Question 6 of 7

Problem 6 (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) Combined vs. separated urban drainage systems (7 marks)

Two main differences: (1) number of pipe networks and what they carry — a combined system conveys sanitary wastewater and stormwater in a single pipe to (usually) a wastewater treatment plant, while a separated system uses two independent networks, a sanitary sewer to the treatment plant and a storm sewer discharging directly to a receiving watercourse; (2) overflow behaviour during wet weather — a combined system's single pipe can be hydraulically overwhelmed by a large storm, requiring combined sewer overflows (CSOs) that discharge a dilute mixture of stormwater and raw sewage directly to the receiving water, whereas in a properly functioning separated system the storm sewer overflow is just stormwater (no sanitary component), so separated-system overflows are far less of a public-health concern.

A combined system is generally preferred (or simply already exists, since it is rarely built new today) in older, dense urban cores where retrofitting two complete pipe networks under existing streets and buildings would be prohibitively disruptive and costly. A separated system is preferred for new development and modern design practice generally, because it avoids CSOs entirely, allows the sanitary flow to the treatment plant to be sized for dry-weather-plus-infiltration flow (not the much larger wet-weather peak), and allows stormwater to be treated for water quality separately/locally before discharge.

(ii) Stormwater pond design measures for TSS removal upstream of a swimming lake (7 marks)

Assumption: the pond is a wet (permanent-pool) extended-detention facility, since quiescent settling time is what removes TSS, and the discharge is to a lake used for primary-contact recreation, so pathogen/microbial risk (often TSS-associated) is a stated design driver, not just an incidental benefit.

Two key design measures: (1) sizing the permanent pool and extended-detention volume for the target particle settling velocity — using a simplified surface-loading (Type I settling) design, a particle of settling velocity $v_s$ is fully captured if the pond's surface overflow rate does not exceed $v_s$: $$v_s \ge \frac{Q}{A_s}$$ where $Q$ is the treated design flow and $A_s$ is the pond's water-surface area; this is why permanent-pool ponds are sized by surface area (not just volume) when the removal target is a specific fine-particle size, and why an extended-detention component (24–48 h drawdown of the temporary storage above the permanent pool) is added on top of the permanent pool to capture the finer, slower-settling fraction that a smaller pool alone would pass through. (2) an inlet forebay with a submerged, baffled outlet — the forebay dissipates incoming velocity and captures coarse sediment near the inlet (concentrating maintenance dredging), while a submerged/baffled outlet draws water from a mid-depth zone rather than the surface (avoiding oil/floatables) or the bottom (avoiding re-suspended, biologically-active sediment), which is important where the discharge reaches a recreational lake since it minimizes re-entrainment of settled microbial-laden solids into the outflow.

(iii) Predicting future stream flow (6 marks)

Two quantitative techniques: (1) statistical/stochastic streamflow forecasting — fitting a time-series model (e.g. an autoregressive model) or a flood-frequency distribution to the historical gauge record and extrapolating or simulating future flow statistically, without explicitly modelling the physical rainfall-runoff process; and (2) deterministic rainfall-runoff (watershed) modelling — using a physically-based or conceptual model (e.g. the rational method for peaks, or a continuous hydrologic model such as HEC-HMS/SWMM) driven by design or forecast precipitation and watershed characteristics (area, land use, soil, slope) to compute the resulting streamflow.

One advantage of the deterministic rainfall-runoff approach over the purely statistical approach is that it can predict streamflow response to conditions that have not yet been observed in the gauge record — a land-use change (urbanization), a climate-driven shift in storm intensity, or a proposed new storm sewer — because it models the physical process rather than just extrapolating historical statistics, which by construction assume the future behaves like the past.