A channel is any open conveyance, natural or man-made, in which water flows with a free surface. A roadside ditch is a man-made channel generally paralleling the roadway surface and distinguished by a regular geometric shape. The design process and analysis requirements for roadside ditches and channels differ. For the purpose of this chapter, "channel" shall refer to all open conveyance facilities not classified as roadside ditches or requiring more than a 2.0 foot base. The design procedure presented is general and intended to present specific criteria and analysis requirements. The Engineer should reference FHWA, HEC-11, (18), HEC-15 (19), and Chapter VI of the AASHTO Drainage Guidelines(1)for more detailed design guidance.

Roadside Ditches

The following is a basic step procedure for evaluating and/or designing roadside ditches.

(1) Establish a ditch plan which shows the proposed ditch locations and flow patterns.

This ditch plan is a part of the drainage plan (Chapter V, Item 7).

(2) Determine the standard or typical ditch cross sections for the project.

This is provided by the roadway plans typical sections. When a ditch is required along the construction limits which is not part of the typical section, the following Criteria are to be followed in establishing a typical section.

• A standard berm ditch section shall be noted at top of cut where required.

• Toe of fill ditches adjacent to shallow fills and flat slopes (4:1 or flatter) shall be formed by continuation of the fill slope to a desired ditch depth, provision of a base width if required, then a stable back slope (2:1 maximum).

• Toe of fill ditches adjacent to high steep slopes shall be constructed with a minimum 2.0 foot berm. A wider berm is desirable for very high fills to prevent embankment from filling the ditch and for maintenance if access is limited from the off roadway side.

(3) Determine the gradient to be used on all proposed ditches.

Roadway ditches included in the typical roadway section will have a grade corresponding to the roadway profile. When the roadway profile grade is less than 0.3%, special roadway ditch grades may be established and noted on the plans.

Ditches along the toe of fill will generally parallel the grade of the natural ground at an established acceptable depth. The approximate grade of these ditches are to be established and plotted on the plan profile view.

(4)Investigate capacity of the established typical ditch.

Roadway ditches are to be designed to contain as a minimum the Q5 flow. The typical roadway ditch section is established with sufficient depth to drain the pavement subbase and flat side slopes for safe vehicle traversability. This generally provides very generous capacity for the design flow requirements. Therefore, actual capacity determination can be done on a selective basis at sites on common project grades to verify adequacy and establish limitations on the length of ditch run.

The size requirements of the project special side ditches along the toes-of-fill will be established based on an analysis of the design flood. This ditch capacity analysis will be performed using Mannings' equation:

Q = (1.49 AR^2/3 S^1/2)/n

Discharge determination shall follow the requirements of Chapter VI - Hydrology. The roadway section including shoulders and slopes shall be considered an urban watershed. This capacity analysis is usually worked in conjunction with the next step of lining evaluation.

(5) Determine the limitations and protection requirements to prevent erosion in the ditch.

The stability of vegetative ditch linings is to be analyzed by use of Charts 1 and 2 (Appendix J). These charts are based on the more frequently used 'V' and base ditch sections. However, a procedure and example are included for evaluating other channel configurations. The stability limitation is based on an established acceptable velocity. When applying the chart, if conditions at a particular site are such that you fall to the left of the stability line, a good vegetative cover would not be expected to erode. Conversely, if you are to the right of the line, the ditch would be expected to be unstable and erode when subjected to design flow even if a good vegetative lining were established; therefore, some type of armoring (rip rap, concrete paving) must be used.

Charts 3 and 4 (Appendix J) are provided to analyze the stability of rip rap ditch linings (Type A,B, and Class I rip rap). They are used in the same manner as Charts 1 and 2 to determine the stability of stone lining under differing ditch shape and flow conditions.

(6) Determine any special measures necessary at or downstream of the ditch outlet.

A check should be made of the transition of flow from a ditch to the receiving outlet. Factors to be considered are:

(a) Is there provision for a smooth transition of flow from the ditch to the outlet?

(b) Will the outlet adequately handle the quantity of flow? Is improvement required?

(c) Is the velocity of flow at the outlet too high for the condition of the receiving channel? Is riprap or other means of velocity reduction justified?

(d) When the receiving outlet is sheet overland flow, is concentration of flow by the ditch a potential problem? Is some form of flow diffusion required?

Channels

Channel analysis differs from roadway ditch analysis in that it involves establishing a channel configuration to meet specific site hydrologic, and geomorphic requirements. The requirements for analysis can range from simple sizing of small ditches constructed adjacent to the roadway fill to intercept and convey discharge to points of acceptable outlet, to complex studies of extensive natural stream and river relocation. In addition to the guidance provided in this document the engineer is directed to FHWA, Hydraulic Engineering Circular #15 (16) and Chapter 8 of the AASHTO Model Drainage Manual (2),for further guidance for small ditch and channel analysis. For larger stream involvement, FHWA "Highways in the River Environment"(20), "Applied River Morphology"(21), by Dave Rosgen and the NC Wildlife Resources Commission, "Guidelines for Stream Relocation and Restoration in North Carolina"(22), are suggested references.

Channels that are realignments of natural streams should be sized and configured to match as near as practicable the natural channel system. For small, "minor relocation" of streams at the inlet and outlet of structures (less than 100 feet total, <50 each side). The engineer is directed to "Stream Relocation Guidelines"; developed jointly by representatives of the NCDOT and the NC Wildlife Resources Commission in 1993 (Appendix M).

If relocation of a stream channel is unavoidable, the design of the replacement channel should provide dimension, pattern and profile that affords natural stability. A proven and accepted method of study for natural channel involvement is through a process of stream classification. The overall objective of classifying a reach of streams is to set categories of types based on morphologic characteristics, so that consistent, reproducible descriptions and assessments of conditions and potential can be developed.

Some specific objectives of a classification system are:

( From "Applied River Morphology ",Dave Rosgen)

• Provide methodology for predicting a streams behavior from its appearance (classification).
• Guide development of specific hydraulic and sediment transport relationships for stream type and state.
• Provide mechanism for comparison of data for stream reaches having similar characteristics.
• Provide a consistent frame of reference for communicating stream conditions and morphology across disciplines.

The recommended sequence of a channel analysis should be as follows (more detailed guidance is provided in the recommended references).

1) Data Collection

Data collection includes office study as well as a field survey. Much of the information needed for initial classification can be obtained from topographic mapping and aerial photography. The field survey provides more detailed information for refinement of the initial classification as well as the analysis and design process. It should include as a minimum the collection of the following data:

Needed for Classification:

• channel width(bankfull)
• channel depth(section mean)
• maximum depth(at bankfull)
• bankfull cross section area
• slope(average for at least 20-30 channel width reach)
• stream length(20-30 bankfull channel widths in length)
• valley length(20-30 bankfull channel widths in length)
• bed material(type, size [D₅₀])
• bank material(type, size [D₅₀])
• width of flood-prone area

   

Needed for analysis and design:

Channel Dimension

• pool depth
• pool width
• pool area
• riffle depth
• riffle area
• maximum pool depth

Channel Pattern

• meander length
• amplitude
• radius of curvature
• belt width

Channel Profile

• valley slope
• riffle slope
• average water surface slope
• pool slope
• pool to pool spacing
• pool length

 

2) Stream Classification

With the data collected and further determination of stream features such as:

• entrenchment ratio
• width/depth ratio
• sinuosity

a stream type classification can be established. (See Reference (18), "Applied River Morphology".)

3) Existing Conditions

It is important to assess the condition of the stream as a stream type classification can be established. (See Reference (18), "Applied River Morphology".) it relates to stability, state and causes of changes, potential future impacts and hydrologic and hydraulic requirements. This assessment process should address:

• the watershed
• flow regime
• riparian vegetation
• bank stability
• bed stability
• meander patterns
• sediment supply and transport
• debris
• aggradation/degradation
• aquatic and terrestrial habitat
• discharge levels and conveyance requirements
• evolutionary trend

Stream condition gathers through the assessment process apply to a reach of the stream and may vary considerably up and down stream as the character of the valley changes. Some sections may be at such an altered state that existing data and conditions are of little value in developing recommendations for a relocated or restored channel. This is when a reference stream of similar classification and morphological characteristics can be used as a guide for developing study proposals.

4) Proposed Plan

The evaluation process should provide the engineer with information and knowledge necessary to develop a recommended channel relocation or restoration proposal that meets hydrological and ecological requirements and provides a natural stable system. Wildlife resource specialist should be consulted for input during the design process.

5) Design Documentation

All information pertinent to the channel design shall be documented in an appropriate design report format.