ssa

B-M-P, Easy as 1-2-3: Model Stormwater BMPs with AutoCAD® Civil 3D® and Autodesk® SSA

Josh Kehs, P.E. – Autodesk, Inc.

CV416-3

AutoCAD Civil 3D and Autodesk Storm and Sanitary Analysis (SSA) are an advanced, powerful, and comprehensive modeling package for analyzing and designing urban drainage systems, stormwater sewers, and sanitary sewers. This course is designed to provide an overview of how to model Stormwater Best Management Practices (BMPs), such as water quality and groundwater recharge technologies, in AutoCAD Civil 3D and analyze their performance using SSA. The class will take a look at the layout of these technologies in AutoCAD Civil 3D as well as the detailed design and analysis that is required as part of the National Pollutant Discharge Elimination System (NPDES) Phase II requirements.

DRAFT

About the Speaker

Autodesk Expert Josh Kehs, P.E., has ten years of experience in civil engineering in both the public and private sectors. He has experience in all phases of civil engineering projects from surveying and GIS mapping to construction and project management. Josh has designed various civil engineering projects including commercial sites, residential subdivisions, and state and local roads and highways. Josh has also designed many complex stormwater management systems meeting local, state, and federal water quantity and water quality requirements for private projects. Josh previously worked for Van Cleef Engineering Associates in Pennsylvania as a Project Manager. He holds a Bachelor of Science Degree in Civil Engineering from Bucknell University.

[email protected]

DRAFT B-M-P, Easy as 1-2-3: Model Stormwater BMPs with AutoCAD® Civil 3D® and Autodesk® SSA

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Introduction

During this 60 minute session, I will discuss and demonstrate the modeling and analysis of stormwater management Best Management Practices (BMPs) in AutoCAD Civil 3D and Autodesk Storm and Sanitary Analysis. We will take a look at some sample measures that address stormwater quality as well as stormwater quantity. The examples reviewed in this class could apply to BMPs such as rain gardens, bio-retention areas, water quality ponds and swales, and infiltration devices.

As you are probably aware, there are an almost endless number of methods of addressing water quality and infiltration requirements for Civil Engineering projects. These methods include structural as well as non-structural technologies. We will start with an overview of stormwter management and reviewing some examples of Stormwater Management BMPs.

The one thing that all of these methods have in common is the need to analyze the proposed measure and produce data that proves that the device will meet regulatory requirements. The next thing that we will look at is how to setup a model in SSA and analyze the model. The examples I show will demonstrate how to do the initial project setup properly and it will identify what hydrographs and data to review to determine whether requirements have been met.

Finally, we will take a look at the reporting options in SSA so that you can create an agency ready report that is ready to be approved in the first submission.

Let’s Get Started!


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Overview of Stormwater Management


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Determining Required Water Quality Volume (WQv) in SSA

Required water quality volume is computed in a number of different ways and is usually dictated by project location and regulating authority. A common equation to calculate water quality volume is:

WQv (in acre-feet) = (P*Rv*A)/12

Where:

P=90% rainfall event (typically 1 inch)

Rv=.05+.009(I), where I = percent impervious area

A= Area in acres

OR

The required volume can be computed in SSA by analyzing a 1 inch storm for the given project site

To calculate the required WQv in SSA, a storm with a total rainfall of 1 inch will need to be analyzed for the given project site.

To avoid having to manually add flow rates or volumes at multiple nodes, a junction or outfall can be created to combine all analyzed areas into one node.


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To create the 1 inch storm, click on the ellipsis next to Time Series in the Rain Gage Dialog Box

Click Add and rename the new Time Series curve WQ Storm (1 inch), then click Rainfall Designer


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Choose the desired storm distribution parameters and change the Rainfall Depth to 1 inch

Close all open dialog boxes and open the Analysis Options. Set the storm to Water Quality (1 inch) storm

Run the analysis by clicking on the running man icon and click on Time Series Plot in the lower left to review the results

Open the results for the outfall or junction that was created to combine all analyzed areas. The total inflow volume to this node is the required WQv.


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Setting Project Options to Account for Infiltration

There are a number of different methods to account for infiltration and these are typically determined by Stormwater Management (SWM) Regulations. In SSA, the terms infiltration and exfiltration are synonymous.

In order to account for exfiltration, a “storage node exfiltration method” must be defined in the Project Options

Tip: Details about the different exfiltration methods can be found in Chapter 6 of the SSA User’s Guide, available from the Help Menu in SSA.


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Modeling Infiltration Chambers in SSA

The following example demonstrates how to setup underground infiltration chambers in SSA. Additional chamber types, including underground pipes, underground arch pipes, and additional manufactured chambers are available within SSA.

In Autodesk SSA, double click on “Storage Curves” in the data tree.

Tip: The storage curves can also be accessed from the Input menu or through the storage node dialog.


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Select the desired chamber type and enter the parameters for the infiltration trench in which the chambers reside. Click Compute to calculate the storage curve and then Close the dialog box.

Select the Storage Node icon from the toolbar, place the storage node in the plan view in the desired location.


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Under Storage Shape, choose Type Storage Curve and under Storage curve, choose the storage curve that was previously created.

Additional input parameters representing the physical and flow properties will need to be entered.

In this example, the option to apply exfiltration at all elevations is chosen. The “Above Elevation” option is typically used for ponds or devices with impermeable liners that are used to maintain a permanent water surface elevation.

Tip: Clicking the ellipses for the Exfiltration rate will launch a lookup table for typical exfiltration rates.


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It should be noted that soil testing is typically required to determine the infiltration rate that is to be used in the analysis.

The storage node (BMP) must be connected with a link to an outfall or another node type. In this example, a 12 inch pipe connects the BMP to an inlet. Tip: Choose Add Conveyance Link to connect two nodes.

Typically a sag inlet will be added upstream of the storage node. This inlet will collect flow and direct it to the storage node via a pipe link.


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If runoff will flow directly to the storage chamber (i.e. an inlet is included as part of the underground chamber design), a link of the type “Direct” can be used.

The system can then be analyzed by choosing the running man icon on the toolbar

If the analysis runs successfully, the following dialog will appear. Tip: An error log will display if the analysis cannot run. Common errors are listed in the Users’ Guide.


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Reviewing Analysis Results

The analysis results can be viewed in a number of ways depending on the type of data that is required.

First, the user can double click the desired model element to view a summary of the analysis data. In this example, the storage node representing the BMP was chosen

To view a graphical animation of the analysis, right click on a node and choose Start Profile Plot.


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Next choose the upstream or downstream node or link you want the profile drawn through, and choose Show Plot

Choosing the play button in the Output Animation dialog will start the animation Tip: The output animation dialog is launched from the Output menu.


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To view time series data, including infiltration data, choose the Time Series button in the lower left Tip: The exfiltration parameter for the BMP node will quantify the amount of stormwater infiltrated during the analyzed storm.

Data from previous analysis runs can be loaded from solution files by right clicking in the Time Series Plot area and choosing Open Solution Tip: Multiple solution files can be compared to determine if requirements have been met. For example, the pre-development and post-development data could be loaded at the same time to compare the difference in flow and quantity values. Multiple storm analysis is set up in Analysis Options.


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Reporting Results

In SSA, the user

can produce reports in a number of ways. Report format can be customized by choosing Custom Report Options from the Output menu.

A report of the entire project can be created in one click by choosing a report button on the toolbar.

Reports can also be created from element dialog boxes

Data can also be exported to Word or Excel from the Time Series Plot data chart


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Additional Information on SSA

More information on SSA from Autodesk.com at http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=15430019

Videos on YouTube:

What's New in AutoCAD Civil 3D 2011 - Storm & Sanitary Analysis - http://www.youtube.com/watch?v=-y4uM0Z0n4I&feature=related

Autodesk SSA Overview – Storm Drain Inlets - http://www.youtube.com/watch?v=tepa7N8U-Js&feature=related

SSA Stormwater Model from Scratch - http://www.youtube.com/watch?v=LR93SC5Xe84

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