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. . . GENERAL FIELD AND OFFICE PROCEDURES FOR INDIRECT DISCHARGE MEASUREMENTS 3 termination of the flow can be made. Every site is a distinct hydraulic problem, and a thorough knowledge of hydraulic principles is essential to proper selection. Ideal conditions for such determinations rarely exist, and judg- ment must be used in choosing the most favor- able of the possible sites by weighing advan- tages and disadvantages of each. It is possible sometimes to preselect indirect- measurement sites for gaging stations. The possible sites might differ depending upon the flood stage. A listing of these sites on the field- station description would keep this information in the most easily accessible place. By such a procedure vital time would be saved following a major flood. Unless it is known th..t favorable conditions for indirect measurement exist near the gage, preliminary selection of sites can usually be most easily made from either topo- graphic maps or aerial photographs. After preliminary selections have been made from maps or aerial photographs, or if the avail- able maps show no definite choice of sites, then field reconnaissance is necessary for making the selection. Under poor conditions, it may be necessary to explore miles of river channel to find a favorable reach. The final selection of site should always be dependent on field in- spection. Where extensive flooding occurs, reconnais- sance by air has been used to locate indirect- measurement sites. As the terrain is viewed from the air, likely sites and access routes may be marked on a map. It is important that no major tributaries enter between the measuring site and the point at which the discharge is desired. Minor tribu- taries may carry negligible flow at the time of the mainstream peak and thus not affect the result. If the measuring site is at some distance from the gaged point, then even with no ap- preciable inflow there may be a significant channel-storage correction. If the storm pro- ducing the flood covers the basin, the peak may increase in a downstream direction; if the storm covers only the upstream part of the basin, the peak may decrease in the main channel. Dis- tance from the gaging point becomes more im- portant for smaller drainage areas and for sudden floods of short duration. Adj ustments can be made, but unless detailed information of the flood wave or inflow rate is recorded, the ad- justments are necessarily arbitrary. Because of these uncertainties, it is desirable to keep the measuring site close to the point at which the discharge is wanted, and it is sometimes prefer- able to accept less favorable conditions at a site nearer to the gage. Field Survey The field survey should be made with a high degree of care, giving particular attention to using all possible checks to avoid error. Various instruments have been used for making the field survey, but experience has shown that an engineer's transit is best suited for the job. It is recommended that a transit be used to make a "transit-stadia" survey. This method combines vertical and horizontal con- trol surveys in one operation, is accurate, simple, and speedy. Surveys have been made by level-and-tape and by planetable, but these are not recom- mended. The first does not provide the exact locations of high-water marks and channel features that are necessary, and the second is not suited for work in rough terrain, in high wind, or in rain. Also, in anyone office indirect measurements are made at infrequent intervals and personnel cannot maintain expertness in all types of instruments and surveys. As the transit-st..dia method is believed best, only this type is recommended. Vertical control If the measuring site is near a gaging station, the survey datum shonld be gage datum, or gage datum plus a convenient constant, such as 10.00,20.00 or 100.00 feet, to avoid possible neg- ative elevations. Otherwise, an arbitrary ele- vation may be assumed either for a reference mark, the first hub, or the first H.J. If the sur- vey datum is not gage datum, reference marks of a permanent nature should be established to permit recovery of the datum years later, if necessary.