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18-Geom-B1 Digital Terrain Modelling · December 2014

Question 7 of 12: DEM for Watershed and Floodplain Boundaries

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format: National Exams, December 2014 — 3 hours, closed book (any non-communicating calculator permitted). TWELVE numbered questions constitute a complete paper; each is of varying value and the schedule totals 100 marks. Most answers are required in essay format, so clarity and organization are graded. All twelve questions are solved below for completeness.

Reference texts: Li, Zhu & Gold, Digital Terrain Modeling — Principles and Methodology (CRC Press, 2005); Maune (ed.), Digital Elevation Model Technologies and Applications: The DEM Users Manual (2nd ed., ASPRS, 2007); Wilson & Gallant, Terrain Analysis — Principles and Applications (Wiley, 2000); Wolf, Dewitt & Wilkinson, Elements of Photogrammetry with Applications in GIS (4th ed., McGraw-Hill); Natural Resources Canada High-Resolution DEM (HRDEM) and CDEM product specifications. Canadian datums throughout (NAD83(CSRS), CGVD2013).

Check / source note. The Question 1 header carries 9 marks but its annotation reads “(3 × 2 marks)” = 6. The header value is authoritative because it is the figure that makes the printed schedule sum to the stated 100 Total marks; the “3 × 2” is a typographic error for 3 × 3. Answers are graded on all three parts of Q1 equally.

Question 7: DEM for Watershed and Floodplain Boundaries (10 marks)

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.

Given. A bare-earth DEM of a drainage basin and its river reach.

Find. How the DEM yields (a) a watershed (catchment) boundary and (b) a floodplain boundary.

(a) Watershed boundary. A watershed is delineated by hydrologic terrain analysis on the DEM: (1) fill spurious sinks/pits so water can drain; (2) compute a flow-direction grid, assigning each cell the direction of steepest descent to a neighbour (the D8 algorithm, eight directions); (3) accumulate flow to build a flow-accumulation grid, whose high-value cells define the stream network; (4) pick an outlet (pour point) on that network; (5) trace upstream every cell whose flow path reaches the outlet—the set of contributing cells is the catchment, and its outer edge is the watershed boundary. The boundary follows the ridge lines (drainage divides) where flow directions diverge on either side.

(b) Floodplain boundary. A floodplain boundary is the intersection of a flood water-surface elevation with the terrain. From a design flood (a given return-period discharge), a water-surface profile is obtained—in the simplest “bathtub” approach a single stage, or more rigorously a hydraulic (HEC-RAS-type) water-surface profile along the channel. The DEM is then queried: every cell whose ground elevation is below the local flood water-surface elevation and hydraulically connected to the channel is flooded. The boundary is the contour where DEM elevation equals the flood stage—i.e. subtract the water-surface grid from the DEM and extract the zero crossing of the connected inundated region. Accuracy hinges on the DEM's vertical accuracy because a small height error moves the boundary a large horizontal distance on flat floodplains.