VIII. STREAMS AND CHANNELS

Open channels may have significant advantages in cost, capacity, multiple use, and potential for reducing flood peaks. This chapter provides standards and criteria for the incorporation of open channels in a drainage system, including both natural & artificial channels.

A. Definitions

This chapter identifies three kinds of open channels: natural, grassed channels, and lined channels. Overlap among these categories exists, and criteria from more than one category may apply to a given channel.

1. Natural channels are characterized by irregular section, alignment, vegetative cover and bottom and side materials. Velocities are usually low in the natural channel, resulting in long concentration times and lower downstream peak flows. Natural channels often have an overbank storage capacity which also tends to decrease peak flows.

Maintenance needs are low if the channel is stable. The natural channel often provides opportunities for multiple uses, including fish and wildlife habitat and recreation and has the best aesthetic qualities of the alternatives.

2. Grassed channels are characterized by a more uniform section and a vegetative cover of grass. They are typically used where velocities are low and erosion may therefore be prevented with a cover of grass. Grassed channels may be natural or, more often, artificial. The advantages of a grassed channel include lower cost and positive aesthetic qualities.

3. Lined channels must be used when there are high velocities due to a reduced section or steep slopes: they may be used with subcritical or supercritical flow. Linings include concrete, stone and other permanent material.

4. Bankfull flow is the flow in a channel that creates a water surface at or near the normal ground elevation, or the tops of dikes or continuous spoil banks that confine the flow for a significant length of a channel reach.

B. Policies

1. Natural Channels Preferred Open channel planning and design objectives are often best met by using natural, or natural-type channels. Therefore, as is stated as a policy in Chapter 2, natural channels shall be used for storm runoff whenever possible. Use of natural channels will be consistent with the floodplains and open space requirements of the area and preserve as much as possible the value of the channel for fish and wildlife habitat, recreation and aesthetics.

2. Channelization Channelization of natural waterways will be avoided: channelizing usually speeds up the flow, causing greater peaks and higher drainage costs downstream.

3. 100-year Capacity Open channels should be capable of carrying the 100-year runoff (a one percent chance of occurring in any single year).

4. Alignment Open channels should follow the natural drainage paths as much as possible.

5. Channels Artificial earth channels, that is, either constructed channels or heavily modified natural channels, shall not be used for drainage because of the potential erosion and damage to those downstream.

6. Compliance With FEMA Standards Where appropriate, channel design criteria shall comply with FEMA standards and the 100-year floodplain shall be designated.

C. Criteria

1. General

a. Water Surface Profile Open channel flow is usually non-uniform because of bridge openings, curves, and structures. Except as specified below, this requires the use of backwater computations for all final channel design work.

A water surface profile must be computed for all channels and clearly shown on the final drawings. Computation of the water surface profile should use standard backwater methods, such as the Corps of Engineers HEC-2 computer program, taking into consideration losses due to changes in velocity, drop structures, bridge openings, and other obstructions. Computations begin at a known point and extend in an upstream direction for subcritical flow and downstream for critical flow. The depth of flow in the receiving stream must be consistent with the level of event being considered.

Extensive cross section data taken for flood insurance purposes may be available from the local jurisdiction or FEMA.

b. Manning n Values Wherever possible, Manning n values should be based on calibrations to observed high water marks and known flows for the same or a similar location. Table 8-1 provides general guidelines for estimating n values for streams and channels in cases where observations are not available. Also see Figures 8-2 and 8-3.

c. Drop Structures Drop structures may be used to decrease the bed slope and to control erosion in natural streams and grassed waterways. Drop structures shall be constructed with reinforced concrete, grouted rock or gabions in accordance with best engineering practice. A low-flow notch shall be provided for drainage and, where applicable, to allow passage of fish and other aquatic life.

d. Riprap and Gabions Riprap may be used to prevent damage to channel bottom and bank upstream and downstream from hydraulic structures, at bends, at bridges, and in other channel areas where erosive tendencies exist. Criteria and guidelines for riprap, grouted riprap, and gabions are presented in Chapter IX.

e. Appurtenant structures The channel design shall include all structures required for proper functioning of the channel and its laterals, as well as travelways for operationand maintenance. Inlets and structures needed for entry of surface and subsurface flow into channels without significant erosion or degradation shall be included in the channel design. The design also shall provide for necessary flood gates, water-level-control devices, bays used in connection with pumping plants, and any other appurtenances essential to the functioning of channels and contributing to attainment of the purposes for which they are built. If needed, protective structures or treatment shall be used at junctions between channels to insure stability at these critical locations.

TABLE 8-1 MANNING N FOR STREAMS AND CHANNELS (24) UNIFORM CHANNELS
Description		n
Concrete Earth Grass Rock, Rubble		0.012 - 0.016 0.017 - 0.022 0.020 - 0.025 0.025 - 0.045
NATURAL STREAMS-CHANNELS Channel n is a composite computed from the component n and k values in the table as follows: n=k (n1+n2+n3+n4 )
Component	Condition	n
Material involved (n1)	Earth Rock Cut Fine Gravel Course Gravel	0.020 0.025 0.024 0.028
Degree of Irregularity (n2)	Smooth Minor Moderate Severe	0.000 0.005 0.010 0.02
Relative effect of Obstructions (n3)	Negligible Minor Appreciable Severe	0.000 0.010 - 0.015 0.020 - 0.030 0.040 - 0.060
Vegetation (n4)	Low Medium High Very High	0.005 - 0.010 0.010 - 0.025 0.025 - 0.050 0.050 - 0.100
Degree of Meandering (k)	Minor Appreciable Severe	1.000 1.150 1.300

TABLE 8-1 (CONTINUED) MANNING N FOR NATURAL STREAMS - FLOODPLAIN
Description	Condition	n
Pasture