18-Env-A2 Hydrology and Municipal Hydraulics Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 18-Env-A2 / Hydrology and Municipal Hydraulics Engineering. 3 hours duration; closed book with an 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 in the work book are marked); all seven Problems are solved below for completeness. Each question is worth 20 marks.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Linsley, Kohler & Paulhus, Hydrology for Engineers (3rd ed.); Chow, Open-Channel Hydraulics; Walski et al., Advanced Water Distribution Modeling and Management; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Guidelines for Canadian Drinking Water Quality (Health Canada); Canadian Council of Ministers of the Environment (CCME) water-quality guidelines.
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.
On-site (source control) SWM reduces runoff volume and peak rate at (or very near) the point where rain falls — infiltration trenches, bioretention cells, permeable pavement, rain gardens and rooftop detention. By keeping water out of the piped network in the first place, on-site controls reduce the peak flow, volume and pollutant load that the downstream conveyance and end-of-pipe facilities have to handle, and they also help maintain pre-development groundwater recharge.
End-of-pipe SWM (wet ponds, dry ponds, wetlands, underground storage) treats and attenuates the combined runoff from an entire drainage area at the outlet, just before it discharges to the receiving watercourse. It provides the last line of defence for both flood control (peak-flow attenuation by temporary storage, releasing at a controlled rate below the pre-development or receiving-channel-erosion-threshold flow) and water-quality control (settling of sediment and particulate-bound pollutants during the extended detention time).
Together, the two levels of control form a "treatment train": on-site controls reduce the volume and peak that reach the pipe network, and the end-of-pipe facility manages what remains before it reaches the receiving water — relying on either one alone typically cannot meet both a peak-flow control target and a water-quality (extended detention/particle removal) target simultaneously.
Two key design principles for a wet pond (or wet pond + wetland): (1) a permanent pool sized for the target water-quality storm, so that incoming runoff displaces "old" water through the outlet at a slow, controlled rate rather than short-circuiting straight to the outlet, giving suspended sediment time to settle; and (2) a forebay at the inlet to capture coarse sediment and floatables close to the inlet, protecting the main pool's storage volume and simplifying maintenance dredging.
Two operational measures: (1) periodic sediment/forebay dredging once accumulated sediment measurably reduces the permanent pool's active storage volume; and (2) routine inlet/outlet structure inspection and debris removal to ensure the control (orifice/weir) that sets the extended-detention release rate is not obstructed, since a blocked outlet defeats the pond's peak-attenuation function exactly when it is needed most (during a large storm).
| Method | Advantage | Limitation |
|---|---|---|
| Rational Method (peak flow only, $Q=CiA/360$) | Simple, fast, well-suited to small (typically <80–100 ha), highly impervious urban catchments with a single dominant time of concentration. | Produces only a single peak flow, not a hydrograph — cannot size a detention/storage facility, and accuracy degrades on large or non-uniform catchments. |
| Unit hydrograph / SCS (NRCS) method | Produces a full runoff hydrograph (shape and timing, not just the peak), so it can be used directly to size storage/detention facilities and route flow through a network. | Requires more input data (loss/infiltration parameters, unit hydrograph shape) and calibration effort than the Rational Method; results are sensitive to the assumed curve number/loss model. |
| Continuous (long-term) simulation model (e.g. SWMM) | Simulates the full rainfall record (not just a single design storm), capturing antecedent-moisture effects, back-to-back storms, and providing a statistically robust performance assessment (e.g. overflow frequency) for water-quality/quantity facilities. | Data- and computation-intensive (needs a long, high-resolution rainfall record and a calibrated model of the whole system); overkill for a single small-site design where a simpler method suffices. |
