Stormwater Design Using the Rational Method
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Stormwater Design Using the Rational Method Although less rigorous than other hydrologic techniques, the Rational Method remains a popular tool for determining peak flows in small catchments. It is based on the theory that the peak runoff occurs during a storm event of duration equal to the travel time of runoff from the top of the catchment to the outlet (time of concentration).This is also referred as the critical catchment response. In this tutorial, the Rational Method is used to develop design flows for a small collection system. Next, XPtools are used to set node and link elevations from the site topography and derive conduit lengths from node coordinates. Finally, the hydraulic calculator and design tools are used to determine pipe sizes and slopes. Part 1 – Rational Method Hydrology The peak discharge is given by the equation: Q = 0.28 ×C × I × A Where: Q = peak discharge, m 3 /s C = runoff coefficient, dimensionless I = rainfall intensity, mm/hr A = catchment area, km 2 Rainfall intensity is obtained from Intensity-Frequency -Duration (IFD) curves specific to the location of the project. An example of a set of IFD curves is shown below. For a given return period and time duration (sometimes referred to as the time of concentration), the intensity is determined for each catchment. 0.01 0.10 1.00 10.00 1 10 100 1,000 10,000 Duration - minutes Intensity - mm/hr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 200-yr Peak flows are used to design gravity collection and conveyance systems. In this tutorial, users will develop peak flow rates using XP’s design tools to determine the required sizes for pipes in the collection system. Level: Novice Objectives: Use the Rational Method to develop peak runoff rates for a collection system Time: 1 hour xp solutions Tutorial 5 - Stormwater Design
Transcript of Stormwater Design Using the Rational Method
Stormwater Design Using the Rational Method
Although less rigorous than other hydrologic techniques, the Rational Method remains a popular tool for determining peak flows in small catchments. It is based on the theory that the peak runoff occurs during a storm event of duration equal to the travel time of runoff from the top of the catchment to the outlet (time of concentration).This is also referred as the critical catchment response.
In this tutorial, the Rational Method is used to develop design flows for a small collection system. Next, XPtools are used to set node and link elevations from the site topography and derive conduit lengths from node coordinates. Finally, the hydraulic calculator and design tools are used to determine pipe sizes and slopes.
Part 1 – Rational Method Hydrology
The peak discharge is given by the equation:
Q = 0.28 ×C × I × A
Where: Q = peak discharge, m3/s C = runoff coefficient, dimensionless I = rainfall intensity, mm/hr A = catchment area, km2 Rainfall intensity is obtained from Intensity-Frequency -Duration (IFD) curves specific to the location of the project. An example of a set of IFD curves is shown below. For a given return period and time duration (sometimes referred to as the time of concentration), the intensity is determined for each catchment.
0.01
0.10
1.00
10.00
1 10 100 1,000 10,000Duration - minutes
Inte
nsity
- m
m/h
r
2-yr
5-yr
10-yr
25-yr
50-yr
100-yr
200-yr
Peak flows are used to design gravity collection and conveyance systems. In this tutorial, users will develop peak flow rates using XP’s design tools to determine the required sizes for pipes in the collection system.
Level: Novice
Objectives: Use the Rational Method to develop peak runoff rates for a collection system
Time: 1 hour
xp solutions Tutorial 5 - Stormwater Design
Tutorial 5 - Stormwater Design
Data files: YarraR0m.xp (starter xpswmm model) Contours.xptin Yarra_Area.dwg (background CAD file)
1. Launch the application. At the opening dialog, navigate to the file YarraR0m.xp. Click on Continue. The file should open with the contours DTM. Set mode to Runoff (Rnf). Use the Layers Control Panel to toggle the display of the CAD files and to adjust the view to display the runoff nodes, catchments, catchment connections, and the DTM as shown below.
2. Activate Rational Method Hydrology. On the Configuration menu, select Job Control and then Runoff. Click on the Rational Formula button to open the Rational Formula Settings dialog. Set the Return Period To Analyze to 5. Click on the Edit button.
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Now, type in “IFD Table” and click on Add button. You will see a database with name IFD Table is added. Left click on this and click on Edit button. Now this is ready for editing. IFD data may be entered in a variety of configurations including tabular or as formulae. Select IFD/IDF Table and click on Edit.
Highlight the IDF Table record in the left panel and click on Edit.
Review the IFD table. Note that the return period ranges from 1 to 100 years and duration ranges from 5 to 4320 minutes (3 days). You need to complete the table as shown below. This data was downloaded from the Australian Bureau of Meteorology website. http://www.bom.gov.au/hydro/has/cdirswebx/cdirswebx.shtml. Follow the instruction given in the web page and you will be able to get an excel table. Just copy and paste the excel table in the dialog box shown below. Alternatively, Australian clients can go for an AR&R 77 or 87 methods as well. In that case you need to get the coefficients from AR&R manual or the above shown web link. To save time you can even copy and paste the data from the excel sheet supplied with the sample files.
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Click OK and return to the previous dialog. Select the Runoff Coefficient Method as Direct and click OK:
Click on Select to choose the IFD Table record, then OK to close the Rational Formula Settings dialog, and then OK to close the Runoff Job Control dialog.
3. Enter node data. Double click on Node 5/4 to open the Runoff Node dialog. Enter 20 in the Imp. (%) box for Sub-Catchments 1. Click on the Sub-catchments 1 button to open the Sub-Catchment dialog. The Rainfall and Infiltration data are ignored when Rational Method Hydrology is invoked. Click the Rational Formula button.
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In the Rational Formula Hydrology dialog enter 0.75 for the Pervious Runoff C. Select Kinematic Wave from the TC Method (Time of Concentration Method) drop list for Pervious. Enter 2 as Pervious Additional Travel Time, 280 for the Pervious Flow Path Length, 1.35 for the Pervious Flow Path Slope, and 0.045 as the Pervious Catchment Roughness.
For the Impervious area, select Kinematic Wave from the TC Method drop list. Enter 280 for the Impervious Flow Path Length, 1.35 for the Impervious Flow Path Slope and 0.035 as the Impervious Catchment Roughness. Enter 60 for Time of Constant Flow.
Data for the remaining runoff nodes may be entered in a similar manner. Alternatively, an XP Table has
been constructed to edit and view the data. Click on the XP Tables tool and click on the Rational tab.
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Enter the data as shown below. Note that the node names shown in grey color are nodes that are not active in the Rnf layer. You don’t need to enter any data for these nodes.
4. Set the time control. On the Configuration menu, select Job Control, Runoff, and then Time Control. Set the Simulation Start to Year 2008, Mth1, Day 1, Hour 0 and the Simulation End to 2008, Mth1, Day 1, Hour 4.
5. Calculate Runoff. On the Analyze menu, select Solve. Right click on node 5/4 and select Review Results
from the popup menu. Note that the runoff hydrograph ramps from a flow of 0 to a rate of 0.078 m3/s at 17 minutes into the simulation. Flow remains constant for 60 minutes and drops to 0.
6. Save your file as YarraR1.xp Questions
1. What is the maximum runoff for nodes: 5/2 ____ 6/1 ____ 5/2 _____ 4/1 ___ 3/2 _____ 2. What is the effect of the Time of Constant Flow on the runoff hydrograph?
3. How does the Additional Travel Time effect peak flows?
Part 2 – Using the DTM to Adjust Inverts
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Level: Novice
Objectives: Use XP Tools to develop conduit lengths and elevations from GIS and DTM data
Time: 1 hour
Data files: YarraR1.xp (completed in Part 1)
1. Launch the program. At the opening dialog, navigate to the file YarraR1. Click on Continue. The file should open with the contours DTM. Set the mode to Hydraulics (Hdr). Use the Layer Control Panel to adjust the view to display the all nodes, all links, and the DTM as shown below. Save the file as YarraR2.
2. Review initial data. Click on the XP Table List tool and click on the Basic Conduit Data tab. Note that the model has been initialized with default data for elevations and pipe geometry.
3. Generate ground elevations from TIN. On the Tools menu, select Generate Ground Elevations From TIN. Note that Node 3/375 and 3/1 are outside of the contoured area. Uncheck the calculate boxes for these nodes. Click on OK to accept the new ground elevations.
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Open the Node Data dialog for Node 3/375, set the spill crest to 1278.5, and 1279.5 for Node 3/1.
4. Modify invert elevations. Click on the Select All Links and Select All Nodes tools to select all objects in
the model. On the Tools menu, select Modify Elevations…. Check the radio button next to Drop Inverts From Node Spill Crest. Check the boxes as shown in the dialog as shown below. This will set node inverts to 2.7 m below the ground and match link inverts to the nodes’ inverts.
Click OK. The number of nodes and links with modified inverts are reported.
5. Calculate conduit lengths from x-y coordinates. On the Tools menu, select Calculate ConduitLengths.
Check the All radio button and click on Calculate. The new lengths will be displayed. Click on OK to accept. In the similar manner, Calculate Conduit Slopes as well.
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6. Review data. Open the Basic Conduit Data table and confirm that the lengths and elevations have been
properly added to the model.
7. Set a run to a constant slope. Sometimes designers set a long section of a network at a constant slope. Right click on node 3/2. Choose Select Downstream Objects from the popup menu. The segment from node 3/2 to 3/375 should be selected.
On the Tools menu, select Modify Elevations. Check the radio button next to Generate Intermediate Inverts. Click OK. The inverts of Junction and 3/1 have been revised so that Pipe07, Pipe08 and Pipe09 have slopes of 2.77%.
8. Adjust Pipe06. Note that the invert of Junction was raised and is above the invert of the downstream end of Pipe06. Double click on Pipe06 to open the Conduit Data dialog. Click on Conduit Profile. Set the invert of the downstream (D/S) end of Link6 to 1283.4. Solve for the Slope. Click on OK twice to exit the link data dialog.
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9. Adjust Node 3/1. When the invert of Node 3/1 was raised the ground elevation was not. Double click on Node 3/1 and set the Spill Crest level to 1282.0 – this will provide appropriate cover for the connecting pipes.
10. Open the Basic Conduit data table and confirm the modifications. Save your file as YarraR2.xp.
Questions
1. What is the slope for Pipe 01?_______.
2. In the Modify Elevations dialog, does the distance that inverts are dropped from the Node Spill Crest affect the slopes?
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Part 3 – Using Dialog Hydraulic Calculators The design tool is used to assist in adjusting conduit cross sections. In this example a circular pipe is used. For a full pipe, there are four variables: flow, diameter, n, and slope. When any three are defined, the fourth is calculated by Manning’s Equation. This calculation assumes a full flowing pipe.
Level: Novice
Objectives: Use Hydraulic Calculator to modify pipe sizes and slopes
Time: 0.5 hour
Data files: Contours.xyz (used to create TIN) YarraR2.xp (completed in Part 2)
1. Open the YarraR2m.xp model created in Part 2 and Save AsYarraR3m.xp. On the Configuration menu, select Job Control Hydraulics. Check the box for Run Hydrology/Hydraulics Simultaneously.
2. Set the mode to Runoff and Hydraulics. On the Configuration menu, select Mode Properties…. Check the boxes next to RUNOFFand HYDRAULICS in the Solve Mode section. Click on OK.
3. Solve the model. On the Analyze menu, select Solve. Now, click on the Dynamic Plan View icon , or alternatively, select the Dynamic Plan View from the Results menu.
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You will see that some nodes are flooded and water is lost from the system. This is due to inadequate diameter of the pipes (remember the default diameter of 0.05 m). Enter an initial diameter of 0.9m for all
links. Enter the value for any pipe and use the Copy tool and click on the data field:
You will see that the data has been copied to the clip board. Select All Links and paste the data either from the Edit Menu or use the <Ctrl> + V of the keyboard. You will see a message box saying that the data has been pasted in to other links. Now simulate the model again and see the Dynamic Plan View. Make sure that the nodes are not getting flooded. We need to design pipes that can carry the flow without flooding.
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4. Using the Design tool. Double click on Pipe 08 to open the Conduit Data dialog. Click on Circular in the Design section. The value in the Flow box is the maximum flow in the conduit during the simulation.
Check the Diameter (B) radio button in the Solve for section and click on Solve. Note that the required diameter is 0.69m. If a 0.5 m diameter pipe is used, the required slope is 23%.
The conduit length and upstream and downstream node elevations may be modified in the data boxes on the right side of the dialog. Click on OK to update the model database. Click on Cancel to discard the edits.
5. Use the design tool to determine if the other conduits in the network can convey the maximum flow.
Questions
1. What is the required pipe diameter for conduits: Pipe 09______ Pipe 08______
2. What slope is required to convey the design flow for Pipe08 in a 0.5 m diameter pipe flowing full?
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Part 4 – Using XPDesign tools The design tool in XP Interface is used to automatically increase the diameter of pipes to meet user defined criteria. The % of depth is applied to the upstream end of the pipe. Depth of flow is calculated by the full dynamic solution (unless an alternative calculation is specified by the user). The algorithm does not decrease pipe sizes. Level: Novice
Objectives: Use XP design tools to size pipes globally and locally in a collection network.
Time: 1 hour
Data files: YarraR3.xp (completed in Part 3)
1. Set up global design criteria. Open YarraR3.xp and Save AsYarraR4.xp. On the Configuration menu, select Job Control and then Hydraulics. Click on Modify Conduits. Use the default settings. Click on OK to close the Modify Conduits dialog.
Click on Design Constraints.
In the Design Constraints dialog, set the Design for to 90% of Depth and the Minimum Cover to 0.6 m. Click on the Available Pipes button.
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In the Available Pipes dialog, uncheck the boxes for Pipe Sizes less than 0.375 m. Click on OK to close the Available Pipes dialog and OK to close the Design Constraints dialog.
2. Set the default head loss coefficient. On the Hydraulics Job Control dialog, double click on Simulation Tolerances. Set the Default Head Loss Coefficient to 0.2. Click on OK to close the Simulation Tolerances dialog and OK to close the Hydraulics Job Control.
3. Size the pipes. Now we will size the pipes. Note that the program will not reduce the pipe diameters. Hence, we need to specify very small diameters for all the pipes. Assign a 0.05 m diameter for all the pipes. Remember to use the copy and paste technique that we adopted before, for a group edit. On the Analyze menu, select Solve. Open the Basic Conduit Data table to view the pipe sizes. You will see the new pipe sizes calculated by the program:
4. Set local criteria. Double click on Pipe 05to open the Conduit Data dialog. Click on Conduit Factors to open the Special Conduit Factors dialog. In the Design For section, click on the% of Full Depth radio button. Enter 60% and analyze the model again. Remember to set the diameters to a lesser value before simulation.
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5. Now review the Basic Conduit Data table again. You will see that the diameter for Pipe 05 is 1.125 m instead of previous 0.6 m.
Questions
1. Does the addition of a tailwater condition to the outfall require a larger pipe diameter for Pipe 09? Why or why not?
2. Determine the required pipe diameters to convey the 100-yr storm with the design constraints described
in Step 1.
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