18-Env-A2 Hydrology and Municipal Hydraulics Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-A2 / Hydrology and Municipal Hydraulics Engineering. 3 hours duration; closed book with an 8×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five questions constitute a complete paper (first five answers marked, 20 marks each, 100 marks total); all seven are solved below for completeness.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — hydrology, stormwater management and water-demand chapters; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — sanitary sewer hydraulics; MWH’s Water Treatment: Principles and Design (3rd ed.) — distribution systems and pumping; Chow, Open-Channel Hydraulics — Manning's n tables and specific-energy theory; Chow, Maidment & Mays, Applied Hydrology — frequency analysis; Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
Approach. Fill the matrix cell-by-cell: storm sewers are open, gravity-fed conveyance already designed for surface water, so their I/I risk is dominated by groundwater; sanitary sewers are meant to carry only wastewater, so both surface inflow (illegal connections, manhole covers in low spots) and groundwater infiltration (through joints/cracks below the water table) are unwanted and regulated.
| Surface Water Inflow | Ground Water Inflow | |
|---|---|---|
| Storm Sewers | Issue: uncontrolled surface inflow through open inlets during extreme events causes hydraulic overload and downstream flooding/erosion beyond the design storm. Solution: provide flow-restrictor inlets or upstream detention (soft: bioretention/rain gardens) so the peak entering the pipe network is attenuated before it exceeds pipe capacity. |
Issue: a storm sewer laid below the water table can act as a French drain, dewatering adjacent groundwater and lowering the local water table (affecting wells, wetlands, tree root zones). Solution: use gasketed, tightly jointed pipe (or line the invert) below the seasonal high water table, and set inverts above the water table where feasible. |
| Sanitary Sewers | Issue: cross-connected downspouts, sump pumps, or low manhole lids let clean stormwater into the sanitary system, consuming treatment-plant capacity and causing sanitary sewer overflows (SSOs) of raw sewage during wet weather. Solution: enforce a downspout/sump-pump disconnection by-law with inspection, and raise/bolt manhole lids in flood-prone low points. |
Issue: groundwater infiltration through deteriorated joints and cracks dilutes influent, increases pumping/treatment energy costs, and can hydraulically surcharge the interceptor during high water-table periods. Solution: a proactive I/I reduction program — CCTV inspection plus cured-in-place pipe (CIPP) lining or joint grouting of the worst-offending reaches, prioritized by flow monitoring. |
(a) Major versus minor systems. The minor system (the piped storm sewer network) is sized for a frequent, moderate design storm (commonly the 2- to 10-year event) and handles day-to-day nuisance flooding. The major system (streets, swales, overland flow routes, and detention facilities) is the overflow path relied upon when a rarer, larger storm (commonly 100-year) exceeds the minor system's capacity; it is deliberately designed — road crowns, curb heights, catchbasin placement — so that the excess water is conveyed safely overland to a receiving watercourse rather than backing up into basements. Recognizing both systems together, rather than sizing pipes alone, is what actually protects the community from the rare extreme event.
(b) Snow-melt runoff prediction. The most widely used method is the degree-day (temperature-index) method, which estimates daily snowmelt depth as $M = C_m(T-T_b)$, where $T$ is mean daily air temperature, $T_b$ is a base (melting) temperature (commonly 0°C), and $C_m$ is an empirical melt-rate coefficient (mm/°C-day) that accounts for solar radiation, wind, and snowpack condition. It is favoured for municipal design because it needs only routinely available air-temperature records, unlike the more physically complete energy-balance method (which sums net radiation, sensible/latent heat and rain-on-snow advection but needs much more instrumentation) used for research-grade snowmelt-flood forecasting.
(a) Sanitary emergency overflow at a pumping station. When a lift station's pumps cannot keep pace with inflow (power failure, pump failure, or a storm surcharging the wet well beyond capacity), an emergency overflow (a weir or standby discharge line to a watercourse, sometimes with an alarm/telemetry trigger) prevents uncontrolled surcharging back through upstream manholes and basement connections, which would otherwise flood streets and private property with raw sewage. It is a last-resort relief that trades an infrequent, permitted/reported bypass for a much worse uncontrolled backup, and its design and use are tightly regulated (reporting to the environmental regulator is normally mandatory).
(b) Storm cyclone grit chamber. A cyclone (vortex) grit chamber uses the centrifugal action of a swirling flow to separate dense, inert grit (sand, gravel) from the lighter organic solids in stormwater before it is discharged or conveyed onward. Removing grit protects downstream pumps and pipes from abrasive wear and prevents heavy sediment from settling out and reducing pipe capacity in flatter downstream reaches; because it relies on flow-induced vortex action rather than long quiescent settling time, it is compact and well-suited to the intermittent, high-flow-rate nature of storm events.
