Shear Strength Based on Soil Properties

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Computer software can be a useful tool for the design of roadside ditches. An effective program will be user intuitive, minimizing the need for personal instruction. Because different theories exist in the design of roadside ditches, users of ditch design software need to be knowledgeable of the theories and assumptions applied in the development of the software. Improper use of software can lead to inaccurate designs, especially when software is applied outside its range of validity.


In an attempt to establish the state of practice of roadside ditch design in Virginia, engineers across Virginia were interviewed during visits to each VDOT construction district. During these visits, engineers were asked which computer programs, if any, are used regularly for ditch design.
To determine the state of practice in surrounding states, contacts were established with design engineers in the states surveyed, as described in Chapter 2. Contacts were initiated based on information available on the Internet. Each engineer contacted was asked which programs, if any, are widely used in their state for the design of roadside ditches. Because at most two designers were questioned from each state, responses obtained reflect the experience of the individual(s) and may not be representative of the practice for the entire state. When available, information concerning recommended computer programs was taken directly from state drainage manuals.
All programs noted by the engineers in the survey were obtained for testing. Each program was evaluated based on theory of design, ease of use, and reliability of results. A variety of design scenarios was tried on each program with the intent of gaining a general idea of how each program responds to changing parameters. Because the intent of this survey was to gain a general understanding of software available for roadside ditch design, extensive testing of each program was not performed. An extensive evaluation of each program would have to be performed to insure the accuracy of results produced by each program.


Because calculations associated with roadside ditch design are not complicated, few programs have been developed with the specific intent for use in designing roadside ditches. Collectively, only four programs were found to be used for ditch design in Virginia and in the various states surveyed. Of these four programs, two were developed based on the Maximum Allowable Velocity Method. The other two programs were based on the Tractive Force Method presented in FHWA HEC-15 (1988). Each program and User’s Manual, where available, were obtained for evaluation. The results are categorized by the design theory employed by the software packages.
The two programs that employ the maximum allowable velocity stability criterion are VDOT’s RDITCH and an Excel spreadsheet developed by Anderson and Associates, a Civil Engineering consulting firm based in Blacksburg, VA. RDITCH was developed by the Virginia Department of Transportation specifically for use to design roadside ditches in Virginia. This is a DOS-based program capable of computing peak storm flows and calculating ditch flow depth and velocity for three lining types: bare earth, lined (synthetic or vegetated), and paved. The Anderson and Associates’ Excel spreadsheet, also intended for roadside ditch design on Virginia highways, provides a more efficient interfacing tool. The spreadsheet is based on guidelines set forth in the Virginia Drainage Manual (1991) and is capable of computing peak design flow and determining ditch stability based on calculated flow depth and velocity.
The Tractive Force Method, developed in HEC-15, has been programmed into two software packages by the FHWA. The program called HY-15 Stable Channel Linings was the first software package developed for the design of roadside channels using the Tractive Force Method. This program can only evaluate stability of simple, straight, lined channels. A more complex model, HYDRAIN, was developed more recently by FHWA. The total HYDRAIN package is an integrated drainage design computer system. Within HYDRAIN, the HYCHL interface can be used for roadside ditch design on more complex ditches, and is capable of computing stability of both bare earth and lined ditches.
Below are brief summaries listing background information about design theory and methods applied in each software package, followed by perceived advantages and disadvantages for each program. The information presented below is also summarized in Table 2. Comments listed reflect the opinions of the evaluator on the research team.

RDITCH developed by VDOT (1989)

Methods and theories used in the program
The Rational Method is used to determine runoff flow for a given rainfall event.
The width-of-strip method of approximating watershed area is utilized by the program for peak flow calculation of the desired storm.
Using the Rational Method, design reach subarea information must be entered by the user.
Manning’s equation is used to determine flow velocity and depth based on design flow.
Values of Manning’s n are standard values recommended by VDOT. Bare earth, n = 0.03; temporary protective lining and vegetated linings, n = 0.05; permanent (paved) lining, n = 0.015. The user can not change these values nor add new values.
The maximum allowable velocity criterion is used to determine the stability of the ditch. The program computes a depth and velocity for both the 2- and 10- year storms for each value of Manning’s “n”. From the output, the user will need to determine if the 10-year storm depth is adequately contained by the given ditch geometry. Then, the user must compare the computed 2-year storm velocity to an accepted maximum allowable velocity for the soil type/temporary lining of the ditch.


The program was developed by VDOT specifically for the purpose of roadside ditch design and encompasses a computational routine calculating peak flow using the Rational Method.
The user can design continuous reaches of roadside ditch with a single run of the program.
IDF (Intensity-Duration-Frequency) curves for each county in Virginia are conveniently incorporated into the program. To use this feature, the user must enter the time of concentration for the initial design reach. Or, the user has the option of reading the IDF curves, independent from the program, and entering rainfall intensity values manually. In either case, intensity for each subsequent reach is incremented from the initial intensity.
It has been used extensively for ditch design on Virginia highways.

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The program is DOS-based, and not Windows interactive. A series of prompts are initiated which require inputs in the form of numbers, even when phrased responses may be more appropriate.
The user cannot see or modify data inputs on a continuous basis. Instead, all data inputs must be entered before the opportunity to edit the entries becomes an option.
For each design reach, all data concerning ditch geometry and subarea information must be re-entered. This may become repetitive when design parameters are the same.
The program does not allow the user to experiment with ditch geometry for a given reach of design with a single run of the program. The user must re-enter all data and run the program again.
Each ditch design segment has to be viewed on separate screens. The user cannot see, on a single screen, a continuous ditch design for a given project. The results must be printed to do this, or otherwise viewed by a different application, such as Notepad.
Some units used in the program can be awkward. The user must use the default units and cannot change them. Example: side-slopes are given in (in/ft).

Anderson and Associates Excel Spreadsheet for Ditch Design Computations

Methods and theories used in the program

Anderson & Associates developed an Excel spreadsheet to aid computations associated with ditch design calculations. The program is developed using the Maximum Allowable Velocity Approach and design criteria specified in the Virginia Drainage Manual (1991).
The Rational Method is used in determining peak flow to the ditch. Drainage area parameters (Area, rainfall intensity, and runoff coefficients) must be entered to compute flow using the Rational Method. An initial time of concentration is entered by the user. Rainfall intensity from both the 2- and 10- year storms is read by the designer from the appropriate IDF curve and then entered into its respective column. The intensity is decreased by 0.1 in/hr for each subsequent design reach until the ditch is emptied.
On a separate worksheet within the file, the designer can define and name ditch cross-sections to be used on a given project. This feature allows the user to enter ditch geometry specifications once. From this list, the user can enter the “name” of the ditch into the column “DITCH TYPE” and the cell then references the correct ditch geometry. When ditch geometry is named, the corresponding description must include intended side-slopes, bottom width, and maximum permissible depth.
Given all necessary data, the spreadsheet computes both depth and velocity for the 2- and 10-year storms using Manning’s equation. The program will first determine if the capacity is adequate for the 10-year storm depth. A message will be sent to the “Comment” column if this requirement is not satisfied. Then, the spreadsheet is programmed to compare the predicted 2-year storm velocity against the maximum allowable velocity for bare earth, currently entered as 3 ft/s. If the predicted velocity exceeds 3 ft/s, the spreadsheet will automatically iterate lining type, using stored data concerning lining stability, until the stability requirement is met. Lining types can be easily added to or deleted from the spreadsheet.
When the program has found a stable lining, a message will be displayed in a column indicating the name of the stable lining.
The file uses color-coded fonts to distinguish user inputs from programs generated outputs. All user inputs are displayed in blue while program calculated outputs are seen in black.


An entire project can be designed and displayed on the same file. The designer can view changes in ditch lining requirements along a design reach and on both sides of the road alignment.
Ditch “Types” can be added to or deleted from the spreadsheet. Ditch “Types” can be either trapezoidal or V-shaped. Also, lining types can be added to or deleted from the spreadsheet. All parameters, for ditch types and lining types, can be adjusted as needed. For example, stability parameters for various linings can be updated.
The spreadsheet can be customized to the individual user’s needs. Additional rows can be inserted, automatically formatted as the previous row, or extra rows can be deleted for more economical screen display.
All data within the spreadsheet can be updated by the user as needed to accommodate changes in design guidelines and/or synthetic lining criteria.
The spreadsheet is intuitive for the experienced Excel user.


The spreadsheet has all calculations performed in English units. The programs would have to be modified to convert to metric units.
When deleting inputs, the user has to be careful not to delete black font cells, which hold programmed expressions for calculations. This may be a concern for color-blind users.
The current spreadsheet uses a default maximum allowable velocity of 3 ft/s for all bare earth conditions.
The spreadsheet is a more recently developed program and has not been used extensively for design on Virginia highways.

Stable Channel Linings (FHWA HY-15)

Methods and theories used in the program
The program employs the Tractive Force Method as described in FHWA HEC-15 (1988).
Stability for a given lining type can be evaluated for an entered discharge.
Manning’s n is calculated as a function of flow depth.
For given ditch parameters and lining types, the program calculates the tractive force generated by the flow, and compares it against a maximum permissible tractive force for the same lining. The maximum permissible tractive force is based on information published in FHWA HEC-15.


The program is one of two software packages based on the tractive force method presented in FHWA HEC-15 and is capable of analyzing both rigid and flexible linings.
The program will compare calculated shear stress with maximum permissible shear and indicate the stability of the selected lining.
Data entries are easily entered and edited. Overall, the program is self explanatory and easy to use.

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The program is DOS-based and not Windows compatible.
The program does not compute peak ditch discharge. The user must calculate this information outside the application of this program.
Only one ditch flow can be entered per execution of the program. The 2- and 10- year storms cannot be analyzed and viewed together.
The bare earth condition cannot be analyzed using this program. This program allows the user to analyze the stability of temporary and permanent linings, only.

HYDRAIN using the HYCHL submenu for ditch design (FHWA 1996)

Methods and theories used in the program
The program uses the tractive force method of analysis as described in FHWA HEC-15.
Manning’s formula is used to compute ditch flow and velocity.
Manning’s roughness, for the bare earth and vegetation cases, is determined as a function of flow depth. For vegetative linings, the average grass height and stiffness are considered in the determination of hydraulic roughness. Vegetal classification must be entered by the user. Hydraulic roughness of bare soil (considered a temporary lining) is independent of soil type and based as a function of flow depth, H. For unlined bare soil with H < 0.15m, n = 0.023 and H > 0.2 m, n = 0.02. Linear interpolation is used for 0.15 m < H < 0.2 m.
When given design flow and channel parameters (slope, shape, and lining type), the program calculates: flow depth, velocity, applied shear stress, permissible shear stress, and maximum ditch discharge.
The program can evaluate a constant flow for the entire ditch length or a variable flow over the ditch length. Variable flow is typical of ditch flow, with increasing discharge occurring along a ditch length due to inflow from the watershed.
Normal depth is calculated using an iterative process beginning with an initial estimate for depth. The iteration continues until the estimated flow calculated from an assumed depth is within 0.1 percent of the given design flow.
Once depth has been calculated, shear stress for the channel bottom is obtained from equation (7).
For noncohesive soil linings, the maximum permissible shear stress, p, is determined from the expression:
p = 800.93 D50. (8)
where units are: [p] = N/m2 and [D50] = m.
(Note: A misprint in the HYDRAIN User’s Manual incorrectly applied the conversion constant of 800.93 as 244.2.)
For cohesive linings, the maximum permissible shear stress is dependent on soil type, plasticity index (PI), and compaction effort. The equations are:
Loose p = 0.1628 PI0.84 (9)
Medium p = 0.2011 PI1.071 (10)
Compact p = 0.2729 PI1.26 (11)
The user may override the permissible shear stress for a lining dictated by the program by entering a different value.
Rigid linings in HYCHL include concrete, grouted riprap, stone masonry, soil cement and asphalt. Flexible linings include those which may be considered permanent and those considered temporary. Permanent flexible linings include vegetation, riprap, and gabions. Temporary linings include woven paper, jute mesh, fiberglass roving, straw with net, curled wood mat, synthetic mats, and bare soil.
The analysis of rigid, vegetative, gabion, and temporary linings in HYCHL is applicable to channels of uniform cross section and constant bottom slope.
The procedure used to analyze temporary linings is identical to that applied for permanent linings. However, since temporary linings are intended to have a shorter service life, the design flow may be lower.
Roadside channels are considered stable when the stability factor (ratio of permissible to calculated shear stress) is greater than one.
Channel side slope shear is analyzed using the parameter of Kside (the ratio of maximum channel side shear to channel bottom shear). It is a function solely of channel shape and is dependent of channel geometry and channel sideslopes. The maximum shear stress on the side slope will always be less than or equal to that on the bottom. Parabolic and V-shaped ditches are assumed to require a Kside = 1 as a conservative estimate. Because Kside is always equal or less than 1, side shear does not limit the design of ditches. It may affect the design of composite linings.
A maximum discharge can be calculated by setting the applied shear equal to the maximum permissible. The maximum allowable flow depth takes the form of:
Hmax   p S
This depth can be used in Manning’s formula to compute the maximum discharge.
Allows analysis of both rigid and flexible linings.
Allows analysis of both straight and curved channel segments.
HYCHL can analyze all linings with a known permissible shear for both stability and maximum conveyance.
The channel shape can consist of regular and irregular profiles.
HYCHL provides for the analysis of all the lining types collectively (temporary and permanent), individual liner analysis, or analysis when two linings are specified together as a composite lining. Composite linings are used when lining side slopes with the same material applied to the bottom is undesirable for reasons of economics aesthetics, or safety.
The program considers differences in maximum permissible shear for cohesive and noncohesive soils.
Visual representations of the ditch geometry and flow depth are shown on the screen.
Once in the CHSHL interface, user inputs are guided to provide an intuitive process.
The user can modify any default calculations made by the program with their own values.
Several choices of ditch shape (triangular, trapezoidal, parabolic, and V-shaped with rounded

Table of Contents
Chapter 1. Review of Literature
1. Factors Influencing Design
1.2 Mechanisms of Erosion
1.3 Shear Strength Based on Soil Properties
1.4 Discussion
Chapter 2. Survey of Design Guidance
2.1 Background
1.2 Methods
1.3 Results
1.4 Discussion
Chapter 3. Review of Computer Programs
3.1 Background
3.2 Methods
3.3 Results
3.4 Discussion
Chapter 4. Collection and Utilization of Data for Design
4.1 Background
4.2 Methods
4.3 Results
4.4 Discussion
Chapter 5. Synthesis of Design Criteria
5.1 Background
5.2 Methods
5.3 Results
5.4 Discussion
Chapter 6. Conclusions
Chapter 7. Recommendations
Chapter 8. Application

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