October 10, 1963


To: District Engineers and Staff Officials (SW)

From: Chief, Surface Water Branch

Subject: Review of indirect measurements and bridge-site reports

A meeting of the Branch Area Chiefs, hydraulics specialists, and other staff officials was held recently in St. Louis to discuss changes in operation procedures required by the recent reorganization of the Surface Water staff. Decisions made at this meeting regarding the review of indirect measurements of flood flow and bridge-site reports are given in this memorandum.

Indirect Measurements

Standard procedures for making indirect measurements of peak flood flow have been developed during the past ten years. During this period of development all indirect measurements have been review by the flood specialists. This review assured the application of the proper methods and techniques and provided training for district personnel in this important area of our work. However, because of the increased number of engineers who have now had experience in making indirect measurement and because the methods have been largely standardized, the review of all measurements by a specialist outside the district is not longer desirable. The following review procedure will be used in the future.

  1. The district engineer will designate the best qualified engineer in his district to serve as a reviewer of all measurements made in the district. The name of the individual so designed should be forwarded to the appropriate hydraulic specialist. If a competent review is not possible at this time, all measurements should be sent to the hydraulic specialist for review, but an effort should be made to train and individual to assume this responsibility as soon as possible. The hydraulic specialist will assist in this training if requested.

    Measurements which are classified as unusual or questionable by the district reviewer should be sent to the hydraulic specialist for comment. In addition, all measurements made using the methods designated below should be forwarded for review because some of these methods have not been fully developed. Measurements made to verify indirect measurements should also be sent to the hydraulic specialist.
  1. The summary of each indirect measurement will be prepared in the district with enough detail to provide a clear indication of the quality of the measurement. The attachment sample illustrates the type of information which should be contained in the summary for all types of measurements. The form is not rigid and should suit the individual measurement.

    Two copies of each summary should be sent to the appropriate hydraulics specialist.
  2. The hydraulic specialist will visit each district office in his area at least once a year. During this visit he will make a sample review of indirect measurements and answer any questions this district may have. He will serve as a consultant on indirect measurements and other hydraulic problems and may be called on for assistance as the need arises.

Bridge-site reports

A copy of all bridge-site reports prepared in the districts will be sent to the appropriate Branch Area Chief. The services of the hydraulic specialist may be utilized as needed in hydraulic analysis or in training district personnel where a new program of this type is initiated.

Although representing the Hydraulics Section in most of his technical activities, the hydraulics specialist is under the administrative direction of the Branch Area Chief.

Melvin R. Williams


A, B, S-2, FO-2, SL


Sample Indirect Measurement Summary

11-5180.5 East Fork Kalihi River at Callahan, California
Kamath River Basin
Par 11 – 1963 – Regular Station
or Crest-gage station
or Miscellaneous Site
Flood of February 12, 1963

Type of Measurement: Contracted opening, Type IV; or Flow through Culvert Type III; or 3-section Slope-Area; or Flow over dam; etc.

Location of Site: Give location with respect to the gaging station or if a miscellaneous site, give lat., long., land line location, distance from nearest tributary and nearest community.

Survey of site: List the following information:

  1. Who selected the site and date.
  2. Who surveyed the site and date or dates.
  3. Datum of the survey and RM’s used.
  4. Date of last two-peg test of instrument.

    Example: Site was selected on February 14, 1963 by S.H. Hoffard. Highwater profiles were surveyed February 17 by R. Chinn, B.R. Davies and R.H. Hansen. X-Sections were run on February 18 by R. Chinn and B.R. Davies. Gage datum + 10 feet was used and survey was referenced to RM2 and RM3 at the station. Instrument was checked by two-peg test on February 7, 1963.

Discharge and gage height: 93,400 cfs; Recorded peak gage height 17.93 feet; HWM in well 17.98 feet; and 18.35 feet from well defined highwater profile past gage (Note this paragraph MUST give both inside and outside gage heights and how they were obtained.)

Drainage area: Give Drainage area at measurement site and at gage site, and if pertinent, how discharge was adjusted to gage site.

Unit discharge: (Discharge in cfs divided by drainage area).

Nature of flood: Very brief description of the storm that caused the flood rainfall (etc.) and any other pertinent remarks.

Field conditions: Describe the reach (dam, culvert, embankment, etc.) with particular emphasis on composition of the bed material, banks, trees, vegetation, overflow. Describe in detail any evidence of scour or fill. Give a general description of the type and quality of highwater marks. List “n” values. Were x-sections subdivided, and if so on what basis? State specifically whether x-sections were located after the highwater profiles were plotted. State how many pictures were taken and type.

For dams describe conditions of the dam crest at the time of the peak (debris, flashboards, gage settings). Also how much water was bypassing the main spillway through power releases, fishways, irrigation diversion, water supply, etc.

For culverts give “n” value used, and reason (size of corrugations, condition of concrete, etc.), presence of debris or fill, entrance conditions and other pertinent facts.

For contracted openings describe approach section, contracted section, “n” values, scour, debris, skew, type of opening, submergence.


A – Slope-Area

  1. Give fall and portions that are velocity head change and friction loss.
  2. List computed discharge for each reach, state whether reach is contracting or expanding (for expanding reaches show % spread between 0% and 100% energy recovery), compute Froude numbers and % spread between subreaches.

    Example: Reach 1-2 85,300 cfs expanding, 15% spread 0% to 100%
    F1=0.75 F2-0.65
    Reach 2-3 105,000 cfs contracting F2=0.78 F3=0.81 3.
  3. Explain how the final discharge figure was determined.

    Example: Reach 1-2 discarded, excessive expansion.
    Reach 4-5 discarded because of poor profile definition and Froude numbers indicate a transition from supercritical to subcritical.
    Reach 2-3-4 103,000 cfs F2=0.75 F3=0.77 F4=0.75. This is considered the most reliable computation to use.

B – Culverts and contracted openings

  1. Give (h1 –z)/D ratio, culvert slope, entrance coefficient and type of flow. State what adjustments were made to the coefficient for wing walls, projections, radius of rounding (etc.), and whether it was corrected for contraction ratio of less than 80%. Give Froude number for the approach section and finally list the proof for the type of flow in question.

    Example: The (h1-z)/D ratio is 1.14, culvert slope .00643, then entrance coefficient is 0.86, corrected for contraction, projection and radius of rounding, and flow was identified as Type III. Froude number in the approach is 0.57 (Describe any other pertinent features) and finally Type III flow was proven because (h1–x)/D<1.5 h4/D<1.00 and h/hc<1.00.

C – Dams, embankments and critical depth.

Describe the dam (or embankment or critical depth section) in detail and state specifically how the discharge coefficient was obtained (Circular 397; USBR Bul No. 3; Kings Handbook, etc.). State what the static head is and the velocity head. Give the percent of submergence. Discuss other factors that affect the computations.

Example: The dam is broad-crested, rock-crib, timber sheathed structure. The crest is 10 feet wide and is horizontal. The upstream face is on a slope of 1 ½ to 1 and the downstream face is on a 2 to 1 slope. The average static-head of 11.53 feet was determined 35 feet upstream, from well-defined highwater profiles along both banks. The velocity head is only 0.34 feet. Tailwater elevation was poorly defined but still good enough to prove that there was no submergence. Base coefficient of 3.63 was determined from Figures 10 and 11 in Circular 397 and then raised to 3.74 because of the 6” radius of rounding of the upstream edge. Froude number in the approach section is 0.43.

Evaluation: Briefly review the conditions affecting accuracy and rate the measurement.

Example: Use 86,400 cfs and consider it a good reliability. Profiles are well defined, reach is slightly contracting, there is little evidence of excessive scour or fill and the results from the two subreaches agree within 8%.

Previous computations: List previous indirects by date of peak, type, and evaluation.

Date of Peak
Type of Measurement
December 21, 1953 Slope-area Poor
February 14, 1959 Contracted opening Fair

Remarks: Discuss how well the indirect looks on the rating and anything else that may be pertinent.


Signature (Part of sample)