Question 3 of 7

Problem 3 (20 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.

(i) Stormwater vs. wastewater collection system design/O&M differences (8 marks)

(1) Design flow basis. Stormwater systems are sized from an intensity-based design storm (Rational Formula or a hydrograph method, at a selected return period), a highly variable, intermittent flow; wastewater (sanitary) systems are sized from population-based average and peak dry-weather flow (per-capita generation rate × peaking factor), a comparatively steady, continuous flow with a predictable diurnal pattern — this fundamental difference in flow character drives every other design decision.

(2) Self-cleansing velocity and flow regime. Storm sewers are typically designed to flow full (or nearly full) only during the rare design storm and otherwise run at low depth; sanitary sewers are designed to maintain a minimum self-cleansing velocity (typically ≥0.6 m/s) at ordinary (not peak) flow, because continuous solids deposition at low velocity causes odour, corrosion (H₂S generation) and blockage risk that a stormwater system, conveying mostly clean runoff, does not face to the same degree.

(3) Public health / containment requirement. Wastewater collection must be a fully closed, leak-tight system (infiltration/inflow and exfiltration are both design failures, since exfiltration risks groundwater contamination and infiltration overloads treatment) with mandatory odour and corrosion control (ventilation, materials resistant to H₂S); stormwater systems, conveying non-sewage runoff, have far less stringent leak-tightness and no treatment-capacity protection requirement, and may discharge directly to a receiving water without further treatment (subject to water-quality controls).

(ii) Reasons to use a sanitary forcemain and pumping station (4 marks)

(1) Overcoming unfavourable topography. Where a continuous gravity grade from the collection area to the treatment plant or a downstream trunk sewer is not achievable without excavation depths that become structurally and economically impractical (deep trenching, dewatering, shoring), a pumping station lifts the wastewater into a pressurized forcemain, allowing the collection system to restart at a shallower, gravity-feasible depth on the discharge side — this is the single most common reason forcemains exist.

(2) Serving a low-lying or isolated development. A subdivision, industrial site or satellite community that sits at or below the hydraulic grade line of the regional trunk sewer cannot physically drain to it by gravity at all; a local pumping station and forcemain are the only way to connect that development to the regional wastewater system without a standalone (and typically inferior) on-site treatment facility, making growth in low-lying areas serviceable within the existing regional infrastructure.

(iii) Predicting runoff from rain versus snow melt (8 marks)

(1) Timing and rate-controlling process. Rainfall-runoff timing is controlled almost entirely by the storm's own intensity-duration pattern (minutes to hours), so peak runoff can be predicted directly from an IDF curve and the catchment's time of concentration; snowmelt runoff instead depends on the pack's energy balance (solar radiation, air temperature, wind, and for rain-on-snow events, advected heat from the rain itself) integrated over days to weeks, so melt-runoff prediction requires a temperature-index or full energy-balance snowmelt model rather than a rainfall-intensity method.

(2) Available water volume. A rain event's total runoff volume is bounded by the storm's own rainfall depth; a snowmelt event's runoff volume is bounded by the ENTIRE accumulated snowpack water equivalent (which may represent weeks or months of accumulated precipitation), so a single warm spell can mobilize a far larger total runoff volume than any single rainstorm of comparable duration, even though its peak INTENSITY (melt rate) is typically much lower than a rainstorm's.

(3) Antecedent state and infiltration availability. For rainfall, antecedent soil moisture is the main infiltration-capacity control; for snowmelt, the ground is frequently still frozen (near-zero infiltration capacity regardless of "moisture") for at least the early part of the melt season, and the snowpack itself acts as a temporary storage/routing reservoir (delaying and attenuating the water released at its base) that has no rainfall analogue — so a snowmelt-runoff model must separately track frozen-ground infiltration blockage and snowpack liquid-water routing, neither of which a rainfall-runoff model needs to represent.