18-Env-B2 Water Resources · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 18-Env-B2 / Water Resources. 3 hours duration; open-book exam (any non-communicating calculator permitted). Six questions are printed; the first five as they appear in the answer book constitute a complete paper and are marked, each worth 20 marks. All six are solved below for completeness.
Reference texts. Chow, Open-Channel Hydraulics; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Linsley, Kohler & Paulhus, Hydrology for Engineers; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Freeze & Cherry, Groundwater; Fisheries Act, Canadian Environmental Protection Act, 1999; Ontario Water Resources Act and Clean Water Act, 2006 (used here as a representative province); CCME, Canada-Wide Strategy for the Management of Municipal Wastewater Effluent.
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.
Point sources discharge at a single, identifiable, typically permitted location: (i) municipal wastewater treatment plant outfalls; (ii) industrial process effluent discharges (e.g. pulp and paper mills, mine-site discharges); and (iii) combined sewer overflows (CSOs), which discharge untreated mixed stormwater and sewage from a specific outfall during large storms. Non-point sources enter diffusely across a landscape, with no single discharge point: (i) urban stormwater runoff from roads and parking lots, carrying road salt, metals, and hydrocarbons; (ii) agricultural runoff carrying fertilizer, pesticide, and manure-derived nutrients and pathogens; and (iii) failing or poorly-sited septic systems and forestry-operation runoff (sediment, nutrients) across a rural watershed.
Downstream contaminant transport in an urban stream is governed by advection (bulk downstream velocity) plus longitudinal dispersion, which lumps together every mechanism that spreads a contaminant cloud faster than molecular diffusion alone. The key controlling factors are: the cross-sectional velocity gradient (shear-driven, or “Taylor”, dispersion — the fastest core flow outruns slower flow near the banks and bed, stretching the contaminant cloud); channel geometry irregularity (pools, riffles, and dead zones behind urban infrastructure like culvert inlets and bridge piers trap and re-release mass, greatly increasing the effective dispersion coefficient); bed roughness and turbulence intensity; discharge and depth (higher flow increases turbulent mixing but shortens residence time); the degree of channelization (a straightened, engineered urban reach disperses a plume less than a natural meandering one, all else equal); and the contaminant’s own properties (sorption to bed sediment, decay rate, buoyancy/density relative to the water column). These combine in the one-dimensional advection-dispersion representation $\partial C/\partial t = -U\,\partial C/\partial x + D_x\,\partial^{2}C/\partial x^{2}$, where the engineer’s task is essentially estimating the dispersion coefficient Dx for the specific reach.
Models used for hydraulic design, environmental assessment, and flood-level prediction (e.g. hydrologic/hydraulic packages of the SWMM/HEC-RAS/HEC-HMS family) share several practical limitations in Canadian practice: calibration-data scarcity — many small urban catchments lack long-term gauged flow records to validate the model against; parameter lumping — imperviousness, infiltration, and land cover are averaged over sub-catchments that are internally quite variable; sensitivity to input resolution — results depend heavily on DEM resolution and on whether rainfall is represented by sparse point gauges versus radar; non-stationarity — design storms and IDF curves calibrated on historical records may understate future extremes under a changing climate; simplified dual-drainage representation — the interaction between the underground minor (pipe) system and the surface major (overland) system during surcharge is difficult to capture accurately; and winter/ice effects — standard open-water hydraulic routing does not represent ice-affected flow and backwater, a distinctly Canadian limitation.
Canada’s water-resources legal framework is deliberately layered across three orders of government. Federal: the Fisheries Act (fish habitat protection, prohibition on deleterious deposits), the Canadian Environmental Protection Act, 1999 (CEPA, toxic substances), the Canada Water Act, and the federal Impact Assessment Act for major projects. Provincial (using Ontario as a representative example): the Ontario Water Resources Act (water-taking permits, water-quality standards), the Clean Water Act, 2006 (mandatory source-water protection planning around municipal drinking-water supplies), the Safe Drinking Water Act, 2002, and the Conservation Authorities Act (watershed-based regulation of development near water). Municipal: stormwater management design standards and bylaws, official-plan policies that overlay provincial source-water-protection areas onto local land-use decisions, and water-conservation/efficiency bylaws (outdoor watering restrictions, low-flow fixture requirements in new construction). The CCME’s Canada-Wide Strategy for the Management of Municipal Wastewater Effluent is a useful example of how the federal and provincial/territorial levels harmonize standards rather than leaving any one level to act alone.
