Part 2Part 2

Some Basic Some Basic Aspects of Aspects of CHANNEL CHANNEL

HYDRAULICSHYDRAULICS

• The volume of water that passes by any given point along a watercourse is called “Q”, for quantity of flow. It is generally expressed in units of cubic feet per second (cfs) or cubic meters per second (m3/s).

• Where:•           Q = Flow Rate, (ft3/s)•           v = Velocity, (ft/s)                •           A = Flow Area, (ft2)•           n = Manning’s Roughness Coefficient•           R = Hydraulic Radius, (ft)•           S = Channel Slope, (ft/ft)

MANNING’S EQUATION for MANNING’S EQUATION for Open Channel Flow (1889)Open Channel Flow (1889)

Hydraulic Depth Hydraulic Depth and Radiusand Radius

• In terms of frictional head losses, the perimeter is important. Hydraulic radius, Rh, is defined as the area of the flow section divided by the wetted perimeter, Pw, which is shown on the figure at left and is written as: Rh = A/Pw

Manning’s n for natural channelsManning’s n for natural channels

• For main channels with clean, straight, full stage, no rifts or deep pools navg avg

= .030

• For mountain streams with channel bed of gravels, cobbles, and few boulders navg avg = .040

• For flood plains with scattered brush, heavy weeds, navg avg = .050

• For excavated earthen channel, clean and recently completed navg avg = .018

• Trapezoidal channels are commonly excavated for Trapezoidal channels are commonly excavated for flood control because they have predictable flood control because they have predictable characteristicscharacteristics

• Over time, these man-made channels can aggrade and Over time, these man-made channels can aggrade and fill with sediment, fill with sediment, diminishing their design capacitydiminishing their design capacity

• Flow data is measured at discrete points along a watercourse, known as gaging stations. Velocity data is usually measured during high flows on stage recorders, like that shown at right. These data are compiled to create statistical databases on runoff and channel flow.

Flow DataFlow Data• Gauging stations usually

record data on channel width, depth and velocity during various flow stages

• These data can be used to calculate the quantity of flow, Q

• If sufficient data exists, a stage record can be constructed for this site which relates Q to flow velocity, depth, and width

• The hydrograph is a graphical plot of Q versus time at a given point along the stream or river. It is influenced by a number of factors, including interflow.

Impacts of Land Use Impacts of Land Use and Impermeable and Impermeable

SurfacesSurfaces

• Changes in land use and vegetation affect runoff by increasing the peak flow, causing erosion of bed and banks

• Hard, impermeable surfaces such as pavement and roofs tend to reduce the time to concentration

Runoff Runoff CoefficientsCoefficients

• The runoff coefficient depends on ground cover, land use, and antecedent moisture

• The time-to-concentration depends on slope, permeability of the ground surface, and distance to an adjacent watercourse

Terrasets caused by compaction of Terrasets caused by compaction of grazing cattle hoovesgrazing cattle hooves

Slopes cleared of vegetation for grazingSlopes cleared of vegetation for grazing

Lag TimeLag Time

• Lag time describes the time interval between the center of mass of rainfall and the runoff

• The lag time diminishes with increasing impermeable surfaces

• The lag time describes the interval between the centroid of the precipitation and the centroid of flow in the hydrograph