Question 3 of 7: Snow Melt in Detention Design, Wastewater Collection Components, and the IDF Curve

Problem 3: Snow Melt in Detention Design, Wastewater Collection Components, and the IDF Curve (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) Incorporating Snow Melt into Detention Pond Design

Method 1 — a dedicated snowmelt design event. Check the pond's active storage and outlet capacity against a separate, longer-duration inflow hydrograph representing a rain-on-snow or pure spring-melt event (e.g., generated with a degree-day snowmelt model), in addition to the usual short-duration rainfall design storm. Spring melt produces a much larger total runoff volume over days to weeks, even though its peak rate is comparatively low, so a pond sized only for a rainfall peak can be overwhelmed by melt volume alone.

Method 2 — extended low-flow drawdown and added storage. Provide a flow-limited low-level outlet (a small-orifice riser, perforated "blanket drain," or slow-release valve) sized for extended drawdown, and increase the pond's total storage volume/freeboard beyond what the rainfall design alone requires, so the larger melt volume can be safely detained and released slowly without surcharging the emergency spillway.

(ii) Wastewater Collection System Components

(a) Sanitary force main. A pressurized pipe downstream of a lift/pump station that conveys sewage where gravity flow is not feasible — flat terrain, a low point, or where the gravity trench would be uneconomically deep. Because it flows full under pump-generated pressure (not by gravity self-cleansing), pipe pressure class, air-release valves at high points, and water-hammer/surge protection are the critical design considerations; it lets the collection system cross a topographic high point that gravity flow alone could not.

(b) Sanitary trunk sewer. A large-diameter gravity collector that receives flow from multiple lateral and collector sewers across a large sub-catchment and conveys the combined flow onward to the treatment plant or a major interceptor. It is the "backbone" of the collection system: its capacity, set for the ultimate build-out population plus an infiltration/inflow allowance, constrains how much upstream development the entire tributary network can serve, and any surcharging in the trunk sewer backs up the whole upstream network.

(iii) Generating and Using the IDF Curve

Generation. An IDF curve is built from a long, continuous rain-gauge record by (1) extracting the annual-maximum rainfall depth for a series of standard durations (e.g., 5, 10, 15, 30, 60, 120 min); (2) fitting a frequency distribution — commonly Gumbel or Log-Pearson Type III — to each duration's annual-maximum series to obtain the depth for chosen return periods (2, 5, 10, 25, 50, 100-yr), and converting each depth to an average intensity (depth/duration); and (3) plotting or curve-fitting the resulting intensity–duration points as a family of curves, one per return period, often smoothed to an equation of the form $i=a/(t+b)^c$.

Three main steps to use it for storm sewer design: (1) select the design return period appropriate to the system classification (typically 5–10-yr for the minor/pipe system, with a 100-yr major-system overland-flow check); (2) estimate the time of concentration $t_c$ for the tributary area (overland flow time plus gutter/pipe travel time) and enter the IDF curve at $t=t_c$ to read the design intensity $i$; and (3) apply $i$ (with the catchment's $C$ and $A$) in the Rational Method — or a hydrograph method for larger areas — to obtain the design peak flow, then size the pipe with Manning's equation to convey that flow at the available slope.