18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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 longshore current forms when waves approach the shoreline at an oblique angle rather than head-on. Refraction bends the wave crests toward parallel with the shore as they shoal, but the crests never become perfectly parallel before breaking, so each breaking wave delivers momentum with a component directed along the shore (in addition to the shore-normal component that drives swash/backwash). This along-shore momentum component drives a steady current confined to the surf zone, flowing parallel to the shoreline in the direction of the net wave-approach angle.
A rip current forms because wave breaking piles up ("sets up") water within the surf zone faster than it can escape by simple backwash; this excess water must return seaward, and it preferentially does so through the lowest, most permeable point in the nearshore bar — commonly where the longshore current itself is locally deflected seaward, or at a gap/embayment in an otherwise continuous breaker bar. The return flow concentrates into a narrow, fast, seaward-directed jet that punches through the breaker zone before dissipating in a head beyond it, rather than spreading evenly along the whole shoreline.
Their effect on sediment dispersal is correspondingly different in direction and geometry. The longshore current produces sustained, one-directional sediment transport parallel to the coast (longshore drift), redistributing beach sand along the shoreline and building depositional features such as spits and barrier islands where the coastline changes orientation or sediment supply exceeds the current's transport capacity. The rip current, by contrast, is the primary mechanism that returns sediment (and water) seaward, out of the surf zone — eroding a narrow channel through the bar, locally lowering the beach profile at the rip's exit point, and depositing a small sediment lobe on the inner shelf just beyond the breaker line where the jet loses velocity.
