Part 2 Some Basic Aspects of CHANNEL HYDRAULICS

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 (m 3 /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 Open Channel Flow (1889)

Hydraulic Depth and 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: R h = A/P w

Manning’s n for natural channels avgFor main channels with clean, straight, full stage, no rifts or deep pools n avg =.030 avgFor mountain streams with channel bed of gravels, cobbles, and few boulders n avg =.040 avgFor flood plains with scattered brush, heavy weeds, n avg =.050 avgFor excavated earthen channel, clean and recently completed n avg =.018

Trapezoidal channels are commonly excavated for flood control because they have predictable characteristicsTrapezoidal channels are commonly excavated for flood control because they have predictable characteristics Over time, these man-made channels can aggrade and fill with sediment, diminishing their design capacityOver time, these man-made channels can aggrade and fill with sediment, diminishing 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 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 and Impermeable Surfaces 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 Coefficients 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 grazing cattle hooves Slopes cleared of vegetation for grazing

Lag 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