Control of Blowing Snow Using SnowMan (Snow Management ... · Control of Blowing Snow Using SnowMan (Snow Management) User Manual NYSDOTIRC Subcontract 28311-5823 Project C-01-67

Control of Blowing Snow Using SnowMan (Snow Management) User Manual

NYSDOTIRC Subcontract 28311-5823 Project C-01-67

revised December 2008

S. S. Chen M. F. Lamanna

Dept. of Civil, Structural, and Environmental Engineering

University at Buffalo State University of New York

ACKNOWLEDGEMENTS

This work was overseen by Project Manager Joseph F. Doherty, MBA, P.E., of the New York State Department of Transportation (NYSDOT). It was sponsored by NYSDOT’s Transportation Infrastructure Research Consortium (TIRC) which is administered by staff at Cornell University. The opinions and conclusions expressed or implied in this report are those of the authors. The advice of Dr. R. D. Tabler of Tabler and Associates and of Mr. D. F. Kaminski, P.E., of NYSDOT, is gratefully acknowledged. The contributions of former graduate students David Schwartz and Xiaotian Wang are also gratefully acknowledged.

DISCLAIMER

The contents of this report reflect the views of the author who is responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the New York State Department of Transportation, the United States Department of Transportation, or the Federal Highway Administration. This report does not constitute a standard, specification, regulation, product endorsement, or an endorsement of manufacturers.

Technical Report Documentation Page 1. Report No. C-01-67

2. Government Accession No.

3. Recipient’s Catalog No. 5. Report Date December 2008

4. Title and Subtitle Control of Blowing Snow using SnowMan

(Snow Management): User Manual

6. Performing Organization Code

7. Author(s) S. S. Chen and M. F. Lamanna

8. Performing Organization Report No.

10. Work Unit No. (TRAIS)

9. Performing Organization Name and Address Dept. of Civil, Structural, and Environmental Engineering

212 Ketter Hall, University at Buffalo, SUNY Buffalo, NY, 14260

11. Contract or Grant No.

13. Type of Report and Period Covered Developers Manual June 2004 – December 2008

12. Sponsoring Agency Name and Address NYS Department of Transportation

50 Wolf Road Albany, New York 12232

14. Sponsoring Agency Code

15. Supplementary Notes Project funded in part with funds from the Federal Highway Administration

16. Abstract

Properly engineered passive snow control measures can significantly reduce the safety hazards and winter maintenance costs associated with the problem of blowing and drifting snow. There are two possible mitigation strategies: roadway (cross section) design and snow fencing. This project developed and deployed a software application, named SnowMan (for Snow Management), to run within the NYSDOT’s MicroStation-based CAD environment to assist highway designers and maintenance users in the design of such passive control measures. This effort thus extends the applicability of the earlier PASCON expert system software (Kaminski and Mohan 1991) and incorporates well-established knowledge regarding snow transport and deposition, evaluating roadway cross sections for drift susceptibility, design of passive and living snow fences, and earthwork modification for reducing drifting (Tabler 2003). The SnowMan software brings the science of engineered mitigation of blowing and drifting snow to the desktop. This manual describes the usage of this software tool for mitigation of blowing and drifting snow problems, while also providing an overview of the relevant data and principal output results produced by the software for blowing and drifting snow mitigation. Benefits of the use of this software include reducing maintenance costs and closure times and improving crash incidence by improving visibility, preventing drifting on the road, and reducing road icing. 17. Key Words Snow fences, blowing snow, drifting snow, snow control, road design, software, MDL, climate, topographic, earthwork, mitigation

18. Distribution Statement No Restrictions

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No. of Pages 80

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SnowMan (Snow Management) User Manual

Table of Contents

Section Page Introduction ........................................................................................................................... 1 Overview: Inputs and Processing.......................................................................................... 2 Inputs ..................................................................................................................................... 2 User Information Input.................................................................................................. 2 Climate Input................................................................................................................. 2 Topographic Input ......................................................................................................... 3 Processing and Outputs ......................................................................................................... 6 Organization of User Manual ................................................................................................ 7 SnowMan User Interface Screens

SnowMan Welcome Screen .......................................................................................... 8 User Input Information.................................................................................................. 9 Climate Data.................................................................................................................. 11 Site-Specific Climate Data Input.............................................................................. 11 Climate Data Inferred from Latitude and Longitude................................................ 12 Open SnowMan Climate Data File .......................................................................... 13 Display Climate Data ............................................................................................... 14 Topographic Data .......................................................................................................... 15 Open SnowMan Topographic Data File................................................................... 15 Enter Starting Data Point along ULOP (in 3D DGN file)........................................ 16 Enter Ending Data Point along ULOP (in 3D DGN file)......................................... 17 Manual Input of 2D Topographic Data .................................................................... 19 Section Information.................................................................................................. 20 Display Topographic Data ....................................................................................... 21 Case Selection ............................................................................................................... 22 Output Options .............................................................................................................. 24 Plotting Output in DGN file .......................................................................................... 25 Example Design File Output Results Prior to Mitigation (Case 1)............................... 26 Snow Fence Solutions ................................................................................................... 28 SnowMan Determines Fence and Setback (Case 2)................................................. 28 User Specifies Fence, SnowMan Determines Setback (Case 3) .............................. 30 User Specifies Maximum Setback, SnowMan Determines Fence and Setback (4). 31 User Specifies Fence and Maximum Setback, SnowMan Determines Setback (5). 33 User Specifies Fence and Setback (Case 6) ............................................................. 35 Example Output Results for a Snow Fence Solution ............................................... 36 Earthwork Design Options ............................................................................................ 38 SnowMan Determines New Ground (Case 7).......................................................... 38 User Specifies Max Setback, SnowMan Determines New Ground ......................... 39

Glossary................................................................................................................................. 40 References ............................................................................................................................. 43

Appendix A Default Values Employed by SnowMan ........................................................ A-1 Appendix B Abbreviated Guide to Essential SnowMan Inputs .......................................... B-1 Appendix C Computer-Aided Design of Passive Snow Control Measures ......................... C-1 Appendix D Introduction to Blowing Snow Mitigation and SnowMan ............................ D-1

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Introduction The SnowMan (Snow Management) software was developed as a MDL (MicroStation Development Language) software application by the University at Buffalo (State University of New York) in collaboration with Dr. R. D. Tabler of Tabler and Associates under contract with NYSDOTIRC (New York State Department of Transportation Infrastructure Research Consortium) under the technical oversight of D. F. Kaminski and J. Doherty of NYSDOT. This software application enables a highway design or maintenance engineer to investigate various options for mitigating blowing and drifting snow by means of snow fence or ground modification measures. A separate paper describes the system (Chen et al. 2008), and a companion Developer’s Manual (Lamanna and Chen 2008) describes the inner workings of the code. Control of blowing and drifting snow on the nation's highways is important to reduce maintenance costs and closure times and to decrease crash incidences by improving visibility, preventing drifting on the road, and reducing road icing. Properly engineered passive snow control measures can significantly reduce these hazards and associated costs (Tabler 2003). The technical complexities involved in design of appropriate preventive or mitigation measures require understanding of the processes of snow transport and deposition, the design of structural and living snow fences, and the design of drift-free sections of roadways. What is needed is a suitable computer-based tool to bring such expertise to the fingertips of those charged with highway design or winter maintenance. SnowMan is just such a tool, although if you have no previous experience with snow control, it is recommended that you become familiar with Tabler (2003). There have been several earlier efforts involved in developing computer-based tools to assist the engineer with the computations involved in the design of passive snow control measures. These efforts include the PASCON system developed by Kaminski and Mohan (1991), the WYDOT Drift Profiler (WYDOT 2007, Tabler 1997), and an interactive web site developed by the University of Minnesota and MNDOT (MNDOT 2007). PASCON and the MNDOT system, unfortunately, are not integrated into the CAD environment utilized by highway designers, and the WYDOT application does not actually design snow fence systems. What SnowMan provides along with the numerical computations of the earlier systems is software-implemented expertise tailored for engineered mitigation using road design and snow fences running in MicroStation for use statewide within the New York State Department of Transportation. As such, SnowMan makes possible the incorporation of snow control design measures directly into the workflow of highway design, although it can also be used for two-dimensional maintenance applications on existing roadways. The scope of SnowMan includes drift prediction (both unmitigated and mitigated, the latter for either user-proposed or automatically generated snow fences), evaluation of roadway sections and determination of trial fence solutions subject to combinations of height, setback, and porosity constraints, and prescription of upwind earthwork (ground modification) solutions. This manual describes the usage of this software tool for mitigation of blowing and drifting snow problems, while also providing an overview of the relevant data and principal output results produced by the software for blowing and drifting snow mitigation.

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The current version of SnowMan uses metric units, is limited to single-fence solutions, and does not provide computational support for living-fence solutions. Overview: Inputs and Processing Passive control of blowing relocated snow is the primary scope and focus of SnowMan, which can be run only as a MicroStation Development Language (MDL) application within the MicroStation environment. User, climate and topographic data are all collected by the input portion of the program. This information is then passed along to the processing portion which handles the evaluations and designs. The processing portion evaluates terrain with or without snow fences, producing snow drift profiles for both. The processing portion also recommends structural snow fences that satisfy the design criteria (if such fences are possible within the constraints of the site) as well as changes to topography to meet the same design criteria. The output options include sending those results and evaluations generated by the processing portion to a MicroStation design file or to a SnowMan data file.

Inputs There are 4 basic types of inputs:

• general user information, • climate data, • topographic data, and • case specific data.

The initial inputs consist of user information, followed by the climate information, then the topographic information, in that order. Case-specific data is requested subsequent to initial evaluation of the site.

User Information Input User information consists of information about the user and the site that is not relevant to the evaluation or design of snow mitigation measures. The data obtained in this portion are the user’s name, the user’s level, the project ID number, the site identification, the method to use for climate input, the method to use for topographic input respectively.

Climate input Climate data is that which affects the computations for snow over the accumulation season and associated transports. Climate data input can be by any of the following:

• site specific data,

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• computation from latitude, longitude and elevation, and • SnowMan data file,

Site-Specific Data Input by site-specific data prompts the user for all inputs. It is recommended that site-specific data be utilized whenever possible. Input of just latitude, longitude, and elevation results in SnowMan computing the remainder of the climate data. Input by data file loads all the climate data from a previous run of the program.

The site-specific data input method prompts the user for the site’s latitude, longitude, and elevation, total annual snowfall during the snow accumulation season, relocation coefficient, and prevailing wind direction. The default value of relocation coefficient (defined in file nym_snowman_defaults.data) is not to be modified by novice users. The value of the exceedance factor is set to the default value of 1.5. Then the dialog box for displaying the climate data is opened. At that point the user can modify data and has the option to save the data to a data file before continuing on to topographic data input.

Computation from Latitude, Longitude, and Elevation

The method of inputting just the latitude, longitude and elevation results in computation by SnowMan for the total annual snowfall, prevailing wind direction, and ambient snow cover. The relocation coefficient and the exceedance factor are both set to the default values. In this method the user is asked to provide only the information about the longitude, latitude and elevation. Once the information is input the remaining values are computed or set by default and displayed to the user. The user will have the option of changing values and saving the new data to a SnowMan climate data file before continuing on to topographic data input.

SnowMan Data File

Input from a SnowMan data file loads the values from a previous run of SnowMan. The previous run may have generated the results from either input by site specific data, or by computation from latitude, longitude and elevation. In this case the user is prompted for the name of a SnowMan climate data file and once provided the file is opened and read in. If the file is corrupted or not a valid SnowMan climate data file the user is prompted again for the name of a valid SnowMan climate data file. Once the file is successfully loaded in, the information is displayed to the user for review and possible modification. The user has the option to save the data to a new file or overwrite the file loaded before continuing on.

Topographic Input Topographic data consists of the ground profiles and corresponding section information. The topographic input methods consist of the following:

• 3D MicroStation design (DGN) file,

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• manual input of 2D sections (transects) and section information, and • input by SnowMan topographic data file.

This topographic information is needed so snow profiles can be generated.

3D MicroStation Design (.DGN) File Input by design file prompts the user for a starting and ending location along the upwind limit of protection line. The ground profiles and the section information are then extracted from the elements in the design file. Input by design file compliant with NYSDOT level standards starts by searching the design file for elements of interest (triangles, ditch, ULOP, DLOP, and fetch). After the elements of interest are found, the user is prompted to enter a data point along the upwind limit of protection. Checks insure the user entered a point along the upwind limit of protection. If they did not then they are prompted until they do. After the starting and ending locations are called, SnowMan determines the number of sections (a.k.a. transects) needed to cover the area the user identified. Each section is drawn and the intersections with the triangle elements determine the ground profile. The intersection with the other lines of interest allow the function to determine the fetch, location of the upwind and downwind limits of protection and the location of the lowest ditch point if there is a ditch. After all the sections are generated, SnowMan displays the data to the user for review. The user can record the data to a file before continuing on to processing. Snowman must be able to find certain information contained in the design file related to the upwind and downwind limits of protection, ditches (if they exist), topography and fetch boundary. All of these elements, except ditches, must exist for SnowMan to execute. SnowMan uses the current NYSDOT CADD Level Standards for these elements. The topography/surface needs to be in the form of triangles. SnowMan does not explicitly differentiate between existing or proposed surfaces/triangles or ditches since the existing and proposed levels are the same for these elements. For these data elements there are corresponding definitions of level, weight, color and style for these lines of interest defined in the file ny_snowman_levels.data. In ny_snowman_levels.data the following definitions are used:

Upwind limit of protection - UPWIND EDGE OF PROTECTION Downwind limit of protection - DOWNWIND EDGE OF PROTECTION Existing and proposed ditch lines - EXISTING DITCH and PROPOSED DITCH. Existing and proposed 3-D surfaces - EXISTING TOPO and PROPOSED TOPO Fetch boundary – LIMIT OF FETCH

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The current NYSDOT CADD Level Standards for these elements are:

Ditches (existing or proposed): level = DD, color = 7, weight = 0, style = 6 Upwind limit of protection: level = LLOPSU_P, color = 5, weight = 2, style = 0 Downwind limit of protection: level = LLOPSD_P, color = 5, weight = 2, style = 0 Topography/triangles (existing or proposed): level = RDEFAULT_P, color = 1, weight =

0, style = 0 Fetch boundary: level = LSFFB, color = 5, weight = 0, style = 3

Manual Input of 2D Sections (Transects)

The manual input option results in the user being prompted for section profiles and section information for each section to be input through a series of dialog boxes. Input by 2-D sections involves repeated gathering of offset/ elevation data points defining the sections (transects). These sections must be parallel to the prevailing transport (wind) direction. Each data point is checked to insure there are no duplicate offsets. Once all the data points of a transect are gathered, the section information is gathered. This consists of the the ditch information (if ditch is present), location of the upwind and downwind limit of protection, fetch, and angle to road. This process is repeated as long as the user clicks the “Next Section” button in the get data point dialog box. The loop ends when the user selects “Done.” At that point the last of the section information is gathered and checked. If at any point the user inputs an invalid value for a field they are prompted for a correct value. Once all data is gathered and checked the information is displayed to the user where the user can review each section and has the option to save the data to a SnowMan topographic data file before continuing on to processing. Input from SnowMan Topographic Data File

Input from SnowMan topographic data file loads all the necessary topographic information from a file that was generated from a previous run of SnowMan. The previous run could have generated the information by either design file or manual input.

Input from SnowMan data file is similar to input from climate data file. That is, all the relevant topographic information is read back in from a previous run and save of topographic data. This includes all the offset / elevation pairs for the ground profile as well as the locations of the upwind and downwind limits of protection, indication if there is a ditch and location of the lowest point of the ditch if present, the value of fetch and the angle between the road and the direction of the prevailing wind. If all the data in the file provided by the user is valid then the data is displayed back to the user where they can review each section. If the file is not valid then the user is prompted again for the name of a valid SnowMan topographic data file. After a valid file is loaded and the user has reviewed the sections they have the option of saving the data to a file before continuing to processing.

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Recurrent Dialog Box Options In a number of input dialog boxes, “OK,” “Cancel,” and “Exit” options will be provided: Click OK to procede further. Click Cancel to go back to User Input Information. Click Exit to exit SnowMan.

Processing and Outputs These modules determine what design options the user wishes to execute and gathers the specific data for those cases. There are 3 main sections: determining what cases to run, collecting the relevant data for those cases, and generating the results (fences, snow profiles, storage computations, depth predictions, etc.)

Case Selection Case selection determines which of the available design options the user would like to run for the current set of sections (transects) and climate data. The available cases are as follows:

1. evaluate existing terrain, 2. user selects fence type (permanent, portable, temporary) and SnowMan determines the

fence height, porosity, and location, 3. user selects fence height and porosity and SnowMan determines location, 4. user specifies fence type and maximum setback and SnowMan determines fence height,

porosity and actual location, 5. user specifies fence height, porosity and maximum setback and SnowMan determines

actual location, 6. user specifies actual fence height, porosity, and actual location and SnowMan determines

the profiles and computes the storage and depths, 7. SnowMan determines new upwind ground surface profile, 8. user specifies maximum setback and SnowMan determines new upwind ground surface

profile. The first time through the processing portion of the program, the set of cases is pre-determined; only the evaluation of existing terrain (Case #1) is run. In all subsequent runs the user selects the cases to run. Cases 2 through 6 constitute the snow fence design options available to the user. Background information is available in Tabler (2003). Cases 7 and 8 constitute the earthwork (roadway section) design options available to the user. Background information is available in Chapter 8 of Tabler (2003). The “Case Selections” menu obtains from the user the set of cases to run. The set is checked to ensure that it is not empty. If the user did not select a set of cases to run they are prompted for a

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set of cases. Once they choose which cases they would like to run the program moves on to collect the specific data related to the cases that were chosen. The data needed for the selected cases is obtained from the user, case by case, and then all selected cases are executed in order.

Generate Results and Output SnowMan prompts the user for where the output should go. There are two options for where to send the results. The results can go either to a SnowMan results data file, or they can go to the design file (screen). The only other output option is to select a scale factor when output goes to the screen. For that, there are only two choices, either 1:1 or 5:1 scale factor for the vertical scale. When output goes to the screen the user is prompted for a data point for the upper left corner where the output is to start. Outputting the results to a data file consists of opening/creating a data file and using the saveResultsDataFile function to output the data to a file. Outputting the results to the screen involves plotting the information to the current MicroStation design file. The user may select further cases to run before exiting SnowMan. Limitations of the snowdrift-generation algorithms employed in SnowMan include the following (Tabler 2003):

• Most of the data used for the development of the equation were from gentle to moderately rolling terrain. The greater turbulence expected in mountainous country could cause slopes steeper than predicted.

• Future research or experience might indicate that the prediction accuracy could be improved by revising the coefficients or mathematical model employed by SnowMan.

• The algorithms are applicable only to two-dimensional terrain features.

Organization of User Manual The main body of this user manual describes the various screen displays encountered by a SnowMan user in approximately the sequence those screen displays would be encountered. A glossary of terms follows. After the list of references, appendices contain the following:

• Appendix A: a list of default values employed by SnowMan • Appendix B: a “cheatsheet” abbreviated guide to essential SnowMan inputs • Appendix C: Introduction to Blowing Snow Mitigation and SnowMan, by R. D. Tabler,

Ph.D., P.E.

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SnowMan User Interface Screens

Snow Management (SnowMan) Welcome Screen

Climate Data: Data specific to the site is preferable. Such data consists of the average yearly snowfall

during the snow accumulation season, the relocation coefficient (whose default value is not to be modified by novice users), prevailing wind direction, and ambient snow cover. If site-specific data is not available, enter latitude, longitude, and elevation; interpolation algorithms based on data from less than 100 sites in NYS will be used by SnowMan to approximate the climate data if site-specific climate data are not available. Site-specific data are preferable due to the coarseness of the NYS weather station data used to generate the interpolation algorithms. CORPSCON software can be used to generate longitude and latitude data from site data in NY State Plane Coordinates.

Topographic Data: Either 2D section data or 3D DGN files are required. Note that MicroStation DGN

files (and associated settings files), if used for topographic data, must utilize the latest NYSDOT CADD Standards and in particular must have the features indicated (RDEFAULT_P, etc.).

Click Accept to continue Click Decline to exit and get the data necessary to run Snowman

User Input Information

User Name: Enter the name of the user running SNOWMAN User Level:

Novice: user with basic knowledge of blowing snow control. Expert: user with good understanding of blowing snow control sufficient to override defaults used in

SNOWMAN (e.g., Relocation Coefficient, etc.). Project ID Number: PIN of the project that is being evaluated for blowing and/or drifting snow problems

and possible mitigation. Site ID: Enter a text description of the site being investigated (40 char max). Climate Data Input Method:

Site specific: Requires the latitude, longitude, and elevation of the site as well as the total annual snowfall for the site, prevailing wind direction, and the relocation coefficient.

Computed from latitude, longitude and elevation: Requires the latitude, longitude, and elevation of the site. SNOWMAN computes the total snowfall, wind direction, and relocation coefficient. CORPSCON software can be used to determine latitude and longitude from NY state plane coordinates.

SNOWMAN climate data file: Load a data file containing the climate data in SNOWMAN Climate Data format.

Topographic Data Input Method: From DGN following NYSDOT Levels Standards: SNOWMAN will use the current design file and

search for elements, relevant to generating cross sections, that meet the level standards (LLOPSU_P, LLOPSD_P, DD, RDEFAULT, LSFFB)

Manual Input of 2D cross sections: SnowMan will prompt the user for relevant information, i.e., the offset and elevation pairs for each data point in a section (transect) as well as the angle to the road, the distance to the downwind limit of protection, lowest ditch point if any, and the distance to the nearest snow transport barrier, or fetch. All of these transects must be parallel with the prevailing wind direction.

SNOWMAN topographic data file: Load a data file containing the topographic data in SNOWMAN Topographic Data format (a text file).

If you have no previous experience with snow control, it is recommended that you become familiar with the following reference by Ronald D. Tabler: Controlling Blowing and Drifting Snow with Snow Fences and Road Design, Technical Report prepared for the National Cooperative Highway Research Program, NCHRP Project 20-7(147), August 2003

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Site Specific Climate Data Input

You will see this dialog box if earlier you specified the Climate Data Input Method as “Site-specific data provided by user”. Enter the data in all 6 fields, with values within the allowable range limits given:

North Latitude: Degrees North Latitude of the site. [40.267, 44.983] West Longitude: Degrees West Longitude of the site. [71.750, 80.183] Elevation: Height in meters above sea level of the site [0.0, 1000.0]. Total Annual Snowfall: mm of snow the site receives annually during the snow

accumulation season. [>0.0] Relocation Coefficient: Ratio of Snow that can be relocated [0.0, 1.0]. Novice users

should use the default value of 0.17 Direction: Degrees true north azimuth of the direction of the prevailing transport

(wind) direction at the site [0.0, 360.0] If you input an invalid value (e.g., 1.1 for Relocation Coefficient), SnowMan will re-display this dialog box until all data entries are within their specified range. Click OK to procede further. Click Cancel to go back to User Input Information. Click Exit to exit SnowMan.

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Latitude, Longitude, Elevation Input

You will see this dialog box if you specified the Climate Data Input Method as “Computed from latitude, longitude, and elevation”. Enter the data in all 3 fields, with values within the allowable range limits given:

North Latitude: Degrees North Latitude of the site. [40.267, 44.983] West Longitude: Degrees West Longitude of the site. [71.750, 80.183] Elevation: Height in meters above sea level of the site [0.0, 1000.0].

SnowMan utilizes an interpolation algorithm to approximate the remaining needed climate data (relocation coefficient, prevailing wind direction) from the above. It is preferable instead to use site-specific data if available. Click OK to procede further. Click Cancel to go back to User Input Information. Click Exit to exit SnowMan.

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Open SNOWMAN Climate Data File

You will see this dialog box if you specified the climate data input method as “SNOWMAN Climate Data File”. Identify the appropriate drive or folder and then select or enter the name of the file containing valid SnowMan climate data in SNOWMAN climate data format.

Normally, this file would have been created in an earlier run through SnowMan.

The default file name extension is .data

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Climate Data

This screen appears subsequent to completion of the climate data input screens. This screen provides the opportunity to edit the climate data and save it to a data file. Note: the Relocation Coefficient (Cr) and Exceedance Factor (K) cannot be edited by “Novice” users. These will appear with the default values shown.

North Latitude: Degrees North Latitude of the site. [Limits: 40.267, 44.983] West Longitude: Degrees West Longitude of the site. [Limits: 71.750, 80.183] Elevation: Height in meters above sea level of the site [Limits: 0.0, 1000.0]. Average Yearly Snowfall: (St) mm of snow the site receives annually during the snow accumulation

season. [>0.0] SnowMan converts this to water equivalent in order to carry out blowing and drifting calculations transparently to the user.

Relocation Coefficient: (Cr) Ratio of Snow that can be relocated. [0.0, 1.0] Exceedance Factor: (K) a.k.a. Design Modulus, the snow transport ratio Qdes/Qt,ave; thus K = 1.5 (snow

transport 50% greater than the long-term average) would be expected to occur 5 years out of 100 (Tabler 2003, Table 4.6). Never design for K < 1.

Wind Direction: Degrees true north azimuth of the direction of the prevailing wind at the site.[0.0, 360.0]

Ambient Snow Cover: Average depth of snow on the ground, prior to blowing and drifting, during the snow accumulation season [>0.0]

No inputs are required, but you may change values in data input fields (with the possible exception of the Relocation Coefficient and the Exceedance Factor). However, the same bounds continue to apply. You may also check the box to save the climate data to a SNOWMAN Climate Data formatted file (text format). Click OK to procede further. Click Cancel to go back to User Input Information. Click Exit to exit SnowMan.

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Open SNOWMAN Topographic Data File

You will see this dialog box if you specified the topographic data input method as “SNOWMAN Topographic Data File”. Identify the appropriate drive and folder and then select or enter the name of the file containing valid SNOWMAN topographic data in SNOWMAN topographic data format. The file contains offset and elevation data on section(s) oriented parallel to the prevailing wind direction. Files may be saved in any user-specified folder. Normally, this file would have been created in an earlier run through SnowMan. If desired, a topographic data file can be created with text editing or word processing software.

A SnowMan topographic data file is a text file that contains, after the first line which specifies the number of sections, the following information line-by-line for each along-wind section (i.e., “slice”) of terrain:

• Number of terrain surface data points, • Terrain offset and elevation pairs (separated by a comma), in meters, one line for

each data point, • Upwind EOP (Edge of Pavement, a.k.a. ULOP Upwind Limit of Protection)

offset and elevation, • Downwind EOP (a.k.a. DLOP Downwind Limit of Protection) offset and

elevation, • Whether there is a ditch on the section (e.g., DITCH_YES), • DITCH: offset and elevation, separated by a comma, • Fetch: in meters, and • Angle (of prevailing wind direction) to road, in degrees, in decimal form

The default file name extension is .data

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Enter Data Point

You are receiving this prompt because you selected “From current 3-D design file (.dgn) following NYSDOT level standards” as your Topographic Data Input Method. The local maintenance residency may have information about “trouble spots” that you can use to determine the starting point for generating snow drift profile sections in SnowMan. Enter the starting point along the upwind limit of protection in the level-standard compliant 3D .dgn file of your site. Typically the upwind limit of protection (ULOP) coincides with the upwind edge of pavement. Keeping in mind the prevailing wind direction (so you know which LOP is the ULOP, i.e., the Upwind Limit of Protection), enter a suitable data point (left-click).

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Enter Data Point

This prompt appears after you selected a starting point. Enter the ending point along the upwind limit of protection in the level-standard compliant 3D .dgn file of your site. Typically the upwind limit of protection (ULOP) coincides with the upwind edge of pavement. Keeping in mind the prevailing wind direction (so you know which LOP is U, i.e., Upwind), enter a suitable data point (left-click). Depending on how many sections (in between the starting point and the ending point) that SnowMan needs to generate, you may need to wait a few moments for SnowMan to produce the next screen.

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The new lines appearing in this screen indicate where SnowMan will generate sections from the 3D DGN file in the direction of the prevailing wind.

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Data Point Entry

You will see this dialog box if you selected “Manual Input of 2-D Cross Sections” as your Topographic Data Input Method.

Offset: In meters, distance (upwind = negative) from the ULOP to the data point being entered.

Elevation: In meters, the elevation of the data point above sea level.

Next Data Point: Select when additional data points are to be input for the specific

section. This records the current data point and gets the next data point.

Next Cross Section: Select when all data points for the indicated section have been entered, and there is another section to input. This records the current data point and prompts the user for section information, after which the user will be prompted for more data points in the next cross section.

Done: Select this when the last data point for the indicated section has been entered

and there are no additional sections to input. This records the current data point and prompts the user for section info, after which the topo data for all sections will be displayed to the user.

Notes:

• Cross-section information will be displayed next, before the next input screen appears.

• 0 (zero) offset occurs at the ULOP (Upwind Limit Of Protection, typically the upwind edge of pavement)

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Section Information

This dialog box will appear after the user selects “Next Section” or “Done” on the Data Point Entry screen.

Ditch Upwind of the Limit of Protection: Check box if section has a ditch and enter ditch offset and elevation data

DITCH Low Point Offset: distance in meters along the direction of the wind from upwind limit of

protection to lowest ditch point. Leave blank if the “Ditch upwind…” box is left unchecked

DITCH Low Point Elevation: elevation of the lowest ditch point. Leave blank if the “Ditch upwind…” box is left unchecked

Downwind Limit of Protection: Offset in meters to the downwind limit of protection.

Fetch: Distance in meters along the direction of the wind from the upwind limit of protection to the first

encountered barrier to transport.

Wind to Road Angle: acute angle between the prevailing wind direction and the road in degrees [between the limits of 10.0, 90.0]

21

Topographic Data

You will see this screen subsequent to topographic data input by either .dgn file, 2D input, or data file. No inputs required; the only allowable edit is the fetch. This screen allows the user to review topographic data. You can also check the box to save the topo data to a SNOWMAN topographic data file for later analysis. If revisions are necessary, click the “Save” checkbox, click Cancel, edit the data file, and re-select it. Notes:

1. Sign convention: offsets upwind of the upwind limit of protection are negative numbers; downwind positive.

2. There is a separate output screen for each section; use the arrow boxes to step through the sections.

3. Fetch is section-specific.

22

Case Selection

This dialog box appears after all climate and topographic input has been obtained. Check the boxes for the type(s) of evaluation/recommendation desired. Evaluate Existing Terrain (Case 1):

It is not typically necessary to check this box explicitly (since it is always run automatically upon the 1st run through SnowMan)

SnowMan Determines Fences and Setbacks (Case 2):

Use when you want SnowMan to determine the shortest fence solution and the closest (i.e., minimum setback) fence solutions to achieve the design objectives.

Permanent Fences Only: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, or 2.1 meters in height.

Temporary Fences Only: SNOWMAN finds solutions for fences that are 2.4, or 2.0 meters in height.

Either Permanent or Temporary: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, 2.1, 2.0, or 1.4 meters in height.

User specifies Fence, SNOWMAN determines Setback (Case 3):

Select porosity and height of the fence from the set of permanent or temporary fence options. You may want to use this case, e.g., if only certain stock fence sizes are available.

User specifies max Setback, SNOWMAN determines fence and setback (Case 4):

Use when suspecting that right-of-way limits constrain viable fence solutions. Permanent Fences Only: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7,

2.4, or 2.1 meters in height.

23

Temporary Fences Only: SNOWMAN finds solutions for fences that are 2.4, or 2.0 meters in height. Either Permanent of Temporary: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, 2.1, 2.0, or 1.4 meters in height.

Enter a maximum setback for the fence. SNOWMAN may place the fence closer than the maximum specified setback (if possible). The range is from -30.0 meters to -100.0 meters or the maximum upwind point - 45.0 meters.

User specifies fence and max setback, SNOWMAN determines setback (Case 5):

Use when both fence options and right-of-way are limited. Select porosity and height of the fence from the set of permanent or temporary fence options.

Enter a maximum setback for the fence. SNOWMAN may place the fence closer than the maximum specified setback (if possible). The range is from -30.0 meters to -100.0 meters or the maximum upwind point - 45.0 meters.

User specifies fence and setback (Case 6):

Use typically when fine-tuning your fence design. Height: In meters the total height of the snow fence [1.0, 10.0] Select porosity from the list. Enter a maximum setback for the fence. SNOWMAN may place the fence closer than

the maximum specified setback (if possible). The range is from -30.0 meters to -100.0 meters or the maximum upwind point - 45.0 meters.

SnowMan Determines New Ground (Case 7):

Use when you want SnowMan to determine the upwind ground cut surface profile needed to achieve snow control design objectives.

User Specified Maximum Setback and SnowMan Determines New Ground (Case 8):

Use when a right-of-way setback constraint must be considered in determining the upwind ground cut surface profile needed to achieve snow control design objectives.

In each case, SnowMan will repeat the process for each section (transect).

24

Output Options

This screen appears after all inputs have been supplied and SnowMan has conducted at least one case of blowing and drifting snow analysis.

Output to file: check if you want output sent to a text file. Output to design file: check if you want output sent to the design file; if so,

SnowMan/MicroStation will prompt you to enter a data point next.

Both output options can be selected if desired.

Vertical Scale ratio: select the horizontal to vertical scale ratio for plotting drift profiles in the design file.

25

Enter Data Point

You are encountering this prompt because you just instructed SnowMan to plot output results to your .dgn file. Find an uncluttered space in your .dgn file and enter a datapoint. The datapoint represents the upper left corner of the plot output. Enter a point to plot the output. Resulting outputs include the following: Graphical Output:

• Ground surface (original, new) • Snow profiles (unmitigated, mitigated)

Textual Output:

• Angle to road • Limit of storage offset (indicated by “X” on the graphical output) • Upwind and Downwind Limits of Protection offsets • Fetch distance • Snow transport (Qave, Qexceedance) • Snow depths at Upwind Limit of Protection (ULOP, typically upwind edge of

pavement) and Downwind Limit of Protection (DLOP, typically downwind edge of pavement)

• Storage (at specified limit of storage and at ULOP) • Whether the site (section) needs mitigation, based on either of the following

considerations: o Whether the average transport (Qave) is stored before the limit of storage,

and o Whether drift depth at either LOP exceeds the allowable drift depth

specified in the defaults file.

26

Example Design File Output: Case 1 Existing Ground

27

Output Results Prior to Mitigation (Case 1) Outputs include the following:

• Ground surface (original, new) • Snow profiles (unmitigated, mitigated), based on wind from the left • Limit of storage offset (indicated by “X” on the plot) • Snow transport (Qave, Qexceedance) per unit length perpendicular to the wind • Snow depths at Upwind Limit of Protection (ULOP, typically upwind edge of


• Storage (at specified limit of storage and at ULOP) • Whether the site (section) needs mitigation, based on either of the following

considerations: o Whether the average transport (Qave) is stored before the limit of storage,



28

Snow Fence Solutions SNOWMAN determines fences and setbacks (Case 2)

This screen appears if you selected “SnowMan determines fences and setbacks” (Case 2) in the “Case Selection” dialog box. In order to discourage the counterproductive placement of snow fences in deposition zones, the “Allowable buried fence ratio” is limited to 0.10 for novice users. Expert users may wish to allow more than 10% of the fence to be buried, e.g., if right-of-way is severely constrained. SNOWMAN uses an iterative guess-and-check approach to design both a shortest-fence solution and a closest-fence (minimum setback) solution. What makes for an acceptable solution are the following: i) the average transport (Qave) is stored before the limit of storage, and ii) the exceedance transport (Qexceedance) is stored before the ULOP or the drift depth at either LOP is acceptable (does not exceed the allowable drift depth).


Portable Fences Only: SNOWMAN finds solutions for fences that are 2.4, or 2.0 meters in height.

Temporary Fences Only: SNOWMAN finds solutions for fences that are 1.4 meters in height.

Either Permanent or Portable: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, 2.1, or 2.0 meters in height.

SNOWMAN will determine and display two snow fence solutions for this case:

• Shortest fence, and • Closest (to ULOP, i.e., roadway) fence.

29

Note: the snowdrift profile that SnowMan plots is the unlimited (a.k.a. equilibrium) drift, which is not likely to occur in New York state; if during climate data input you use the NYS default K = 1.5 then the exceedance probability is only 5%, i.e., snow transport will exceed Qexc only 5 years in 100 (Tabler 2003, Table 4.6).

30

User specifies fence, SNOWMAN determines setback (Case 3)

This screen appears if you selected “User Specifies Fence and SnowMan Determines Setback” in the “Case Selection” dialog box. In order to discourage the counterproductive placement of snow fences in deposition zones, the “Allowable buried fence ratio” is limited to 0.10 for novice users. Expert users may wish to allow more than 10% of the fence to be buried, e.g., if right-of-way is severely constrained. Select porosity and height of the fence from the set of permanent or temporary fence options. You may want to use this case, e.g., if only certain stock fence sizes are available. Some relevant considerations include the following:

• Permanent fences are structural fences that require more substantive post anchoring than the alternatives. If you need a tall fence, you need a permanent fence. But the right-of-way and/or land use restrictions (e.g., during the growing season) may restrict the use of permanent fences.

• Temporary fences are readily available (with 50% porosity only) and easily removed (e.g., for the growing season) but are generally not as effective (e.g., in preventing icing of the roadway, although this deficiency is subjective and not reflected in the algorithms used in SnowMan). They also tend to get buried.

• Portable fences provide an intermediate set of snow fence options between the other two fence options.

30

User specifies fence, SNOWMAN determines setback (Case 3)

This screen appears if you selected “User Specifies Fence and SnowMan Determines Setback” in the “Case Selection” dialog box. In order to discourage the counterproductive placement of snow fences in deposition zones, the “Allowable buried fence ratio” is limited to 0.10 for novice users. Expert users may wish to allow more than 10% of the fence to be buried, e.g., if right-of-way is severely constrained. Select porosity and height of the fence from the set of permanent or temporary fence options. You may want to use this case, e.g., if only certain stock fence sizes are available. Some relevant considerations include the following:

• Permanent fences are structural fences that require more substantive post anchoring than the alternatives. If you need a tall fence, you need a permanent fence. But the right-of-way and/or land use restrictions (e.g., during the growing season) may restrict the use of permanent fences.

• Temporary fences are readily available (with 50% porosity only) and easily removed (e.g., for the growing season) but are generally not as effective (e.g., in preventing icing of the roadway, although this deficiency is subjective and not reflected in the algorithms used in SnowMan). They also tend to get buried.

• Portable fences provide an intermediate set of snow fence options between the other two fence options.

31

User specifies max Setback, SNOWMAN determines fence and setback (Case 4)

This screen appears if you selected “User Specifies Max Setback and SnowMan Determines Fences and Setbacks” (Case 4) in the “Case Selection” dialog box. This screen is similar to the one used for “SnowMan Determines Fences and Setbacks” (Case 2) except that it also prompts for Max Setback. Setback is the distance between the snow fence and the Upwind Limit of Protection, as measured parallel to the direction of the prevailing wind. Note: setback must be a negative number due to the sign convention of upwind negative. In order to discourage the counterproductive placement of snow fences in deposition zones, the “Allowable buried fence ratio” is limited to 0.10 for novice users. Expert users may wish to allow more than 10% of the fence to be buried, e.g., if right-of-way is severely constrained.


Portable Fences Only: SNOWMAN finds solutions for fences that are 2.4, or 2.0 meters in height. Temporary Fences Only: SNOWMAN finds solutions for fences that are 1.4 meters in

height. Either Permanent or Portable: SNOWMAN finds solutions for fences that are 3.6,

3.0, 2.7, 2.4, 2.1, or 2.0 meters in height.

32

Enter a maximum setback for the fence. SNOWMAN may place the fence closer than the maximum specified setback, if possible. The range is from -30.0 meters to -100.0 meters or the maximum upwind point - 45.0 meters.

Note that for all cases with user-defined setback, the setback must be ≥ 15H, where H = fence height, or SnowMan will not run.

33

User specifies fence and max setback, SNOWMAN determines setback (Case 5)

This screen appears if you selected “User Specifies Fence and Max Setback and SnowMan Determines Setback” in the “Case Selection” dialog box. Select porosity and height of the fence from the set of permanent or temporary fence options. Setback is the distance between the snow fence and the Upwind Limit of Protection, as measured parallel to the direction of the prevailing wind. Note: setback must be a negative number due to the sign convention of upwind negative.

Enter a maximum setback for the fence. SNOWMAN may place the fence closer than the maximum specified setback, if possible. The range is from -30.0 meters to -100.0 meters or the maximum upwind point - 45.0 meters.


34

In order to discourage the counterproductive placement of snow fences in deposition zones, the “Allowable buried fence ratio” is limited to 0.10 for novice users. Expert users may wish to allow more than 10% of the fence to be buried, e.g., if right-of-way is severely constrained.

35

User specifies fence and setback (Case 6)

This screen appears if you selected “User Specifies Fence and Setback” (Case 6) in the “Case Selection” dialog box. Use to check a go/no-go trial fence. You would normally be an experienced user to select this case.

Height: In meters the total height of the snow fence [1.0, 10.0]

Select porosity from the list. Setback is the distance between the snow fence and the Upwind Limit of Protection, as measured parallel to the direction of the prevailing wind. Note: setback must be a negative number due to the sign convention of upwind negative.

Enter a maximum setback for the fence. SNOWMAN may place the fence closer than the maximum specified setback (if possible). The range is from -30.0 meters to -100.0 meters or the maximum upwind terrain data point - 45.0 meters.


36

Example Output Results for a Snow Fence Solution (shown for Case 2)

37

Outputs include the following:

• Ground surface (original, new) • Snow profiles (unmitigated, mitigated), based on wind from the left • Limit of storage offset (indicated by “X” on the plot) • Snow transport (Qave, Qexceedance) per unit length perpendicular to the wind • Snow depths at Upwind Limit of Protection (ULOP, typically upwind edge of


• Storage (at specified limit of storage and at ULOP) • Whether the site (section) needs further mitigation, based on either of the

following considerations: o Whether the average transport (Qave) is stored before the limit of storage,



38

Earthwork Design Options SNOWMAN determines new ground (Case 7)

This screen appears if you selected “SnowMan Determines New Ground” (Case 7) in the “Case Selection” dialog box. This is an earthwork solution whereby upwind earth is removed in order to form a topographic catchment to “trap” blowing snow upwind of the roadway. Enter the back slope and bottom slope (left and right slopes shown below from Tabler).

1 3

Wtop = 29 + (Sin α) 5.8 Hc

≥ 1.2

14

Guidelines for Drift-Free Cuts (dimensions in meters)

WIND WIND

Either Direction

≥ 15

Hc

39

User Specifies Max Setback and SNOWMAN determines new ground (Case 8)

This screen appears if you selected “User Specifies Max Setback and SNOWMAN Determines New Ground” (Case 8) in the “Case Selection” dialog box. This is an earthwork solution whereby upwind earth is removed in order to form a topographic catchment to “trap” blowing snow upwind of the roadway. Enter the back slope and bottom slope (left and right slopes shown below from Tabler). Maximum Setback is the distance between the top of upwind cut and the Upwind Limit of Protection, as measured parallel to the direction of the prevailing wind. Note: setback must be a negative number due to the sign convention of upwind negative.

1 3

Wtop = 29 + (Sin α) 5.8 Hc

≥ 1.2

14

Guidelines for Drift-Free Cuts (dimensions in meters)

WIND WIND

Either Direction

≥ 15

Hc

40

SNOWMAN Glossary (some adapted from Tabler 2003) Ambient snow cover: Average depth of snow on the ground, prior to blowing and

drifting, during the snow accumulation season [>0.0] Angle of attack: the angle between the prevailing snow transport direction (= prevailing

wind direction) and the alignment of the roadway or snow fence. Denoted as “angle to road” in screen output.

Average annual snow transport (Qt,ave): the amount of snow transport that has a 50%

probability of being exceeded in any given year. Average yearly snowfall: the amount of snow in mm that the site receives annually

during the snow accumulation season. Bottom gap: the space between the ground and the bottom edge of a snow fence. Capacity (or Storage): the most snow that a fence or other type of barrier can hold

(store), measured in metric tons per meter of fence length, or tons per foot. Design Modulus (K): the ratio Qdes/Qt,ave of design transport to the average annual

snow transport. Same as Exceedance Factor. Design Transport (Qdes): the quantity of snow transport used for designing snow

mitigation measures, expressed as a weight per unit of width (kg/m). Typically equal to Qt,ave (K = 1.0).

Downwind limit of protection (DLOP): the downwind limit of the area to be protected

by the passive snow control measure, typically the edge of pavement downwind of the centerline of the roadway

Equilibrium drift: the snowdrift (profile) formed by a snow fence, terrain feature, or

other barrier when filled to capacity for the existing wind condition. The drift profile plotted by SnowMan is the equilibrium drift.

Exceedance Factor (K): the ratio Qdes/Qt,ave of design transport to the average annual

snow transport. Same as Design Modulus. Fence height (a.k.a. effective fence height): vertical height of snow fence above the

surrounding snow surface, including the bottom gap. Fetch: the length of the area, in the prevailing wind direction, that is a source of blowing

snow to a downwind location. The upwind end of the fetch is any boundary across which there is no snow transport, such as forest margins, deep gullies or stream

41

channels, rows of trees, ice pressure ridges, and shorelines of unfrozen bodies of water.

Limit of Protection: a feature (typically edge of pavement) delimiting the region to be

protected by passive snow control measures such as snow fences. Limit of Storage: the offset location (in meters) where computation of snow storage for

design transport is computed, measured from the most upwind point (limit of fetch). In SnowMan, this is compared against the offset in meters measured along the direction of the wind of the 7 meter perpendicular to the road setback required for storage of plow cast.

Living snow fence: trees, shrubs, or crops (e.g., rows of standing corn) that are used to

control blowing snow. Porosity, or Porosity ratio: ratio of area of openings to total frontal area of a snow fence,

excluding the bottom gap. Prevailing transport (wind) direction: the mean wind direction that corresponds to the

mean (average) annual snow transport Relocation coefficient (Cr): the proportion of (water equivalent) winter snowfall that is

relocated (drifted) by the wind. Section: A 2D vertical plane parallel to the prevailing wind directioin. Same as transect. Setback: the distance between the snow fence and the upwind road shoulder, as

measured in the direction of the prevailing wind. Snow accumulation season: the season of drift growth, beginning with the first blowing

snow event that causes drifts that persist through the winter, and ending when snowdrifts reach their maximum volume for the winter.

Snow fence: structural barrier that protects an area from wind-transported snow Snow transport: the mass of blowing snow that is transported by the wind over some

specified period of time, per unit of width across the wind. Snow transport normally refers to the total within the first 5 m (16 ft) above the surface, per meter of width across the wind.

Transect: Same as section. Upwind Limit of Protection (ULOP): the upwind limit of the area to be protected by

the passive snow control measure, typically the edge of pavement upwind of the centerline of the roadway

42

Wind Direction: Same as prevailing transport (wind) direction. Common Acronyms Cr: Relocation Coefficient DLOP: Downwind limit of protection (typically, downwind edge of pavement). Qdes: Design transport. Q(exceedance): K * Qdes Qt,ave: average annual snow transport ULOP: upwind limit of protection (typically, upwind edge of pavement).

43

References Chen, S. S., Lamanna, M. F., Tabler, R. D., and Kaminski, D. F., “Computer-Aided Design of Passive Snow Control Measures,” manuscript prepared for submittal to TRR/TRB (Transportation Research Record, Transportation Research Board), July 2008. Kaminski, D. F., and S. Mohan, ”PASCON: An Expert System for Passive Snow Control on Highways,” Transportation Research Record #1304, 1991, pp. 193-201. Lamanna, M. F., and Chen, S. S., “Control of Blowing Snow Using SnowMan: Developer’s Manual,” Department of Civil, Structural, and Environmental Engineering, University at Buffalo, State University of New York, revised July 2008. Minnesota Department of Transportation, “Living Snow Fences: Control of Blowing and Drifting Snow,” http://www.dot.state.mn.us/environment/livingsnowfence/ (accessed Dec. 2007)

Tabler, R. D., Computer-Aided Design of Drift Control Measures, Technical Report for Research Project # FHWA-WY-97/02, Federal Highway Administration and Wyoming Department of Transportation, 1997. Tabler, R. D., “Controlling Blowing and Drifting Snow with Snow Fences and Road Design,” Final Report prepared for the National Cooperative Highway Research Program, Transportation Research Board of the National Academies, NCHRP Project 20-7(147) August 2003. Wyoming Department of Transportation, “Controlling Blowing and Drifting Snow – Snow Drift Profiler,” http://www.heepweb.org/ProgramsReports/Reports/HEEPAgencyReports/tabid/96/EntryID/46/Default.aspx (accessed Dec. 2007).

A-1

Appendix A

Default Values Employed by SnowMan

A-2

Default values employed by SnowMan are contained in the ascii text files nym_snowman_defaults.data (metric units) and nyu_snowman_defaults.data (U.S. units), which must be in the path to the directory referenced by the MicroStation Environment Variable “SNOWMAN_HOME”. Typically this setup is arranged by your system manager. The default values are as follows:

• Section Spacing: 20.0 m • Cr: 0.17 • Maximum Fence Offset: 1000.0 m • Minimum Fence Offset: 20.0 m • Allowable Buried Fence Depth Ratio: .2 • Maximum Natural Accumulation Offset: 130.0 m • LOCACCM Offset: 7.0 m (perpendicular to road) • Allowable Snow on Road: 0.0762 m • Minimum Upwind Terrain: 100.0 m • Minimum Downwind Terrain: 45.0 m • Minimum Fetch: 60.0 m • Maximum Fetch: 10000.0 m • Ditch Bottom Slope: 0.02 (for earthwork solutions, cases 7 & 8) • Ditch Back Slope: 0.5 (for earthwork solutions, cases 7 & 8)

Appendix B

Abbreviated Guide to Essential SnowMan Inputs


B-2

USER INPUT INFORMATION User Name:

Name of user running SnowMan. User Level:

• Novice - User with basic knowledge of blowing snow control. • Expert - User with advanced understanding of blowing snow control.

Project ID Number:

Project Identification Number (PIN) of the project that is being evaluated for blowing and/or drifting snow problems and possible mitigation.

Site ID:

Description of the site being investigated under the PIN. Climate Data Input Method:

• Site-specific data provided by user (RECOMMENDED): User inputs the latitude, longitude, and elevation of the site; the total annual snow fall for the site and wind direction; and the relocation coefficient to use.

• Computed from latitude, longitude and elevation: User inputs the latitude, longitude, and elevation of the site and SNOWMAN computes the total snowfall, and wind direction as well as the relocation coefficient.

• SNOWMAN Climate Data File: Load a data file containing the climate data to use in SNOWMAN Climate Data format.

Topographic Data Input Method:

• From current 3-D design file (.dgn)NYSDOT level standards (RECOMMENDED): SNOWMAN will use the current design file and search for elements relevant to generating cross sections for analysis (LLOPSU_P, LLOPSD_P, DD (optional), RDEFAULT_P, and LSFFB).

• Manual Input of 2-D cross sections: Snowman will prompt user for relevant information. That is the offset and elevation pairs for each data point in a cross section as well as the angle to the road, the distance to the downwind limit of protection, lowest ditch point if any, and the distance to the nearest barrier to fetch. All of this is along the direction of the wind.

• SNOWMAN Topographic Data File: Load a data file containing the topographic data in SNOWMAN Topographic Data format.

CLIMATE DATA INPUT METHODS: Site Specific Climate Data Input:

User inputs climate date specific to the site under investigation. This is the recommended method of providing climate date. • North Latitude: Degrees North Latitude of the site. (40.267, 44.983) • West Longitude: Degrees West Longitude of the site. (71.750, 80.183) • Elevation: Height above sea level of the site. [0.0, 1000.0) • Total Annual Snowfall: mm of snow the site receives annually. (>0.0)


B-3

• Relocation Coefficient (Expert User only): Ratio of Snow that can be relocated. (0.0, 1.0)

• Direction: Degrees true north azimuth of the direction of the prevailing wind at the site.(0.0, 360.0)

Latitude, Longitude and Elevation Input:

Climate date is interpolated from a database based on location and elevation. • North Latitude: Degrees North Latitude of the site. (40.267, 44.983) • West Longitude: Degrees West Longitude of the site. (71.750, 80.183) • Elevation: Height above see level of the site. (0.0, 1000.0)

Open SNOWMAN Climate Data File:

Uses a previously saved climate data file. Select or enter the name of the file containing valid snowman climate data in SNOWMAN climate data format.

Climate Data:

No inputs required but user may edit climate data values. The user may also check the box to save the climate data to a SNOWMAN Climate Data formatted file.

TOPOGRAPHIC DATA INPUT METHODS: Enter Data Point:

For 3-D design file analysis. This is the Prompts the user to enter a data points along the upwind limit of protection (LLOPSU_P) for the beginning and end of the site to be investigated.

Data Point Entry:

For input of 2-D cross sections for analysis. • Offset: In meters, distance from the ULOP to the data point being entered. • Elevation: In meters, the elevation of the data point above sea level. • Next Data Point: records the current data point and gets next data point. • Next Cross Section: records the current data point and prompts user for

section information. After which the user will be prompted for more data points in the next cross section.

• Done: records the current data point and prompts user for section info. After which the topo data for each section will be displayed to the user.

Cross Section Information:

• Ditch upwind of the limit of protection: Check box if section has a ditch (must be on level DD).

• Ditch low point offset: distance in meters along the direction of the wind from upwind limit of protection to lowest ditch point. Use point closest to limit of protection if more than one ditch point with the low point elevation.

• Ditch low point elevation: elevation of the lowest ditch point. • Downwind Limit of Protection: Offset in meters to the downwind limit of

protection. • Fetch: Distance in meters, measured parallel to the direction of the

prevailing wind, from the upwind limit of protection to the first encountered barrier to fetch.

• Wind to Road Angle: Angle between the prevailing wind direction and the road [10.0° min., 90.0° max.].


B-4

Topographic Data:

No inputs required, but user may change values. The user can also check the box to save the topo data to a SNOWMAN topographic data file.

SNOW FENCE SOLUTION EVALUATION TYPE SELECTION: Evaluate Existing Terrain:

This evaluation runs automatically during the first iteration of SnowMan for the project site.

Snowman Determines Fences and Setbacks:

• Permanent Fences Only: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, or 2.1 meters in height.

• Temporary Fences Only: SNOWMAN finds solutions for fences that are 2.4, or 2.0 meters in height.

• Either Permanent of Temporary: SNOWMAN finds solutions for fences that are 3.6, 3.0, 2.7, 2.4, 2.1, 2.0, or 1.4 meters in height.

User Specifies Fence, SNOWMAN Determines Setback:

Select height of the fence from the permanent or temporary column and porosity. Typically used if a specific height fence is on hand.

User Specifies Max Setback, SNOWMAN Determines Fences and Setbacks:

Typically used if right-of-way is a constraint. • Permanent Fences Only: SNOWMAN finds solutions for fences that are 3.6, 3.0,

2.7, 2.4, or 2.1 meters in height. • Temporary Fences Only: SNOWMAN finds solutions for fences that are 2.4, or

2.0 meters in height. • Either Permanent of Temporary: SNOWMAN finds solutions for fences that are

3.6, 3.0, 2.7, 2.4, 2.1, 2.0, or 1.4 meters in height. • Enter a maximum setback for the fence. SNOWMAN may place the fence closer

than the maximum specified setback if that is a viable solution. The range is from -30.0 meters to -100.0 meter or the maximum upwind point minus 45.0 meters.

User Specifies Fence and Max Setback, SNOWMAN Determines Setback:

Typically used if a specific height fence is on hand and right-of-way is a constraint. • Select height of the fence from the permanent or temporary column and

porosity. • Enter a maximum setback for the fence: SNOWMAN may place the fence closer

than the maximum specified setback if that is a viable solution. The range is from -30.0 meters to -100.0 meters or the maximum upwind point minus 45.0 meters.

User Specifies Fence and Setback:

Typically used for analyzing a specific snow fence design. • Height: In meters the total height of the snow fence [1.0, 10.0]. • Select porosity from the list. • Enter a maximum setback for the fence: SNOWMAN may place the fence closer

than the maximum specified setback if that is a viable solution. The range is from -30.0 meters to -100.0 meter or the maximum upwind point minus 45.0 meters.


B-5

OUTPUT: Output Options: • Output to file: Check if you want output sent to a text file. • Output to design file: Check if you want output sent to the design file. • Vertical Scale: Select vertical scale ratio, 1H:1V or 1H:5V.

D-1

Appendix C

Computer-Aided Design of Passive Snow Control Measures

By S. S. Chen, Ph.D., P.E., M. F. Lamanna, M. S.,

R. D. Tabler, Ph.D., P.E., and D. F. Kaminski, M.Eng., P.E.

Computer-Aided Design of Passive Snow Control Measures

Submission Date: 31 July 2008

Words: 10,200 Authors: Stuart S. Chen, Ph.D., P.E. Michael F. Lamanna, M.S. Dept. of Civil Engrg., Univ. at Buffalo Dept. of Civil Engrg., Univ. at Buffalo 226 Ketter Hall 212 Ketter Hall Buffalo, New York 14260 Buffalo, New York 14260 Tel: (716) 645-2114 ext. 2428 Tel: (716) 645-2114 Fax: (716) 645-3733 Fax: (716) 645-3733 Email: [email protected] Email: [email protected] Ronald D. Tabler, Ph.D., P.E. Darrell F. Kaminski, M. Eng., P.E. Tabler and Associates New York State DOT P. O. Box 483 100 Seneca St. Niwot, CO 80544 Buffalo, NY 14203 Tel: (303) 652-3921 Tel: (716) 847-3214 Email: [email protected] Email: [email protected] ABSTRACT Control of blowing and drifting snow on the nation's highways is important to reduce maintenance costs and closure times and to improve crash incidence by improving visibility, preventing drifting on the road, and reducing road icing. Means of engineered mitigation using road design and snow fences have been incorporated into a software tool, SnowMan (for Snow Management) that has been deployed for use statewide within the New York State Department of Transportation. The software has been developed as a MDL application to run within the Bentley MicroStation CAD software environment used in highway design projects. The scope of SnowMan includes drift prediction (mitigated and unmitigated), evaluation of roadway cross sections and determination of trial fence solutions subject to combinations of height, setback, and porosity constraints, and prescription of upwind earthwork solutions. This paper describes the development and implementation of this software tool for mitigation of blowing and drifting snow problems and illustrates its usage, while also providing an overview of the relevant data, underlying algorithms, and engineering approaches to blowing and drifting snow mitigation as implemented in SnowMan.

Chen, Lamanna, Tabler, and Kaminski C-1

BACKGROUND AND INTRODUCTION Blowing and drifting snow, in addition to being hazardous to motorists through icing of pavements and reduced visibility, can add significantly to the cost of winter maintenance. It forms drifts on roads, increases snow removal costs, and leads to pavement damage. Properly engineered passive snow control measures, however, can significantly reduce these hazards and costs ([1] – [2]). The empirical snow drifting prediction approaches with and without snow fencing that are described in these references have been widely applied, being utilized in a variety of studies, e.g., simulating wind-related evolution of snow depth distributions for various global topographic and climate enviroments [3], comparing predictions to field measurements of blowing and drifting snow [4], and incorporating in spatial tools for highway planning and analysis (e.g., understanding accident history and investigating mitigation treatments) [5], the latter utilizing snow drifting predictions implemented in software available as an ArcView extension [6].

The technical complexities involved in design of appropriate preventive or mitigation measures require understanding of the processes of snow transport and deposition, the design of structural and living snow fences, and the design of drift-free cross sections of roadways. What is needed is the incorporation of some of this understanding in the tools available to highway design and maintenance personnel. There have been several earlier efforts involved in developing computer-based tools to assist the engineer with the computations involved in the design of passive snow control measures. These efforts, in addition to [6] mentioned above, include the PASCON system developed by Kaminski and Mohan [7], the WYDOT Drift Profiler [8, 9], and an interactive web site developed by the University of Minnesota and MNDOT [10]. PASCON and the MNDOT system, unfortunately, are not integrated into the CAD environment utilized by highway designers, and the WYDOT application [8] does not actually design snow fence systems. The purpose of this paper is to describe the development and application of a tool for computer-aided design of passive control measures that incorporates key aspects of state-of-the-practice guidelines (primarily, reference[2]) for snow fence design and roadway design into the MicroStation CAD environment used by highway design personnel in most states. This tool makes possible the incorporation of snow control design measures directly into the workflow of highway design, although it can also be used for two-dimensional maintenance applications on existing roadways. PROJECT OBJECTIVES AND DEVELOPMENT The SnowMan (Snow Management) tool described herein was developed to provide highway design and maintenance personnel with readily available software that can be incorporated into a familiar CAD software usage environment to design passive measures for control of blowing and drifting snow. There are two possible mitigation strategies: roadway (cross section) design and snow fencing. A suitably designed roadway cross section will facilitate deposition of blowing and drifting snow in ditches rather than on the roadway or be a drift-free design with no additional upwind storage, while snow fences capture blowing snow upwind of a problem area and store that snow over the winter season.


Knowledge is now well-established regarding snow transport and deposition, evaluating

roadway cross sections for drift susceptibility, design of passive and living snow fences, and earthwork modification for reducing drifting [2]. By incorporating key aspects of this knowledge into a software application, transportation design and maintenance personnel not traditionally trained in this area can conduct informed evaluations of actual or proposed sites and design mitigation measures as appropriate. It was decided to develop and deliver this software as a MDL (MicroStation Development Language) application for maximum flexibility. Initial SnowMan software development was conducted jointly between Brookhaven National Laboratory and the authors, although the version of the software currently in use was developed solely by the authors.

Specifically, the scope of SnowMan was defined to encompass the following use cases: • Predict existing snow transport, natural upwind storage and snow drifting (unmitigated) for

various roadway sections, • Evaluate proposed roadway cross sections, • Evaluate trial snow fence solutions, • Prescribe (design) snow fence placement using systematic iterative approaches subject to a

variety of possible constraints on fence porosity, height, and/or setback, subject to limiting drift and storage criteria, and

• Prescribe upwind earthwork solutions. ALGORITHMS FOR GENERATING SNOW FENCE DRIFT PROFILES The following algorithms are used to predict the profiles of snowdrifts, as measured parallel to the wind, formed by snow fences in irregular terrain. Input data consist of distances and elevations along a transect parallel to the prevailing wind direction, and the structural height, porosity, and location of one or more snow fences - although it should be noted that the current implementation of SnowMan does not evaluate multiple snow fences. An essential requirement is that the 45 m of the topographic section farthest upwind must not be in a snow deposition area; i.e., the snowdrift depth over the first 45 m of the transect must be known to be zero (Figure 1). The generated snowdrift profile is that resulting from an unlimited supply of blowing snow so that terrain features and snow fences have been filled to capacity; i.e., the snowdrifts are at equilibrium. Because of the downwind progression of drift growth, whereby an upwind sink for blowing snow must be filled before a drift begins to form in the next deposition area downwind, the equilibrium snowdrift profile is generated incrementally in a downwind direction. It is therefore possible to truncate the generated profile when the accumulated mass, i.e., deposition, of blowing snow equals the estimate for actual snow transport at the site being analyzed.


PREDICTION STARTS HERE

45 m

NO SNOWDEPOSITION

45 m

GROUND PROFILE DATA REQUIRED

DEPOSITIONAREA

WIND

REGION OFINTEREST

ROAD

Figure 1 Ground Profile required to generate the snowdrift profile in the region of interest [1]

The procedure described here is a modification of the method developed in 1976 by R. D. Tabler [11] for the Wyoming Snowdrift Prediction System, as described by R. H. Christensen [12]. The changes from the original version are noted in the narrative below. The step-wise procedure is as follows:

1. Generate the snowdrift profile caused by terrain features, without snow fence(s), at least up to and including the anticipated location of the (first) snow fence from the upwind end of the transect.

2. For a given structural fence height (Hs) at a specified location, determine the effective

height of the fence (He) above the snow surface generated in Step 1 (calculated as Hs minus the snowdrift depth at the specified location).

3. Generate the drift on the upwind side of a snow fence of height He (this drift is

generated on top of the topographic drift from Step 1). 4. Generate the drift on the downwind side of the fence (this drift is generated over the

ground surface, not the topographic drift surface).

5. For a second row of fence (not currently implemented in SnowMan), calculate the effective fence height relative to the snowdrift surface generated by the first row of fence, and generate the upwind drift on top of the drift formed by the first fence. The downwind drift is generated over the ground surface. Additional fences can be analyzed by repeating the same procedure.

STEP 1: Topographic Snow Surface The first step is to generate the snowdrift profile that would result from terrain features without snow fences, here referred to as the “topographic snow surface.” As described in the publication Design Guidelines for the Control of Blowing and Drifting Snow [1], this procedure utilizes the


algorithm for estimating the slope of the snow surface over an increment of distance (typically 0.1 m) downwind from a point

m5 = 0.25m1 + 0.55m2 + 0.15m3 + 0.05 m4 (1)

if calculated m2, m3, or m4 < -0.20, set m2, m3, or m4 = -0.20

where m5 = slope of the snow surface over the next increment of distance,

m1 = the mean slope from 45 m upwind to the point where the next increment of the drift surface is being generated,

m2 = the mean slope from the point to a point 15 m downwind, m3 = the mean slope from the point 15 m downwind to a point 30 m downwind, m4 = the mean slope from the point 30 m downwind to a point 45 m downwind. Slopes upward in the direction of the wind are taken as positive, and downward slopes as negative. The mean slope is calculated as the difference in elevation from the initial point to the terminal point divided by the horizontal distance between the points. Slope m1, referred to as the approach slope because it is upwind of the point at which the surface slope is being computed, is computed from the last calculated point on the snow surface to the generated snowdrift surface 45 m upwind (Figure 2). Slopes m2, m3, and m4 are referred to as “exhaust slopes.” Slope m2 is computed from the last calculated point on the snow surface to the ground, and slopes m3 and m4 are determined from the ground (topographic) surface. The maximum negative exhaust slope is –0.20; i.e., if the computed value is less than this value, -0.20 is used in determining the slope of the snow surface. The slope of the snow surface, m5, multiplied by the horizontal incremental distance, is added to the snow surface elevation at the previous point to determine the elevation of the next point on the surface. This value is compared to the ground elevation at the point and if the computed surface would fall below the ground surface, it is set equal to the ground surface. Total mass of snow along a transect is calculated by accumulating the mass of snow contained in each increment of distance using the relationship

ρave = 522 – [304/(1.485Yave)][1-exp(-1.485Yave)] (2) where ρave is average snow density (kg/m3) over the increment, Yave is the average vertical depth over the increment, and exp(-1.485Yave) is the base of the natural logarithms (2.718…) raised to the (-1.485Yave) power [1]. The notation convention exp(X) ≡ eX will be used throughout the remainder of this narrative. By comparing the accumulated mass to the estimated snow transport at the site ([13], [1]) it is possible to estimate the probable end of the drift where the estimated snow surface could be truncated rather than generating the equilibrium profile for unlimited snow transport.


45 m 15 m 15 m 15 m

m = -8%

m = -38% -20%

m = -2%m = +3%

1

2

3

4

WINDP

m = -13.2%

DRIFT SURFACEGENERATED UP TO

m = 0.25m + 0.55m + 0.15m + 0.05m1 2 3 4(LIMIT m , m , m to -20% minimum)2 3 4

5

5

POINT P

Figure 2. Examples of distances and slopes used in Equation (1) to estimate the slope of the next

snow profile increment [1] STEP 2: Calculate Effective Fence Height The effective height (He) of a snow fence at a specified location is calculated by subtracting the snow depth (Y) determined in Step 1 from the structural fence height (Hs):

He = Hs – Y (3)

Where Y>Hs, the fence is buried by the topographic snowdrift and is ignored in subsequent calculations. In practice, locations for snow fences should be selected to avoid burial that can cause structural damage to the fence as well as reducing the snow storage capacity of the fence. In SnowMan, the default, which can be overridden by an expert user, is to allow no more than 10% fence burial in deposition zones, STEP 3: Generate Drift on Upwind Side of Fence The algorithm for estimating the upwind drift has been completely revised from the original version described by Christensen [12]. In the present algorithm, the approach slope for a snow fence, ma, is defined as the mean slope of the topographic snow surface between the fence and a point 45 m upwind (Figure 3). An uphill approach is the case where the wind is blowing uphill toward the fence (ma positive), and a downhill approach is where the terrain slopes in the opposite direction. The snow profile on the windward side of the fence having effective height He is approximated by a polynomial equation Yu/He = γAu + Bu(ds) + Cu(ds)2 + Du(ds)3 + Eu(ds)4 + Fu(ds)5; ds < Limit (4)


where Yu is the drift depth above the topographic snow surface measured normal to the approach slope; ds is (distance from the snow fence measured along the approach slope plane)/He; and the coefficients Au, Bu, Cu, Du, Eu and Fu, and the limit of applicability, vary with fence porosity as given in Table 1. γ is a slope correction factor that is unity for flat terrain or a downhill approach, but varies with an uphill approach (i.e., wind blowing uphill toward the fence) according to

γ = exp(-6mau); mau > 0 (5) The coefficient for mau (6) was determined empirically from measured drift profiles. The original algorithm described by Christensen [12] assumed a linear approximation to the profile starting at a point 10H upwind of the fence, and extending to the midpoint or top of the fence, depending on the approach slope. If Yu/He, as calculated from Equation (4), is negative, it should be set equal to zero.

m i

m al

LEE DRIFTPROFILE

SNOW FENCE

LOCI OF m i

30 m

45 m

WIND

m e

α a

TOPOGRAPHIC SNOW SURFACE

m m at ith POINT

d

d i

ith POINT

α e

GROUND SURFACE

Tabler & Associates

UPWIND DRIFT

m au

Figure 3. Illustration of slopes used to generate the snow fence drift profiles (revised)

Table 1. Coefficients and limits of applicability for upwind drift polynomials (Equation 4) for fences with indicated porosities.

% Porosity Au Bu Cu Du Eu Fu Limit for ds

0 9.13E-01 -3.610E-01 1.0050E-01 -1.8790E-02 1.7830E-03 -6.4000E-05 <10

25 6.30E-01 -1.450E-01 1.9240E-02 -1.2975E-03 7.5800E-06 1.8028E-06 <12

37.5 5.75E-01 -7.600E-02 4.4025E-04 6.8276E-04 -5.9656E-05 1.5934E-06 <15

50 5.20E-01 -5.540E-03 -2.1701E-02 3.5524E-03 -2.2153E-04 4.8560E-06 <16

The vertical snow depth above the topographic snow surface is given by

Y = Yu / cos αa (6)


where αa is the approach slope angle (αa = tan-1 mau). The elevation of the snow surface at a given point is computed as the vertical snow depth plus the elevation of the topographic snow surface at the point. The effect of snow fence porosity on drift geometry is illustrated for the case of flat terrain in Figure 4. The original version of this algorithm ([12]) was restricted to a fence porosity of 50%.

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

0

1

2

3

4

5

DRIFT PROFILES FOR 0-, 25-, 37.5-, AND 50% POROSITIES

0%25%

37.5%50%

WIND

(DISTANCE FROM FENCE) / HEIGHT

DE

PTH

/ H

EIG

HT

Tabler & Associates09/19/00 09:38

Figure 4. Effect of snow fence porosity on shape of upwind and lee drifts on flat terrain, as given by Equations (4) and (10) and the values presented in Tables 1 and 3 [1]

STEP 4: Generate Drift on Downwind Side of Fence The leeward drift is generated over an “effective slope” that combines the effect of the approach slope upwind of the fence and the exhaust slope from the last point on the generated drift surface to the ground 30 m downwind. As with the exhaust slopes in Equation (1), me is limited to values ≥ -0.20. In addition, however, me is also limited to values less than, or equal to, zero. For each increment of distance downwind from the fence, di, an effective slope, mi, is computed as

mi = me(1-t) + mal t (7)

where me is the mean slope from the last point on the generated snow surface to a point on the ground 30 m downwind, mal is the approach slope 45 m upwind from the fence determined from points on the upwind drift profile generated in Steps 1 and 3, and t is an exponential decay function

t = exp[-d/(εHe)] (8) where d is horizontal distance downwind from the fence (Figure 3), and ε is a coefficient that varies with fence porosity as shown in Table 2. As distance d increases, the effect of the approach slope diminishes in a fashion analogous to a fading memory, and the influence of the exhaust slope increases. Values for ε were determined empirically as that providing the best fit to measured snowdrift profiles.


Table 2. Values for ε in Equations (8) and (11) for different fence porosities.

Porosity (%) ε

0 3.5 25 17

37.5 20 50 20

The average of the incremental effective slopes from the fence to the point being estimated, mm, defines the plane of the effective slope upon which the next increment of the lee drift profile is constructed, and is calculated as

∑=

=j

iim m

jm

1

1 (9)

where j is the number of increments over distance d. The depth of the lee drift at the end of the increment of surface being generated is estimated from a polynomial equation similar to Equation (4):

YL/He = δAL + BL(ds) + CL(ds)2 + DL(ds)3 + EL(ds)4 + FL(ds)5; ds < Limit (10) where YL is the drift depth above a plane defined by the mean effective slope, mm, measured normal to the slope; ds is (distance from the snow fence measured along the plane of the mean effective slope)/He; and the coefficients AL, BL, CL, DL, EL and FL, and the limit of applicability, vary with fence porosity as given in Table 3. δ is a slope correction factor similar to γ that is unity for flat terrain or a downhill approach, but varies with an uphill approach (i.e., wind blowing uphill toward the fence) according to

δ = 1 – t + exp[-6mau – d/(εHe)]; mau > 0 (11) The original algorithm described by Christensen [12] did not include an adjustment for uphill approach, and was limited to a fence porosity of 50%. Distance ds is given by

ds = d/(Hecos αe) = d/[Hecos(arctan mm)] (12) If YL/He, as calculated from Equation (10), is negative, it should be set equal to zero. Snow depth, YL, is converted to a vertical dimension and adjusted for the height of the effective slope from the relationship

Sp = Zf + YL/cos αe +diΣmi (13)


where Sp = the elevation of the snow surface at the point, Zf = the ground elevation at the fence, αe = the effective slope angle Σmi = the summation of incremental effective slopes downwind of the fence to the point. The resultant value is compared to the ground elevation at the point and if the computed surface would fall below the ground surface, it is set equal to the ground surface. When the limit of applicability for Equation (10) is reached, e.g., when ds = 34 for the case of a 50%-porous snow fence, the snow surface over the remainder of the transect is generated using Equation (1). If a second row of snow fence is to be analyzed, the snowdrift profile is terminated at the location of the second fence, provided that the second fence is not buried by the upwind fence’s profile (see STEP 2). STEP 5: Generate Drift for Multiple Rows of Fence, If Applicable. Although not yet implemented in SnowMan as of this writing (July 2008), to predict the snowdrift formed by a second snow fence downwind from the first, the drift on the downwind side of the first fence is terminated at the location of the second fence. The snow depth at this point is used to calculate the effective height of the second fence, and the upwind drift formed by the second fence is generated on top of the drift resulting from Step 4. The lee drift formed by the second row of fence is generated over the ground surface. If a third row of fence is involved, the lee drift behind the second fence must be terminated at the third fence, and so on.

Table 3. Coefficients and limits of applicability for lee drift polynomials (Equation 10) for fences with indicated porosities.

% Porosity AL BL CL DL EL FL Limit for ds

0 1.00E+00 -8.100E-02 -3.2520E-02 5.8280E-03 -3.2840E-04 5.7400E-06 <13.2

25 5.80E-01 2.218E-01 -2.9048E-02 1.0150E-03 -1.4489E-06 -3.4199E-07 <24

37.5 5.02E-01 2.689E-01 -3.7588E-02 1.9275E-03 -4.4983E-05 3.9880E-07 <31.6

50 4.30E-01 3.016E-01 -4.1203E-02 2.1930E-03 -5.4209E-05 5.1050E-07 <34

SNOWMAN IMPLEMENTATION Initial Input Data Requirements Certain climatological and topographic data are required in order to evaluate blowing and drifting snow situations. Climatological data includes snowfall over the snow accumulation


season (and snowfall water-equivalent), the snowfall relocation coefficient (the proportion of winter snowfall water-equivalent that is relocated by the wind), and the prevailing wind direction. The climatological data, most ideally, should be site-specific. Where site-specific data is not available, SnowMan implements an interpolation algorithm developed for New York State based on input of site latitude, longitude, and elevation. Either way, climatological data is stored in a climate data file. Topographic data in turn can be provided from a NYSDOT level-standard-compliant 3D DGN file of the site or by 2D section (offset and elevation) data for sections taken in the prevailing wind direction. Topographic data, like climatological data, is stored in a text-format data file for later use. Figure 5 shows the SnowMan user input dialog box coded in MDL to discern the user’s intent and available data sources. “Novice” users cannot override defaults (e.g., relocation coefficient). Topographic data is stored in 2D sections and can be input manually as 2D sections (e.g., from users at a maintenance residency with rod and level at a problem site), although input from file (2D or 3D) will typically be preferred in order to avoid tedious error-prone data entry.

Figure 6 illustrates an example of using a 3D surface file for purposes of topographic data input. In this case, the user must specify start and end stations and distance between sections. Fetch distance (i.e., the length of upwind terrain that is a source of blowing snow to the location of interest) must also be present according to level standards in the DGN file or explicitly input. From a 3D surface file, SnowMan obtains 2D sections (transects) at a regular spacing between the user-defined start and end station limits that are input graphically.

Fig. 5 Climate and Topographic Data Input in SnowMan


Fig. 6 3D Design File Being Used for Topographic Input

Mitigation Options (Strategies)

Figure 7 shows the various cases that the user can direct SnowMan to analyze. The first case, “Evaluate existing terrain,” is automatically executed by SnowMan the first time it is run. An iterative design approach described below can be taken, with the “User Specifies…” cases. Otherwise, the user can direct SnowMan to determine a solution (in the “SnowMan Determines” cases). Note that the last two choices are for cross-section redesign and not snow fence solutions.

Fig. 7 SnowMan Analysis Cases


The algorithms presented above are augmented with design expertise and implemented in SnowMan to enable the user to explore various mitigation options. A glimpse of the resulting pseudocode is provided below in Figs. 8 and 9. Fig. 8 describes the approach used to compute snow storage and depths and check for blowing or drifting problems. A drifting snow problem is defined to exist when the exceedance transport is not stored upwind of the upwind limit of protection (ULOP) and the drift depth at either the upwind limit of protection or the downwind limit of protection (DLOP) is greater than the allowable snow depth (75 mm default) specified for these locations. A blowing snow problem is defined to exist when the average transport is not stored upwind of the upwind limit of storage (LOCACCM) located at a setback needed for plow cast. foreach data point in the depth profile if the current data point is in the storage range (i.e. upwind of the ULOP and downwind of the fetch barrier) then if the current data point is in the storage range of just storageAtULOP compute the average depth for the current point and the previous point compute the average density over this increment compute the density of the snow stored upto the current point if the density is > 0.0 update storage for storageAtULOP check if this is where the average and/or exceedence transports are stored and update accordingly else the current data point is in the storage range of both stroageAtULOP and StorageAtLOCACCM then compute the average depth for the current point and the previous point compute the average density over this increment compute the density of the snow stored upto the current point if the density is > 0.0 update storage for storageAtULOP and storageAtLOCACCM check if this is where the average and/or exceedence transports are stored and update accordingly else we are past the point where storage matters (i.e. downwind of the ULOP) break out of loop determine the depths at the limits of protection and if the site has problems get unlimited drift depths at ULOP and DLOP if storage at the upwind limit of protection (storageAtULOP) is less than the exceedence transport then drift at the ULOP and DLOP will be the same as the unlimited drift depths else there is no drifting to compute so set predicted drift at ULOP and DLOP equal to 0.0 if storage at the upwind limit of storage (storageAtLOCACCM) is > the average transport (Q50) then indicate this section does not have a blowing snow problem else the average transport is not stored upwind of the limit of storage so indicate the section has a blowing snow problem. if storage at ULOP (storageAtULOP) is greater than the exceedence transport (Q20) indicate the section does not have a drifting problem else the site may have a drifting problem if the actual drift depths at either the ULOP or DLOP exceed the allowed drift at that location then indicate the section has a drifting snow problem else the actual drift depths are less than the allowable so indicate the section does not have a drifting problem if the site has either a drifting or blowing snow problem then indicate the section needs further mitigation else there are no problems detected indicate the section does not need further mitigation

Figure 8 Pseudocode for Computing Storage & Depth & Checking for Problem(s)


Figure 9 describes the procedure employed by SnowMan to determine fences and setbacks. For this case, SnowMan generates drift depths for two snow profiles (closest fence and shortest fence). Providing these two solutions allows the user to select between a more economical (shorter) fence and a fence that minimizes setback (closest). for each section if section needs mitigation determine limit for fence offset (min of terrain or max allowable fence offset) determine number of fences possible for fence types allowed

start with the shortest fence then select each additionally taller fence based on fence types allowed.

foreach fence that is allowed determine height, actual height, effective height, if buried set porosity to 50% and set fence location check if starting location is valid if not then fence's setback is increased until it runs out of room or a suitable location is found if we have a suitable location then get the profile for that fence check for problems (blowing and/or drifting) while an acceptable solution is not found and there is room to move the fence back increase the setback and check again find shortest and closest solutions else section doesn't need mitigation do nothing

Figure 9. Pseudocode for “SnowMan Determines Fences and Setbacks” Outputs from SnowMan Outputs from SnowMan include the following:

• Ground surface (original, new) • Snow profiles (unmitigated, mitigated) • Limit of storage offset • Snow transport (Qave, Qexceedance) • Snow depths at Upwind Limit of Protection (ULOP, typically upwind edge of pavement)

and Downwind Limit of Protection (DLOP, typically downwind edge of pavement) • Storage (at specified limit of storage and at ULOP) • Whether the site (section) needs mitigation, based on either of the following

considerations: o Whether the average transport (Qave) is stored before the limit of storage, and o Whether drift depth at either LOP exceeds the allowable drift depth specified in a

defaults file.

Graphical outputs can be plotted (with or without exaggerated vertical scale) to a design (DGN) file, and report outputs can be plotted to a report file. Examples of “SnowMan Determines Fences and Setbacks” output (graphical, unmitigated, and mitigated, respectively) are shown in Figures 10 – 12. In Fig. 10, the ultimate drift is shown graphically along with the


point indicating the limit of storage (marked with “X”) and the upwind and downwind limits of protection.

Figure 10. Sample “SnowMan Determines Fences and Setbacks” Graphical Output from

SnowMan

Figure 11. Sample Unmitigated Output from SnowMan


Figure 12. Sample Mitigated Output from SnowMan

A user can also use a “trial and check” approach to try other candidate snow fence solutions using different heights, setbacks, or porosities, as seen in Figs. 13 and 14.

Figure 13. User-Specified Trial Fence Case


Figure 14. Trial Fence Parameters and Options

Earthwork Solutions Earthwork (roadway cross section design) solutions (the last two cases in Fig. 7) can also be performed by SnowMan. These implement Tabler’s guidelines [2] as depicted in Fig. 15.


Fig. 15 Earthwork Solutions

VERIFICATION AND STATUS During its development, SnowMan has been checked against approximately a dozen cases of measured snow depths to verify reliability of the encoded algorithms and against other drift-profile-plotting software deemed to be reliable. SnowMan results were also checked with sites where snow fence mitigations have successfully been previously installed. In addition, user testing of earlier versions of SnowMan was conducted, and changes to the software and accompanying user manual were incorporated accordingly. The software has been distributed for use within New York State. SUMMARY AND CONCLUSIONS

The SnowMan software brings the science of engineered mitigation of blowing and drifting snow to the desktop. The present paper has described an overview of this software tool for CAD – integrated design of passive control measures for blowing and drifting snow. Highlights of this software include the following:

• provides a connection between passive snow control and both highway designers (for new or remediary designs) and maintenance personnel (for existing problem sites),

• potentially improves safety for travelers on highways that have blowing and drifting snow problems,

• does not require the user to have a complete knowledge of blowing and drifting snow expertise,

• runs within MicroStation (a familiar design environment for technical staff in NYSDOT and many other states),

• will help reduce maintenance costs associated with blowing and drifting snow, • improves design capability for design engineers,


• automates computation of drift profiles, • allows designers to quickly check and design roadway alignments to avoid or minimize

blowing and drifting snow problems, and • enables maintenance residencies to locate and size snow fencing to reduce blowing and

drifting snow on roadways and therefore reduce the amount of plowing and de-icing required.

Aspects of SnowMan specific to New York State are the interpolations of climatological data when site-specific climatological data are not available and the CAD level standards employed for surface features specific to snow control (e.g., fetch barrier, upwind limit of protection) when a 3D DGN file is used as the source of surface topographic data. Thus, the SnowMan software, which has been implemented as an MDL application for use in the MicroStation environment, could be adapted and extended for use in other regions beyond New York State having similar concerns with blowing and drifting snow.

ACKNOWLEDGMENTS This work was overseen by Project Manager Joseph F. Doherty, MBA, P.E., of the New York State Department of Transportation (NYSDOT). It was sponsored by NYSDOT’s Transportation Infrastructure Research Consortium (TIRC) which is administered by staff at Cornell University. The opinions and conclusions expressed or implied in this report are those of the authors. The contributions of former graduate students David Schwartz and Xiaotian Wang are gratefully acknowledged.


REFERENCES

1. Tabler, R. D., Design guidelines for the control of blowing and drifting snow. Strategic Highway Research Program, Report SHRP-H-381 (1994).

2. Tabler, R. D., Controlling Blowing and Drifting Snow with Snow Fences and Road Design, Technical Report prepared for the National Cooperative Highway Research Program, NCHRP Project 20-7(147), August 2003.

3. Liston, G. E., Haehnel, R. B., Sturm, M., Hiemstra, C. A., Berezovskaya, S., and R. D. Tabler, “Instruments and Methods: Simulating complex snow distributions in windy environments using SnowTran-3D,” Journal of Glaciology, Vol. 53, No. 181, 2007, pp. 241 – 256.

4. Hershey, B. W., and L. F. Osborne, Jr., “The Physical Nature and Prediction of Blowing Snow within the Roadway Environment,” paper # Snow08-018, Fourth TRB National Conference on Surface Transportation Weather and Seventh International Symposium on Snow Removal and Ice Control Technology, Indianapolis, IN, June 2008.

5. Perchanok, M. S., McArdle, S., Grover, P., and A. Naumov, “Spatial Modeling for Evaluation and Remediation of Snow Drifting on Ontario Highways,” paper #Snow08-036A, Fourth TRB National Conference on Surface Transportation Weather and Seventh International Symposium on Snow Removal and Ice Control Technology, Indianapolis, IN, June 2008.

6. Ontario Ministry of Transportation, SNOWDRIFT V2 Extension for ArcView 3.x Snow Drifting Simulation Program, http://www.xfer.mto.gov.on.ca/PTASapps/index.htm (accessed June 2008).

7. Kaminski, D. F. and S. Mohan. “PASCON: An expert system for passive snow control on highways,” Transportation Research Record #1304, 1991, pp. 193-201.

8. Wyoming Department of Transportation, “Controlling Blowing and Drifting Snow – Snow Drift Profiler,” http://www.heepweb.org/ProgramsReports/Reports/HEEPAgencyReports/tabid/96/EntryID/46/Default.aspx (accessed Dec. 2007).

9. Tabler, R. D., Computer-Aided Design of Drift Control Measures, Technical Report for Research Project # FHWA-WY-97/02, Federal Highway Administration and Wyoming Department of Transportation, 1997.

10. Minnesota Department of Transportation, “Living Snow Fences: Control of Blowing and Drifting Snow,” http://www.dot.state.mn.us/environment/livingsnowfence/ (accessed Dec. 2007)

11. Tabler, R. D., “Predicting profiles of snowdrifts in topographic catchments,” Western Snow Conference (Coronado, Calif.; April 23-25, 1975), Proceedings 43: 87-97.


12. Christensen, R. H. User Manual: Snowdrift Prediction System for EWSD. State of Wyoming, Data Services, System Analysis and Design, Cheyenne, 1976.

13. Tabler, R. D., Estimating the transport and evaporation of blowing snow. Symposium on Snow Management on the Great Plains (Bismarck, N. Dak.; July 1975) Proceedings, Great Plains Agricultural Council Publication 73: 85-104.

14. Lamanna, M. F., and Chen, S. S., “Control of Blowing Snow using SnowMan: Developer’s Manual,” Dept. of Civil, Structural, and Environmental Engineering, University at Buffalo, State University of New York, July 2008.

15. Chen, S. S., and Lamanna, M. F., “Control of Blowing Snow Using SnowMan: User Manual,” Dept. of Civil, Structural, and Environmental Engineering, University at Buffalo, State University of New York, July 2008.


LIST OF FIGURES FIGURE 1: Ground Profile required to generate the snowdrift profile in the region of interest FIGURE 2: Examples of distances and slopes used in Equation (1) to estimate the slope of the

next snow profile increment FIGURE 3: Illustration of slopes used to generate the snow fence drift profiles (revised) FIGURE 4: Effect of snow fence porosity on shape of upwind and lee drifts on flat terrain, as

given by Equations (4) and (10) and the values presented in Tables 1 and 3 FIGURE 5: Climate and Topographic Data Input in SnowMan FIGURE 6: 3D Design File Being Used for Topographic Input FIGURE 7: SnowMan Analysis Cases FIGURE 8: Pseudocode for Computing Storage & Depth & Checking for Problem(s) FIGURE 9: Pseudocode for “SnowMan Determines Fences and Setbacks” FIGURE 10: Sample “SnowMan Determines Fences and Setbacks” Graphical Output from

SnowMan FIGURE 11: Sample Unmitigated Output from SnowMan FIGURE 12: Sample Mitigated Output from SnowMan FIGURE 13: User-Specified Trial Fence Case FIGURE 14: Trial Fence Options FIGURE 15: Earthwork Solutions LIST OF TABLES TABLE 1: Coefficients and limits of applicability for upwind drift polynomials (Equation 4) for

fences with indicated porosities TABLE 2: Values for ε in Equations (8) and (11) for different fence porosities TABLE 3: Coefficients and limits of applicability for lee drift polynomials (Equation 10) for

fences with indicated porosities

D-1

Appendix D

Introduction to Blowing Snow Mitigation and SnowMan

By R. D. Tabler, Ph.D., P.E.

Documents

Control of Blowing Snow Using SnowMan (Snow Management ... · Control of Blowing Snow Using SnowMan (Snow Management) User Manual NYSDOTIRC Subcontract 28311-5823 Project C-01-67