same length and grade as the existing culverts is only 80 cfs, thus 160 cfs for a dual culvert system. Based on the relatively low amount of cover over the existing culverts and the amount of flow that needs to be conveyed under the parkway, a much larger hydraulic structure needs to be installed. Table 7 provides a summary of preliminary options for addressing the drainage problems.

TABLE 7. COLONIAL PARKWAY STORM SEWER INFRASTRUCTURE RECOMMENDATIONS

Location Type Deficiency Recommendation

Option 1 - Replace existing 24" HDPE with

30" RCP

Option 2 - Add one parallel 24" RCP Option 1 - Parallel Culvert System - Double 9x5 RCBC Option 2 - CON-SPAN, 4'x28'

Option 1 - Install one parallel 24" RCP

Option 2 - Replace existing 15" RCP with one

24" RCP

2 24-inch HDPE / 30-inch

HDPE

Does not convey Future 25-yr storm

4 15-inch RCP Does not convey Existing or Future Condition 25-yr Storm Event

3 Double 36-inch RCP Does not convey Existing or Future Condition 25-yr Storm Event

STORMWATER MANAGEMENT

As part of this study, it was determined that the National Park Service should not be required to implement any stormwater improvements associated with the Colonial Parkway. Because the NPS is not engaging in new construction activity or improvements, nor adding impervious area to the parkway, which would increase runoff and flows to the watershed, their responsibility is to convey upstream flows.

Ultimately, any development upstream from the parkway must adhere to current state and local stormwater management regulations and requirements. In adhering to these laws, any increase in runoff resulting from new development must be mitigated and reduced to existing flow rates before release downstream. Despite this requirement for upstream development, there is still an increase in the overall volume of runoff due to the increased imperviousness of any new development. Runoff from new development is essentially metered out over a longer period of time to meet the pre-development requirement. The following recommended options were developed to accommodate the increased peak flow volume for the 25-year design storm event.

Option 3 is provided for attenuating the peak flow from a post-developed 2-year design storm event (245 cfs) to a pre-developed peak flow rate (205 cfs). This allows the upstream development to comply with state stormwater management regulations.

EROSION CONTROL

Indications of erosion and potential scouring exist at the outlets of the large culvert outfalls due to the outlet velocity of the culverts. The velocity measured at the downstream end is typically higher than the maximum natural stream velocity. This higher velocity causes stream bed scour and bank erosion for a limited distance downstream from the outlet. Outlet protection or an energy dissipation device is recommended at the outfalls where noted.

Sediment deposits usually occur within the culvert barrels at flow rates smaller than the design flow. The deposits may be removed during larger floods depending on the relative transport capacity of flow in the stream and the culvert, compaction and composition of the deposits, flow duration, ponding depth above the culvert, and other factors. Culvert location in both plan and profile is of particular importance to the maintenance of sediment-free culvert barrels. Deposits occur in culverts because:

The sediment transport capacity of flow within the culvert is less than in the stream.

At moderate flow rates, the culvert cross section is larger than that of the stream, so the flow depth and sediment transport capacity is reduced.

Point bars form on the inside of stream bends and culvert inlets placed at bends in the stream are subjected to deposition in the same manner. This effect is most pronounced in multiple-barrel culverts with the barrel on the inside of the curve often becoming plugged with sediment deposits.

Abrupt changes to a flatter grade in the culvert or in the channel adjacent to the culvert induce sedimentation. Gravel and cobble deposits are common downstream from the break in grade because of the reduced transport capacity in the flatter section.

EROSION AND SEDIMENT CONTROL IMPROVEMENTS

In implementing the necessary culvert improvements to meet the established criteria for this analysis, the required outlet protection improvements should be installed with the associated culvert improvements.

Outlet velocities were reviewed as part of this analysis and improvements are recommended where applicable. Implementing the recommended improvements at the outfalls addresses the protection of the downstream channel. In most cases, improving culvert conveyance reduces the outlet velocities.

The outlet velocity of a culvert is the velocity computed at the downstream end of the culvert and is typically higher than the maximum natural stream velocity.