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Regional Hydraulic Geometry Curves Procedure

Note: The following are detailed instructions on data needed to complete a stream geometry survey and assumes the user knows how to set up and run their own surveying equipment.

Small Exit SignPreliminary Activities

(Most of the links are external)

Select sites with a USGS gaging station:

Site information on USGS gaging stations may be obtained from the USGS Site Information for USA web page. Site information may be pulled by Location (County, Hydrologic Unit, or Lat-Long box), Identifier (Site Name, Site Number, or Agency Code) or Attribute (Site type, Altitude, Drainage area, or Data type). Unless the site has a drainage area of less than 10 mi2, select sites with a minimum of 20 years of record. USGS gaging stations with an area of less than 10 mi2 are rare and can contribute important data to the regional curve development, thus you should always attempt to survey such sites.

Pull USGS flow data:

Once possible sites have been selected, download Peak Annual Discharges data, sort data in ascending order and plot using a Weibul Distribution (see Weibul Distribution in the GLOSSARY). From the Weibul Distribution plot, identify the discharge at the 67% exceedance interval (1.5 year recurrence interval). USGS Peak Annual Discharge data may be downloaded for the USGS Peak Streamflow for USA web page. The USACE has developed a free mathematic graphic tool that includes multiple scaling types, including linear, logarithmic, and probability scales, called DPLOT (Note: this site is sometimes difficult to attach to; you may need to click this link several time before it connects). DPLOT is a useful tool for creating Weibul Distributions and other hydrologic plots.

Request USGS rating tables, discharge measurement notes, and station descriptions:

From the State's USGS District Office request the Discharge Rating Curve, a Measurement Note made at a discharge near the 67% exceedance interval identified on the Weibul Plot, and the Station Description for selected stations. The Rating Curve will allow you to determine discharge for a given gage height and vice versa. From the Measurement Notes you can calculate width, depth, area, etc. for a flow near the bankfull discharge. Knowing the geomorphic geometry at bankfull discharge is helpful when identifying bankfull indicators. Station Descriptions contain a variety of important ancillary information including, reference (bench) marks with gage heights, low- and high-flow controls, specific gage location, driving directions to the gage, station history, and more. Phone numbers and addresses for local USGS District Office may be obtained from the USGS Local Offices for Water Resources web page.

Obtain aerial photography

Digital aerial photography may be printed before going into the field and is helpful when deciding on reachs to survey and may be marked with survey information including the locations of

  • the gaging station
  • instrument set-up(s)
  • cross-section locations
  • reference marks
  • location and direction of photographs
  • etc.

 

Site Survey Procedures

Identify reference marks at gage

The reference (bench) marks described in the USGS station description provide vertical control for the site survey. Locate the reference (bench) marks described in the USGS station description. A known elevation referenced to gage datum should be associated with each location. Assume a north direction or measure with a compass and measure distances between reference marks using a steel tape and note the north-south (X-Y) distances between marks.

Walk site and sketch map

Walk several meander lengths both up and down stream of the gage, keeping in mind the reach selection criteria listed below. This is a good time to start identifying bankfull indicators (see discussion below). Select your survey reach, cross-section locations, and total station instrument set-up locations. This is also a good time to begin Taking photos and drawing a site map. Draw your site map in a notebook or directly on aerial photography. A good site map should include the following:

  • Station Name
  • Date
  • Survey Party
  • Location of benchmarks
  • Direction of streamflow
  • North Arrow
  • Scale
  • General site elevation
  • Landmarks near stream
  • Location and direction of photos
  • Gage reading or depth of flow
  • Explanation of map symbols
  • Terraces (height, vegetation)
  • Features (Trees, Rocks, Debris)
  • Pool / Riffle sequences
  • Gravel and sand bars
  • Gage, bridge, and road locations
  • Abandoned channels
  • Floodplain boundaries
  • Cross-section Locations
  • Weather conditions

Selection of survey reaches

  1. Choose sites with evident natural features. Features of most interest include those involved in developing and maintaining the channel, floodplains, terraces, bars, and natural vegetation.
  2. Look for evidence of physical impact on the stream, banks, or in the floodplain from fords, roads, bridges, structures, etc.
  3. The reach should include an entire meander (i.e., two bends) if possible. The length should be at least 20 times the bankfull width of the channel. Given the fairly constant relation between the width of the channel, the radii of meander bends, and the sequence of pools and riffles, this length will include a range of features sufficient to accurately characterize the stream.
  4. Locate cross-sections on straight, riffle segments between bends.
  5. Keep total station set-up locations in mind when selecting survey reaches. Good reaches are a compromise between comfort and integrity. Use your best scientific judgment.

Longitudinal Profile Survey

The longitudinal profile measures points up and down the stream channel and is important for calculating the slope of the water surface, channel bed, and bankfull indicators. Profiles should extend approximately 20 times the channel's bankfull width. Start at the gage, surveying all reference (bench) marks and the gage orifices. From the gage move either up or down stream towards your selected survey reach. All profile survey shots should be made in the thalweg, noting water depth with each shot. Take enough survey shots to define channel meandering, extremes in water depth (riffle crest and the deepest points in pools), and breaks in channel bottom slope. This is also a good opportunity to survey in bankfull indicators. Cross-Section Survey

The cross-section is the location for measuring channel form and particle size distribution. Measure all significant breaks in slope, left and right water's edge, and important bank and floodplain features. Ideally, cross-sections should be measured from valley wall to valley wall. At a minimum, cross-sections should measure two times the maximum channel depth in the cross-section at bankfull flow. Features to look for when locating the cross-sections include

  • A straight riffle reach between two meander bends
  • Clear indicators of the active floodplain or bankfull discharge
  • Presence of one or more terraces
  • Channel section and form typical of the stream
  • A reasonably clear view of geomorphic features.

Bankfull Indicators

Like stream-types, bankfull indicators vary. The active floodplain is the flat, depositional surface adjacent to many stream channels, and is the best indicator of bankfull stage. Floodplains are most prominent along low-gradient, meandering reaches. They are often hard or impossible to identify along steeper mountain streams. They may be intermittent on alternate sides of meander bends or may be completely absent. Steep, confined streams in rocky canyons often lack distinguishable floodplains, so other features must be used. Recently disturbed systems may give false indications of bankfull.

To assist in identifying bankfull indicators it is useful to know the approximate gage height (elevation) of the bankfull stage. From the material collected before coming into the field, use the 67% exceedance interval from the Weibul Distribution and the USGS Rating Table to estimate an approximate gage height of the bankfull stage.

Where floodplains are absent or poorly defined, other indicators may serve as surrogates to identify bankfull stage. The importance of specific indicators varies with stream type. Several indicators should be used to support identification of the bankfull stage; use as many as can be found. Useful indicators include

  • the height of depositional features (especially the top of the point bar, which defines the lowest possible level for bankfull stage);
  • a change in vegetation (especially the lower limit of perennial species);
  • slope or topographic breaks along the bank;
  • a change in the particle size of bank material, such as the boundary between coarse cobble or gravel with fine-grained sand or silt;
  • undercuts in the bank, which usually reach an interior elevation slightly below bankfull stage; and
  • stain lines or the lower extent of lichens on borders.

Pebble Count

The composition of the stream bed (substrate) is an important factor in how streams behave. With a pebble count you can document a quantitative description of the bed material. The most efficient basic technique is the Wolman Pebble Count (1954). A pebble count is made on a riffle bar at the cross-section surveys. The b axis (intermediate axis) is measured and recorded for 100 rocks chosen at random. Rocks (particles) are tallied by using the pebble count size classes given in the table below and then plotted by size class and frequency.

 

PEBBLE COUNT SIZE CLASSES
Inches Size Class Millimeter Inches Size Class Millimeter
  Silt/Clay <0.062 2.5-3.8 Small Cobble 45-64
  Very Fine Sand 0.062-0.125 3.8-5.0 Small Cobble 90-128
  Fine Sand 0.125-.025 2.5-3.8 Large Cobble 128-180
  Medium Sand 0.25-0.50 7.6-10 Large Cobble 180-256
  Coarse Sand 0.50-1.0 10-15 Small Boulder 256-362
0.04-0.08 Very Coarse Sand 1.0-2.0 15-20 Small Boulder 362-512
0.08-0.16 Very Fine Gravel 2-4 20-40 Medium Boulder 512-1024
0.16-0.24 Fine Gravel 4-5.7 40-80 Large Boulder 1024-2048
0.24-0.31 Fine Gravel 5.7-8 80-160 Very Large Boulder 2048-4096
0.31-0.47 Medium Gravel 8-11.3   Bedrock  
0.47-0.63 Medium Gravel 11.3-16  
0.63-0.94 Coarse Gravel 16-22.6
0.94-1.26 Coarse Gravel 22.6-32
1.26-1.9 Very Coarse Gravel 32-45
1.9-2.5 Very Coarse Gravel 45-64

Photography

Photos are one of the best methods for documenting the current stream conditions. Note the location and direction in which photos are taken on the field map and in the field notebook. Photos should be made of the following

  • Gaging station & bridge
  • Up- and down-stream views of reference reaches
  • Cross-sections
  • Typical stream bars and banks
  • Bankfull indicators

References

Dunne, T. and L. Leopold, 1978, Water in Environmental Planning: W.H. Freeman, New York, 818 pp.

Emmett, W.W., 1975, Hydrologic evaluation of the upper Salmon River area, Idaho: U.S. Geological Survey Professional Paper 870-A, U.S. Govt. Printing Office, Washington, D.C.

Harrelson, C.C., C.L. Rawlins, and J.P. Potyondy, 1994, Stream Channel Reference Sites: An Illustrated Guide to Field Technique: USDA Forest Service, General Technical Report RM-245, 61 pp.

Leopold, L.B., 1994, A View of the River: Harvard Univ. Press. Cambridge, MA.

Rosgen, D.L., 1996, Applied River Morphology: Printed Media Companies, Minneapolis, MN.

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