city of gaston storm drainage master plana0e7bb59-ba39-470a-b0f2... · this master plan update was...

200 S.W. Market St. Suite 500Portland, Oregon 97201

503-295-4911503-295-4901 (Fax)

City of Gaston Storm Drainage Master Plan

08 December 2008

Prepared for

City of Gaston PO Box 129

Gaston, Oregon 97119

K/J Project No. 0891012.10

City of Gaston, Storm Drainage Master Plan i y:\projects\08proj\0891012.10_gastonplanning\09-reports\stormwater master plan\final 12_08\sw master plan_final_12_08.doc

Table of Contents

List of Tables................................................................................................................................ iii

List of Figures............................................................................................................................... iii

List of Appendices........................................................................................................................ iii

Section 1: Executive Summary...................................................................1-1 1.1 Purpose...............................................................................................1-1 1.2 Study Area ..........................................................................................1-1 1.3 Planning Projections ...........................................................................1-1 1.4 Conveyance System Analyses............................................................1-1 1.5 Capital Improvement Program ............................................................1-1

Section 2: Introduction ............................................................................... 2-1 2.1 Background .........................................................................................2-1 2.2 Authorization .......................................................................................2-1 2.3 Purpose for Study ...............................................................................2-1 2.4 Scope of Work.....................................................................................2-1

Section 3: Study Area Characteristics.......................................................3-1

3.1 General ...............................................................................................3-1 3.2 Existing Environment...........................................................................3-1

3.2.1 Topography .............................................................................3-1 3.2.2 Geology/Soils ..........................................................................3-1 3.2.3 Climate ...............................................................................3-1 3.2.4 Hydrology ...............................................................................3-1

3.3 System Mapping .................................................................................3-2 3.4 Service Area and Basin Boundaries....................................................3-2

3.4.1 Storm Drainage Basins............................................................3-2 3.4.2 Conveyance System................................................................3-4

Section 4: Basis of Planning....................................................................... 4-1

4.1 Introduction .........................................................................................4-1 4.2 Land Use and Development Area Projections ....................................4-1 4.3 Flow Analysis ......................................................................................4-2 4.4 Guiding Principles ...............................................................................4-2

Section 5: Stormwater System Analysis .................................................... 5-1 5.1 Introduction .........................................................................................5-1 5.2 Spreadsheet Analysis Method.............................................................5-1

5.2.1 Runoff coefficient for the drainage areas.................................5-1

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5.2.2 Equivalent Impervious Runoff Area .........................................5-2 5.2.3 Time of Concentration .............................................................5-2 5.2.4 Rainfall Intensity ......................................................................5-2 5.2.5 Runoff Discharge.....................................................................5-2 5.2.6 Hydraulic Analysis ...................................................................5-3

5.2.6.1 Cottonwood Street .................................................5-3 5.2.6.2 Park Street .............................................................5-3 5.2.6.3 Carol Drive.............................................................5-4 5.2.6.4 Trail Street .............................................................5-4

Section 6: Master Plan................................................................................ 6-1 6.1 Non-Structural Recommendations ......................................................6-1 6.2 Structural Recommendations..............................................................6-2

6.2.1 Cottonwood Street...................................................................6-2 6.2.2 Park Street...............................................................................6-2

Section 7: Funding Issues........................................................................... 7-1 7.1 State/Federal Grants and Loans .........................................................7-1 7.2 Debt Financing ....................................................................................7-1 7.3 System Development Charges............................................................7-1 7.4 Fee-In-Lieu of On-Site Detention ........................................................7-2 7.5 Improvement Districts and Special Assessments ...............................7-2 7.6 Plan Review and Inspection Fees.......................................................7-2 7.7 Stormwater Service Charges ..............................................................7-2 7.8 Ad Valorem Taxes...............................................................................7-2

References.................................................................................................................................7-3

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List of Tables

3.1 Storm Drainage System Inventory 4.1 Population Forecast 5.1 Runoff Coefficients By Land Use 5.2 Rainfall Duration and Intensity

List of Figures

1 Existing Storm Drainage Conveyance System 2 Existing Storm Drainage Conveyance System with Basins

List of Appendices

A FEMA Floodplain Map B Storm System Evaluation for Cottonwood, Park Street, Trail Street, and Carol Drive C CIP Cost Estimates

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Section 1: Executive Summary

1.1 Purpose This Master Plan update was prepared to evaluate the City of Gaston’s (City) storm drainage system. The master plan is a key component in the decision process for both continued maintenance and upgrade improvements to the storm drainage system. The scope of this document includes an evaluation of current and future needs, a projection of current and future flows, proposed system improvements, and a Capital Improvement Plan (CIP).

1.2 Study Area The City’s storm drainage system serves nearly all of the homes and businesses within the city limits as well as a portion of the developed area outside the city’s limits but within the urban growth boundary (UGB).

1.3 Planning Projections Storm flow simulations based on land use analysis, City wide zoning, and future land use projections have been developed for system modeling and evaluation. City provided data on land use and zoning was incorporated into the flow estimation process.

1.4 Conveyance System Analyses Current conveyance system conditions and future system requirements have been evaluated through hydrologic and hydraulic simulation. The current conditions have been summarized from information provided by the City. Various solutions addressing these storm system problems are evaluated.

1.5 Capital Improvement Program Future conveyance has been evaluated using an Excel spreadsheet model. Flow projections for existing land use and buildout conditions indicate capacity issues on Cottonwood Street and Park Street.

A total of 3 CIPs have been developed based on the system evaluations. Two of these are estimated as medium priority projects, to be completed on a 6-10 year timeframe. The other is an annual budget cost for routine inspection of the existing system.

• Cottonwood Street. A replacement of the existing 8-inch diameter storm drainage line along Cottonwood Street between 3rd street and Front Street with 645 feet of new 12-inch diameter storm pipe is recommended. A budget-level estimate of $186,000 is provided to allow for replacement of this line.


• Park Street. A replacement of the existing 12-inch diameter storm drainage line along Park Street from the Gaston School east to Front Street with 1,000 feet of new 18-inch diameter storm pipe is recommended. A budget-level estimate of $233,000 is provided to allow for replacement of this line.

• Annual TV inspection. An annual budget of $10,000 is recommended to allow for annual inspection and/or documentation of storm system infrastructure, condition, and construction details. There are portions of the system that do not have manhole access for inspections and maintenance. This annual budget would facilitate inspection of these areas.


Section 2: Introduction

2.1 Background The City’s storm drainage system provides service to more than 630 residents covering more than 90 acres of area, including all businesses within the city limits. The City operates and maintains all of the storm drainage system within the City limits including roadside ditches and storm piping systems within County roads. Currently, there are little to no stormwater improvements outside the City limits but within the urban growth boundary (UGB).

The original Storm Drainage Master Plan was completed in 1989, although a complete copy has not been located. Since its completion, ongoing changes have occurred throughout the City that warrant a new Storm Drainage Master Plan that will encompass the City’s needs both today and in the future.

The goal in developing this 20-year Master Plan is to give the City a usable, living document that can actively address storm drainage needs. This Plan will provide direction for existing and future capacity needs, and the type of system that is intended to meet those needs. Additionally, by tying the Master Plan to the rate of development or pending regulations, it will be flexible enough to account for changing conditions.

2.2 Authorization In May 2008, Kennedy/Jenks Consultants was authorized by the City of Gaston to prepare this Storm Drainage Master Plan.

2.3 Purpose for Study The purpose of this study is to analyze the City’s existing stormwater system for current hydraulic adequacy, as well as the system’s hydraulic performance under future land use requirements. Through this process, this Master Plan has been developed. This updated Plan will allow the City to address key issues required to keep the storm drainage system working as efficiently as possible and continue to meet the needs of its residents.

2.4 Scope of Work The scope of work needed to accomplish this study has been defined by the project team. These objectives will guide the project and can be used as a measure of project accomplishment.

• Gather and review stormwater system inventory data pertinent to this study.

• Review the updated system map and expand upon the existing system service areas and basin boundaries.


• Review the comprehensive plan and zoning code for consistency with stormwater standards outlined in this plan.

• Describe the existing stormwater system, identify drainage basins, and inventory of known problem areas.

• Develop land use projections as a basis for formulating flow projections.

• Develop, validate, and analyze sections of the stormwater system through an Excel based model program. This is to include evaluation of current flow and pipe conditions.

• Identify and prioritize capacity deficiencies and needs, forming the foundation for the Capital Improvement Program (CIP), if needed.

• Describe the general funding mechanisms and programs available.

• Prepare a storm drainage master plan report summarizing the findings and recommendations.


Section 3: Study Area Characteristics

3.1 General The City of Gaston is located 30 miles west of Portland. State highway 47 intersects the City, with low flat lands on the east and rolling hills transitioning into the coastal range to the west. The City covers approximately 156 acres, with a total UGB area of approximately 290 acres. The 2007 population estimate was approximately 650 citizens.

3.2 Existing Environment

3.2.1 Topography The study area is situated in the western part of the Tualatin River Valley. The area is typical of the Tualatin River Valley, with wide areas of lowlands encroached by steeper, more prominent slopes to the west. The City is bordered by the Tualatin River to the north and Wapato Creek to the east; each of which are fed by naturally occurring drainage paths and hard piped systems. An unnamed intermittent stream flows to the east parallel to Olson Road. Areas to the north along the Tualatin River and east along Wapato Creek have the lowest elevations at approximately 170 feet mean sea level (MSL) while the western areas near elevations as high as 460 feet MSL.

3.2.2 Geology/Soils Soils in the area are classified as Laurelwood Silt Loam in the City area west of Front Street, Woodburn Silt Loam in the area of East Main Street, and Carlton Silt Loam along SW South River Road (Source: Soil Survey of Washington County, United States Department of Agriculture Soil Conservation Service). These soil types percolate slowly and are not conducive for stormwater infiltration into the subsurface. They typically exhibit low bearing strengths.

3.2.3 Climate The climate is typical of the Pacific Northwest; moderate seasons with few temperature extremes. The spring season typically begins in March while winter has begun by November. The average annual rainfall is approximately 44 inches with an average yearly snow fall of 10 inches. The heaviest of the rains typically falls between November and February. Temperatures of 30 to 45 and 53 to 83 degrees Fahrenheit are average winter and summer temperature ranges, respectively.

3.2.4 Hydrology The City’s UGB abuts the Tualatin River to the north and Wapato Creek and Wapato Lake to the east, and Blackjack Creek to the south. Since the study area varies in topography, some of the City has sufficient relief, while areas in the lowlands can exhibit occasional flooding. The U.S. Corps of Engineers has developed flood plains for the Tualatin River. These flood plains describe the areas in immediate proximity to Wapato Lake as having some flooding possible. The Wapato


Lake area is hydraulically controlled with manmade series of diversion structures to manage the water levels in the lake; allowing flooding in the winter and then draining the lake bed for farming in the spring and summer (US Fish and Wildlife, April 2006).

3.3 System Mapping As part of the Master Plan, system maps have been developed using the most current Washington County geographic information system (GIS) coverages. These maps incorporate many aspects central to the Master Planning effort including, roadways, rivers, wetlands, parcels, land uses, zoning, and other local information. Additionally, more storm specific information was added such as existing pipes, manholes, pipe sizes, and pipe material. The GIS mapping network also includes information that was used to develop the individual flow patterns within each delineated flow basin.

3.4 Service Area and Basin Boundaries The major storm drainage basins were delineated based on the existing drainage system layout and topographical contours. A map illustrating the service area is included as Figure 1 and the drainage basins as Figure 2. Generally, the northern portions of the City drain to the Tualatin River, while southern and eastern portions of the City drain to a ditch along Front Street just north of the Olson Road intersection. The currently undeveloped area south of Hedin Terrace, drains south towards the unnamed creek just south of Olson Road. Development in this area is likely. Undeveloped areas in the northwest portion of the City, west of Costelloe Drive, drain toward the Tualatin River.

3.4.1 Storm Drainage Basins The City of Gaston is comprised of 15 major drainage basins with a total combined drainage area of approximately 290 acres. These basin boundaries are shown in the infrastructure map on Figure 2. General descriptions of each drainage basin are included below:

• Carol Drive. This 9.9 acre basin drains the Carol Drive right-of-way, from the southern end of the road north toward SW South Road. Land use in this area is single-family residential. Much of this basin is undeveloped, therefore stormwater runoff is expected to increase. Stormwater from this basin enters a ditch along SW South road and flows west approximately 200 feet where it crosses to the north via a 16-inch concrete culvert. The northern end of this culvert is an outfall. Stormwater that flows in this basin enters the Tualatin River.

• Trail Street. This 19.9 acre basin drains the Hedin Terrace and Vista Ridge areas, Salter Street west of 6th Street, most of Trail Street, and Church Street west of Trail Street. Land use in this area is single-family residential, with no development expected to significantly increase stormwater runoff in the basin. There are two areas available for future subdivision, each approximately 1 acre in size. There are two below-ground detention systems; the first consisting of a 36-inch diameter steel pipe located in the Hedin Terrace development, and the second consisting of two 60-inch steel pipes located west of the Trail Street and Salter Street intersection. Stormwater in this basin flows north and across


Church Street, where it is conveyed via 15-inch pipe approximately 200 feet. Flows are then split to two separate outfalls via a ditch. The western end of the ditch has a 24-inch pipe at the outfall and the eastern end of the ditch has a 12-inch pipe at the outfall. The 12-inch pipe outfalls to an area that has been enhanced for infiltration. Stormwater that flows in this basin enters the Tualatin River, located several hundred feet to the north, as surface flows from the 24-inch outfall and shallow subsurface flows from the 12-inch outfall.

• Central Church Street. This 6.1 acre basin consists of a short section of Church Street, west of Trail Street. Land use in this area is single-family residential, with no appreciable development expected to increase stormwater runoff in the basin. Stormwater drains via a series of 8-inch pipes, then across Church Street to an outfall. Stormwater that flows in this area enters the Tualatin River.

• 3rd Street. The northern end of 3rd Street is a cul-de-sac. This cul-de-sac area drains into a 12-inch stormwater line that outfalls into the Tualatin River. This drainage basin area is approximately 5.8 acres. Land use in this area is single-family residential, with no appreciable development expected to increase stormwater runoff in the basin.

• Mill Street. This 6.2 acre basin extends from the intersection of Front Street and Main Street north to Mill Street. Flows from the northern end of 3rd Street are also included in this basin. An outfall enters the Tualatin River just north of Mill Street. Land use in this area is a mix of single-family residential and commercial, with no appreciable development expected to increase stormwater runoff in the basin.

• West Main. This area encompasses West Main Street to 3rd Street and Church Street approximately 400 feet beyond the 3rd Street intersection. This basin includes flows north of Park Street along 2nd Street. Flows in this basin join the Front Street trunk line at the intersection of Main Street and Front Street, where flows continue south toward Olson Road. This basin is approximately 8 acres. Land use in this area is a mix of single-family residential and commercial, with no appreciable development expected to increase stormwater runoff in the basin.

• Park Street. This 18.1 acre basin area begins on Salter Street east of 6th Street. Stormwater from Gaston Schools, stormwater along Park Street, and a 330 foot line along 3rd Street are also included. This basin drains to the Front Street trunk line at the intersection of Park Street and Front Street, where flows continue south toward Olson Road. Land use in this basin is primarily single-family residential, with additional use by the school which includes impervious services of buildings and parking lots.

• Cottonwood Street. This 6.6 acre basin collects flows along Cottonwood Street just west of 3rd Place. Flows in this basin join the Front Street trunk line at the intersection of Cottonwood and Front Street, where flows continue south toward Olson Road. Land use in this basin is primarily single-family residential, with additional use by the school’s open space and playing fields.

• Front Street. This basin collects flows generated along Front Street as well as connections from West Main, Park Street, and Cottonwood Street basins. Land use in this


area is primarily commercial. The drainage area is approximately 7.4 acres. The combined basin flows in Front Street, opens to a ditch south of Olson Road and then crosses Front Street using two 36-inch culverts. Flows then enter Wapato Creek.

• Gaston Heights. This area of approximately 59.7 acres is currently undeveloped. This area is defined by Hedin Terrace to the north, Olson Road to the south, UGB to the west, and Upper Olson Road basin to the east. The natural drainage pattern off this hillside is south to Olson Road. There is currently no stormwater infrastructure in this basin except possibly a few culverts that cross under Olson Road.

• Lower Olson Road - North. This area of approximately 20.1 acres is currently undeveloped. It is defined by Cottonwood Street to the north, Olson Road to the south, Gaston Heights basin to the west, and Front Street to the east. The natural drainage pattern from this area is south to Olson Road, then easterly along the north ditch of Olson Road. There is currently no stormwater infrastructure in this basin.

• Lower Olson Road - South. This undeveloped area is bounded on the west, south, and east by the UGB and Olson Road to the north. Stormwater drainage in this area flows north to the unnamed creek on the south side of Olson Road. This basin is approximately 45.2 acres. Flows from the City drain into this basin, then cross under Front Street (HWY 47) via 2 36-inch culverts. There is currently no other stormwater infrastructure in this basin.

• SW South Road - West. This 22 acre undeveloped area is north of Gaston Heights, in the northwest corner of the UGB. Relatively steep slopes in this area drain to the north to SW South Road and on to the Tualatin River. There is currently no stormwater infrastructure in this basin.

• SW South Road – East. This 24.4 acre undeveloped area is adjacent to the basin above, however flows are toward the north-eastern direction. It is bounded by Gaston Heights basin to the south and the Carol Street and Trail Street basins to the east. As with the western portion of SW South Road – West basin, there are relatively steep slopes in this basin. Surface flows also drain toward SW South Road and on to the Tualatin River. There is currently no stormwater infrastructure in this basin.

• East Main Street. The 30.8 acre area east of Front Street and inside the UGB is incorporated within this basin. Surface water flows on the northern side of this basin toward the Tualatin River. The area south of East Main Street, including the City park, drains to Wapato Creek. There is currently no identified stormwater infrastructure in this basin.

3.4.2 Conveyance System The conveyance system is composed of open drainageways, ditch systems, and gravity storm drainage pipelines. Approximately 13,000 feet of storm pipeline has been identified in the system inventory and is the responsibility of the City. Table 3.1 (provided at the end of the document) summarizes the system by size and material. Approximately 1% of the system contains no documented records of pipe size or material properties.


Section 4: Basis of Planning

4.1 Introduction Regional stormwater quantity and quality facilities within the City limits is the responsibility of the City. The City is also not obligated to accept any responsibility for any programs or activities outside of the City limits. The Phase II municipal separate storm sewer system (MS4) program, regulated by the U.S. Environmental Protection Agency and administered in the State of Oregon by DEQ, is not a pertinent requirement to the City. The City however, has adopted Clean Water Services programs, rules, policies and standards regarding stormwater application and design. Additionally, any private development must meet the requirements set forth by the City code, including Clean Water Services Design Standards. All improvements to the City’s stormwater system are to comply with the most current version of Clean Water Services Design Standards.

4.2 Land Use and Development Area Projections Conveyance system flows are based on land use information provided by the Counties and the City zoning map and population projections provided by Portland State University.

Population projections through the 20-year planning period were estimated using a mixture of techniques. The population through 2018 assumes the new Gaston Heights subdivision, located in the northern section of the Gaston Heights drainage basin, will include as many as 300 homes at an average density of 2.61 people per home, resulting in a population increase of 783 between 2008 and 2018. The 2028 population assumes the historical Washington County growth rate for the City of Gaston between 2018 and 2028, resulting in a 2028 population of 1728. While storm flow estimates are governed predominantly by land use and impervious coverage, the population information is provided here to substantiate the expected timing of changes to land uses within the UGB.

Table 4.1 presents population estimates for Gaston in 2006, as well as forecasted populations for the City between the years 2008 and 2028, on ten-year increments. The City’s comprehensive plan states a target population of 2600 people inside the UGB, which will occur beyond the 20-year planning period of this study.

Table 4.1 Population Forecast

Population Projection Year Population 2006 630 2008 639 2018 1422 2028 1728


4.3 Flow Analysis Hydrologic and hydraulic analysis for selected trunk stormwater systems was conducted by applying spreadsheet methods to the Rational Method (peak flow determination) and Manning’s equation (conveyance capacity) under gravity flow conditions.

4.4 Guiding Principles The following guiding principles are to be applied to the development of stormwater solutions and to the development of the overall master plan for the City of Gaston:

● Establish an inventory and map of Gaston’s stormwater infrastructure system.

● Establish the adequacy of the Gaston’s existing stormwater system through interviews with City staff and selective analysis of trunk storm systems.

● Establish or confirm policy guidance and standards for the design of Gaston’s public stormwater infrastructure system. Consider Clean Water Services stormwater guidelines in development of the City’s stormwater system, including detention requirements.

● Consider and encourage the use of low impact development (LID) principles for application to new development and redevelopment.

● Consider Goal 5 implications (open space/riparian corridors) in development of the overall master plan. Goal 5, part of Oregon’s statewide planning goals and guidelines, is focused on protecting natural resources and conserving scenic and historic areas and open spaces.

● Establish an equitable means of implementing stormwater system expansion using an appropriate combination of City funds, system development charges, and developer provided improvements.

● Follow Oregon Civil Law Doctrine of Drainage, which stipulates that adjoining landowners are entitled to have the normal course of natural drainage maintained. The lower owner must accept stormwater which naturally comes to his land from above, but he is entitled not to have the normal drainage changed or substantially increased. The lower landowner may not obstruct the run-off from the upper land, if the upper landowner is properly discharging the water.


Section 5: Stormwater System Analysis

5.1 Introduction City wide stormwater infrastructure has been investigated through a combination of existing document review, site visits, and discussions with City staff. Through this process, areas requiring further analysis have been found. While a number of issues such as current City staff concerns, current areas experiencing flooding events, ongoing problematic maintenance needs, and areas of expected growth could generally indicate a need for further review, areas of expected growth and potential redevelopment, were the primary reason for further review. Areas requiring further review have been included in the spreadsheet analysis. The spreadsheet analysis evaluates both the current and future capacity of the system. Throughout this section, the preparation of the spreadsheet inputs and analysis results will be discussed.

5.2 Spreadsheet Analysis Method The storm system hydrologic analysis involved the determination of the following parameters:

5.2.1 Runoff coefficient for the drainage areas The basis of the generated stormwater runoff is a function of the impervious surfaces within each of the design basins. An impervious surface is one in which water is unable to infiltrate. These impervious surfaces are typically man-made systems such as roadways, parking lots, and structure rooflines. When rainfall lands on these types of surfaces, runoff occurs.

The runoff coefficient is related to land use under build out conditions. In this analysis, a percent of the total area that is covered with impervious surface (percent impervious) was defined for each type of land use, as shown in Table 5-1. After determining the amount of each type of land use in a drainage area (the actual amount for existing conditions and the maximum amount the zoning allows for future conditions), the percent impervious was used to calculate the total pervious and impervious surface in that drainage area. The runoff coefficient for each area was calculated by applying a coefficient of 0.20 to its percent of pervious area and a coefficient of 0.98 to its percent of impervious area.

Table 5.1: Runoff Coefficients By Land Use

Land Use Runoff Coefficient Open Area/Undeveloped 0 - 0.10 Single Family Residential 0.3 - 0.5

Commercial 0.5 – 0.8 Industrial Park 0.5 – 0.8


5.2.2 Equivalent Impervious Runoff Area The equivalent impervious runoff area for each drainage area was calculated by multiplying its runoff coefficient by its total acreage. The higher the runoff coefficient number is, the greater the impervious surface and amount of surface water flow generated.

5.2.3 Time of Concentration The time of concentration for a drainage area is defined as the time it takes for storm runoff to travel to the storm inlet from the most hydraulically distant point in the drainage area. Along the length of a storm sewer system, the time of concentration was calculated as the sum of the initial time of concentration and the travel time along the length of the system.

5.2.4 Rainfall Intensity Rainfall intensity is a function of the duration of a storm. The shorter the duration of a given frequency storm, the higher the rainfall intensity. Time of concentration was used as an estimate of duration, and rainfall intensity was estimated from a chart developed by the Oregon Department of Transportation (ODOT). ODOT used historical Oregon rainfall information to develop a series of intensity-duration-frequency curves for zones with the same rainfall characteristics; the City of Gaston is in Zone 8 under this system. Table 5-2 summarizes the data for this zone.

Table 5.2: Rainfall Duration And Intensity

Storm Duration Rainfall Intensity (inches/hour)

(minutes) 10-Year Storm 25-Year Storm

5 2.50 2.65

10 1.78 2.05

15 1.50 1.75

20 1.30 1.50

25 1.15 1.35 Source: Oregon Department of Transportation Hydraulics Manual, Appendix A

5.2.5 Runoff Discharge The runoff discharge for a drainage area was calculated by multiplying the equivalent impervious runoff area by the rainfall intensity. Manholes were used as collection points because this was an evaluation of main lines; inlet spurs were not investigated. Runoff discharges were calculated along the length of each system.


5.2.6 Hydraulic Analysis Storm tabulation spreadsheets were used to evaluate the storm sewers for existing and future development conditions. The full-flow gravity capacity and velocity of each pipe segment were calculated, based on the segment’s material, slope, diameter and length, the pipe invert elevation at the upstream and downstream ends, and the elevation of manhole tops.

This spreadsheet model assumes that all available stormwater flow has the ability to make it into the storm drain system; meaning there is no ponding in streets or yards and no impediments to stormwater entering the pipe network.

5.2.6.1 Cottonwood Street The Cottonwood Street system drains an area of approximately 6.6 acres. Existing conditions in this system assumed primarily single-family residential, with a small amount of infill development expected to impact future flows. Due to the lack of available stormwater system design data, the slope of the stormwater pipe was assumed to be parallel to the surface grade, but at a depth of 4-feet below ground surface. Drainage from this basin includes flow generated along Cottonwood Street. A small portion of the uppermost portion of this collection system is 12-inch concrete sewer pipe (CSP). Just west of the intersection of 3rd Street, the stormwater pipe changes to 8-inch PVC. This pipe continues east to the intersection with Front Street.

The 8-inch sections of pipe along Cottonwood Street starting at approximately 3rd Street appear to be at or over capacity for flows assumed entering the system during a 25-year storm event and at capacity during a 10-year storm event. Due to relatively steep surface slopes in this area, it is possible that stormwater flows are bypassing catch basins along Cottonwood and are entering the storm system in the catch basins at Front Street, where there are three catch basins collecting stormwater flows. These catch basins connect to the Front Street trunk line that continues south along Front Street.

5.2.6.2 Park Street The Park Street system drains an area approximately 18.1 acres. Pipe invert elevations were obtained from design drawings (Robert Meyer Consultants, 1985). The uppermost portion of this basin includes an 8-inch pipe along Salter Street, 10-inch concrete culvert pipe (CCP) pipe along the north-western property line of the Gaston School, which then connects to a 12-inch CCP down (east) Park to the intersection with Front Street, where it connects with the 18-inch reinforced concrete pipe stormwater line. There is also one catch basin located at the intersection of Park and Trail Street that contributes to this basin.

Existing conditions in this system assumed primarily single-family residential, with commercial-type development at the school. Commercial development areas have a higher amount of impervious surfaces, and therefore typically produce more stormwater runoff. Future development plans for Gaston are not anticipated to impact flows within the Park Street basin.

The lower (eastern) sections of Park Street, from the school to Front Street appear to be at or over capacity for the flows assumed entering the system during a 25-year storm event. Similar to Cottonwood Street basin, under full-flow pipe conditions, it is possible that stormwater flows


are bypassing catch basins along Park Street, flowing along the surface, and entering the storm system in the catch basins at Front Street.

5.2.6.3 Carol Drive The Carol Drive system drains an area approximately 9.9 acres. Due to the lack of available stormwater system design data, the slope of the underground stormwater pipe was assumed to be parallel to the surface grade, but at a depth of 4-feet below ground surface. Flows in the basin are collected using 12-inch diameter CSP.

Existing conditions in this system assumed primarily open space, with some single-family residential contribution. Existing development conditions also assumed complete development of the basin along Carol Drive with single-family residential homes, as already approved for development by the City. Developer provided stormwater infrastructure has been installed to support this development effort. Future conditions assumed no infill development that would contribute to the existing stormwater infrastructure.

Both existing development conditions and assumed future development conditions produce flows that are within the existing capacity of the system during a 25-year storm event.

5.2.6.4 Trail Street The Trail Street system drains approximately 19.9 acres. Recent development in this system included a series of onsite detention pipes to mitigate downstream impacts of new residential development, including Hedin Terrace and Vista Ridge Court. The goal of this evaluation was to determine if the system was adequately sized before the construction of the system expansion and detention system. The detention system connected to an existing 12-inch pipe. This pipe collects additional flows from Salter Street. From the Salter Street intersection, the pipe reduces to an 8-inch PVC pipe down to the 12-inch line at Church Street. There is also a 12-inch pipe that collects flows along the western side of Trail street that also connects at Church Street.

This evaluation included estimating the capacity of the 8-inch diameter pipe along the eastern side of Trail Street, the 12-inch pipe along the western side of Trail Street, and the downstream 15-inch line that runs along Church Street. The stormwater line along Church is relatively unknown beyond size. There is no manhole where the Trail Street connections are made, therefore invert elevations were assumed to be 4 feet below ground surface. This evaluation assumed that the detention system provides flows equal to predevelopment conditions of Hedin Terrace and areas south of the Salter Street intersection.

Conditions in this system produce flows that are within the existing capacity of the system during a 25-year storm event. The flows in the 15-inch line along Church Street are at or slightly over capacity using the assumptions of upstream pipe depth and the resulting slope of the pipe. An addition of a manhole, for access to connecting pipes, would provide additional detail that would refine the model and possibly determine excess capacity.


Section 6: Master Plan

There are two types of control measures the City should employ with the future expansion of the stormwater infrastructure system: non-structural controls (e.g. planning and studies) and structural controls (e.g. pipe upgrade). These are summarized in the following subsections.

6.1 Non-Structural Recommendations Almost all new development within the UGB will not connect to existing stormwater infrastructure. The new stormwater infrastructure will be provided by the developer. The following non-structural recommendations should be applied to the development of stormwater solutions in the City.

● Apply Clean Water Services (CWS) Design and Construction Standards for Sanitary Sewer and Surface Water Management (June 2007 or most current update) for new development and infill development.

● Consider and encourage LID standards and practices consistent with CWS Design Standards for new development, infill development, and City street improvements. Implementation of LID methods can reduce impervious surfaces, reduce stormwater runoff volume and velocity, and provide treatment to reduce pollutants entering receiving water bodies and streams. While steep slopes and unstable soils may prohibit infiltration, other technologies may be feasible (e.g. landscaping, treebox filters, swales).

● Consider incorporating CWS Design and Construction Standards into the City’s Development Code.

● Address the following conditions when evaluating development in basins currently contributing to Blackjack Creek south of Olson road (Gaston Heights, Lower Olson Road – North and South):

♦ Conveyance of stormwater across Olson Road,

♦ Flows and velocities in the receiving stream, and

♦ Impacts at the culverts crossing Front Street and into Wapato Creek.

● Undeveloped basins (SW South Road – East and West) currently contributing to the Tualatin River, and basins (Gaston Heights, Lower Olson Road – North and South) currently contributing to the intermittent stream south of Olson road, should be implemented with developer-provided improvements consistent with the flow directions on the City’s master plan map to preserve existing conditions. Development in these areas should be implemented with LID standards and practices consistent with CWS Design Standards, which include:

♦ Evaluation of downstream impacts,


♦ Evaluation of pre- and post-development flow conditions,

♦ Evaluation for the need for on-site detention, and

♦ Preservation of natural flow direction.

● The City should continue with the management and maintenance of the City’s stormwater infrastructure.

● The City should allocate funds for routine TV inspection and/or documentation of storm system infrastructure, condition, and construction details, particularly for areas where design and construction information is currently unknown. There are portions of the system that do not have manhole access for inspections and maintenance.

6.2 Structural Recommendations Our hydraulic analyses identified potential deficiencies in sections of two drainage basins, Cottonwood Street and Park Street. Sections of pipe in these areas are at or above capacity in a 25-year storm event. While interviews with City staff indicate that these areas are not currently causing problems with flooding, it is recommended that attention be paid to these areas and a potential upgrade made in the 6 to 20 year time period if problems arise or flooding occurs.

Additionally, there are two locations along the Trail Street basin that would benefit from installation of manholes. Manholes provide access for pipe inspection and cleanout.

These potential future improvements are summarized below:

6.2.1 Cottonwood Street Based on the analysis presented in Chapter 5, a replacement of the existing 8-inch diameter storm drainage line along Cottonwood Street between 3rd street and Front Street with 645 feet of new 12-inch diameter storm pipe is recommended. A budget-level estimate of $186,000 is provided to allow for replacement of this line.

6.2.2 Park Street Also based on the analysis presented in Chapter 5, a replacement of the existing 12-inch diameter storm drainage line along Park Street from the Gaston School east to Front Street with 1,000 feet of new 18-inch diameter storm pipe is recommended. A budget-level estimate of $233,000 is provided to allow for replacement of this line.


Section 7: Funding Issues

This section describes the range of alternative funding sources that municipalities have used in implementing drainage improvements. Stormwater service charges and system development charges have typically been viewed by municipalities as the most equitable and reliable methods for funding stormwater capital and maintenance needs. When used in combination, these methods also distinguish both public and private responsibilities

7.1 State/Federal Grants and Loans Various grant/loan programs are available at both the federal and state level. However, no single grant/loan program is available on a consistent, on-going basis for funding of local stormwater management. With communities competing on both a state-wide and even nation-wide basis, and with constraints on how grant/loan money is to be used, these sources can only serve to supplement an existing local funding program for stormwater management.

7.2 Debt Financing General obligation bonds and revenue bonds are two commonly used forms of debt financing for public infrastructure improvements. General obligation bonds, primarily used for major capital improvements, are subject to voter approval and are backed by the full credit of the government issuing them. Revenue bonds, on the other hand, may be sold and secured only by those specific revenue sources which are earmarked for their payment.

7.3 System Development Charges These charges are imposed on new development as a means to allocate planned new facility or upsizing costs required to accommodate growth to these new connections. In addition, these charges can also be used to recover a proportionate share of the City’s historical investment in existing facilities which also provides system capacity to new connections. System development charges address the question of who should pay for required up-sizing of the stormwater system due to new development and how historical payers into the system can recover their costs in providing facilities that accommodate future growth.

These charges may be applied to new development that is connecting to the existing stormwater system which is owned and paid for by the City, provided that the new connection exceeds predevelopment stormwater flow rate by NOT providing on-site detention. Development providing on-site detention and/or mitigation from the effects of newly generated impervious coverage to predevelopment standards would not create a capacity demand on the City’s existing stormwater system and would thereby not be required to pay the reimbursement portion of the system development charge.


7.4 Fee-In-Lieu of On-Site Detention These fees afford a land developer the option of either constructing an on-site stormwater detention facility in accordance with established design criteria, or paying a fee into a fund dedicated to the construction of an off-site or regional stormwater detention facility serving multiple properties. These fees tend to promote siting and construction of regional versus on-site detention facilities. However, cash flow necessary for a regional stormwater detention facility may not necessarily coincide with the required construction timing.

7.5 Improvement Districts and Special Assessments The concept of deriving funding from local improvement or special assessment districts is founded on quantifying benefits. For water, sewer or street improvements, these benefits can often be easily identified and thus quantified. However, drainage differs in the respect that upstream or hillside properties that are major contributors of runoff may not be specific recipi-ents of benefits.

7.6 Plan Review and Inspection Fees These fees are intended to recover the expense of examining development plans to ensure consistency with comprehensive land use and stormwater master plans, and to ensure that construction standards and regulations are met at the construction site. These fees are not intended to be a primary revenue generating source.

7.7 Stormwater Service Charges Another method gaining popularity for financing stormwater management is the utility-based service charge. Historically, the concept of considering stormwater as a public utility attracted very few communities. However, as other more conventional funding sources became difficult to obtain, and as federal requirements increase, the service charge concept has generated greater appeal. Service charges for stormwater management reflect a rationale that those who contribute to stormwater problems should logically contribute to the costs of providing mitigative services.

7.8 Ad Valorem Taxes Ad valorem taxes are taxes levied on a property as a direct result of "value added" to the subject property. However, with stormwater, there is no clear correlation between property value and contribution of runoff. Ad valorem taxes could provide a significant source of revenue, however with the apparent lack of equity, should not be considered a primary source for funding stormwater programs.


References

ODOT 2005. Oregon Department of Transportation Hydraulics Manual. Appendix A – Rainfall Intensity Duration Recurrence Intervals, 2005

Robert Meyer Consultants 1985. City of Gaston, Community Development Block Grant Project #4348. 1985

Soil Survey of Washington County, United States Department of Agriculture Soil Conservation Service

US Fish and Wildlife 2006. Tualatin River National Wildlife Refuge Proposed Wapato Lake Unit, Draft Land Conservation Plan and Environmental Assessment. U.S. Fish and Wildlife Service. April 2006

Tables

Table 3.1Storm Drainage System Inventory

Pipes: Material & Length (ft)Size (in) CSP CMP CONC RCP PVC ADS AS CCP Unknown TOTALS

8 33 1,438 210 1,055 2,736 10 87 1,204 360 1,651 12 721 300 60 273 877 1,360 1,510 5,101 15 445 520 965 18 44 983 160 630 1,817 21 725 72524 60 60 36 180 180 60 180 180

Unknown 528 528

Total Lineal Feet 754 180 344 1,828 2,403 877 180 3,404 3,973 13,943 Total Miles 0.14 0.03 0.07 0.35 0.46 0.17 0.03 0.64 0.75 2.64

Culverts: Material & Length (ft) % of "Unknown, Unknown" Pipes: 3.8%Size CONC CCP Laminated Wood Box Unknown TOTALS % of total material "Unknown" Pipes : 28.5%

12" 75 7516" 60 6036" 120 120

6' W x 4' H 60 60

Total Lineal Feet 60 120 60 75 315Total Miles 0.01 0.02 0.01 0.01 0.06

Trench Drains: Length (ft)Size (in) RCP

10 20

Total Lineal Feet 20Total Miles 0.00

Ditches: Length (ft)Size Earthen

N/A 1,010

Total Lineal Feet 1,010Total Miles 0.19

Legend:

ADS = Polyethylene Corrugated HDPE PipeAS = Aluminized SteelCCP = Concrete Culvert PipeCMP = Corrugated Metal PipeCONC = Concrete, UnspecifiedCSP = Concrete Sewer PipePVC = Polyvinyl ChlorideRCP = Reinforced Concrete Pipe, C76

Figures

Appendix A

FEMA Floodplain Map

Appendix B

Storm System Evaluation for Cottonwood, Park Street, Trail Street, and Carol Drive

25-YEAR STORM; EXISTING CONDITIONS FOR COTTONWOOD STREET

System Labels Runoff Area Hydrologic Calculations System InventoryStation Spur Area Runoff Equiv. Spur Total Time of Travel Rainfall Design Invert Pipe Pipe Full Flow Full Flow Length Pipe Invert Top of or MH Coeff. Area Sum Sum Conc. Time- Intensity Discharge Slope Size Mat'l Capacity Velocity Elevations U/S MH or CB

No. A C CA CA CA Tc Pipe I Q S D Qf Vf L U/S D/S Elev.(acres) (3)x(4) (acres) (min) (min) (in/hr) (cfs) (%) (in.) (cfs) (fps) (ft.) (ft) (ft) (ft)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 1.0 0.35 0.4 0.0 0.4 10.0 0.1 2.05 0.7 1.21 8 CSP 1.3 3.8 33 223.3 222.9 228.22 1.0 0.35 0.4 0.0 0.7 10.1 0.4 2.05 1.4 2.00 12 CSP 5.1 6.4 143 222.6 223.0 227.83 3.0 0.28 0.8 0.0 1.5 10.5 0.8 2.05 3.1 7.19 8 PVC 3.2 9.3 420 223.5 193.3 228.24 1.6 0.35 0.6 0.0 2.1 11.3 0.4 2.00 4.2 7.48 8 PVC 3.3 9.5 225 193.3 176.5 198.0

6.6

RAINFALL INTENSITYLook-up Table

Notes:Tc 25-yr pipe section 2 - assumed a 2% slope, surface grade is flat.

Due to lack of IE on pipes, depth assumed 4' bgs to top of pipe. CSP assumed 3" thickness.5.0 2.65 MH or CB elevations from Topographic Map, City of Gaston, Spencer B Gross, 19846.0 2.507.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

Filename: Cottonwood; 25-year; Exist. Cottonwood Page 1 of 3 Print Date: 12/9/2008

25-YEAR STORM; FUTURE CONDITIONS FOR COTTONWOOD STREET



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 1.0 0.35 0.4 0.0 0.4 10.0 0.1 2.05 0.7 1.21 8 CSP 1.3 3.8 33 223.3 222.9 228.22 1.0 0.35 0.4 0.0 0.7 10.1 0.4 2.05 1.4 2.00 12 CSP 5.1 6.4 143 222.6 223.0 227.83 3.0 0.35 1.1 0.0 1.8 10.5 0.6 2.05 3.6 7.19 12 CSP 9.6 12.2 420 223.2 193.0 228.24 1.6 0.35 0.6 0.0 2.3 11.1 0.3 2.00 4.6 7.33 12 CSP 9.7 12.3 225 193.0 176.5 198.0

6.6 Total Area


Notes:Tc 25-yr pipe section 2 - assumed a 2% slope, surface grade is flat.

Due to lack of IE on pipes, depth assumed 4' bgs to top of pipe. CSP assumed 3" thickness.5.0 2.65 MH or CB elevations from Topographic Map, City of Gaston, Spencer B Gross, 19856.0 2.50 Future conditions assumed an upsize in pipe sections 3 and 4 to 12 inch pipe7.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

10-YEAR STORM; EXISTING CONDITIONS FOR COTTONWOOD STREET

System Labels Runoff Area Hydrologic Calculations System InventoryStation Spur Area Total Runoff Equiv. Spur Total Time of Travel Rainfall Design Invert Pipe Pipe Full Flow Full Flow Length Pipe Invert Top of or MH Check Coeff. Area Sum Sum Conc. Time- Intensity Discharge Slope Size Mat'l Capacity Velocity Elevations U/S MH or CB


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 1.0 100% 0.35 0.4 0.0 0.4 10.0 0.1 1.78 0.6 1.21 8 CSP 1.3 3.8 33 223.3 222.9 228.22 1.0 100% 0.35 0.4 0.0 0.7 10.1 0.4 1.78 1.2 2.00 12 CSP 5.1 6.4 143 222.6 223.0 227.83 3.0 100% 0.28 0.8 0.0 1.5 10.5 0.8 1.78 2.7 7.19 8 PVC 3.2 9.3 420 223.5 193.3 228.24 1.6 100% 0.35 0.6 0.0 2.1 11.3 0.4 1.70 3.5 7.48 8 PVC 3.3 9.5 225 193.3 176.5 198.0

6.6 100% Total Area


Notes:Tc 10-yr 25-yr pipe section 2 - assumed a 2% slope, surface grade is flat.

Due to lack of IE on pipes, depth assumed 4' bgs to top of pipe. CSP assumed 3" thickness.5.0 2.50 2.65 MH or CB elevations from Topographic Map, City of Gaston, Spencer B Gross, 19856.0 2.507.0 2.458.0 2.259.0 2.15

10.0 1.78 2.0511.0 1.70 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.50 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.30 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.15 1.35

25-YEAR STORM; FUTURE CONDITIONS FOR PARK STREET

System Labels Runoff Area Hydrologic Calculations System InventoryStation Spur Area Runoff Equiv. Spur Total Time of Travel Rainfall Design Invert Pipe Pipe Full Flow Full Flow Length Pipe Invert or MH Coeff. Area Sum Sum Conc. Time- Intensity Discharge Slope Size Mat'l Capacity Velocity Elevations

No. A C CA CA CA Tc Pipe I Q S D Qf Vf L U/S D/S(acres) (3)x(4) (acres) (min) (min) (in/hr) (cfs) (%) (in.) (cfs) (fps) (ft.) (ft) (ft)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Salter 1.8 0.35 0.6 0.0 0.6 10.0 0.3 2.05 1.3 9.88 10 CCP 6.9 12.7 252 271.0 246.12 4.8 0.53 2.5 0.0 3.1 10.3 1.2 2.05 6.4 0.61 12 CCP 2.8 3.5 264 246.1 244.53 8.3 0.45 3.7 0.0 6.9 11.6 0.7 2.00 13.7 3.06 18 CCP 18.4 10.4 408 244.5 232.04 3.3 0.35 1.1 0.0 8.0 12.2 0.5 1.90 15.2 8.93 18 CCP 31.5 17.8 545 232.0 183.3

18.0

RAINFALL INTENSITY Notes:Look-up Table US elevation of School line - Project #5 design dwg

US and DS elevation of 2, 3, and 4 segments: Project #5 design drawingTc 25-yr Future conditions assume upgrade of pipe segments 3 and 4 to 18 inch CCP

5.0 2.656.0 2.507.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

25-YEAR STORM; EXISTING CONDITIONS FOR PARK STREET

System Labels Runoff Area Hydrologic Calculations System InventoryStation Spur Area Runoff Equiv. Spur Total Time of Travel Rainfall Design Invert Pipe Pipe Full Flow Full Flow Length Pipe Invert or MH Coeff. Area Sum Sum Conc. Time- Intensity Discharge Slope Size Mat'l Capacity Velocity Elevations

No. A C CA CA CA Tc Pipe I Q S D Qf Vf L U/S D/S(acres) (3)x(4) (acres) (min) (min) (in/hr) (cfs) (%) (in.) (cfs) (fps) (ft.) (ft) (ft)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Salter 1.8 0.35 0.6 0.0 0.6 10.0 0.3 2.05 1.3 9.88 10 CCP 6.9 12.7 252 271.0 246.12 4.8 0.53 2.5 0.0 3.1 10.3 1.2 2.05 6.4 0.61 12 CCP 2.8 3.5 264 246.1 244.53 8.3 0.45 3.7 0.0 6.9 11.6 0.9 2.00 13.7 3.06 12 CCP 6.3 8.0 408 244.5 232.04 3.3 0.35 1.1 0.0 8.0 12.4 0.7 1.90 15.2 8.93 12 CCP 10.7 13.6 545 232.0 183.3

18.0

RAINFALL INTENSITY Notes:Look-up Table US elevation of School line - Project #5 design dwg

US and DS elevation of 2, 3, and 4 segments: Project #5 design drawingTc 25-yr

5.0 2.656.0 2.507.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

Filename: Park; 25-year; Exist. Park Page 2 of 2 Print Date: 12/9/2008

CITY OF GASTON - STORM DRAINAGE MASTER PLAN

25-YEAR STORM; PREDEVELOPMENT CONDITIONS FOR TRAIL STREET



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 8.0 0.13 1.0 0.0 1.0 10.0 0.1 2.05 2.1 6.02 12 CSP 8.8 11.2 60 282.3 278.7 290.6Salter 1.5 0.23 0.3 0.0 1.3 10.1 1.0 2.05 2.7 8.18 8 PVC 3.5 9.9 600 276.1 227.0

3 1.2 0.35 0.4 0.0 1.8 11.1 0.1 2.00 3.5 13.60 10 CSP 8.1 14.9 80 227.0 216.14 0.0 1.8 11.2 1.4 2.00 3.5 0.73 12 CSP 3.1 3.9 335 216.1 213.7

10.7


Notes:Tc 25-yr Dual detention system is assumed to provide predevelopment condition flows at 90% open space, 10% residential

Section 1 MH, U/S, D/S elevations and U/S elevation for section 2 from Hedin Terrace development drawings.5.0 2.65 Salter: Drainage area includes Salter St. (Flows along Trail drain to Park St basin.) 6.0 2.50 US/DS elevation estimated from Topo elevation and depth bgs field measured.7.0 2.45 Section 3: Area includes flows collected in 12" line along west side of Trail and small 8" pipe at Church St.8.0 2.25 DS elevation assumes pipe 4 feet bgs. No manhole available for measurement9.0 2.15 Section 4: No additional inflows into this pipe section. US elevations assumed 4 feet bgs.

10.0 2.05 Elevations from Topographic Map, City of Gaston, Spener B Gross, 198511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

Filename: Trail Street; 25-year; Pre-dev Trail to 8" Page 1 of 2 Print Date: 12/9/2008

25-YEAR STORM; PREDEVELOPMENT CONDITIONS FOR TRAIL STREET - 14-INCH EQUIVALENT



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 8.0 0.13 1.0 0.0 1.0 10.0 0.1 2.05 2.1 6.02 12 CSP 8.8 11.2 60 282.3 278.7 290.6Salter 2.3 0.29 0.7 0.0 1.7 10.1 0.7 2.05 3.4 8.18 14 PVC 15.4 14.4 600 276.1 227.0

3 0.4 0.35 0.1 0.0 1.8 10.8 0.1 2.05 3.7 13.60 10 CSP 8.1 14.9 80 227.0 216.14 0.0 0.00 1.8 10.9 1.4 2.05 3.7 0.73 12 CSP 3.1 3.9 335 216.1 213.7

10.7 Total Area


Notes:Tc 25-yr Dual detention system is assumed to provide predevelopment condition flows at 90% open space, 10% residential

Section 1 MH, U/S, D/S elevations and U/S elevation for section 2 from Hedin Terrace development drawings.5.0 2.65 Salter: Drainage area includes Salter St. (Flows along Trail drain to Park St basin.) 6.0 2.50 Single pipe equivalent of 1-14" PVC pipe replacing 8" and 12" existing7.0 2.45 US/DS elevation estimated from Topo elevation and depth bgs field measured.8.0 2.25 Section 3: Area includes flows collected in 12" line along west side of Trail and small 8" pipe at Church St.9.0 2.15 DS elevation assumes pipe 4 feet bgs. No manhole available for measuremen

10.0 2.05 Section 4: No additional inflows into this pipe section. US elevations assumed 4 feet bgs.11.0 2.00 Elevations from Topographic Map, City of Gaston, Spencer B Gross, 198512.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

25-YEAR STORM; EXISTING CONDITIONS FOR CAROL DRIVE



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 3.5 0.35 1.2 0.0 1.2 10.0 0.2 2.05 2.5 10.59 12 CSP 11.6 14.8 170 246.8 228.8 252.02 3.2 0.35 1.1 0.0 2.3 10.2 0.7 2.05 4.8 2.00 12 CSP 5.1 6.4 254 228.8 204.8 234.0

6.7


Notes:Tc 25-yr Due to lack of IE on pipes, depth assumed 4' bgs to top of pipe. CSP assumed 3" thickness.

MH elevations from Topographic Map, City of Gaston, Spencer B Gross, 19845.0 2.656.0 2.507.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

Filename: Carol Drive; 25-year; Exist. Carol Page 1 of 2 Print Date: 12/9/2008

25-YEAR STORM; FUTURE CONDITIONS FOR CAROL DRIVE



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 3.5 0.35 1.2 0.0 1.2 10.0 0.2 2.05 2.5 10.59 12 CSP 11.6 14.8 170 246.8 228.8 252.02 3.2 0.35 1.1 0.0 2.3 10.2 0.7 2.05 4.8 2.00 12 CSP 5.1 6.4 254 228.8 204.8 234.0

6.7


Notes:Tc 25-yr Due to lack of IE on pipes, depth assumed 4' bgs to top of pipe. CSP assumed 3" thickness.

MH elevations from Topographic Map, City of Gaston, Spencer B Gross, 19845.0 2.656.0 2.507.0 2.458.0 2.259.0 2.15

10.0 2.0511.0 2.0012.0 1.9013.0 1.8514.0 1.8015.0 1.7516.0 1.7017.0 1.6518.0 1.6019.0 1.5520.0 1.5021.0 1.4822.0 1.4323.0 1.4024.0 1.3825.0 1.35

Appendix C

CIP Cost Estimates

City of GastonStormwater CIPCottonwood Street

Item Unit Unit cost Quantity TotalStorm Pipes 4-inch diameter LF $73.51 $0 6-inch diameter LF $80.52 $0 8-inch diameter LF $105.76 $0 10-inch diameter LF $114.11 $0 12-inch diameter LF $119.89 645 $77,000 15-inch diameter LF $133.38 $0 18-inch diameter LF $145.02 $0 21-inch diameter LF $159.38 $0 24-inch diameter LF $183.48 $0 27-inch diameter LF $241.81 $0 30-inch diameter LF $284.28 $0 33-inch diameter LF $284.27 $0 36-inch diameter LF $304.77 $0 39-inch diameter LF $332.38 $0 42-inch diameter LF $352.99 $0 45-inch diameter LF $383.38 $0 48-inch diameter LF $405.59 $0 54-inch diameter LF $457.85 $0Subtotal, sewers $77,000

Open ChannelNew Channel LF $120.00 $0Restoration LF $80.00 $0Subtotal, forcemains $0

Catchbasins EA $2,500.00 4 $10,000

Manholes a EA $3,500.00 $0

Forcemains 4-inch diameter LF $84.98 $0 6-inch diameter LF $126.27 $0 8-inch diameter LF $99.90 $0 10-inch diameter LF $110.92 $0 12-inch diameter LF $121.84 $0 16-inch diameter LF $144.93 $0 18-inch diameter LF $163.90 $0 20-inch diameter LF $217.57 $0 24-inch diameter LF $247.49 $0 30-inch diameter LF $274.49 $0Subtotal, forcemains $0

Roadway Allotment $40,000

Grand SubTotal $117,000Contractor O&P 25% $29,250

Engineering and Admin 25% $29,250Estimated Construction Cost @ ENR = 8,449 $176,000 April 2006Estimated Construction Cost @ ENR = 8,626 $180,000 Sept 2007Estimated Construction Cost @ ENR = 1.035 $186,000 August 2008

Notes:aUnless otherwise noted, the cost of manholes has been incorporated into the unit cost of new sewers by assuming that a manhole will be placed every 400 to 500 feet, on average.

City of GastonStormwater CIPPark Street

Item Unit Unit cost Quantity TotalStorm Pipes 4-inch diameter LF $73.51 $0 6-inch diameter LF $80.52 $0 8-inch diameter LF $105.76 $0 10-inch diameter LF $114.11 $0 12-inch diameter LF $119.89 $0 15-inch diameter LF $133.38 $0 18-inch diameter LF $145.02 1,000 $145,000 21-inch diameter LF $159.38 $0 24-inch diameter LF $183.48 $0 27-inch diameter LF $241.81 $0 30-inch diameter LF $284.28 $0 33-inch diameter LF $284.27 $0 36-inch diameter LF $304.77 $0 39-inch diameter LF $332.38 $0 42-inch diameter LF $352.99 $0 45-inch diameter LF $383.38 $0 48-inch diameter LF $405.59 $0 54-inch diameter LF $457.85 $0Subtotal, sewers $145,000

Open ChannelNew Channel LF $120.00 $0Restoration LF $80.00 $0Subtotal, forcemains $0

Manholes aEA $3,500.00 $0

Forcemains 4-inch diameter LF $84.98 $0 6-inch diameter LF $126.27 $0 8-inch diameter LF $99.90 $0 10-inch diameter LF $110.92 $0 12-inch diameter LF $121.84 $0 16-inch diameter LF $144.93 $0 18-inch diameter LF $163.90 $0 20-inch diameter LF $217.57 $0 24-inch diameter LF $247.49 $0 30-inch diameter LF $274.49 $0Subtotal, forcemains $0

Grand SubTotal $145,000Contractor O&P 25% $37,000Engineering and Admin 25% $37,000

Estimated Construction Cost @ ENR = 8,449 $220,000 April 2006Estimated Construction Cost @ ENR = 8,626 $225,000 Sept 2007Estimated Construction Cost @ ENR = 1.035 $233,000 August 2008

Notes:aUnless otherwise noted, the cost of manholes has been incorporated into the unit cost of new sewers by assuming that a manhole will be placed every 400 to 500 feet, on average.

city of gaston storm drainage master plana0e7bb59-ba39-470a-b0f2... · this master plan update was...

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