18-Geol-B2 Terrain Analysis · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Geol-B2 Terrain Analysis. Three-hour, open-book exam; approved Casio/Sharp calculator permitted. The paper prints SIX questions; per the instructions only the first five as they appear in the answer book are marked (100 points total), but all six are answered below so the set stands as a complete study resource.
Reference texts: Lillesand, Kiefer & Chipman, Remote Sensing and Image Interpretation (7th ed.) — primary reference for spectral/spatial/radiometric resolution, radar imaging geometry, Landsat sensor comparisons, and image-interpretation elements (Q1, Q2, Q3, Q5); Sabins, Remote Sensing: Principles and Interpretation (3rd ed.) — radar depression-angle geometry, albedo, atmospheric correction, Landsat 8 TIRS (Q1, Q2, Q5); Mollard, J.D. & Janes, J.R., Airphoto Interpretation and the Canadian Landscape (Energy, Mines and Resources Canada, 1984) — the exam's own required reference for the stereopair interpretation questions (Q4, Q6); Van Zuidam, Terrain Analysis and Classification Using Aerial Photographs — slope-form and karst terrain-classification context (Q1, Q6).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
a) River flow direction and evidence [2.5 marks]. On a meandering alluvial river, flow direction is read from several independent, mutually reinforcing clues rather than any single feature: (i) point bars (light-toned, low, sparsely vegetated depositional sand/gravel bars) form on the inside (convex) bank of each meander bend, while the outer (concave) bank shows a fresh-cut scarp — tracing the point-bar/cut-bank asymmetry along successive bends and connecting them gives the downstream sense; (ii) tributary streams join the main channel at an acute angle opening in the downstream direction ("confluence angle" rule); (iii) elongated mid-channel bars and islands taper to a point on their downstream end, where flow reconverges and deposits a narrowing sediment tail; (iv) any visible sediment plume or turbidity front points downstream from its source. Applying these criteria to the specific bends, tributary junctions, and islands visible on the candidate's copy of Plate 4-19 fixes the flow direction unambiguously.
b) Island stability over time and evidence [2.5 marks]. Island stability is read from vegetation maturity and margin sharpness. An unstable, recently reworked island shows bare or sparsely vegetated sediment, pioneer species only (willow/alder shrub thickets, no mature forest canopy), a low relief close to bankfull stage, and sharp, unvegetated, actively eroding or depositing margins that mimic the adjacent active channel bars. A stable, long-lived island shows a closed mature forest canopy matching the surrounding floodplain terrace in species composition and crown texture, a well-developed soil/vegetation-stabilized surface standing distinctly above the active channel, and smoothly vegetated margins with no fresh scarps or bare point-bar sediment — i.e. it reads on the photo essentially the same as the adjacent, undisputedly stable floodplain terrace rather than as an active bar.
c) Erosion evidence along the slope adjacent to the river [5 marks]. Because this plate specifically depicts bottleneck earthflows, the expected erosional signature is the classic bottleneck-earthflow morphology superimposed on the valley wall: a bowl-shaped source area at the top of the slope bounded by a fresh, arcuate headscarp (a sharp break in the tonal/textural pattern where undisturbed forest cover is abruptly truncated); a constricted, narrowed "neck" below the headscarp where the failed mass is funnelled through a topographic pinch point (giving the feature its "bottleneck" name and appearing as a hummocky, disturbed band narrower than the source bowl above or the toe below); and a lobate, hummocky toe that spreads out again as it reaches the valley floor, often encroaching on or directly loading the active river channel. Other corroborating evidence of active slope erosion/mass movement: disturbed, hummocky micro-relief and disrupted drainage within the flow mass (contrasted with smooth, undisturbed forest terrain outside it); tonal contrast between bare/disturbed ground and surrounding vegetated slope; tilted or "jack-strawed" trees within the moving mass; secondary scarps and pressure ridges within the flow body; and a fresh, unvegetated toe actively depositing sediment into or against the river, consistent with (b)'s sediment-plume evidence.
d) Points # 3–4, # 10–14, and # 15–16 [10 marks]. Without the plate itself, exact point-by-point identification cannot be certified (see the source note above); the defensible exam-day approach is to apply the earthflow zonation established in part (c) to whichever position each numbered point occupies on the actual photo: points positioned near the top of the slope, inside the bowl-shaped scarp area, mark the source/headscarp zone — describe the arcuate scarp, disturbed ground, and truncated vegetation there. Points positioned along the narrow, elongated mid-slope band mark the transport ("bottleneck") zone — describe the constrained, channelized, hummocky texture and any lateral shear margins or levee-like ridges bounding it. Points positioned at the base of the slope, in the broad lobate area reaching the river, mark the toe/depositional zone — describe the spreading, hummocky debris apron and its contact with the active channel (fresh sediment intrusion, deflected flow, or a new point bar forced by the encroaching toe). A candidate working from the real plate reads which of these three zones each numbered point falls in and names the corresponding earthflow element and its diagnostic photo-tone/texture signature, following exactly the framework above.
