Civil Engineering Design
Posted: August 25th, 2021
West Buckeye Community
Prepared By: Sponsored by:
Entellus, Inc.
Discipline: Drainage Contact:
Team #14, ANOFA, Inc. Email:
Drainage Draft Report Phone:
Address:
Prepared For:
Mr.
Civil Engineering Design
Submitted: October 25th, 2019
Executive Summary
Studies done by the United States Census Bureau show that the city of Goodyear is increasing in yearly population and therefore, the ANOFA LLC agrees that development of this land is vital for the success of the city (2019). The project shall expand businesses by developing a new commercial building, new neighborhoods, recreational parks, and schools, all that is signifies a growing community.
The West Buckeye Community is a proposed 335-acre multi-use land development project. The land is located on the West side of S Rainbow Valley Road and East of S 183rd Avenue, also North of W Chandler Heights Road and South of W Queen Creek Road. In the South West of the site location, a small portion of the land falls under the city of buckeye jurisdiction. To continue, team ANOFA requested to annex the property to the city of Goodyear following the Arizona revised status title 9, chapter 4, article 7, under appendix E. The target area is divided into eight (8) main parts which include the high-end area occupying land size of 73.5 acres, the middle-class area on land size area of 41.3 acres, park 1 and 2 in an area of 11.2 and 11.4 acres respectively as well as the community residential on land size of 139 acres. The site has a set commercial area of land size 23.3 acres and a space ground of 35.2 distributed in the south and northern part of the site. Besides, there are three channels cutting across the site. The summary division of the land size is shown in appendix E.
The proposed site is surrounded by agricultural land and established golf courses. This project aims to repurpose the area to increase the services offered to the users and accommodate the rising population as well as ensure that quality of life is high for the residents of the area. Particularly, the residential setup will include both low end and high-end structures, a community park, commercial space, and high school. The land is fully owned by the provider for development. Besides, the new design plot adheres to all requirements, both the existing standards and the ordinances. The construction design has put into consideration community sustainability, which is the main constraint in the whole process.
Table of Contents
1.1 Executive SummaryPAGEREF _Toc22848962 \h3
1.2 IntroductionPAGEREF _Toc22848963 \h5
1.2.1 PurposePAGEREF _Toc22848964 \h6
1.2.2 Proposed Site LocationPAGEREF _Toc22848965 \h6
1.2.3 Existing Site ConditionPAGEREF _Toc22848967 \h6
1.2.4 Proposed Project OverviewPAGEREF _Toc22848969 \h6
1.2.5 FEMA Flood PlainPAGEREF _Toc22848970 \h6
1.2.6 Soil ConditionsPAGEREF _Toc22848972 \h7
1.2.7 Proposed ZoningPAGEREF _Toc22848973 \h7
1.3 Drainage Design CriteriaPAGEREF _Toc22848976 \h8
1.3.1 West Buckeye Community Hydrologic CriteriaPAGEREF _Toc22848977 \h8
1.3.2 West Buckeye Community Hydraulic CriteriaPAGEREF _Toc22848983 \h11
1.4 Hydrological AnalysisPAGEREF _Toc22848988 \h13
1.4.1 On-Site AnalysisPAGEREF _Toc22848990 \h14
1.4.2 Off-Site AnalysisPAGEREF _Toc22848992 \h15
1.5 Hydraulic StructuresPAGEREF _Toc22848994 \h15
1.5.1 Retention BasinsPAGEREF _Toc22848995 \h15
1.5.2 CulvertsPAGEREF _Toc22848999 \h18
1.6 Cost AnalysisPAGEREF _Toc22849005 \h21
1.6.1. Stormwater Collection Network CostsPAGEREF _Toc22849006 \h21
1.6.2. Excavation CostsPAGEREF _Toc22849009 \h23
1.6.3. Bedding CostsPAGEREF _Toc22849013 \h25
1.6.4. Backfill CostsPAGEREF _Toc22849015 \h25
1.64. Manhole CostsPAGEREF _Toc22849017 \h25
1.7 SustainabilityPAGEREF _Toc22849019 \h26
1.8 Value AddedPAGEREF _Toc22849020 \h26
1.9 Conclusions and RecommendationsPAGEREF _Toc22849021 \h27
2.0 Works CitedPAGEREF _Toc22849022 \h28
Appendices A: Land Use PlanPAGEREF _Toc22849023 \h29
Appendices B: FEMA Floodplain Designation MapPAGEREF _Toc22849024 \h29
Appendices D: City of Goodyear Design Manual: Retention VolumePAGEREF _Toc22849025 \h32
Appendices E: Manning EquationPAGEREF _Toc22849026 \h32
Appendix E: Rational EquationPAGEREF _Toc22849027 \h33
Appendix F: Offsite Flow CalculationsPAGEREF _Toc22849028 \h34
Appendix G: Onsite Flow CalculationsPAGEREF _Toc22849029 \h35
Appendix G: Onsite Flow CalculationsPAGEREF _Toc22849029 \h36
List of Figures
Figure 1: West Buckeye Vicinity Map……………………………………………………………………………………………………………………………………… 4
Figure 2: Topography Site Map……………………………………………………………………………………………………………………………………… 5
Figure 3: FEMA Site Map……………………………………………………………………………………………………………………………………… 6
Figure 4: Proposed Land Usage Map……………………………………………………………………………………………………………………………………… 7
Figure 4.1: Onsite Watersheds (left) and Concentration points (red)………………..……………9
Figure 5. Off-site contributing……………………………………………………………………………………………………………………………………. 10
Figure 7 (c). North East Watershed……………………………………………………………………………………………………………………………………. 13
Figure 7 (d). For North Watershed……………………………………………………………………………………………………………………………………. 13
Figure 7 (d): Location of the retention basin……………………………………………………………………………………………………………………………………. 17
Figure 8: General Design of the channel……………………………………………………………………………………………………………………………………. 20
List of Tables
Table 1: Goodyear City zoning codes……………………………………………………………………………………………………………………………………… 8
Table 2: summary of off-site flow criteria……………………………………………………………………………………………………………………………………. 10
Table 4: Runoff coefficients……………………………………………………………………………………………………………………………………. 14
Table 5: Land use in West Buckeye city……………………………………………………………………………………………………………………………………. 15
Table 6: Off-site peak flows for watersheds……………………………………………………………………………………………………………………………………. 15
Table 7: Retention area basin estimates……………………………………………………………………………………………………………………………………. 16
Table 8: Channel Calculations……………………………………………………………………………………………………………………………………. 18
Table 10: Culverts Design Dimensions……………………………………………………………………………………………………………………………………. 19
Table 12: Cost per unit for RCP and CMP pipes……………………………………………………………………………………………………………………………………. 23
Table 14: Cost per linear cubic yard……………………………………………………………………………………………………………………………………. 25
Table 16: Costs of a precast concrete manhole……………………………………………………………………………………………………………………………………. 26
1.1 Introduction
The proposed site for development, West Buckeye Community, is located in the South-West part of Goodyear City and enclosed by South Rainbow Road on the East, S 183nd Avenue on the West, W Chandler Heights Road on the South and W Queen Greek Road on the North as shown under Figure 1. The Goodyear City aspires to develop a community resident in the area as an initiative to improve the urban setting and residential structure distribution.
1.1.1 Purpose
Team 14 was assigned to create a layout plan for the targeted construction area. Some of the aspects that would be affected in the implementation of the development include; planning, that encompasses the land size, roadway designs, and zoning, the second is the water systems, where the focus will be on water delivery routes for the residential area and wastewater planning to design sewage outlay. Further, the team is tasked to design drainage systems to route stormwater. Structures will also be affected whereby a layout of pedestrian bridges and finally, the geotechnical systems, that encompasses retaining wall designs.
1.1.2 Proposed Site Location
The proposed site will be divided into 6 parcels. On north side of the location will develop a High School with consisting a total area of 75 Acers, following that toward the south side will have a middle school with an area of 41 acres, between the residential parcels and middle school ANOFA, LLC is designing a two recreational parks and gold course of 11 acres connected by a pedestrian bridge. The residential area is going to be for a single-family house design with total area of 140 acres. Finally, all that following a commercial district of 23 acres area.
Figure 1: West Buckeye Vicinity Map
1.1.3 Existing Site Condition
The proposed area in undeveloped semi-arid terrain characterized by relatively well-developed roads along with the site that is run by small washes. The land and the washes run from the north to the southwest part of the property surface on an approximate slope of 0.006 Feet/Feet at an elevation of 1,080 feet high point and 1,017 feet low point. The report will examine two main washes. The washes will require permitting according to City of Goodyear Engineering Design Standards and West Maricopa County Water Policies, which will be utilized in designing culverts. The half part of the northern region is occupied by desert and landscaping companies. It also hosts a small residential area on the southern part of the S 183RD Avenue. A canal runs east to west, separating the golf club from the small commercial portion of north Baseline Road. Figure 2 below shows the topography site map for the proposed site;
Figure 2: Topography Site Map
1.1.4 Proposed Project Overview
The proposed development site is in Section 33, Township 3 South Range 5 in West Buckeye area, Goodyear City, Arizona. The land size consists of a total of 335 acres. The area is currently developed with medium residential housing, commercial office spaces, and Country Golf Clubhouse. Almost three-quarters of the Southern part of the demarcated area for development is found within limits of the West Buckeye area and the remaining South-West section extends into the Palo Verde area, within Goodyear City.
1.1.5 FEMA Flood Plain
West Buckeye area is located on Flood Insurance Rate Map (FIRM) 0113C205M of the National Insurance Program. The development site is categorized as Zone X, implying that it is an “area that has 0.2% annual chance of flood, Areas of 1 % annual chance of flood with an average depth not above 1 foot or with a drainage area below one (1) square mile. It is among the areas that are protected from 1% annual chance flood” (Luke et al. 1097). The land has been designated by FEMA in a 500-year flood plain which is characterized by floodwaters below one (1) foot. Further, based on the categorization of the site in Zone X, special permits will not be needed before executing the project. The development of culverts in the area is planned to be done across all the sections from which the washes run under the roadways. Further, retention basins are also designed for every parcel of land designated in the development area. The off-site drainage system will be developed to help contain water that runs through the development site. To effectively develop proper drainage systems, the developer will use a rational method in conducting a hydraulic analysis. The following map under figure 3 below highlights the FEMA designated site.
Figure 3: FEMA Site Map
1.1.6 Soil Conditions
Team 14 Consulting Engineers performed a primary geotechnical assessment of the development area on October 5, 2019. It was indicated that the content of the soil is majorly characterized by stony clay loam that overlaps the basalt bedrock. Following these assessments, the assumption was made that the off-site conditions of the soil are similar to onsite ones (Ghafari 9). This suggestion follows an establishment from previous studies that most of the soil around the area are consistent with the same features (Ghafari 9). Further, an evaluation of both onsite and offsite vegetation established the existence of a moderate set of densely populated Alligator, Shaggy Juniper, and Ponderosa Pine trees alongside the field that is covered over by native grasses.
1.1.7 Proposed Zoning
The West Buckeye Park project established is designed for mixed community use located in South-East of Goodyear City region. The community area will be made up of residential housing and commercial areas distributed in the southern part of the area, along with two parks located in the northern area. Northern area doubles up as an educational site with a high school and middle school just next to W Queen road. Further, the area will also accommodate a transit center that is next to the north W Queen Creek road to provide access in and out of the proposed development area as the streets are originally established for bike use. The zoning designations are following the Goodyear City zoning codes. However, little adjustments have been made to ensure that they fit on the ground. Figure 4 below is a display of the proposed land usage for the project
.
Figure 4: Proposed Land Usage Map
Table 1 below is a summary of the zoning ordinances for the City of Goodyear
| Zone | Land Use |
| C | Commercial |
| WW | Wastewater plant |
| TC | Transit Center |
| R | Residential |
| E-1 | Elementary School |
| M | Mixed-Use |
| G | Greenspace |
| P | Parking garage |
Table 1: Goodyear City zoning codes
1.2 Drainage Design Criteria
It is expected that the whole of the designated West Buckeye/Goodyear area will be fully developed. According to the policies of the Goodyear City, wastewater discharge from the development should be in tandem with the run-off rate of flow of the prior developments. Hence, the proposed development should detain the run-off of the stormwater before it is discharged beneath the drainage systems within the residential area. The subsequent part of the discussion looks in detail, at the drainage design criteria that will be adopted in completing the project.
1.2.1 West Buckeye Community Hydrologic Criteria
The hydrologic criteria will be completed using the West Buckeye County Engineering Standards and City of Goodyear, Arizona Policies and the Drainage Design Manual, Hydrology. According to The National Oceanic and Atmospheric Administration (NOAA), the proposed development site is characterized by approximated 1.15 inches of rainfall for a 100-year, 1-hour storm (Saba 119). Besides, an estimated precipitation frequency point of about 1.35 inches for the 100-year, 6-hour storm while in the 100-year, 24-hour storm experiences amount to 2.43 inches (Saba 119). These estimates were sourced from NOAA Atlas 14 Point Precipitation Frequency Estimate tool found on the official website of NOAA, link: https://www.noaa.gov/. Detailed information can be located in Appendix B and C.
Further, both the onsite and offsite watersheds were delineated to facilitate the calculation of the peak flows. Offsite watersheds employed in this study were selected based on the direction of flow on and around the development site. The direction of flow of water in the proposed site is northwest across the area, hence the watersheds southeast of the development area were used in the delineation. Moreover, the development site is unique since there is a canal that flows across the southern portion. As stated earlier about water flow direction, it is critical to note that the water flows into the site southeast of the canal would be directed into the canal. Hence, the onsite watershed located on the south of the canal has no impact against the accumulation of flow the north canal portion of the site.
The examination of off-site criteria was conducted using the Flood Control District of Maricopa County FLO-2D Model Web Tool. With the aid of the model, it was possible to provide information on the county watersheds about the 100-year, 24-hour storm. The proposed development site location is between the Chandler Heights to the South East, the Queen Creek road to the North and 183rd Avenue to the West. The sub-watersheds of Rainbow Valley usually referred to as the Lower Rawhide and Reach 12. Accordingly, the sub-watershed on the Chandler Heights tunnel collects water that flows from the surrounding sub-watersheds from the Upper S Jack Rabbit Triangle, W Queen Creek area and Nymphernberg Area. Besides, inflows from the W Queen Creek South that is located to the immediate east of Palo Verde area watershed are also collected into the drainage. S Jack Rabbit Triangle is located to the south end of the development site and it collects water coming from all the watersheds (Wida 15). The summary information on watershed drainage criteria can be located in Appendix 4. Further, an evaluation study was also conducted on the two watersheds that are located on the off-site, entering the projected development area. The calculations for the off-site flows were made from the two watersheds that enters the proposed site from the north region. Rational Method was applied.
According to the assessment, the total off-site flow for the two watersheds was estimated at 900 cubic feet per second. As such, three differently sized channels were required to help route the flows through the development area. The first channel is made up of a bottom width measuring 15 feet with a top width of about 39 feet, the second channel is made up of width measuring 100 feet with 116 feet for the top width and the third channel consisted of a bottom width of about 10 feet and 34 feet for the top width. These dimensions were acquired using Manning’s equation. On the other, the collected onsite water would be stored before being infiltrated using a retention basin. Each of the twelve parcels is allocated a single basin, however, with a unique size. The estimated volume was 2,600,320 cubic feet for the whole area. Equally, the combined required area for the twelve basins is 20 acres. Finally, the project incorporates two different culverts to give direction to the site-flow under the arterial streets that the channels cross beneath. The culverts will be designed to allow the flow of the 100-year storm runoff and position at a slope almost similar to the position of channel slope. It is expected that the final length of culverts covers the 100 feet to permit the flow from the roadway width. Essentially, the structure will be made of two-barrels of circular culverts of 4” and 4.5” diameters for channel one and channel two respectively. However, the third channel crossings will utilize box culverts measuring 6” by 6”.
Figure 6.1 below gives a picture of the two watersheds. Table 1 below is a summary of the off-site flow criteria, for the inlet drainage systems to the site.
| Watershed | C | Inches per hour | Area size in acres | Q (CFS) | |
| No. 1 | 0.5 | 1.35 | 251.25 | 169.6 | |
| No. 2 | 0.5 | 1.35 | 83.75 | 56.4 | |
Table 2: summary of off-site flow criteria
Figure 5. Off-site contributing
| Contributing Area | Acreage, A | Area (SF) | Runoff Coefficient, C | Rainfall, Intensity, I (ft/hour) | Peak Flow Rate, Q (CFs) |
| Undeveloped Residential Area, North East Watershed | 251.25 | 64,0125 | 0.5 | 1.35 | 169.6 |
| North Watershed (Undeveloped School Area) | 83.55 | 6, 980.60 | 0.5 | 1.35 | 56.4 |
| Total for Northern area | 226 |
Table 3 below summarizes the off-site peak flow rate
Figure 6 below shows the main channels for collecting the watersheds.
Figure 6. Main Channel and offsite drainage systems
Main Channels
1.2.2 West Buckeye Community Hydraulic Criteria
The existing washes utilized in draining the off-site stormwater flowing through the proposed site will be linked with the proposed roadway on four separate occasions. Two of these systems will be enhanced through the construction of a culvert while the third and the fourth runway will be directed through the underway vehicular bridges. The culverts are key in ensuring that there is a natural flow of the stormwater once it enters and leaves the development site using the same locations without disrupting traffic flow (Flanagan 23). Subsequently, the design of the culverts will be in a such a way that they allow proper flow of the river during the 100-year, 2-hour storm with no instances of overtopping, that is, the velocities are designed such that they are greater than the natural flow of the washes entering through the culverts (Flanagan 24). The two washes will require at least two culverts for every intersection to the offsite roads. The following figure shows the sizing of the culverts (Flanagan 25). The length measures 70 inches while the diameter is sized to 4.5 inches. The details about the calculations of the culverts and how the values were attained can be found in the appendices F. Both the Northeast and North watershed have a unique set of culverts.
| 4.5 |
Figure 7 (a). Main culvert design for North East Watershed
| 4.5 |
Figure 7 (b). Main Culvert Design for North Watershed.
From figure 7 (b), the North culvert design consists of about 70 inches in length and 4 inches of diameter.
Besides, it is also important to understand the main channel design for both watersheds. The major existing washes will carry the off-site flow through the site during the period of development. It is necessary to develop the channel design that can withstand the 100-year, 2-hour occasional storms (Basumatary and Sundar 56). To ensure that natural flow is sustained throughout for the washes and remain in control of the water flow velocity, there will be bank protection added to the two washes. Also, a rip rap will be installed along the banks and buried in the rocks and enhanced with natural dirt. The subsequent figures show the wash sizes and their respective calculations.
| 5 FT |
Figure 7 (c). North East Watershed
| 5 FT |
Figure 7 (d). For North Watershed
The North East watershed wash size measures 30.00 feet by length and a height of 5 feet. On the other hand, the North watershed measures 20.00 feet by 5 feet.
1.3 Hydrological Analysis
The availability of various methods for the analysis of hydrological data allowed the group members to choose the most appropriate technique. The most known techniques include infiltration method, rational method, overland flow hydrograph, unit hydrograph method, empirical formulae, curves and tables, and coaxial graphical correlation and API. The group settled on the rational method for both on-site and off-site hydrological analysis. The method assumes a constant and uniform intensity of rainfall is spread over an area, and the effective rain falling on the most remote part of the catchment takes a specific time, referred to as the time of concentration (Tc), to get to the outlet. Tc is determined using Kirpich’s equation as shown
Tc= 0.01947L0.77s-0.385 Equation 1
Where
L=Maximum length of travel of water
s=
slope of drainage basin = H/L
H= difference in elevation between the most remote point of the basin and its
outlet
Tc =Time of concentration in minutes
Therefore, the peak runoff, Qp is expressed as shown in equation2
Qp=CIA Equation 2
Where;
Qp= Peak runoff rate from a given area in CFS
C = Runoff coefficient
I= Intensity of rainfall in inches/hr lasting for a time of concentration man
A= Drainage area in acres
The runoff coefficients used were incorporated into the study as shown in table 3
| Type of catchment | Value of C |
| Rocky and impermeable | 0.8–1.0 |
| Slightly permeable, bare | 0.6–0.8 |
| Cultivated or covered with vegetation | 0.4–0.6 |
| Cultivated absorbent soil | 0.3–0.4 |
| Sandy soil | 0.2–0.3 |
| Heavy forest | 0.1–0.2 |
Table 4: Runoff coefficients
Taking into account that the area of study is covered with vegetation, therefore, a uniform runoff coefficient of 0.5 was chosen. The latest precipitation data of West Buckeye city from The National Oceanic and Atmospheric Administration (NOAA), showed 1.15 inches of rainfall for the 100-year, 2-hour storm. For the 100-year, 6-hour storm, the estimated point precipitation frequency is 1.35 inches and for the 100-year, 24-hour storm the estimate is 2.43 inches. These estimates were found from the NOAA Atlas 14, volume 8 version 2 Point on the NOAA website shown in appendix C. The initial time of concentration guess is taken as 20 minutes which corresponds to a 6-hour storm intensity i of 1.35.
1.3.1 On-Site Analysis
This part provides details required in determining the amount and size of the retention basins that are needed for each of the twelve parcels in the proposed development site. According to Goodyear City Water Policies and Standards and the Drainage Manual for Maricopa County, equations for establishing the retention volume were developed and applied in the development process. Accordingly, the 100-year, 6-hour criteria were used in designing facilities for storing stormwater in the off-site flow areas of Maricopa County. During the estimation of the retention volumes, runoff coefficient C is used. The computation for the onsite peak was done using the Rational Method for each of the watersheds by (Raghunath, 122). Emphasis was placed on the accumulation of flows directed into the northwest portion of the proposed site. The onsite flow estimations are summarized under the table below.
Notably, peak flow for the watersheds 2 and 3 is computed based on concentration points of the two watersheds as a combined flow rate for watersheds 2 and 4, 3 and 5. The Runoff Coefficient Table below exhibits the Runoff Coefficient method. Details about the same table can be located in the Drainage Design Manual for Maricopa County. Runoff Coefficients were based on the type of zoning and land use. The estimated Runoff Coefficient, 2.29 is provided by the NOAA (See Appendix C) and applied in obtaining calculations shown in the following table. The on-site drainage will use the gutter and curb roadways that would be used in conveying flows into the retention basins all over the proposed site by a storm drainage design system. The purpose of the on-site analysis is for estimating the sizes of retention basins required. Various factors such as land use contribute to the retention volume of any drainage basin. The on-site retention volume was obtained from the equation
V=CAd
Where;
V = required retention volume, in cubic meters
C = weighted runoff coefficient
A = Area of land in square ft
d = intensity of rainfall for 6-hour storm over a period of 100 years
The selected retention basins were selected in such a way that their location was at a low location where stormwater could move through gravity.
| Land use | Area (acres) | Runoff coefficient | Volume (acre-ft) | Volume (cubic ft) |
| High | 73.5 | 0.5 | 4.134375 | 180093 |
| Middle | 41.3 | 0.5 | 2.323125 | 101195 |
| Park 1 | 11.3 | 05 | 0.635625 | 27688 |
| Park 2 | 11.4 | 0.5 | 0.64125 | 27933 |
| Residential | 139 | 0.5 | 7.81875 | 340584 |
| Commercial | 23.3 | 0.5 | 1.310625 | 57091 |
| Space | 35.2 | 0.5 | 1.98 | 86249 |
Table 5: Land use in West Buckeye city
1.3.2 Off-Site Analysis
To effectively analyze the off-site hydrology, the Flood Control District of Maricopa County FLO-2D Model Web Tool was used to execute the process. The model offers information obtained for the two-county watersheds in accordance to the 100-year, 6-hour stormwater. The location of the development site is within the two watersheds. The group used the two watersheds already selected to find the off-site peak flow as shown in table 5.
| Watershed | Area (Acres) | Coefficient, C | Intensity, (in/hr) | Peak flow (CFS) |
| #1 | 251.25 | 0.5 | 1.35 | 169.6 |
| #2 | 83.75 | 0.5 | 1.35 | 56.4 |
Table 6: Off-site peak flows for watersheds
1.4 Hydraulic Structures
Notably, the hydraulic structures require to address the safety aspects of the proposed site. The main purpose of these structures is to ensure that there is proper drainage throughout the development area and its adjacent land to ensure a safe and comfortable living of the people in the West Buckeye Community. Hydraulic structures require to provide strong withholding of the peak discharge of a 100-year storm. The West Buckeye Community will have two washes/channels, one retention basins, and 2 culverts for each entrance and exit on each water shed.
1.4.1 Retention Basins
The retention basins are the hydraulic designed storage facilities aimed at controlling the storm drainage effects of the urbanization. The objective is to help reduce peak discharges. Also, the basins can reduce the volume of the storm on the storm drainage based on existing conditions. In most cases, retention basins require an adequate commitment of the land resources by the land developer or the community. As such, the emphasis is often placed on the planning for the basins which are key amenities and sometimes incorporate multiple concepts about the usage.
According to Goodyear City regulations, the criteria for designing facilities for stormwater storage in independent areas of Maricopa County is 100-year, 6-hour storm. These basins should have the capability to drain the stormwater completely within an estimated period of 36 hours immediately after the end of the storm runoff. Volumes used are provided under table 2, which displays the retention volume for each of the twelve parcels was calculated. In the case of a 100-year, 6-hour storm, the allowable depth for the stormwater basin is sixteen (16) feet. The following table displays the required basin area following a 3-foot maximum allowable depth.
| Basin Area |
| Land use | Area (acres) | Runoff coefficient | Volume (acre-ft) | Volume (cubic ft) | |
| 1 | High School | 73.5 | 0.5 | 4.134375 | 180093 |
| 2 | Middle | 41.3 | 0.5 | 2.323125 | 101195 |
| 3 | Park 1 | 11.3 | 5 | 0.635625 | 27688 |
| 4 | Park 2 | 11.4 | 0.5 | 0.64125 | 27933 |
| 5 | Residential | 139 | 0.5 | 7.81875 | 340584 |
| 6 | Commercial | 11.35 | 0.5 | 1.310625 | 26090 |
| 7 | Commercial | 11.35 | 0.5 | 1.310625 | 57091 |
| 8 | Space | 35.2 | 0.5 | 1.98 | 86249 |
| 334.4 | 8.5 | 20.154375 | 846923 |
Table 7: Retention area basin estimates
Although the site topography is relatively flat, there are washes identified in the area as displayed in figure 7(d). Further, the existence of several small streets in the proposed development will require that the greenspace is utilized as the main means to route the flow through the site. Most of the washes run through the greenspace except the canal which routes flows from onsite watershed 6 to the offsite flows as a predevelopment criterion. The northern retention basin in the greenspace site will be 16-acre feet. This size will allow the basin to perform multiple functions such that when it is not retaining rainfall, it serves as a park. Otherwise, the basin will provide retention service for onsite runoff from the watersheds one up to the fifth. Subsequently, there will be no need for the retention basin running through the south canal portion of the site because there is no increase in the runoff volume. According to the City of Goodyear, it is required that the retention basins are located in such a way that they can intercept the flows from the entire site. As such, the area will be dominated by open green space compared to other development in the vicinity. Such ground coverage will ensure limited runoff. The following figure summarizes the location of retention basins.
Figure 7 (e): Location of the retention basin
Figure 7 (e) shows the location of retention basins that would aid in mitigating the effect of urbanization by reducing the peak discharges around the West Buckeye Community development area.
1.5.2. Channels
According to the City of Goodyear drainage design manual, an open channel refers to a conveyance system through which water flows with a free surface depending on the atmospheric interface. In this case, three trapezoidal open channels to convey water flow through the proposed site. From Figure 6 above, two main channels collect water from the northern watersheds running through the site. The third watershed stems from the north-eastern wash. These channels will be engineered with the main objective being to ensure that that they are as natural as possible. Besides, rain gardens and bioswales will be constructed alongside the roads about the curbs to provide routing for stormwater through the development site (Gabriel, 122-213). Natural landscaping would be embraced to ensure that the swales do not require the significant extra watering from outside of the level provided by the rainfall in the area.
Equally, a perforated system of pipe network will be developed across the underneath swales purposely to route stormwater through the proposed site. Subsequently, given that the proposed construction area is made up of different road connections, the green space will be utilized as the main channel for routing the runoff through the site. A set of perforated pipes will be designed in the area such that it drains the collected water into the greenspace. To ensure that this drainage is effective, the green space will be designed such that it has a lower elevation gradient than the adjacent areas to enhance routing of the flow. Since the area is characterized by high off-site flow, it will be critical to construct a rock lining and natural vegetation on the channel bottom to help control the stormwater and prevent erosion effects on the channels. The drainpipes would be positioned below the ground surface at every point parallel and along the roadways going through the site before draining into greenspace. The selected model of the pipe is the 20 in HDPE to ensure that it complies with the City of Goodyear stormwater standards. To obtain measurements for the channels, Manning’s equation was used to calculate the values for all the channels (See Appendix D).
| Depth (ft) | Area (ft^2) | Wetted Perimeter (ft) | Hydraulic Radius (ft) | Top Width (ft) | Velocity (ft/s) | Q Peak flow (ft^3/s) |
| 3 | 81.2 | 39.7386 | 2.03842 | 39 | 8.3589 | 677.1166 |
| 2 | 217 | 116.5924 | 1.8652 | 117 | 7.9484 | 1695.19971 |
| 3 | 67 | 34.7383 | 1.8999 | 35 | 7.9876 | 526.4549 |
Table 8: Channel Calculations
1.4.2 Culverts
The culverts will be constructed underneath the roads that cross the onsite washes to ensure that the roads are kept the drain and maintained from water erosion. Besides, the culverts will still be useful in allowing the peak flows to be routed through the channels. Table 5 below summarizes the number of culverts needed for each of the sites in the proposed development. The design for the culvert’s barrels is corrugated metal 4.5 ft, the distance between each barrel is 2 ft, the depth of each culvert as instructed is 4.5 ft, channel 1 depth is 6 ft and channel 2 is 5 ft look at figure 9 for barrels location.
| Channel | Number of Culverts Crossing |
| 1 | 2 |
| 2 | 2 |
Table 9: Summary of culverts needed in each part of the onsite wash
The culverts under each channel will be designed in a circular shape depending on the peak flow that requires to be accounted for throughout that particular channel. Notably, the length of the culvert should be equal to the road width they are crossing under. In this case, it is estimated to be 24 feet for all the roads crossing the designated white space. See appendix F for the City of Goodyear culvert design criteria. According to the City of Goodyear stormwater standards, it requires that the box form culverts are designed such that the minimum heights is 6 feet, hence the heights with 5 feet are still within the six feet design heights. The standards also provide that one feet allowance should be given between the road and the box. Therefore, the designed channels will be at least seven (6) feet. Table 6 below shows the culverts design dimensions;
| Channel | Width (ft) | Design Depth (ft) | Headwall | Depth Used (ft) | Material | Control | Passes City of Goodyear Criteria |
| 1 | 20 | 5 | 1.5 | 6 | 208 | Concrete | Yes |
| 2 | 30 | 5 | 1.5 | 5 | 322 | Concrete | Yes |
Table 10: Culverts Design Dimensions
As shown in Table 6 above, all culvert designs should pass the City of Goodyear criteria before being approved in the construction. Concrete is used as control components that help compact the culverts. Also, it is necessary when linking up the culverts during implementation (Stephenson, 103). Subsequently, the greenspace leading northward to the retention basin will be designed into a series of channels to help route the flow across the site. As mentioned earlier the channel designs are determined using the Manning’s equation for a normal depth and the trapezoidal channel with a slope-side ration of 2:1. Significantly, it is necessary to consider a less steep slope because of maintenance and public safety requirements for the site. Since the channels are designed to almost natural and straight, Manning’s n = 0.026 was used. The design of the surface sloping is of keen interest to help reduce water velocity, hence the erosive power from the stormwater. At points where it is impossible to reduce the velocity power of water, the channels will be covered with concrete to ensure it is as hard as possible. Figure 8 is an exhibition of the schematic design of the channel cross-sections, while Table 8 summarizes the dimensions for each of the channels. To cater for overflow safety, at least one foot of the freeboard is added above the normal depth of the channel (City of Goodyear, 33-34). However, in the cases when the depth needs of the intersecting culverts are greater, the depth of the culvert design was used for the corresponding channel.
Figure 8: General Design of the channel
| Wash (Channel) | Q (CFS) | Normal Depth (ft) | Total Depth (ft) |
| 1 | 208 | 3 | 6 |
| 2 | 320 | 2.5 | 5 |
Table 11: The normal and total depth values for each channel
The minimum 7 feet depth is used to comply with the culvert design standards according to the City of Goodyear standard provisions. Since channel is the existing canal, the depth will not be modified beyond the current provision. The proposed hydraulic structures will be located in the sites as shown in figure 9.
Figure 9: Schematic of the proposed hydraulic structures
1.5 Cost Analysis
Under this part, an analysis of the construction costs for the 100-year, 6 – hour stormwater facility is made based on the quality and quantity standards provided under the City of Goodyear control devices and methods, including the collection, control and development and maintenance of the treatment systems. Available data obtained from different reports that have conducted a wide review of the costs of developing a similar facility was reviewed to come up with the best estimate for the West Buckeye Community development. The information is presented in the form of various report types, including tables, figures, and equations besides making references to a variety of sources of estimates made in the analysis (Mubarak et al., 163). Also, the comparison is made with different costs for typical applications besides making a review of the Engineering News-Record (ENR) cost index which could be critical in adjusting costs for different years and the existing locations to the current conditions of the proposed site. Finally, a summary of the excel spreadsheet of the projected costs for the stormwater network was established.
The total costs as used in this analysis include the capital expenditures, that is, land and construction as well as the yearly operations and related maintenance costs. The capital costs are apportioned only for the first year during the installing of the stormwater unless there are some retrofits or upsizing of the facility (Mubarak et al., 177). However, capital costs depend on financing costs. As a result, they are amortized over the project’s lifecycle while the operation and maintenance costs are frequent expenditures throughout the stormwater project life. Capital costs encompass the cost of land, construction, and the proposed site. These costs are distributed across various subsets of labor, material costs, equipment, grading and excavation as well as land, landscaping, erosion control, control structure and appurtenances. Further, expenditures on technical or professional services necessary to support the development of the proposed stormwater facility are categorized under capital costs.
1.6.1. Stormwater Collection Network Costs
To adequately estimate the costs of stormwater collection network, the team relied on the following formula;
(Source. Narayanan & Pitt 5)
Where; C refers to construction cost, D the pipe diameter and X the average excavation depth. Accordingly, Narayanan & Pitt (2) also manipulated the formula to come up with a graphical relationship about the costs for constructing the stormwater pipe as shown below;
Whereby;
C => total capital
K= fixed costs
L= pipe length in meters (m)
D = diameter, in meters (m)
While a and b are parameters whose values range from 1.2 to 1.5.
(Source. Narayanan & Pitt 2).
Moreover, an examination of different types of stormwater pipe materials was made, comparing among the reinforced concrete with a service life of 75 years, asphalt – coated galvanized steel with the service life of 20 years and aluminum-coated steel with the service life of 25 years. Since the project requires 100-year stormwater, the suitable material selected for development was the reinforced concrete. However, further reinforcement would be made on the material to enhance its durability and abrasiveness to ensure it sustains the project over the desired period before any maintenance or adjustments are made. Equally, the additional reinforcements are meant to enable the pipe network to withstand the erosive and corrosive conditions of stormwater. This implies that costs for construction of the stormwater network would have to increase depending on the extra materials incorporated in the construction. The following figure is a sample plot showing cost variation of the stormwater pipeline as provided by the R.S Means estimates. This includes other services during design and development such as material receiving, handling, site movement, mobilization, clean up and breaks as well as material costs, equipment costs and labor costs.
Figure 10: Cost estimates for stormwater network
As revealed in the figure, a non-linear function, fitted with the second polynomial function was employed to help estimate the costs for corrugate metals pipe (CMP) and reinforced concrete pipeline (RCP) based on the RS Means data set, as the formula below:
C and D refer to construction cost per feet and pipe diameter in inches respectively. Different pipe diameters will be used in the construction as earlier mention. The following is a lookup table, revealing the estimated costs per unit of each of the two types of pipe. Although, the required material size for this project 48 inches however, further material cost estimates were used as a proposal for future construction purposes.
| Diameter, inches | Corrugated Metal Pipe Cost, $/LF | Reinforced Concrete Pipe, Cost, $/ LF |
| 10 | 21.5 | 28.6 |
| 15 | 30 | 33 |
| 18 | 35.5 | 36 |
| 24 | 43 | 50.5 |
| 30 | 64.5 | 74 |
| 48 | 116 | 144 |
Table 12: Cost per unit for RCP and CMP pipes
(Source Narayanan & Pitt 2)
1.6.2. Excavation Costs
The excavation costs refer to the expenditures incurred when developing trenches for the laying of stormwater pipes. Notably, these costs will vary based on the external diameter of the pipe. On the other hand, the side slopes will be defined depending on the soil type and whether there is the use of sheeting. Although, the required material size for this project 48 inches however, further material cost estimates were used as a proposal for future construction purposes.
| External diameter size, inches (in) | The bottom width of the trench, feet (ft) |
| 24 | 4.2 |
| 30 | 5.0 |
| 36 | 5.7 |
| 42 | 6.4 |
| 48 | 7.1 |
| 60 | 8.6 |
| 72 | 10 |
Table 12: Diameter size and proposed bottom width of the trench
(Source: Means Estimating Handbook, R.S Means Company 2003)
Based on the above estimates, the following figure is a graphical explanation of the relationship between the bottom width of a trench as a function of the diameter of the pipe.
Figure 11: bottom width of a trench as a function of the diameter of the pipe
The external diameter of the pipe about the bottom width will be determined using the following equation;
Whereby, W is the width size of the trench bottom and D is the external diameter of the pipe, all measured in inches. The equation would then be used in estimating the costs of the width of the trench bottom based on the diameter size. More so, it is critical to note that the costs of trench excavation are affected by the inherent fixed costs such as equipment, material and labor costs. However, these fixed costs vary based on the depth and the back-hole bucket size (Narayanan & Pitt 4). The estimated excavation costs are as displayed under Table 11. For additional future plans , more sizes were calculated.
| Depth, in feet (ft) | Size of the backhoe, (Cubic Yards) | Cost ($/Cubic Yard) | |
| 1-4 | 3/8 CY tractor loader per Backhoe | 6.4 | |
| 4-7 | 1/2 CY tractor loader per Backhoe | 4.86 | |
| 5/8 CY tractor loader per Backhoe | 4.86 | ||
| 3/5 CY tractor loader per Backhoe | 4.28 | ||
| 7-10 | 1/2 CY tractor loader per Backhoe | 5.8 | |
| 1 CY tractor loader per Backhoe | 3.30 | ||
| 1.5 CY tractor loader per Backhoe | 2.60 | ||
| 10-15 | 0.75 CY tractor loader per Backhoe | 6.5 | |
| 1 CY tractor loader per Backhoe | 3.70 | ||
| 1.5 CY tractor loader per Backhoe | 2.88 | ||
| 15-20 | 1.5 CY tractor loader per Backhoe | 4.16 | |
| 1 CY tractor loader per Backhoe | 3.24 | ||
| 2.5 CY tractor loader per Backhoe | 2.70 |
Table 13: Estimated excavation costs per cubic yard
(Source: Means Estimating Handbook, R.S Means Company, 2003)
1.6.3. Bedding Costs
Bedding is essential as it provides for sufficient compacting of materials to help protect the pipe network from peripheral loading forces (Akan 217). There are variations in the costs of pipe bedding in terms of diameter, the trench side slope and the type and quality of materials used in bedding. The cost of bedding was calculated based on the following,
| Bedding material | Cost per linear cubic yards |
| Sand, bank or dead | 13.8 |
| Crushed stones 0.75 inches to 0.5 inches | 40.0 |
| Screened or crushed bank run gravels | 32.0 |
Table 14: Cost per linear cubic yard
(Source: Means Estimating Handbook, R.S Means Company, 2003)
1.6.4. Backfill Costs
These costs are subject to the size of the backhoe, the hauling distance of the backfill materials in feet (ft) and the depth of the backfill in inches. The following table is a summary of the costs per linear cubic yard for the backfill of a trench. These costs include equipment, labor and 12% overhead costs.
| 1 Cubic yard bucket | Minimum haul | 1.49 |
| 1 Cubic yard bucket | 100 inches haul | 2.94 |
| 2.25 cubic yard bucket | Minimum haul | 1.19 |
| 2.25 Cubic yard bucket | 100-inch haul | 2.37 |
Table 15: Backfilling costs per linear cubic yard
(Source: Means Estimating Handbook, R.S Means Company, 2003)
1.64. Manhole Costs
The following equation was applied in estimating costs for an individual manhole cost;
Where;
Cm = manhole cost
H = manhole depth
(Source: Means Estimating Handbook, R.S Means Company 2003)
These costs are related to the depth and diameter of the manhole, that is, the maximum difference between the invert and ground elevation of stormwater sewer that enters a manhole. The following table shows the costs of a precast concretes for the manholes, including fixed costs, labor, materials, and equipment cost installations. More values added for future constructions.
| The internal diameter of the riser in feet (ft) | Depth, feet (ft) | Cost per unit |
| 4 | 4 | 1200 |
| 4 | 7 | 1575 |
| 5 | 6 | 2890 |
| 5 | 15 | 3490 |
| 5 | 10 | 4080 |
| 6 | 12 | 5345 |
| 6 | 14 | 6255 |
Table 16: Costs of a precast concrete manhole
(Source: Means Estimating Handbook, R.S Means Company 2003)
1.6 Sustainability
The proposed project will provide sustainability for the development site by maintaining about 25 feet on each side of the washes of the natural existing habitats along the drainage wash-downs. This is based on understanding sustainability is key to the project especially in the high traffic area. As such, there will be the implementation of sustainable infrastructures that are site-specific. Further, bioswales that are consistent with street curbs will be developed to help maximize the infiltration of stormwater and the execution of the substantial green space within the site. Greenspace will be used through the site to improve the infiltration of stormwater conditions and prevent flooding as well as decrease the runoff. Additionally, regular implementation of rain gardens will be made particularly along the streets to provide verges between roads and houses. At the same time, the gardens will serve as natural barriers separating the bike lanes and pedestrian roads. Subsequently, barricaded routes will be implemented along the northern walls next to the open fields that are frequented by animals to allow the animals to maintain the natural routines without interfering with the developed area.
1.7 Value Added
The West Buckeye Community does not have a bike and walkability. Also, the area is often affected by frequent flooding due to poor drainage systems in the area. Occasionally, the area has been affected by outbreaks of water-borne infections. In some cases, the area has experienced water disasters during the long rain season. More so, the poor transport system has often affected the movement of people as they engage in their daily lives. Besides, the area lacks a robust public transportation system. Daily, most commuters waste time jammed upon traffic, a situation that prominently contributes to pollution besides impacting negatively on the quality of life of the population in the area. Although many residents have resigned to the situation of life, the West Buckeye area provides a great opportunity for infrastructural development in the City of Goodyear. The key area for development is green space, which has the potential to optimize drainage and increase community sustainability as well as contribute to the overall quality of life for the community. Also, the existence of important mixed-use areas in the proposed development site could facilitate interaction among different uses within the site. Accordingly, attaining these objectives underpins the goal of fostering enforcing attainment of affordable housing while improving the quality of life among the community members.
1.8 Conclusions and Recommendations
The report has critically assessment both the hydrology and hydraulic structures that are necessary for the proposed development site. It was established that the successful development of a 100-year, 6-hour stormwater drainage system would be achieved through understanding the existing structures that are key to the drainage system. It was noted that there are two watersheds and the corresponding washes existing on the site. This is besides the greenspaces and canals which can be significant in enhancing proper management of the site. Subsequently, both onsite and offsite peak flows were computed, and channels, culverts and detention basins designed to ensure that they effectively help manage the peak flows and prevent overflows into the surrounding watersheds. Besides, cost analysis for the area was made while referring to the established sources that have embraced the development of similar drainage structures both in the adjacent areas and internationally.
Finally, following the examination of the area, the report also came up with several recommendations that could be input in the development to help enhance the achievement of quality work and objectives of development as listed below;
- Offsite flows from the north and northeast will pass through the development through two existing washes and returned to natural flow once the flow exists the site.
- Finished floors are 15 inches above outfall or 6.5 inches above the maximum 100 – year water surfaces elevation
- Retention basins are developed to contain stormwater from 100p-year, 6 hours storm events and drain the whole water within 60 hours.
- Existing City of Goodyear Utility Structure Tie-in Locations
- n.
2.0 Works Cited
Akan, A O. Urban stormwater hydrology: a guide to engineering calculations. Lancaster, Pa: Technomic Pub. Co, 1993. Print.
Basumatary, Violina, and B. SUNDAR Sil. “Generation of rainfall intensity-duration-frequency curves for the Barak River Basin.” Meteorology Hydrology and Water Management. Research and Operational Applications, vol. 6 (2018).
City of Goodyear, 2012 Edition. Engineering Design Standards and Policies Manual, Retrieved: http://goodyear.granicus.com/MetaViewer.php?view_id=8&event_id=91&meta_id=154006
Flanagan, Shea E., et al. “Buffer Options for the Bay: Exploring the Trends, the Science, and the Options of Buffer Management in the Great Bay Watershed Key Findings from Available Literature.” (2017).
Gabriel, Lester H., and Eric T. Moran. The service life of drainage pipe. Washington, D.C: National Academy Press, 1998. Print.
Ghafari, Heidar, et al. “Identification and prioritization of critical erosion areas based on onsite and offsite effects.” Catena, vol. 156, 2017, 1-9.
Ghosh, S. N. Flood control and drainage engineering. Rotterdam Brookfield, VT: A.A. Balkema, 1997. Print.
Goodyear (Ariz). Engineering Design Standards and Policies Manual. City of Goodyear, 2012. Print
Luke, Adam, et al. “Going beyond the flood insurance rate map: insights from flood hazard map co-production.”, 2018, 1097-1120.
Means estimating handbook. Kingston, MA: Reed Construction Data, 2003. Print.
Mubarak, Saleh A., and R. S. Means. How to Estimate with RS Means Data. Hoboken: John Wiley & Sons, 2012. Print.
Narayanan, A. & Pitt, R. Cost of Urban Stormwater Control Practices. The University of Alabana, 2006. Retrieved:http://unix.eng.ua.edu/~rpitt/Class/International%20urban%20water%20systems/Arvind%20Costs%20of%20Urban%20Stormwater%20Control%20Feb%2005%202006%20clean%20copy.htm
Raghunath, Hassan Manjunath. Hydrology: principles, analysis, and design. New Age International, 2006.
Saba, Vincent S., et al. “Enhanced warming of the Northwest A Atlantic Ocean under climate change.” Journal of Geophysical Research: Oceans, vol. 121, no. 1, 2016, 118-132.
Wida, Waode Asryanti, Azwar Maas, and Junun Sartohadi. “Pedogenesis of Mt. Sumbing Volcanic Ash above the Alteration Clay Layer in The Formation of Landslide Susceptible Soils in Bompon Sub-Watershed.” Ilmu Pertanian (Agricultural Science) vol. 4, no. 1, 2019, 15-22.
Stephenson, David. Stormwater hydrology and drainage. Amsterdam New York New York: Elsevier Scientific Pub. Co. Distributors for the U.S. and Canada, Elsevier/North-Holland, 1981. Print.
Appendices A: Land Use Plan
Appendices B: FEMA Floodplain Designation Map
Note that the proposed development site is outlined in a blue color. The purple line represents the canal which runs through the proposed site and the light blue area Zone X FEMA floodplain designation.
Appendices C: C CoefficientFigure 1: Precipitation data for West Buckeye City from NOAA website
Appendices D: City of Goodyear Design Manual: Retention Volume
| The required retention basin volume shall be obtained by using the following equation: Whereby; V = Calculated volume in acre-feet C = Runoff coefficient (See Table 3) P = 100-year, 6-hour rainfall depth in inches A = Drainage area in acres Appendices E: Manning Equation Appendices F: Culvert Design |
Appendix E: Rational Equation
| The rational Equation relates to the rainfall intensity, a runoff coefficient and the watershed size to the generated peak discharge. The following shows this relationship Where; Q = the peak discharge, in CFS, from a given area C = a coefficient relating to the runoff to rainfall I = average rainfall intensity, in inches per hour, lasting for Tc A = drainage area, in acres |
Appendix F: Offsite Flow Calculations
The Rational Method was employed in computing the offsite flow. The method was the most appropriate considering that almost all the watersheds were below or equal to 160 acres. The rainfall intensity, i was obtained through an iterative computation about the time of concentration, Tc. Initial guess of Tc = 26 minutes for each of the three watersheds. The following is a sample calculation of rainfall intensity for watershed 1.
Watershed 1:
Initial Tcguess is 26 minutes that corresponds to an intensity of 26 minutes, 100-year design storm, that yields i =8.0426 inches. The next iteration of Tcvalue was calculated as;
For this watershed, the longest flow path was estimated at 1.61 miles, Kb is 0.3 for the watershed is based on City of Goodyear standards, while S is 39.6 feet per mile. Thus, Tcis as follows:
= 0.810 ͠or 1 hour that is equivalent to 60.10 minutes. Based on the 60 minute and 120-minute intensities for the 100-year storm, the intensity is interpolated to find the value of I corresponding to Tcof 60.10 minutes.
Where Ta and Tb represent known design storm duration jus less than and just greater than Tc. Hence, Q can be calculated using the Rational Method.
Since
Then,
Watershed 2:
I= 4.35 in, Tc = minutes
Then,
Watershed 3:
I = 2.66 in. Tc = minutes,
Then,
Appendix G: Onsite Flow Calculations
The onsite peak flow rates were computed using the Rational Method for the combined watersheds. The peak flow was calculated for each of the three watersheds individuals, after which the accumulated flow at each concentration point ascertained. To obtain onsite flow rates, the same model as used in the offsite flow rate was applied.
For example, Watershed 1
Stand-alone watershed 1:
, C1 = 0.6
The concentration and the rainfall intensity were calculated iteratively, same as the offsite watershed flow computations (Appendix F).
L => the longest flow path = 0.18 miles
Kb => watershed coefficient = 0.03
S => Site slope = 39.6 ft per mile
Tc => 24.46 minutes
Intensity, I = 5.95 inches per hour.
The flow for each concentration point was calculated using the weighted averages of C and Kb for respective watersheds, along with the combined longest flow path of the contributing watersheds.
Concentration point 1;
Concentration point 2;
Appendix F: Set Up of The Study Area
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