There is some vegetation in the channel in places, brush and trees on the banks are submerged at high stages. At the survey location, Dry Creek can be considered a mountain stream with steep banks. Here, you will use this published table provides ranges of n for different stream descriptions. Estimate ‘n:Manning’s Roughness Coefficient’.‘ΔElevation avg‘ between the UPstream and DOWNstream water surface elevationsįor each of the cross-sections in its corresponding red table, complete the following.Calculate the ‘Cross-sectional Area (m 2)’ of the flood flow by multiplying the width of each section and the average flood water depth of each section.Calculate the ‘Wetted Perimeter (m) of the channel by using Pythagorean theorem at each channel section as illustrated in figure on the right.Calculate the ‘Flood Water Depth (m) for each station in column G. ![]() Enter the calculated value in the ‘Average FLOODline Elevation (m)’ of the red table of the appropriate UPstream/DOWNstream cross-section. Calculate the average elevation of the high water marks on the right and left banks. High water marks (a.k.a FLOODline) are identified in the ‘Field Notes’ column.In the workbook provided, these cross-sections are automatically plotted in the “FloodFlow Calculation” tab. The field crew identified the flood water marks on each bank and then, two locations were used to survey the shape of the flooded channel resulting in an UPstream cross-section and DOWNstream cross-section. The instructions below outline specific steps to complete the exercise based on the fieldwork which was conducted near Lower Gauge. The distance and elevation drop between cross-sections along the stream were also surveyed to calculate channel slope.Įstimate the peak flood flow at DCEW Lower Gauge from the Janustorm event. Each cross-section survey was completed by setting a survey instrument in line with the cross-section, and then measuring the elevation of the ground surface or stream bed at increasing distances away from the station, perpendicular to the channel. The field crews then selected two locations along the channel reach and measured the shape of the flooded cross-sections. Indicators of high water included debris clinging to brush, erosion scours, and anything else that gave clues to the highest height that water reached. To do so, field crews walked a section of the stream near the Lower Gauge and identified high water marks. Instead of using the velocity-area gauging method, we used Manning’s equation in a forensic sense to estimate discharge. We know the stage from our instream datalogger, but nobody was in the stream making measurements during the flood. This flood presented an opportunity to add a very high water point to the rating curve. The combined rain and snowmelt produced a depth of streamflow approximately 2 ft higher than the highest stage for which we had a discharge measurement, rendering the stage-discharge relationship for the Lower Gauge unreliable (see Streamflow Measurement Exercise for an explanation of stage-discharge relationships). On Januapproximately 2 inches of rain fell on the Dry Creek Experimental Watershed, much of which was covered by snow. Ultimately, n is a site-specific factor that is determined by the scientist/engineer. An internet search will reveal many different ways to estimate n. Manning’s roughness, n, is a factor that characterizes the channel resistance. ![]() The wetter perimeter is the distance along the channel bottom between water levels at each bank. The hydraulic radius is computed as the cross-section area, divided by the wetted perimeter, P as indicated in equation 3: ![]() The channel discharge can be computing by multiplying U by the channel cross section area ( A), Schematic showing the different variables used in Manning’s Equation Where R is the hydraulic radius (L), S is the channel slope (L/L), n is Manning’s roughness coefficient, and k is a unit conversion factor ( k = 1 if R is in meters and U is m/s k = 1.49 if R is in feet and U in ft/s). Manning’s equation calculates the average velocity, U, of uniform flow in an open channel based on channel properties:
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