Question 1 of 7

Problem 1 (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) Critical functions of the minor and major stormwater systems (8 marks)

The minor system is the piped storm sewer network (catch basins, pipes, manholes) sized to convey the frequent, lower-intensity design storm (typically a 2- to 10-year event). Two of its critical functions are: (1) everyday nuisance-flow conveyance — it removes the runoff from common storms quickly enough that streets, sidewalks and low points do not pond during routine rainfall, keeping the road network serviceable; and (2) water-quality and debris control at the source — catch-basin sumps trap sediment and coarse debris before it reaches receiving waters, so the minor system is also the first stage of pollution control.

The major system is the overland flow path (streets acting as shallow channels, swales, floodplains) that conveys runoff once a storm exceeds the minor system's capacity (typically the 25- to 100-year event). Two of its critical functions are: (1) flood-protection conveyance for rare, large storms — it provides an engineered, predictable overland route (following road crowns/gutters to a receiving watercourse) so that when the minor system surcharges, water still flows to a safe outlet instead of pooling unpredictably against buildings; and (2) defining safe building elevations and rights-of-way — because the major system's flow path and depth are established at the design stage, it sets minimum opening elevations for basements and doors along the route, protecting property even though the major system is only called on rarely.

Integration during large storms. The two systems are always operating together, not sequentially: the minor system continues to carry its full piped capacity throughout the storm, while the major system carries only the excess that the minor system cannot accept once its inlets and pipes surcharge. In other words, total design flow is split between the two paths by capacity, not by time — the major system is the deliberately engineered "overflow" for the minor system, and both must be sized together (not the major system as an afterthought) so that the combined capacity meets the municipality's flood-protection design storm.

(ii) Water hammer in pumped systems (7 marks)

What it is. Water hammer (hydraulic transient/surge) is a sharp pressure wave that travels through a pipeline when the velocity of the flowing water is changed very rapidly — the kinetic energy of the moving water column is converted almost instantaneously into a pressure pulse that propagates at the pipeline's acoustic wave speed (typically 900–1300 m/s for water in steel/ductile-iron pipe), reflecting back and forth between boundaries (valves, pump, reservoir) until it is damped by friction.

When it occurs. It occurs whenever flow velocity changes abruptly: rapid valve closure, sudden pump trip/power failure (the check valve slamming shut against reversing flow), or rapid pump start-up against an empty or air-filled main. The severity depends on how fast the change occurs relative to the pipeline's characteristic period ($2L/a$, where $L$ is pipe length and $a$ is the wave speed) — a closure time shorter than $2L/a$ produces the full "instantaneous closure" surge pressure, $\Delta p = \rho a \Delta V$.

One mitigation. Installing a surge (anti-water-hammer) relief valve or an air/surge tank near the pump discharge absorbs the transient by allowing a controlled, cushioned release (or admission) of water/air as the pressure wave arrives, capping the peak surge pressure well below the pipe's rating; slowing valve closure times and using pump control valves that open/close on a programmed ramp are the complementary operational measure.

(iii) Pumps in series vs. parallel (5 marks)

[Figure not reproduced: Fig. 1 — Pump performance curves (as printed on the exam): single pump, two pumps in parallel (combined flow at a given head), and two pumps in series (combined head at a given flow), referenced to the single pump's design point (100% flow, 100% head). See the official exam paper.]

Parallel operation is chosen when the system needs a higher flow rate at roughly the same head — e.g., matching a fluctuating demand curve with duty/standby units, or simply because no single pump is available at the required capacity. Advantage: operational flexibility and redundancy — units can be staged on/off to track demand, and one pump can be serviced while the other continues to run at partial capacity. Challenge: because the pumps discharge into a common, rising system curve, adding a second pump in parallel gives markedly less than double the single-pump flow (the parallel curve is flatter and the operating point rides up the system curve), so the flow gain diminishes as more units are added, especially on a friction-dominated (steep) system curve.

Series operation is chosen when the system needs a higher head at roughly the same flow — e.g., a long transmission main or a large static lift that exceeds any single available pump's shut-off head. Advantage: a large head can be developed from smaller, standard/off-the-shelf pump units instead of a single custom high-head pump. Challenge: every unit in the series train must pass the full system flow, so the downstream pump casing and seals must be rated for the combined discharge pressure of all upstream units, and taking one unit offline interrupts the whole train (no redundancy without a bypass).