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. . -e GENERAL FIELD AND OFFICE PROCEDURES FOR INDIRECT DISCHARGE MEASUREMENTS 13 be obtained in the gage well if the recorder was not operating during the flood. It is often important to obtain high-water marks at sites where gaging stations have been discontinued. This may make it possible to ex- amine the consistency of a computed discharge at another location with respect to the old stage- discharge relation. It may also provide an additional figure of peak discharge at the dis- continued site, if the rating curve is defined to that stage. Cross Sections Cross sections should be identified as section 1, 2, 3,4, etc., in downstream order. Locate cross sections as nearly as possible at right angles to the direction of flow. On large streams it may be necessary to break the cross section at one or more points to maintain the section roughly perpendicular to the flow. In slope-area measurements, the conveyance is assumed to vary unifoTIU!y between cross sections; therefore take cross sections at major breaks in the high-water profiles. Plot the high- water marks and the profile in the field before surveying the sections. Rough plotting is ade- quate for this purpose providing high-water marks have been surveyed separately on each bank along lines roughly parallel to the flow. A better method is to use a field drafting ma- chine for making the plan and either to plot the profiles by simple projection or, as for the final plotting, to refer the stationing to a base line. If the profiles appear to represent a series of somewhat regular waves, locate the cross sec- tions at each end of the selected reach at com- parable parta of the waves-both at the crest or both at the trough. It is important that enough high-water marks be available near the ends of cross sections to define the high-water elevations there. Plot- ting profiles in the field will assure that sections are located where the profiles are well defined. It may be possible to obtain additional marks, if needed, where the plottings indicate sections to be desirable. In extremely rough channels, locate the cross sections 90 as to represent average or typical conditions. Where large scattered boulders are present, the cross sections should not wholly avoid them or include a disproportionate num- ber of them. Survey The first step in defining cross sections is to drive stakes to be used as auxiliary hubs, at both ends of a cross section, and to tie the eleva- tions and locations of these hubs into previously established transit stations. In surveying the cross section, set up the transit over a hub at one end and measure the distance from the top of the stake to the horizontal axis of the tele- scope to compute the H.I. The line of sight is fixed by a sight on the hub at the opposite bank. Take rod reading to tenths of a foot at interme- diate points to define the cross section, establish- ing temporary turning points on the other side. If the cross section is short enough that a tag- line may be stretched across it, determine the depths by setting up on one of the transit sta- tions in the regular traverse, rather than by setting up over a hub on the cross section. A tagline should be used whenever possible. Take enough readings to define the major breaks in the bottom, with a minimum spacing such that not more than about 5 percent of the total area will be between any 2 sounding points. Only a few depth observations are needed in shallow overflow portions containing only a small percentage of the total area and discharge. At the edges of the stream, take rod readings to hundredths of a foot on the water surface. Determine elevations of the streambed either (1) by direct rod readings on the bottom, (2) by sounding down from the water surface and adding these distances to the average rod read- ings to water surface, or (3) by deducting the soundings from the water-surface elevation. Soundings Soundings from the water surface may be made from a boat by using a weighted line, a wading rod, or an old level or stadia rod. When sounding a rough or boulder-bed stream, do not set the rod consistently either at the top of boulders or between them at each point. Be- cause the average bed elevation is required, set;