18-Env-A2 Hydrology and Municipal Hydraulics Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 04-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.
(1) Understanding the snowpack/snowmelt component of the cycle shows that flood risk in a mountain valley is driven by two coincident processes — the accumulated snow-water-equivalent (SWE) available to melt, and a rain-on-snow event that both melts the pack rapidly and adds direct rainfall — so a warning system needs SWE and freezing-level monitoring (snow pillows/courses, remote sensing) upstream of the valley, not just rain gauges in the valley itself, because the trigger event can be entirely upstream and hours ahead of the flood peak. (2) Understanding basin travel/concentration time (time of concentration, channel routing) tells the designer how much lead time the warning actually has: a steep mountainous watershed has a short time of concentration, so the warning system must be automated (real-time telemetered gauges triggering an automatic alert) rather than relying on manual observation, because the lag between an upstream rainfall/melt event and the flood wave reaching the valley may only be on the order of an hour or two.
(a) Stormwater catch basin. A catch basin is a below-grade inlet structure (grate + sump) that collects surface runoff from a roadway or paved area and conveys it into the storm sewer, while its sump also traps coarse sediment, grit and debris before they can enter and clog the downstream pipe network — it is therefore both a hydraulic inlet and the first stage of pretreatment.
(b) Minor system. The minor system is the piped storm sewer network (catch basins, pipes, manholes) sized to convey the frequent, lower-intensity design storm (typically a 2- to 10-year event) without surcharging; larger, rarer storms exceed its capacity and are instead conveyed overland by the "major system" (streets, swales), so the two systems together, not the minor system alone, provide the full level of flood protection.
(c) Inlet control device (ICD). An ICD is a flow-restricting orifice or valve installed at (or just downstream of) a catch basin/manhole inlet that throttles the rate at which runoff enters the sewer, forcing temporary surface ponding upstream of each ICD during large storms; this distributes storage across many small points in the catchment (source control) instead of relying on a single downstream detention pond, which reduces the peak flow the trunk sewer must be sized to carry.
(a) Sanitary vacuum sewer. A vacuum sewer uses a central vacuum station to draw wastewater from small collection sumps through shallow, small-diameter pipes laid on a sawtooth profile rather than continuous gravity grade; it is used where flat terrain, high water table, or rock would make a conventional deep gravity sewer impractical or very costly, since the vacuum sewer can be laid at a shallow, constant depth and self-cleans by drawing air-water slugs at high velocity.
(b) Inverted syphon. An inverted syphon is a sealed, pressurized section of sewer (typically two or more parallel pipes) that dips below an obstruction — a stream, railway cut, or utility crossing — that the sewer cannot cross on grade; because the pipe runs full and under pressure through the dip, self-cleansing velocity must be checked explicitly (often using multiple barrels so at least one stays velocity-self-cleansing across the full range of flows), since a syphon has no access for inspection along its length the way a normal gravity manhole run does.
(c) Sanitary forcemain. A forcemain is the pressurized pipe downstream of a sanitary pumping station that conveys wastewater (under pump head, not gravity) to the point where it can re-enter a gravity sewer or reach the treatment plant; because it is pressurized it can run uphill and does not need to follow a continuous downhill grade, but it must be designed for a minimum scouring velocity across the pump's operating range and for surge/water-hammer protection at pump start-stop, since trapped air or debris cannot be relieved through a manhole the way it can in a gravity main.
