Methods of Data CollectionCross Section LocationCross sections along the North Fork Toutle, South Fork Toutle, Green, and Toutle Rivers were initially monumented and surveyed during the summer and fall of 1980. Cross section sites were selected using several criteria. Within the Toutle River drainage basin, many cross sections were located at bridge sites or other locations where pre-eruption survey data are available. One or more cross sections are usually located at or near gaging stations. Many cross sections were located to coincide with cross sections measured photogrammetrically from imagery taken during June and July 1980. Where either pre-eruption or photogrammetric cross section locations were reoccupied, or where cross sections were located at gaging stations, additional sections were added to form clusters of cross sections with a spacing of two to three sections per kilometer. Clusters of sections were used to monitor changes in pattern and slope (as well as cross-section shape) within specific reaches, and to sample different environments within the reaches. Thirty-four clusters, comprised of 160 cross sections are included in this report. The North Fork Toutle River drainage dominates the Toutle basin in terms of size, volcanic impact, runoff, and sediment production. Therefore, 49 percent of the channel cross sections and 71 percent of the individual surveys presented in this report are from the North Fork Toutle River basin.The network of cross section measurement sites established during the summer and fall of 1980 was subsequently expanded. Cross sections have been added as channels shifted, new channels and alluvial fans formed, or conditions developed that require more detailed measurements. Along the Bear Creek and Lower Avalanche reaches of the North Fork Toutle River accelerated bank erosion has been so extensive that all monuments for some cross sections were destroyed. When this occurred, new cross sections were monumented close to the same location but the relocation of cross sections is only approximate, with questionable elevation control. These sections are plotted separately, one survey to a plot, in order to avoid potentially erroneous comparisons. The entire network of cross sections was surveyed at the end of the rainy season from WY 1983 through WY 1985. A subset of 35 indicator cross sections is surveyed more frequently, following significant winter storm flow and volcanic events. Only the 35 indicator cross sections were surveyed during the summer of 1985.
Surveying MethodsCross sections usually span the valley floor, and are located by establishing a line of sight perpendicular to the channel, or perpendicular to the valley floor where the channel is braided. Because of severe channel shifts, cross sections may not remain perpendicular to the channel. Endpoints are monumented with metal fenceposts or sections of steel reinforcing bar. Channel cross sections are usually surveyed using electronic distance measuring (EDM) equipment and theodolite. The instruments are set up on the line with the monuments, which serve as reference marks for horizontal and vertical control. Deep gullies or vertical banks occasionally preclude precise surveying measurements within the available time, and a few points on the cross section are measured with hand level and tape. In cases where the channels cannot be waded on line with the cross section, estimates of maximum depth of the channel are made. During high flows of winter and spring, channels along much of the Toutle River and the lower North Fork Toutle River are surveyed using an air boat. The above-water portions of the cross section, and portions of the section that can be waded are surveyed as described above. Measurements of horizontal distance to the boat from reference monuments are made using either an EDM, or a tag line. Depths are sounded manually from the boat. Cross sections that are located at bridges are often measured by sounding from the bridge. Commonly, this entails use of a bridgeboard, A-reel, and 35 or 45 kg sounding weights.Field data are entered into a computer, converted to relative horizontal distance and elevation, and referenced to monuments with fixed station distance and elevation. Elevation is referred to National Geodetic Vertical Datum (NGVD) of 1929. Surveys are plotted using digital plotters, and superimposed on earlier surveys of the same cross section. Accuracy and precision of surveys are dependent on the nature of the channel, weather at the time of the survey, and surveying techniques. Surveys of wide channels are not as precise as surveys of narrow channels. Selection of surveyed points introduces a large degree of uncertainty in reproducing rough surfaces that have not changed. Inadequate visibility and atmospheric temperature and pressure fluctuations affect the accuracy of EDM measurements, and wind and saturated ground occasionally cause difficulties in precise leveling. When at least two monuments remain from previous surveys to establish the cross-section bearing, and points that are common to previous surveys are measured consistently, both horizontal and vertical precision is generally within 0.05 meters, regardless of adverse weather conditions. |
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