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) Three main components of a stormwater collection system (8 marks)

(1) Inlets/catch basins. These are the entry points that capture overland flow from streets and yards into the piped system; their main function toward flood prevention is intercepting runoff quickly enough that it does not pond and back up onto the roadway, and toward erosion control their sump traps coarse sediment/debris before it can scour the downstream pipe or receiving channel.

(2) Conveyance piping/conduits (storm sewer network). Sized by the Rational Method or a hydrograph method for the design storm, its main function is carrying the collected runoff at a non-silting, non-scouring velocity from each inlet to the outfall or a detention facility — preventing flooding by providing adequate hydraulic capacity, and preventing erosion by keeping velocities within the self-cleansing-but-non-scouring range (typically 0.6–3 m/s) so the pipe neither silts up nor erodes at pipe/manhole transitions.

(3) Outfall / energy-dissipation and detention structure. At the point the system discharges to a receiving watercourse (directly, or via a detention/retention facility), its main function is controlling the discharge rate and velocity so the receiving channel is not overwhelmed (flooding) or scoured (erosion) — typically an outlet control structure (orifice/weir) that throttles peak discharge, paired with riprap or an energy dissipator at the pipe outlet to prevent local scour where concentrated flow re-enters a natural channel.

(ii) Sanitary pumping station: functions and O&M issues (6 marks)

Two important functions. (1) Lifting wastewater from a low-lying collection area, where a continuous gravity grade to the treatment plant or downstream trunk sewer is not achievable without excessive excavation depth, into a forcemain or a higher gravity sewer. (2) Equalization/buffering of flow — the wet well provides short-term storage that smooths out the peaky inflow pattern (diurnal peaks, wet-weather surges) into a more even pumped discharge, protecting downstream treatment processes from hydraulic shock loading.

Two O&M issues to prevent basement flooding/overflow. (1) Standby power and duty/standby pump redundancy — a power outage or single pump failure at a station serving a low-lying area can surcharge the upstream gravity network within minutes, backing sewage up into connected basements; a backup generator (or dual utility feed) and an automatically-alternating duty/standby pump pair, tested regularly, prevent this. (2) Wet-well high-level alarm and regular check-valve/screen maintenance — debris fouling the check valve or intake screen (rags, grease) can silently reduce effective pump capacity long before a full failure occurs, so a telemetered high-level alarm (giving operators warning before the wet well surcharges into the collection network) combined with a scheduled cleaning/inspection program for screens, check valves and impellers keeps the station's real capacity matched to its design capacity.

(iii) Watershed conditions affecting flooding potential with concurrent rain-on-snow (6 marks)

(1) Antecedent snowpack water equivalent (SWE) and areal extent. A large, ripe (isothermal, near 0 °C) snowpack already holding significant liquid water can release its full water equivalent rapidly once rain adds energy and moisture, so the pre-event SWE directly sets the additional volume the rain event can mobilize on top of the rainfall itself.

(2) Ground/soil frost state and antecedent soil moisture. If the ground is frozen or already saturated from prior wet-season precipitation, infiltration capacity is near zero, so both the rainfall and the snowmelt are forced to become surface runoff almost immediately rather than being buffered by soil storage — this single factor can turn an otherwise moderate rain-on-snow event into a severe flood.

(3) Air temperature, wind and the resulting melt rate (energy balance of the snowpack). A warm, humid, windy rain event delivers much more energy to the snowpack (via rain's sensible/latent heat and turbulent convective/latent-heat transfer) than a cold, still one, driving a much higher snowmelt rate and hence a larger combined rain-plus-melt runoff volume and peak — this is why the most severe rain-on-snow floods in Canada are associated with warm frontal systems ("Pineapple Express"-type events) rather than cold-season rain.