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a. Cross Sections. Stream cross sectional data acquired from a field survey at the site are highly desirable. At least three cross sections should be taken to establish the stream slope, the culvert inlet, the culvert outlet, and the configuration of the natural channel. (figure II -6) Sections should be taken (I) about 100 ft (30 m) upstream from the crossing, (2) at the centerline of the roadway, and (3) about 100 ft (30 m) downstream from the crossing. The natural streambed width and side slopes, and the floodplain width may be obtained from these cross sections. The cross-sectional data will also help to verify the accuracy of existing topo- graphic maps. If significant ponding is , / /" j sary to establish downstream water level (tailwater) conditions. j i .. If only one cross section' of the natu- ral channel is available, it will be used as the typical cross section. This assumption should be checked using topo- graphic maps and aerial photos. Addition- al information on stream slope and up- stream storage volume should also be obtained from the topographic maps. ~ , j J I . . 1 .. j b. Stream SloDe. The longitudinal slope of the existing channel in the vicinity of the proposed culvert should be defined in order to properly position the culvert in vertical profile and to define flow characteristics in the natural .. j . i " j j . , . '~HIGHWAY . .. ~ J ~ . ~ i . . CROSS ...... SECTION, ........... J/ ""'-/ l' ,,' '", ,. " , Figure 1I-6.--Cross section locations. likely, additional sections may be neces- sary to determine the storage capacity upstream of the culvert. Likewise, addi- tional downstream sections may be neces- stream. Often, the proposed culvert is positioned at the same longitudinal slope as the streambed. Cross sections will provide streambed elevations at the deep- 20