North Carolina Department of Environment and Natural Resources
Division of Water Resources
Instream Flow Unit


( We strongly recommend that you read all of these notes before undertaking a wetted perimeter instream flow study. They contain suggestions pertaining to field data collection as well as running the model and analyzing results. )

  1. Roles/Responsibilities

  2. Wetted Perimeter

  3. Incremental Wetted Perimeter..........example

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These guidelines describe the roles and responsibilities of the developer and/or consultant, as well as the agencies involved in conducting instream flow studies utilizing the Wetted Perimeter Method. The agencies include: the Division of Water Resources (DWR), the Wildlife Resources Commission (WRC), the U.S. Fish and Wildlife Service (USFWS), and others depending on the relevant instream uses at a particular location.

  1. Study site and cross-section (transect) locations will be selected by the agencies. The developer or consultant is encouraged to participate.
  2. The developer or consultant will provide the following equipment and supplies for conducting the study:
    • steel re-bar or galvanized pipe for staking out transects, two stakes, each at least 18" long, per transect;
    • both 1 and 3 inch PK masonry nails (3/16" diameter) for benchmarks, one per transect;
    • survey flagging and paint;
    • data books;
    • surveying equipment, including: automatic level or transit, tripod, rod, 150'fiberglass tape marked in tenths of a foot;
    • machete and/or bush axe for clearing line of sight and access;
    • Price AA flow meter or equivalent.
  3. The developer or consultant will have to provide at least two persons with surveying experience for each data collection team.
  4. Transect profiles will be surveyed by the developer or consultant with one or more of the agencies in attendance. Either DWR, WRC, or USFWS will determine the location of all points to be surveyed on each cross-section.
  5. Photographs of each transect will be taken, with tape in place, by DWR during the initial data collection. Photos with identical orientation will be taken during subsequent data collections by the developer or consultant. DWR will provide documentation of photos to be taken.
  6. At least three discharge/water surface elevation data sets will be collected by the developer or consultant. The range and distribution of discharges will be specified by DWR. Stage-discharge curves should also reflect the stage at zero flow for each transect.
  7. The developer or consultant will notify either DWR, WRC, or USFWS at least five working days in advance of any planned data collections.
  8. DWR, WRC, and USFWS staff will map instream habitat to assign weighting factors to each transect.
  9. The consultant will provide DWR with copies of all raw data - survey notes, discharge measurements, staff gage readings, etc. - within 15 days after the third data set is collected.
  10. The Division of Water Resources will provide the wetted perimeter and discharge calculation programs to the developer or consultant by the best available technology. An IBM compatible personal computer is necessary to run this program. Further documentation of the software will also be provided by DWR.
  11. The final report submitted by the developer or consultant should include the following:
    1. Listings of the output files for discharge calculation.
    2. Listings of the output files from running the wetted perimeter program for each transect.
    3. A table showing discharges and the water surface elevations for each transect by the collection dates. Also include the stage at zero flow for each transect.
    4. The following plots:
      • Transect profiles - distance (x-axis), bottom elevation (y-axis)
      • Arithmetic stage versus discharge - discharge (x-axis), water elevation (y-axis)
      • Wetted perimeter - wetted perimeter (x-axis), water elevation (y-axis)
    5. A diskette containing the input data files for the wetted perimeter program.
    6. Labeled copies of all slides and/or photos.
    7. Work-up of the overall wetted perimeter recommendation, including explanation of how points of inflection were selected.

Additional information related to these guidelines or the Wetted Perimeter Method can be obtained by contacting Fred Tarver (919/715-5442) or Jim Mead (919/715-5428), Division of Water Resources, Department of Environment and Natural Resources, 1611 Mail Service Center, Raleigh, North Carolina 27699-1611.

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(The Division of Water Resources assumes no liability for the output, results, subsequent analysis or support of these programs.)


The WETTED, QNEW, and Q-CALC programs are written in FORTRAN and should be run on an IBM compatible PC.

Use any line editing software to create input files for either program. Data can be entered in free format. The input file name must be filename.DAT or the models will not recognize it. Creating the input files using spreadsheet software such as LOTUS will also work if the finished input file is exported as a .PRN or ASCII file before attempting to use it as input.

View the Q-CALC.DOC or QNEW.DOC files for instructions for creating input files and running the Q-CALC and QNEW programs.

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Structure of input files for the Wetted Perimeter model:

Note: All distances and elevations are assumed to be in feet.

You may enter the height of instrument used during the survey on line 2 and then foresights for the y variables. Or, if you wish to enter actual elevation as the y variable, enter 0.0 as the instrument height on line 2.

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( Click here to see an example of Wetted Perimeter input (*.DAT) file. )

To run the Wetted Perimeter model:

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Wetted Perimeter output file structure:

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( Click here to see an example of Wetted Perimeter output (*.OUT) file. )

This model was developed by the Division of Water Resources, North Carolina Department of Environment and Natural Resources, 1611 Mail Service Center, Raleigh, NC 27699-1611

Contact Jim Mead at 919/715-5428 or Fred Tarver at 919/715-5442 if you have any questions.

We assume no liability for the output, results, subsequent analysis, or support of any of the models provided.

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These notes refer to the Division of Water Resources work-up sheet. The sheet should be completed as follows:

Transect - #1 through however many comprise the study site.

Elevation - This is a crucial part of the analysis. This elevation is for the water surface at the POI for the wetted perimeter versus water elevation relationship (from the *.OUT file). It should be determined to the nearest .01 foot. It helps to graph this, and the DELTA WP column in the *.OUT file is also a good indicator. Look for the elevation where the change in wetted perimeter drops off significantly.

What if there is no clear POI or what if it is at a very low flow? The 7-day, 10-year low flow (7Q10) is used as the lower cut-off point at each cross-section. If the cross-section is a pool and there is no POI, the elevation at 7Q10 is used. Also, if the POI is very low, the elevation at the 7Q10 flow is used in its place, regardless of whether the cross-section is a pool.

Sometimes there will be more than one POI, or no POI, or a gradual drop-off in DELTA WP that is more of a "range of inflection." If the cross-section is not a pool (where the elevation would default to the 7Q10) the investigator must use his or her professional judgement as to what water level is needed to adequately maintain aquatic habitat. Two pieces of information are useful here. The first is substrate data collected in the field at each point on the cross-section. This can be used to determine what portions of the channel need to be inundated and how deeply. For example, is that broad flat area which results in a POI an important gravel spawning bed, or is it a bedrock slab?

The other important factor in selecting the water elevation is the discharge at that elevation. Is this flow, in cubic feet per second (cfs), below the 7Q10, or very high, or within the range of reasonable flows?

If selecting a water elevation is difficult for a particular cross-section, we may run through the calculations more than once, using different water elevations to see what effect they have on the overall recommendation for the site.

Q (discharge) - This is the flow in cfs at the water elevation selected. Developing a good elevation vs. discharge relationship for each cross-section is very important to assure validity of the wetted perimeter analysis. There should be at least 3 water surface elevation vs. discharge data points for each cross-section. These should be spread over a range of flows that preferably encompass the water elevations recommended for each cross-section (it is better to interpolate than extrapolate). A fairly low flow data point is often needed to determine the elevation at 7Q10 flow.

Note: once the bottom profile is surveyed at each cross-section, the WETTED.EXE program can be run and preliminary elevations can be selected for each section. These preliminary elevations can help guide the selection of discharges at which to measure the remaining water elevation versus discharge data points.

Discharge at the study site should be measured at the most suitable cross-section. We look for a cross-section that has a fairly uniform bottom profile, with an even distribution of velocities, and the current all moving in the same direction (no eddies or angled flows). It is also preferable to find a section where velocities are not very slow or very fast, to reduce measuring error. If none of the habitat cross-sections are suitable, it may be necessary to locate a separate one just for measuring discharge. If no such cross-section is available, then it is preferable to measure discharge at more than one habitat cross-section and average the results.

Both discharge measurement and surveying of water surface elevations must be done under steady flow conditions.

Wetted Perimeter - This is the amount of wetted perimeter, in feet, at the water elevation selected. If the elevation selected is between two elevations in the *.OUT file table, just interpolate. For example, the table may list level = 96.00 with wetted perimeter = 10 feet, followed by level = 96.10 with wetted perimeter = 20 feet. If the elevation selected was 96.05, then the wetted perimeter would be 15 feet.

Q' - This is 80% of the discharge recommended for a particular cross-section. Or more simply, 0.8 times the discharge in the Q column.

Elevation' - This is the water elevation at a discharge equal to Q'. It is determined from the elevation versus discharge curve constructed for each cross-section (to the nearest .01 foot).

Wetted Perimeter' - This is the amount of wetted perimeter at elevation'. It is determined from the *.OUT file table and can be calculated by interpolation if necessary.

Sensitivity Coefficient - The purpose of this coefficient is to weight each cross-section according to how sensitive (how much wetted perimeter is lost) if the discharge is reduced. The lower the sensitivity, the lower this coefficient and the less weight the cross-section is given in calculating the overall recommended flow. The coefficient should be determined to 4 decimal places.

The formula for calculating the sensitivity coefficient (SC) is:

SC = ( Wetted Perimeter - Wetted Perimeter' ) / Wetted Perimeter

Representative Stream Coefficient - This coefficient is a weighting factor for how much of the stream segment being studied is represented by each cross-section. The sum of these coefficients for all cross-sections must equal 1.0. Each should be carried to four decimal places.

The coefficient is calculated by measuring the length of stream associated with each cross-section and dividing by the total length of the study site. Depending on how the site was set up, these lengths may be the distance upstream and downstream from a cross-section that "goes with" that cross-section. Alternatively, there may be scattered portions of a diverse stream which are all represented by a particular cross-section. The total length of these scattered pieces divided by the site total yields the coefficient. We refer to this latter approach as habitat mapping.

If a very long stream segment is being studied, or if there are physical difficulties with actually measuring stream lengths, we may use other approaches. These include measuring from the side of the stream or from roads which closely follow the stream. Or having done a reconnaissance of the stream to select the site and cross-sections, we may use notes, photos, and recollections to assign percentage weights to each cross-section which total 1.0.

Composite Factor - This is obtained for each cross-section by multiplying the Sensitivity Coefficient by the Representative Stream Coefficient. Carry to four decimal places. This is part of the algebraic process for calculating the overall recommended flow which properly weights each cross-section.

Q - Recommended discharge in cfs for each cross-section. This number is the same as in the first Q column.

C x Q - This is the Composite Factor multiplied by the recommended discharge. Carry to four decimal places.

Overall Discharge - This is the recommended flow for the site in cfs. It is calculated as follows:

( Click here to view and retrieve Wetted Perimeter work-up sheet. This is a .pdf file and will require Adobe Acrobat Reader to view.)

The wetted perimeter computer spreadsheet, STRAITWP.123, is a Lotus program. The spreadsheet is convenient if there is a large number of transects, or if there are alternative POIs to be considered.

The columns "Transect #", "Elev @ POI", "Transect Length", "Q", "Elev'", "Pre-Step Elevation", "Pre-Step WP", "Post-Step WP", "Pre-Step Elevation'", "Pre-Step WP'", and "Post-Step WP'" are entered by the user from field notes, discharge calculations, wetted perimeter output file, and plots of water elevation versus wetted perimeter. The remaining columns are calculated with equations which use values in "Pre-Step" and "Post-Step" columns.

The "Pre-Step Elevation" values are found in the wetted perimeter *.OUT file on the line above the value that matches the "Elev @ POI". If no value matches the "Elev @ POI", then use the value on the line above where the "Elev @ POI" would be inserted. The "Pre-Step WP" contains the wetted perimeter value that corresponds to the elevation in the "Pre-Step Elev" column. The "Post-Step WP" column contains the wetted perimeter value that corresponds to the elevation on the line below the "Pre-Step Elev" value. If the "Elev @ POI" matches a value in the "Pre-Step Elev" column, then its corresponding value in the Wetted Perimeter column would be used as the "Post-Step WP".

The "Pre-Step Elevation'", "Pre-Step WP'", and "Post-Step WP'" columns are completed using the same procedure except that the values in the "Elev'" column are used as the points of reference.

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( Click here to see an example of a Wetted Perimeter spreadsheet file. )

Once the overall recommended flow has been calculated it is desirable to check the effect of this flow on each cross-section. Determine the water elevation at each cross-section with the elevation versus discharge curves at the overall recommended flow. How does the cross-section appear at this water elevation? What depths of water are provided? Are any areas of valuable habitat de-watered?

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The Incremental Wetted Perimeter Method makes use of three types of information:

  1. Wetted perimeter versus water surface elevation for each cross-section. These are taken from the output files (*.OUT) from the wetted perimeter model.
  2. Water surface elevations at the discharges, in cubic feet per second (cfs), of interest for each cross-section. The list of discharges must be the same for each cross-section. Note: It is important to remember that the range of water surface elevations in these tables must be within the range of water surface elevations in the corresponding water elevation versus wetted perimeter table. The values for these tables are determined from the water elevation versus discharge curves developed for each cross-section during wetted perimeter analysis. The "flows of interest" should reflect the negotiating range, recommendations from other methods, and a range to fully define the wetted area versus flow relationship and curve.
  3. The length of stream, in feet, represented by each cross-section.

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The finished spreadsheet calculates tables of wetted area in square feet at each cross-section at the flows of interest. A table of the total wetted area for the entire site at the different flows is also produced.

( Click here to see a clipping of output from an incremental wetted perimeter study. )

A plot of this last table for the entire site shows the effect of increments of flow on the total wetted area available. This can be used to evaluate the effects of increasing or decreasing flows - a useful tool in considering trade-offs and negotiating a recommended flow regime. In contrast, the wetted perimeter model alone produces a single recommended flow.

Remember to also check the effects of increments of flow on the wetted area at each individual cross-section. The table/curve of total wetted area for the site may mask undesirable effects at cross-sections which are more sensitive to reduced flows or have a relatively short length of associated stream.

Before beginning, make sure that directory for study site exists on your hard drive, format a blank computer disk, have hard copies of all *.OUT files (wetted perimeter output files) on hand, and have stage/discharge curves for all transects.

Open the Incremental Wetted Perimeter spread sheet Increm.123 in Lotus 1-2-3. Move to cell B25 into which the first transect's *.OUT file is copied.

Please send questions or comments to the Instream Flow Unit, which created and maintains this site.

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