Design Report
Storm Water Sewers and Outlet for Prairie City, Iowa
Prepared for
The City of Prairie City, Iowa
DWY Engineering
Nathan Dingles, Project Manager
Nicholas Whitmore, Report Production
Tianjiao Yin, Technology Services
Executive Summary
Prairie City Iowa, is located twenty miles west of the capital Des Moines, with a population of just over 1,600 people. The city is just over one square mile in area and has a mixture of residential, commercial, and some small scale industrial buildings. The focus of this design project was to analyze the existing storm-water sewer network in the city, which is broken into Northern and Southern sections, and to design mitigation systems to prevent the localized flooding and channel erosion that the city has been experiencing during intense rainfall.
The current system starts with an open ditch system that runs alongside roads in the main housing district and collects rainfall from the housing lots and diverts the water into a storm-water sewer pipe underneath the roads. The network of pipes meets at an underground structure and the storm-water is channeled to an outlet that empties into a nearby stream. The current issues with this network include the open ditch system decaying and being filled in by residents, the pipe network not having a high enough capacity to handle the flow of water during periods of intense rainfall, and the high outlet flows into the stream causing erosion of the stream banks during periods of high flow.
The Client (The city of Prairie City, Iowa) asked our group, DWY Engineering, to develop a plan that will help mitigate the localized flooding experienced by the town during rainfall events. In addition to this, it was requested that we also mitigate the bank erosion in the outlet stream. Traditionally, removing the existing pipe network and replacing it with a more modern and higher capacity network would be ideal, but due to monetary constraints, our team opted to go with a more cost effective plan. In order to determine what mitigation methods that we were going to implement, a site evaluation had to be completed.
DWY Engineering evaluated the site in question by using the Rational Method, topographically mapping and modeling the watershed areas using Geographical Information Systems (GIS), and by conducting physical inspections during site visits. AutoCAD was also used in the design of the mitigation systems deemed most realistic and effective given the issues at hand. This proposal includes a comprehensive summary of our experience as a design team and the how we developed our strategies in solving the issues of storm-water management on site.
Our team decided on several possible mitigation systems that could be put in place to help mitigate the flooding that the area is experiencing. One design solution that was deemed necessary by our team was the cleaning out of the current drive pipe system that is currently in place. The other possible options that could be implemented are placing a detention basin on the Northeast corner of State Street and 2nd Street in downtown Prairie City to hold water during rainfall events, placing a form of permeable PVC pipe underneath the open channel ditch system to help with the soil infiltration, placing drainage tiles in the bottom of the open channels to help direct the runoff to the pipes, and placing rain gardens throughout the city. These mitigation systems can be used individually or all used together to help solve the flooding problems that Prairie City is currently experiencing. In regards to the bank erosion, there are a couple proposed solutions. One design solution that our team deems necessary is cutting and filling the banks to a 4:1 horizontal to vertical (H:V) slope, which is the optimum slope to prevent bank erosion. Other possible options that could be implemented are lining the channel with concrete, placing rock riprap in the channel, lining the channel with vegetation and grass, or placing a soil retention blanket onto the banks of the channel.
The total cost of the channel bank erosion mitigation and the standing water mitigation comes to $17,720. The other possible mitigation practices are also priced out in this report. The DWY design team believes that the suggested actions could easily be constructed within a weeks’ time.
DWY Engineering has greatly appreciated the opportunity to provide some insight and solutions to the problems at hand in Prairie City and very much looks forward to the process and completion of the this project.
Introduction
DWY Engineering consists of three senior Civil and Environmental Engineering majors at the University of Iowa. The members of this group have extensive experience with analysis and design software such as ArcMap, AutoCAD, Civil3D, and HEC-RAS. Additionally, members of DWY Engineering have taken hydrology design classes which involved design and analysis techniques that were directly used in this design project.
This design was completed for the city of Prairie City, Iowa. The city came to us because the city is having flooding problems in the residential district of the town during intense rainfall events. DWY Engineering created this design for the purpose of mitigating these flooding problems.
The first portion of the report discusses the problem statement. The problem statement includes the design objectives, approaches that were taken during the design project, the constraints and challenges that were faced during the design project, and the societal impacts. The design objectives section discusses what the design project is expected to achieve. The approaches section discusses the steps that were taken while completing the design. It also includes any permits that would be required while completing the project. The constraints section deals with all of the “hard” and “soft” constraints that were encountered while completing this project. The challenges section discusses any particularly difficult challenges that were encountered in the process of completing the project. In the societal impacts section, the environmental and economic impacts of the project are discussed.
The second portion of the report is the preliminary development of alternative solutions. Here, all of the mitigation methods that DWY Engineering decided upon to deal with the flooding are discussed in detail. The third portion of the report is the selection process. In this section, the process and reasons for the final design selection are given in detail. The next section of the report is the final design details. In this section, the final design for the project is presented and explained. The detailed calculations and complete design sheets can be found in the appendices. The next section of the report is the cost and construction estimates. In this section, the final materials and construction schedule are laid out in detail. It then goes on to explain the reasoning for those choices and the cost of construction labor and materials.
The next two sections of the report are the conclusion and bibliography. In these sections, the conclusions that DWY Engineering arrived at during this project are discussed. In addition, the research sources that were used during the design process are listed and discussed.
The final section of the report are the appendices. Here, detailed calculations that were performed during the project in addition to the final design sheets can be found.
Problem Statement
Design Objectives
The city of Prairie City currently has 1,685 residents and, due to its convenient location near Des Moines, is experiencing rapid growth. The current storm-water mitigation system needs to be improved upon in order to help accommodate the current and future population of the town. During heavy rainfall events, there is occasionally standing water for between twelve and fourteen hours in the roadside swales. Due to the high flows from rainfall events, the outlet of the storm-water network is currently eroding the banks of the outlet stream. The two major objectives of this project were to analyze the current storm-water pipe network that is in place and design mitigation systems to help deal with the generated flows that the pipe network cannot handle, and to help mitigate the erosion of the outlet stream.
Approaches
In order to adequately determine the runoff generated during rainfall events and the capacity of the pipes, several methods were used. In order to determine the runoff generated during rainfall events, the rational method was used. A runoff coefficient of 1.0 was used to ensure a factor of safety while performing these calculations. The runoff flow that was calculated using the rational method was then compared to the capacity of the pipes to determine the volume of detention basin that was necessary. In order to determine the flow in the full pipes, Manning’s equation was used. In order to use Manning’s equation, the slopes of the pipes were needed. These slopes were attained by dropping a tape measure down the manholes and measuring the elevation of the inlet and exits of each of the pipes in the network. All of the collected data was then input into Arc GIS to create a map with contour lines that was used during the calculation and design process. In order to model the detention basin and open channel system, AutoCAD was used.
Constraints
There are two types of constraints that can be faced during a project, “hard” and “soft” constraints. Hard constraints are constraints that absolutely have to be adhered to while soft constraints are constraints are not mandatory to follow, but it would be advised that they be followed. During this project, both hard and soft constraints were encountered. One hard constraint that was encountered was a monetary budget. Because of this budget, it was deemed that removing the current pipe network and replacing it with a pipe network with a larger capacity was not an option. Another hard constraint that was faced was time. The time period from the first site visit where the problem was described to the final presentation was only a little over three months. This required data collection, calculations, and the design to be created in that amount of time. One soft constraint that was encountered was making the design be aesthetically pleasing. The client made it clear that it would be preferred if the final design was appealing to the eye, but that it was not mandatory.
Challenges
All projects experiences challenges along the way, and this project was no different. Some challenges that were encountered during this project were a lack of data provided by the city of Prairie City,
difficulty gathering data due to weather constraints, and the state of the existing infrastructure in Prairie City. Because Prairie City is a pretty small town, the public records that they have are not as expansive or detailed as those a bigger city would have. Because of this, the only data that we were able to gather from the city was the map of their current underground pipe network. Interestingly enough, while examining the pipe network during one of the site visits, it was discovered that the map that they did have was not even totally correct. Because of the lack of provided data, it caused the design team to have to gather much of the data ourselves during the site visits. This led to another challenge that was encountered during this project which was the difficulty gathering data due to weather constraints. During the first site visit which took place in February, there was roughly six inches of snow covering the ground. This made it difficult to gather any solid data because it was impossible to get elevations of pipes, see the roadside open channels, or see the true topography of the land. Because of this, the first site visit was mainly focused on completely understanding the scope of the problem that the design team was being asked to solve. The final major challenge that was faced during this project was the state of the existing infrastructure in the city. This was a challenge because the conditions of the pipes could not really be determined because there was no way to go down and see the insides of the pipes. Information on the condition of the pipes would have been very helpful when the pipe flow and capacity calculations were being carried out. Also, since it was deemed not feasible to remove the current pipe system and replace it with a new one, we had to design a storm-water mitigation system without knowing for sure what type of flows the underground pipe network can handle.
Societal Impact
Even though this was a relatively small-scale project, it will still has a large societal impact for Prairie City. The current storm-water network that is in place in Prairie City is not adequate to handle the flows that the area experiences during rainfall events. Because of this, localized neighborhood flooding occurs in the area which leads to a significant amount of basements flooding, costing the residents sizeable amounts of time and money. If the final design that was proposed were to be implemented in the city, the flooding problem would no longer exist. This decrease in flooding will save the community time and tax dollars that they would regularly expend towards dealing with the effects of frequent flooding. Tax dollars will also be saved due to the correcting of the erosion of the outlet stream bank. If the erosion problem were not fixed, sediment would have continued to erode away, which would have eventually forced the city to build an entire new channel. This project would be both time intensive and very expensive. Because of the decreased flooding and standing water in the city, it will now be possible for more expansion in the area because of the improved land conditions. This is a benefit because the city has currently been experiencing significant growth due to its close proximity to the city of Des Moines. Because of the significant drop in flooding, the city will now be able to expand to accommodate those people who are looking to commute from Prairie City to Des Moines, bringing a significant amount of money to the area. This could lead to increased businesses in the area in addition to more jobs available for the residents of the city.
Preliminary Development of Alternative Solutions
Due to the fact that the main problem to be addressed in this project was extensive standing water, our design goal was to help mitigate the standing water. The possible solutions that were formulated by the design team to help mitigate standing water include the following: placing drainage tiles at the bottom of the roadside open channels, placing permeable PVC pipes underneath the open channels to help with infiltration, placing rain gardens at the problem areas within the open channel system, and creating a detention basin to help hold the water for an extended period of time. Other possible solutions included laying new pipes in certain areas that are experiencing excessive standing water. This option was discarded, however because of the lack of funding for this project. The drainage tiles would have lined the bottom of the channel and helped the water make its way to the pipe network instead of sit in the open channels. In order to install the PVC pipe, the open channels would have to be dug up and the PVC channel would be laid in the ditch that was created. This would help more water be able to infiltrate into the ground instead of sitting in the open channels. The rain garden option would also be an easy option to implement. This would require plants to be planted in areas of severe standing water. These plants would soak up the water and keep it from standing in the open channels. The final option that was debated was the creation of a detention basin. The purpose of the basin would be to hold the water generated during extreme rainfall events and hold it until the pipe network would be able to accommodate the flow. All of these mitigation options can be seen in the figures below.
Figure 1: Drainage Tiles Figure 2: Permeable PVC Pipe
Figure 3: Rain Garden Figure 4: Detention Basin
In regards to the outlet channel erosion, the design team formulated one main solution. It was decided that the best way to help stop the erosion at the outlet channel would be to place riprap for the first 200 feet of the channel where the flows would be the most turbulent. In order to accomplish this, it is also recommended that a soil retention blanket be placed underneath the rock in order to keep the soil from eroding during the placement of the rock.
Selection Process
The necessary design alternative that our team decided on to mitigate the localized standing water was the clearing of the drive pipes underneath the driveways and roadways. As can be seen in Figure 5, most of the drive pipes in the focus area are quite clogged, which is causing most of the runoff to not be able to reach the underground pipe network which is leading to standing water. Clearing the pipes will allow water to flow more easily and quickly through the system, decreasing the time of concentration of the watershed and leading to less sever standing water in the focus area. A small amount of equipment would be needed to accomplish this and the total cost of completing this part of the mitigation will make up only a small proportion of the total cost of the project. Due to these factors, we believe that the clearing of the drive pipes is essential to the mitigation process of the standing water problem.
Figure 5: Clogged Drive Pipe
Along with the necessary design alternative for standing water mitigation, our team selected one of the optional design alternatives that we thought would be the most appropriate for Prairie city given the site specific constraints. We chose the design of a detention basin on the N.E. corner of intersection of S State Street and E 2nd Street as the best of the optional choices. Its location adjacent to an inlet structure in the S.W. corner of the existing parking lot that experiences surcharging during extreme rainfall events makes it an ideal site. The general location and layout of the proposed detention basin can be found in Figure 6. Surcharging at that inlet structure is caused by the water coming from the North part of the city traveling south through the 24” pipe on State Street. The standing water area contributes largely to this flow and our calculations found that the pipe would be over capacity for anything over a 10 year return period rainfall event, meaning that surcharge at the inlet location would be relatively common. The details for the detention basin including dimensions and exact location can be found in the Final Design Details section of the report. Our group thought that this was the best of the optional design alternatives because it is easier to construct that the other choices, requires little to no maintenance, and already has an adequate site available that would not require any invasive construction work.
Figure 6: Map of the focus area and location of proposed detention basin
Next, for the channel bank erosion, our team chose cutting and filling the banks of the stream to a 4:1 horizontal to vertical side slope. 2:1 is commonly used for vegetation and 5:1 is commonly used for rock and boulder sections of streams. Being conservative, our group chose 4:1 as it would be relatively suitable for either application. The process of cutting and filling channel banks is labor and intensive but is likely the most effective simple mitigation method for its cost.
The additional option for mitigation of channel bank erosion that our group chose was rock riprap installation along the worked banks. Riprap is used to create a strong and durable physical barrier between the stream banks and the water flowing through the stream. Riprap has little to no maintenance costs, is easy to install and is one of the tried and true methods for erosion mitigation. The slopes should be cut to an appropriate side slope for the rocks to grip the soil and each other tightly and not allow the fastest water in the stream away from the banks. The process of installation of the rocks onto the stream bank is not time consuming and can be done in just a day with a crew of a handful of people. This was chosen for its effectiveness of mitigation and relatively low cost.
Final Design Details
For the first design objective of mitigating the standing water in the localized flooding area, the design includes a set of instructions rather than calculations and AutoCAD drawings for the necessary clearing of the drive pipes. Clearing drive pipes is something that can be done manually with regular yard equipment and a significant amount of manual labor, or can be contracted out to pipe clearing companies who have specialized equipment for these types of jobs. If it is desired to contract out the work, the DWY design team suggests Smith’s Sewer Service which is located in Johnston, Iowa. The main task in clearing the pipes is removing the soil and debris that is inside the pipe, and the secondary task is to bring the ground level at the entrance and exits of the pipes to the same level as the bottom lip of the drive pipe. This will allow water to flow freely through the entire area of the pipe and not have to move vertically as it enters and exits the pipes. When this is complete, the time of concentration of the watershed will be significantly decreased as the water will be able to flow through the network of open channels and pipes without being dammed by clogged pipes, and a significant amount of the water that previously has been left standing will make it successfully into the network of storm-water sewer pipes.
The next part of the design for the first design objective was the design of a detention basin to collect some of the excess water that flows through the 24” pipe down State Street and detain it, preventing the buildup of water upstream of the 24” pipe where there has historically been standing water. We designed the detention basin to account for the difference between the flow capacity of the 24” pipe when it’s running full and the peak flow in the pipe due to a 100 year return period, six hour duration storm. This difference in these values is the amount of flow that the pipe cannot handle and will cause surcharging and backup in the pipe. The detention basin will be located on the N.E. corner of the intersection of State Street and 2nd Street in the S.W. corner of the existing parking lot adjacent to the inlet structure that experienced the surcharging and thus will be able to immediately collect any excess water. The site area is 90 feet East to West and 50 feet North to South, thus leaving a depth of about 3.5 feet to account for the maximum volume from discharge at the inlet structure of 15,000 cubic feet. Drawings, site location, and calculations for this design can be found in Appendices B and C.
For the second design objective of mitigating the channel bank erosion near the outlet structure, the first part of the final design our team chose was using cut and fill to stabilize the channel banks, which is more of a set of instructions like the clearing of drive pipes. Cutting and Filling the banks of the channel to a 4 to 1, horizontal to vertical, side slope will require excavation and the use of either a front-end loader or yard tools to compact the soil banks of the channel. When finished, the bed of the channel should remain relatively unchanged in width and the banks will have to be widened wherever the bank does not already have at least a 4:1 side slope. We believe that the majority of earthwork will need to be removing soil, not placing addition soil. As well the distance downstream that the cutting and filling should be done is to be at least 200 feet from the outlet structure in order to protect the part of the stream bank that is most seriously exposed to the turbulent flow of the water. In addition to the channel bank slope, the team feels that it would also be appropriate to place riprap on the first 200 feet of the channel. In order to do this, it is also suggested that a soil retention blanket be placed underneath the riprap to hold the soil in place during the placement of the rocks.
The channel bank erosion mitigation construction should also only be done during times of very low flow in the channel and no near-future rainfall events. This will prevent the erosion of the newly exposed soil. Once the bank slopes have been cut and filled to the adequate slope, the additional erosion mitigation
can be performed. Rock riprap of the size specified by the city can be carried from the truck by a front end loader and dumped on the newly worked soil on the channel banks. The final placement of the rocks will need to be done by hand but simply involves moving the rocks around to cover the full channel banks and making sure that the bottom layer of rocks is partially settled into the soil to create a bond between the rocks above and the soil below. Once placed along the full length of the newly worked banks of the channel the riprap will prevent any more significant erosion of the banks.
Cost and Construction Estimates
After considering all of the possible mitigation practices for the given design requirements, DWY Engineering formulated a cost and constructing planning estimate for the project. The cost of the project was calculated using the 2013 Cost Estimating Guide for the state of Iowa. The cost of the project was broken down into subtotals for each individual mitigation practice that was suggested. These subtotals were then added together to obtain a grand total if every suggested mitigation practice were to be implemented. The spreadsheet of the cost analysis can be found in Appendix (A).
The first task that the design team deemed necessary to deal with the standing water was the cleaning of the drive pipes. It was estimated that this project could be completed with twenty-four hours of labor at $25 per hour. Therefore, the total cost of clearing the drive pipes came out to $600. There were several optional mitigation practices suggested for dealing with the standing water, the cheapest of which is creating a detention basin. The final price of the designed detention basin came out to $4,981.00. This price was calculated by adding the cost of the cut and fill, the concrete work, and the riprap. The price of the cut and fill was estimated using $2.39 per cubic yard of earth moved at 575.36 cubic yards, coming to a cost of $1,375. The price of the concrete work was estimated using $134.72 per cubic yard of concrete at 17.78 cubic yards of concrete needed, coming to a cost of $2,395. Finally, the cost of the riprap was estimated using $44.76 per cubic yard of rock at 27 cubic yards coming to a cost of $1,210.
The perforated PVC pipe was another suggested alternative to help mitigate the standing water, the total cost of which came to $24,627. This price was calculated by adding the cost of the necessary cut and fill and the cost of the pipe itself. The cost of the earthwork was estimated using $2.39 per cubic yard of earth moved at 27.7 cubic yards coming to a cost of $66. The PVC pipe cost was estimated using $5.47 per linear foot of pipe at 4490 linear feet coming to a cost of $21,560.
A third suggested alternative for standing water mitigation was using drainage tiles under the open channel system. This option is very similar in cost to the PVC pipe option, equaling a total of $23,729. This price was calculated by adding the cost of the necessary earthwork to the cost of the drainage tiles themselves. The cost of the necessary earthwork was estimated using $2.39 per cubic yard of earth moved at 27.7 cubic yards, coming to a cost of $66. The cost of the drainage tiles was estimated at $5.27 per linear foot of tile at 4490 linear feet, coming to a cost of $23,662.
The last suggested option to help mitigate the standing water was the planting of rain gardens. This was the second cheapest of the suggested options, equaling a total of $6400. It was suggested that eight rain gardens be planted throughout the city at roughly two hundred square feet each. The price of the rain gardens was estimated using a cost of $4 per square feet at 1600 square feet.
For the erosion control of the outlet, the total estimated price came out to $11,761. This price was calculated by adding the cost of the earthwork, the soil retention blanket, and the riprap. The cost of the earthwork was estimated using $2.39 per cubic yard of earth moved at 122 cubic yards, coming to a cost of $292. The cost of the soil retention blanket was estimated using $1.44 per square yard of blanket at 366.67 square yards, coming to a cost of $528. The cost of the riprap of the outlet was calculated using $44.76 per cubic yard of rock needed at 244.44 cubic yards, coming to a cost of $10,941.
As can from the above numbers, the total cost of the proposed mitigation practices (the cleaning of the drive pipes along with the detention basin and outlet erosion control) comes to a total cost of $17,342 for the entire project. Due to uncertainties in cost estimating and unforeseen challenges during the construction process, the design team feels that $30,000 for the project would be a very conservative estimate.
The design team also created a construction time estimate for this project, the graphical representation of which can be found in Appendix A. The team feels that this project can be completed within a week. The construction time estimate was completed assuming a crew of five people that work an eight hour day. Day one of the project would entail the starting and completion the cleaning of the drive pipes and the starting and completion the earthwork for both the outlet channel and detention basin. Day two of the project would include laying the soil retention blanket on the outlet channel, beginning the earthwork for the PVC pipe/drainage tiles, and the concrete work for the detention basin. Day three of the project would entail the riprap of the outlet channel, finishing the earthwork for the PVC pipes/drainage tiles, and the riprap of the detention basin. Days four and five of the project would focus on laying the PVC pipe/drainage tiles. Day six of the project would be planting the rain gardens in the specified locations, and day seven would include any miscellaneous finishing work that needed to be completed.
Conclusion
The DWY Engineering design group considered many possible mitigation options to deal with the standing water in the residential district of the city. Due to the high cost, the permeable PVC pipe and drainage tiles were ruled out. Because of this, the recommended course of action to deal with the standing water is to clean out the drive pipes and implement the detention basin design. In addition to this, rain gardens would also help with the standing water, although it is not deemed mandatory to implement them.
Appendix A: Cost and Construction
Cost Analysis
Task Unit Price ($)
Quantity Needed
Total Cost Per Task ($)
Soil Blanket Erosion Control (per square yard of blanket) 1.44 366.67 528.0048
RipRap of Outlet (per cubic yard of rock) 44.76 244.44 10941.1344
Cut and Fill for PVC pipe laying (per cubic yard of earth) 2.39 27.7 66.203
Perforated PVC pipe laying (per foot of pipe) 5.47 4490 24560.3 Drainage Tiles (per linear foot of tile) 5.27 4490 23662.3 Drainage Tile Cut and fill (per cubic yard of earth) 2.39 27.7 66.203
Detention Basin Cut and Fill (per cubic yard of earth) 2.39 575.36 1375.1104
Detention Basin Concrete Work (per cubic yard of concrete) 134.72 17.78 2395.3216
Detention Basin Rip Rap (per cubic yard of rock) 44.76 27.037 1210.17612 Detention basin Outlet Pipe (per linear foot of pipe) 8.12 40 324.8 Cleaning drive pipes (per hour of labor required) 25 24 600
Channel Cut and Fill Earthwork (cost per cubic yard of earth) 2.39 122.2 292.058
Rain Garden Planting (per square foot) 4 1600 6400 Cost for Drivepipe Cleaning Only 600 Cost of PVC Pipe mitigation system 24627 Cost of Drainage Tiles 23729 Cost of Detention Basin 5305 Cost of Outlet Erosion Control 11761 Cost of Rain Gardens 6400 Cost of PROPOSED ITEMS 17667 Total Cost of All Mitigation Practices 72422
Table 1: Cost analysis breakdown
Gantt Chart
Task Subtask Duration
(days) Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Drive Pipe Clearing 1
Outlet Channel Work 3
Cut and Fill 1
Soil Retention
Blanket 1
Riprap of Channel 1
Perforated PVC
Pipe/Drainage Tiles 4 Earthwork 2 Laying PVC Pipe 2
Planting Rain Gardens 1
Detention Basin 3 Cut and Fill 1 Concrete Work 1
Riprap of Basin 1 Misc. Finishing Work 1
Table 2: Gantt chart of construction breakdown
Appendix B: Calculations for Pipe Flowrates and Geometric Design of Detention Basin
Table 3: Pipe Capacities calculated using Manning’s Equation
CAPACITY Diameter
(ft) Area (ft^2)
Hydraulic Radius (ft)
Wetted Permieter (ft) Slope
Max Flowrate (cfs)
Max Velocity (ft/s)
18 in 1.5 1.77 0.38 4.71 0.01 11.37 6.44
24 in 2 3.14 0.50 6.28 0.01 24.52 7.80
48 in (1) 4 12.57 1.00 12.57 0.01 156.04 12.42
48 in (2) 4 12.57 1.00 12.57 0.01 156.04 12.42
72 in 6 28.27 1.50 18.85 0.01 460.68 16.29
Table 4: Flow in pipes due to a 100 year return period – 6 hour duration storm
18 in pipe
Drainage Area
(acres) Runoff
Coefficient Intensit
y Flowrate
(cfs) Available Capacity
Velocity (ft/s)
10 year 27.5 1 0.52 14.30 -2.93 8.09
25 year 27.5 1 0.6 16.50 -5.13 9.34
50 year 27.5 1 0.66 18.15 -6.78 10.27
100 year 27.5 1 0.72 19.80 -8.43 11.20
18 in pipe
(downstream)
10 year 39.5 1 0.52 20.54 -9.17 11.62
25 year 39.5 1 0.6 23.70 -12.33 13.41
50 year 39.5 1 0.66 26.07 -14.70 14.75
100 year 39.5 1 0.72 28.44 -17.07 16.09
24 in pipe
10 year 45 1 0.52 23.40 1.12 7.45
25 year 45 1 0.6 27.00 -2.48 8.59
50 year 45 1 0.66 29.70 -5.18 9.45
100 year 45 1 0.72 32.40 -7.88 10.31
48 in pipe (1)
10 year 81.59 1 0.52 42.43 113.61 3.38
25 year 81.59 1 0.6 48.95 107.08 3.90
50 year 81.59 1 0.66 53.85 102.19 4.29
100 year 81.59 1 0.72 58.74 97.29 4.67
48 in pipe (2)
10 year 83.66 1 0.52 43.50 112.54 3.46
25 year 83.66 1 0.6 50.20 105.84 3.99
50 year 83.66 1 0.66 55.22 100.82 4.39
100 year 83.66 1 0.72 60.24 95.80 4.79
72 in pipe (outlet)
10 year 109.33 351.35 3.87
25 year 126.15 334.53 4.46
50 year 138.77 321.91 4.91
100 year 151.38 309.30 5.35
Total Downstream
10 year 129.87 330.81 4.59
25 year 149.85 310.83 5.30
50 year 164.84 295.84 5.83
100 year 179.82 280.86 6.36
Table 5: Detention Basin Volume based on difference in capacity and actual flow experienced in the 24” pipe during a 100 year return period – 6 hours duration storm
TR-55, Type II Rainfall Vs / Vr = C0 + C1 (qo / qi) + C2 (qo / qi)2 + C3 (qo / qi)3
C0= 0.682 C1= -1.43 Vs/Vr= 0.088 C2= 1.64 C3= -0.804 Vr (ft^3)= 170208 q0= 1 Storage Volume: Vs (ft^3)= 14978.304 q1= 1
Vr=Q*t Vs=Vr*(Vs/Vr) E.W. Length (ft)= 90 Q= 7.88 N.S. Length (ft)= 60 t= 21600 Min. Depth (ft)= 2.77376
Table 6: Broad crested overflow weir design calculations
Q=Cw*L*H^1.5 Q (cfs)= 7.88
Cw= 3 H (ft)= 0.75 L (ft)= 4.04
Table 7: Outlet drain design calculations using Manning’s equation. Designed to drain full basin in 12 hours.
Volume to drain (ft^3)= 15000 Time to drain (hr)= 12 Q necessary (cfs)= 0.347222
Slope (between striaght drop and horizontal to pipe)= 0.5
Min Diameter (ft)= 0.079 Min Diameter (in)= 0.948
Actual Diameter (in)= 6
Appendix C: Full AutoCAD Designs
Figure 7: Detention Basin technical drawing, top dow
n view
Figure 8: Detention Basin technical drawing, side view
, looking South to North into the page
Figure 9: Detention Basin technical drawing, side view
, looking East to West into the page
Figure 10: Outlet drain design
Figure 11: Channel cross-section with retention blanket and riprap
Figure 12: Channel after retention blanket and riprap are placed, top view
Appendix D: Provided Data
Appendix E: References
Chin, D. A. (2014). Water-resources engineering. Upper Saddle River, NJ: Pearson Education.
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