
Engineered Channel Design CriteriaFlow Rate/Design Flows

• Engineered channel design flows- 25-year flow – conveyed in channel

• In addition- 100-year flow- Must be contained within drainage easement

• Design flows – must be calculated for fully developed watershed condition- Based on current/anticipated land uses

• Upstream detention storage may be considered- If ownership and maintenance responsibilities of facility are clearly defined- To get flow reduction credit, detailed modeling/basin routing must be completed

100-year

25-year


Engineered Channel Design CriteriaFroude Number and Flow Regime

Fr < 1.0 Flow is subcritical

Fr = 1.0 Flow is critical

Fr > 1.0 Flow is supercriticalSupercritical flows should be avoided if possible

- Best to have Fr < 0.8(near 1.0 – flows are unstable)

- Fr should be checked for all unique channel sections for 25-year flow

- Checked using minimum “n” value for channel lining

- If supercritical necessary, then Fr should be > 1.2

- Concrete lining required


Engineered Channel Design CriteriaVelocity

Channel TypeMinimum Velocity

Maximum Velocity

Grass, seed and mulch 2 ft/s 4 ft/s

Grass, sod 2 ft/s 6 ft/sGrass, TRM 2 ft/s 8 ft/sGrass, pre-

vegetated TRM 2 ft/s 10 ft/s

Manufactured hard lining 5 ft/s 12 ft/s

Riprap 5 ft/s 12 ft/s

Concrete 5 ft/s 18 ft/s

Table OC-6 – Design Velocity Limitations for Open Channel With Different Lining Types

Increasing allowable velocity


Engineered Channel Design CriteriaLongitudinal Slope

Acceptable maximum longitudinal slope generally dictated by criteria for:

- Flow regime (i.e., Fr < 0.8)

- Flow velocity (i.e., type of channel lining)

Acceptable minimum slope (to minimize ponding):

- 0.4 percent – for natural linings

- 0.2 percent – for concrete linings


Engineered Channel Design CriteriaLongitudinal Slope

If slope is too steep, options to reduce slope include:

Integrate drop structure into channel

Integrate curvature into channel

- With careful consideration- Centerline of curve should have minimum radius of 2 x top width (25-yr)(radius no less than 100 feet) Every design is

site-specific


Engineered Channel Design CriteriaCurvature and Superelevation

• Solve for Superelevation

cgrTVy

2

2

=∆

Where:Δy = difference in water surface elevation

(inside vs. outside of curve)(ft)V = Mean flow velocity (ft/sec)T = Top width of channel (design flow conditions)(ft)g = gravitational constant (32.2 ft/sec2)rc = radius of curvature (ft)

Superelevation on outside of bend

Channel x-sectionChannel curve –Plan view

For velocity, use minimum range of “n” value (to get max. velocity)


Engineered Channel Design CriteriaFreeboard

The required freeboard for engineered open channels is dependent on the type of channel:

Concrete channels Required freeboard6 inches above 25-year water surface

Other types of channels

Required freeboard12 inches above 25-year water surface

(Except for channels where 25-year flow depth is < 12 inches, then freeboard is only 6 inches above 25-year surface)


Engineered Channel Design CriteriaGrass Channel Example

Design of Trapezoidal Grass-Lined Channel

Open“Open Channel Design” file

Click on“Grass Channel” tab

Project:Channel ID:

*** DRAFT ***

Channel Input ParametersDesign Discharge Qd = 75.00 cfsDesign Discharge Return Period Yeard = 100 yearsExisting Ground Slope Along Channel Centerline So = 0.030 ft/ft25-Year Discharge Q25 = 50.00 cfs2-Year Discharge Q2 = 30.00 cfs

Warning 01 Left Side Slope (H:V) Z1 = 3.00 ft/ftWarning 01 Right Side Slope (H:V) Z2 = 3.00 ft/ft

Min Manning's n (max velocity and Fr # check) nmin = 0.030 See "Design Info" tab forWarning 02 Max Manning's n (min velocity and capacity check) nmax = 0.045 recommended Manning's n values.

Grass Channel Lining Type (sets max velocity) Select One

User-Defined Bottom Width B = 5 ft

Maximum Allowable Longitudinal Slope Smax = 0.0225 ft/ft (based on Q25, nmin, & channel lining)User-Defined Longitudinal Slope Suser = 0.0200 ft/ft (min of 0.004 to prevent ponding)Drop Height per 100 ft D = 1.000 ft/ 100 ft

Proposed Channel Conditions (Calculated)25-Year Discharge Flow Depth Required(using Q25 and nmin) (using Q25 and nmax)

25-Year Flow Depth Y= 1.06 ft Y= 1.31 ft25-Year Flow Velocity Vmax= 5.75 fps V= 4.29 fps25-Year Top Width T= 11.37 ft T= 12.84 ft25-Year Flow Area A= 8.69 sq ft A= 11.65 sq ft

Warning 04 25-Year Froude Number Frmax = 1.16 Fr = 0.7925-Year Wetted Perimeter P= 11.72 ft P= 13.26 ft25-Year Hydarulic Radius R= 0.74 ft R= 0.88 ft

2-Year Discharge(using Q2 and nmax)

2-Year Flow Depth Y= 1.01 ft2-Year Flow Velocity Vmin= 3.72 fps2-Year Top Width T= 11.04 ft2-Year Flow Area A= 8.07 sq ft2-Year Froude Number Fr = 0.772-Year Wetted Perimeter P= 11.36 ft2-Year Hydarulic Radius R= 0.71 ft

Design Discharge(using Qd and navg)

Design Discharge Flow Depth YD= 1.46 ftDesign Discharge Flow Velocity VD= 5.47 fpsDesign Discharge Top Width TD= 13.77 ftDesign Discharge Flow Area AD= 13.71 sq ft

Warning 04 Design Discharge Froude Number Fr D= 0.97Design Discharge Wetted Perimeter PD= 14.24 ftDesign Discharge Hydarulic Radius RD= 0.96 ft

Click here for important information

regarding this spreadsheet

sod, Vmax = 6 fps


Engineered Channel Design CriteriaChannel Linings


Engineered Channel Design CriteriaChannel Linings – Soil Bioengineered Lining

Soil bioengineering integrates native vegetation as a structural element of streambank stabilization

Provides:- effective streambank protection- effective energy dissipation- aquatic/terrestrial habitat benefits

Selection of plants and planting method specifications must be prepared by a professional with expertise in stabilization properties of plants


Engineered Channel Design CriteriaChannel Linings – Grass Lining

The design velocity in the channel will dictate the type of grass lining to use

If design velocities exceed the allowable for seed and mulch, the channel will be lined with sod, approved TRM, or approved reinforced sod Up to level

of 25-year water surface


Engineered Channel Design CriteriaChannel Linings – Manufactured Hard Lining

Up to level of 25-year water surface

Similar to reinforced turf, manufactured hard linings must reach


Engineered Channel Design CriteriaChannel Linings – Riprap sizing

• Solve for K-value

66.0

17.0

)1( −=

SGVSK

Where:K = Riprap sizing constantV = Mean channel velocity (ft/sec)S = Longitudinal slope (ft/ft)Gs = Gravitational constant (32.2 ft/sec2)

K Value Rock Type

< 3.3 VL**(d50 = 6 inches)

> 3.3 to < 4.0 L**(d50 = 9 inches)

> 4.0 to 4.6 M**(d50 = 12 inches)

> 4.6 to 5.6 H**(d50 = 18 inches)

> 5.6 to 6.4 VH**(d50 = 24 inches)


Engineered Channel Design CriteriaRiprap Example

Open“Open Channel Design” file

Click on“Riprap” tab

Project:Channel ID:

*** DRAFT ***

Design Information (Input)Channel Invert Slope So = 0.0200 ft/ftBottom Width B = 0.0 ft Left Side Slope Z1 = 3.0 ft/ftRight Side Slope Z2 = 3.0 ft/ftSpecific Gravity of Rock (minimum of 2.6) Gs = 2.60Radius of Channel Centerline (enter 0 for straight channel) rc = 100.0 ft

Design Disharge Q = 3.0 cfs

Flow Condition (Calculated)Riprap Type (Straight Channel) Type = MIntermediate Rock Diameter (Straight Channel) D50 = 12 inchesCalculated Manning's n (Straight Channel) min-n = 0.038 Riprap Type (Outside Bend of Curved Channel) Type = MIntermediate Rock Dia. (O.B. of Curved Channel) D50 = 12 inchesCalculated Manning's n (O.B. of Curved Channel) min-n = 0.038 Water Depth Y = 0.64 ftTop Width of Flow T = 3.8 ftFlow Area A = 1.2 sq ftWetted Perimeter P = 4.1 ftHydraulic Radius (A/P) R = 0.3 ftAverage Flow Velocity (Q/A) V = 2.5 fpsHydraulic Depth (A/T) D = 0.3 ftFroude Number (max. = 0.8) Fr = 0.780Channel Radius / Top Width rc /T = 26.04

Riprap Design Velocity Factor For Curved Channel Kv = Riprap Sizing Velocity For Curved Channel VKv = fps

Minimum Riprap Sizing Paramenter for Straight Channel K = 0.94Minimum Riprap Sizing Paramenter for Outside Bend of Curve Kcurve =Superelevation (dh) dh = ftRequired channel depth (based on max n value) Yrequired= 0.66 ftTotal channel depth including superelevation Ytotal= 0.66 ft

Discharge (Check) Q = 3.1 cfs

Design of Riprap Channel Cross Section


Engineered Channel Design CriteriaChannel Cross-Sections

• Most desirable cross-section- Relatively wide- Primarily vegetated

• Benefits- Recreation - Maintenance- Safety- Water quality- Downstream impacts- Habitat


Engineered Channel Design CriteriaLow-Flow Channels

• A low-flow channel is necessary for:

• Vegetated channels where:

- Baseflow exists- High peak runoff from developed

area may cause channel erosion- 2-year flow > 5 cfs

(for unreinforced grass only)

• For all channels where erosion is likely to occur (such as downstream from point discharges)


Engineered Channel Design CriteriaChannel Outfalls

• Vegetated channels generally cannot withstand point discharges

• Some type of energy dissipation is necessary to minimize bank, channel, or wall erosion

• Protrusions into the channel (e.g., pipes) should be trimmed flush with the main channel wall or bank


Agenda









Hydraulic StructuresLarge Hydraulic Structures

The Open Channels chapter does not provide detailed information on the design of large hydraulic structures

In cases where a large/complex hydraulic structure is proposed, a preliminary meeting must be held with City staff.

Text references are provided for several specific types of hydraulic structures


Agenda









Small Grade Control StructuresFive Types of Small Drop Structures Presented

Newberry-style structure

Grouted sloping boulder

Sloping concrete structure

Sculpted concrete structure

Vertical hard basin


Small Grade Control StructuresDesign Conditions

Design Parameter Criteria

Peak flow rate 200 cfs

Typical channel longitudinal slope 0.3 to 0.8 percent

Normal flow depth (maximum) 3 feet

Flow velocity (maximum) 6 ft/s

Froude number (maximum) 0.8

Geotechnical condition (assumed) Moderate strength clay with chert

Table OC-12 – Design parameters for small grade control structures

For drops of 2 feet or less


Small Grade Control StructuresDrop Structure Selection Process – Evaluation Matrix

Primary Considerations- Public Safety- Functional Hydraulic Performance

Grade Control Structure TypeConsiderations for Grade Control

Structure DesignGrouted

Sloping BoulderConcrete Sloping

Vertical Hard Basin Newbury-style Sculpted

Concrete

Primary Considerations

Public Safety More preferred for areas with high public usage

Less preferred for areas with high public usage

Less preferred for areas with high public usage, (though the potential for reverse rollers and backflow eddies is reduced with drop of 2 feet or less)

More preferred for areas with high public usage

More preferred for areas with high public usage

Functional Hydraulic Performance

Good flexibility of layout options. Roughness effective for dissipating kinetic energy

Maximum vertical drop limited by public safety concern

Maximum vertical drop limited by public safety concern (see above)

Good flexibility of layout options. Roughness effective for dissipating kinetic energy

Good flexibility of layout options


Small Grade Control StructuresDrop Structure Selection Process – Evaluation Matrix2ndry - Cost, Ecological Impacts, Aesthetics, Maintenance, Environ. Permitting

Secondary Considerations

Land Use(Contextual Design)

Provides natural channel appearance

Best suited for urban setting

Best suited for urban setting



Cost Costs comparable for all options

Costs comparable for all options




Ecological Impacts Creates larger footprint than vertical drop

Creates larger footprint than vertical drop. Has no habitat or water quality benefits and may be detrimental.

Creates smallest footprint of any option. Has no habitat or water quality benefits and may be detrimental.

Provides refuge for macroinvertebrates and small fish, fish passage, simulates naturally occurring riffle. Provides refuge during flow event.

Creates larger footprint than vertical drop. Has no habitat or water quality benefits and may be detrimental.

Aesthetics Natural-appearing, aesthetic option

Less aesthetic relative to other options

Less aesthetic relative to other options

Natural-appearing, aesthetic option

Natural-appearing, aesthetic option

Maintenance Potential for scour erosion at downstream end

Potential for scour erosion at downstream end

Sediment deposition in impact basin, scour erosion at downstream end of basin

Little potential for scour erosion at downstream end

Potential for scour erosion at downstream end

Environmental Permitting

More disturbed area than other options


Less disturbed area than other options

More disturbed area than other options. Simulates naturally occurring riffle. No concrete in stream. When vegetated, supports both terrestrial and aquatic species.


Sculpted ConcreteNewbury-styleVertical

Hard BasinConcrete Sloping

Grouted Sloping Boulder

Grade Control Structure TypeConsiderations for Grade Control

Structure Design


Small Grade Control StructuresGrouted Sloping Boulder

Pros• Preferred in areas with high public

usage• More aesthetic than vertical hard

basin drop• Low maintenance

Cons• Dependent on availability of rock


Small Grade Control StructuresGrouted Sloping Boulder – Design Criteria (p. 1 of 2)

General Feature Design Parameter Parameter Value

Approach Approach length (La)

8 feet(armored with grouted rock-see below for boulder sizing)

Boulder sizing—nominal size

18 inches(acceptable range: 17 to 20 inches)

Boulder placement—crest

and cutoff

Grouted boulders must cover the crest and cutoff and extend downstream through the energy dissipating basin

Boulder placement—through drop

Boulders must be carefully placed to create a stepped appearance, which helps to increase roughness.

Boulder placement—basin

end

Boulders must be placed at basin end to create a sill transitionto downstream channel invert elevation.

Grout Grout thickness (Dg)

½ mean diameter of boulders (Dr)

Crest Crest width(minimum)

Minimum width same as upstream channel bottom width

Longitudinal slope of drop

Maximum slope 4H:1V(slopes flatter than 4:1 promote increased safety, enhanced

structure stability, and improved appearance)

Boulders


Small Grade Control StructuresGrouted Sloping Boulder – Design Criteria (p. 2 of 2)

General Feature Design Parameter Parameter Value

Basin length (Lb) 20 feet

Basin width (B) Same as crest width

Basin depression

1 foot

Upstream configuration

Trickle or low-flow channel should extend through the drop crest section

Downstream configuration

Trickle or low-flow channel protection should extend downstream from the main channel protection

Trickle zone protection width

below drop

Smaller of:i) 3 times trickle zone channel width, or ii) trickle zone channel width, squared

Energy dissipation

Install large boulders in center basin zone to dissipate energy of high flow stream

Downstream channel Downstream channel armoring

Buried riprap zone shall be installed for a minimum of 10 feet downstream of the drop basin sill

Note: Design guidelines contained in this table are for channels that meet the threshold criteria for maximum allowable flow depth and velocity.

Low-flow Zone(if necessary)

Basin Geometry


Small Grade Control StructuresGrouted Sloping Boulder – Conceptual Drawing (Plan View)


Small Grade Control StructuresGrouted Sloping Boulder – Conceptual Drawing (Profile)


Small Grade Control StructuresSloping Concrete Structure

Pros• Allows for sloped drop if boulders unavailable

• Relatively straightforward to construct

Cons• Lack of energy dissipation results in greater potential for scour at toe of structure – increased maintenance • Less aesthetic • Larger relative disturbance area


Small Grade Control StructuresVertical Hard Basin

Pros• Can construct if boulders unavailable

Cons• Less preferred in areas with high public usage• Less aesthetic


Small Grade Control StructuresNewberry-Style Structure

Pros• Interstitial rock spaces provide refuge for macroinvertebrates and small fish• More aesthetic than other hard basin structures• Simulates natural riffle• Provides energy dissipation• Naturalizes over time

Cons• Shallow slope of rock ramp results in large footprint from structure• Live stake must be planted during dormant period• More difficult to achieve shape in very small streams.


Small Grade Control StructuresSculpted Concrete Structure

Pros• Preferred in areas with high public usage• Allows for sloped drop if boulders unavailable• More aesthetic than vertical hard basin structure

Cons• Requires contractor labor force with specialized skill and/or more field oversight by designer• Less energy dissipation than grouted boulders• Does not provide habitat for macroinvertebrates and small fish


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