18-Env-A2 Hydrology and Municipal Hydraulics Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 04-Env-A2 / Hydrology and Municipal Hydraulics Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five questions constitute a complete paper (only the first five answers as they appear in the work book are marked); all seven are solved below for completeness. Each question ("Problem") is worth 20 marks, with sub-part weights shown in brackets.
Reference texts. Chow, Open-Channel Hydraulics; Linsley, Kohler & Paulhus, Hydrology for Engineers (3rd ed.); Walski et al., Advanced Water Distribution Modeling and Management; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.).
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.
The four processes form a closed loop. Evaporation and transpiration remove moisture from open water, soil and plant surfaces, replenishing atmospheric humidity and restoring the soil's infiltration capacity between storms. Precipitation is the loop's input event, delivering water to the ground surface. Infiltration/percolation then competes with surface runoff for that incoming water, diverting a share to soil moisture and, ultimately, groundwater storage. Whatever exceeds the soil's infiltration capacity becomes surface runoff, which collects in the stream/ocean shown at the base of the schematic, closing the loop back to the evaporation term.
A stormwater pumping station is best located at or immediately upstream of a point where the receiving channel already has adequate erosion resistance (a lined reach, a natural rock control, or a section with a stable, vegetated bed) rather than discharging directly onto an erodible natural channel — this places the highest-velocity jet where the channel can already tolerate it. It should be operated with staged pump capacity (multiple smaller pumps brought on and off in sequence rather than one large unit switching between fully off and fully on) so the discharge — and hence the receiving channel's velocity — ramps up and down gradually rather than surging instantaneously to full capacity; pairing this with a stilling basin, riprap apron or energy-dissipating outlet structure at the discharge point further reduces the outlet velocity before it reaches the natural channel bed, directly protecting against the scour that an unattenuated pumped discharge would otherwise cause.
A typical application: a 15 ha residential subdivision with a runoff coefficient $C=0.55$ and a time of concentration $t_c=20$ minutes is to be served by a new storm sewer designed for the 10-year event. The designer enters the local IDF curve family at $t=20$ min on the $T=10$-year curve to read the design intensity (e.g. $i_{10}\approx90$ mm/hr), then applies the Rational Formula $Q=CiA/360 = (0.55)(90)(15)/360 \approx 2.06$ m³/s as the peak design discharge used to size the pipe via Manning's equation. The same IDF curve, read at the same duration but the 100-year curve, would instead be used to check that the major (overland) system can safely convey the excess once the minor system's 10-year capacity is exceeded.
(1) Scale and ownership of control. On-site systems (e.g. lot-level bioretention, permeable pavement, rooftop detention) are designed and typically maintained at the level of the individual parcel, sized to that parcel's own contributing area; off-site systems (a regional detention pond, a trunk storm sewer) are designed for the cumulative flow from many parcels and are owned/maintained by the municipality or a drainage authority. (2) Point of control relative to runoff generation. On-site design controls runoff at its source, before it leaves the property, reducing both the peak rate and the volume that the downstream conveyance system must ever handle; off-site design instead accepts runoff already generated upstream and manages it further downstream (conveyance sizing, regional storage), so its design must anticipate the combined, already-concentrated flow from the whole tributary catchment rather than a single lot's hydrograph.
