Question 1 of 5: Short-answer concepts

Question 1: Short-answer concepts (25 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.

(1) Population growth model for a built-out downtown. A logistic (saturation) growth model is appropriate. Where land is scarce and expensive the population approaches a carrying capacity (saturation) fixed by the available building stock, so growth decelerates as that ceiling is neared. A geometric/exponential model has no upper bound and would badly over-predict; the declining-growth or logistic curve $P(t)=\dfrac{P_{sat}}{1+m\,e^{-a t}}$ captures the S-shape as growth flattens toward $P_{sat}$.

(2) Major vs. minor stormwater systems. The minor (convenience) system is the piped network — gutters, catch basins/inlets and storm sewers — designed for frequent, nuisance-level events (typically a 2- to 10-year return period). The major system is the set of overland flow paths — streets, swales, ravines and channels — that safely conveys the rare extreme event (up to the 100-year storm) once the minor system surcharges, keeping flood water away from buildings. Good design provides both, with a defined major-system route for the excess.

(3) Four measures to limit urbanization impacts on stream water quality. (i) Low-impact development / green infrastructure (bioretention cells, rain gardens, bioswales) to infiltrate and treat runoff at source; (ii) stormwater detention/retention ponds and constructed wetlands sized for a water-quality volume; (iii) permeable/pervious pavement to reduce effective imperviousness; (iv) vegetated riparian buffer strips together with erosion-and-sediment control during construction. Each reduces peak flows, thermal loading and pollutant (sediment, nutrient, metals) export.

(4) Why distribution is looped but storm collection is not. A water distribution network is pressurized and must stay reliable: looping provides redundancy so supply and pressure are maintained if one main is isolated for a break or repair, avoids dead-ends where water stagnates and loses chlorine residual, and lets flow reach any node from more than one direction to meet fire flow. A storm collection system is gravity-driven: water only has to reach a single downstream outfall, and gravity forces one-directional, convergent (dendritic/branching) flow — a loop would serve no redundancy purpose and could not reverse against the grade.

(5) Effect of storage on WTP pumps. Elevated and ground storage decouples production from instantaneous demand. The plant pumps can then be sized close to the maximum-day (average) demand rather than the peak-hour demand, because storage floats on the system and supplies the diurnal peaks and fire flow. The pumps therefore run steadily near their best-efficiency point with fewer starts/stops, which lowers energy use and extends equipment life; storage also sustains supply during a power or pump outage.

(6) Controlling distribution water losses. Combine (i) active leakage control — district metered areas (DMAs) with continuous night-flow monitoring and acoustic leak-detection surveys; (ii) pressure management — reduce excess pressure (leakage scales with pressure), the single most cost-effective real-loss measure; (iii) infrastructure renewal — prioritized main replacement/relining based on break history; and (iv) reduce apparent losses through meter replacement/calibration and curbing unauthorized use. A water-balance/IWA audit quantifies real vs. apparent losses to target the program.

(7) Reducing combined-sewer overflows (CSOs). Structural: provide off-line storage (deep tunnels or storage tanks) that captures the first-flush combined flow for later treatment, or progressively separate the storm and sanitary sewers. Non-structural: apply source controls — green infrastructure, downspout disconnection and real-time control of regulators — to keep stormwater out of the combined system and shave the overflow frequency; public education and inflow reduction supplement this.

(8) Decentralized water and sewer systems. Decentralized (distributed) systems collect, treat and often reuse water/wastewater at or near the point of generation — individual on-site units (wells, septic systems), cluster/community package plants, or satellite water-reclamation facilities — rather than conveying everything to one large central plant. They suit remote or low-density areas, allow staged/incremental growth without huge trunk mains, shorten conveyance, and enable local non-potable reuse (greywater, reclaimed irrigation).

(9) Two rational-method assumptions. (i) The rainfall intensity is assumed uniform over the whole catchment and constant for a duration equal to the time of concentration $t_c$; this is only realistic for small areas, since real storms vary in space and time. (ii) The runoff coefficient $C$ is assumed constant — a fixed linear fraction of rainfall becomes runoff regardless of storm depth, antecedent moisture or depression storage — whereas $C$ actually rises during a storm as the ground saturates. (A third: the computed peak has the same return period as the rainfall.) These limit the method to small urban catchments (commonly < 80–200 ha).

(10) Peak factors vs. serviced-area size. As the serviced population/area grows, the maximum peaking factor decreases and the minimum factor increases (both move toward unity), so the hydrograph flattens. The reason is aggregation: flows from many dwellings and from distant reaches arrive out of phase, so individual diurnal peaks and lows are desynchronized and partly cancel in the trunk. Empirical formulas capture this — e.g. Harmon $M = 1+\dfrac{14}{4+\sqrt{P}}$ (with $P$ in thousands) gives a peak factor that falls as $P$ rises.