18-Env-A2 Hydrology and Municipal Hydraulics Engineering · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 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 (first five answers marked); all seven are solved below for completeness. Each question ("Problem") is worth 20 marks.
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 hydrologic equation (HE) is a water-balance statement: gross precipitation input less the abstraction processes (interception, depression storage, infiltration, evapotranspiration) leaves the effective input that becomes surface runoff, i.e. $P - (I_a + F + E) = Q$, with the peak runoff flowrate obtained by routing the resulting effective-rainfall hyetograph through the watershed's response (unit hydrograph / time of concentration). To account for snowmelt, the precipitation term $P$ must be replaced with a combined liquid-water-input term that adds a melt release $M$ to any concurrent rainfall $R$: $P_{\text{eff-input}} = R + M$, where $M$ is estimated by a temperature-index (degree-day) method, $M = C_m(T - T_{\text{base}})$, or an energy-budget method for a more rigorous watershed. The abstraction terms must also be re-evaluated for snow-covered/partially frozen conditions rather than a bare watershed: interception is negligible once the canopy and ground are snow-covered, while infiltration capacity $F$ is sharply reduced by frozen or saturated soil beneath the pack, so a much larger fraction of the melt (and any concurrent rain) becomes rapid surface runoff than the same water input would on an unfrozen watershed. Finally, because snow accumulates before it is released, the timing of the HE's input must be shifted from the precipitation event itself to the melt-release period — with the critical design case typically being a rain-on-snow event, where warm rainfall accelerates melt and the combined $R+M$ input produces peak runoff well after (and often much larger than) what the rainfall record alone would predict.
(a) Trunk sewer. A trunk sewer is a large primary collector that receives flow from numerous lateral and branch sewers across a drainage catchment and conveys the combined wastewater onward to an interceptor or treatment works. It is sized for the cumulative peak flow of everything tributary to it and establishes the hydraulic grade line that all upstream sub-collectors must discharge into, making it the backbone of the collection network for that catchment.
(b) High level water alarm in a pumping station. This is a float or level switch set above the normal lag-pump-on elevation in the wet well that annunciates locally and via SCADA/telemetry when the well is rising toward overflow. Its importance is early warning: it alerts operators to a developing surcharge (pump failure, power loss, blockage, or inflow exceeding capacity) in time to intervene — starting a standby pump, dispatching a crew, or activating an emergency bypass — before the station overflows to the environment or the surcharge backs up into the upstream collection system.
(c) Dry well associated with a pumping station. The dry well is the separate, normally-dry chamber adjacent to (or concentric with) the wet well that houses the pumps, valves, and discharge piping in accessible, ventilated, non-submerged conditions. Its importance is operational and safety: it keeps electrical and mechanical equipment out of the raw-sewage wet well, allowing routine maintenance and inspection in a dry, confined-space-managed environment, and is the defining feature that distinguishes a "dry-pit" station from a submersible-pump station.
The storage vault is an underground detention chamber that intercepts the inflow sewer before it reaches the downstream trunk. During a high-flow storm event, incoming flow enters the vault and is throttled on its way out by a small controlled-flow orifice at the base of the vault (rather than being passed through at full sewer capacity); the difference between the inflow rate and the throttled outflow rate is temporarily stored within the vault volume, flattening and delaying the peak that the downstream sewer must convey and so directly reducing downstream surcharging. Where the vault has an open, gravel-filled bottom wrapped in geotextile, stored water is also held in contact with the surrounding native soil for an extended period (rather than draining away within minutes through a sealed pipe), which promotes infiltration into the ground — the geotextile keeps fines out of the gravel void space so the infiltration surface does not clog, while the gravel itself provides both structural support and a high-void-ratio temporary reservoir. Should inflow exceed the vault's storage and infiltration capacity, a manhole/overflow outlet set at a controlled high-water elevation passes only the excess to the downstream sewer, so the vault behaves as a combined "first flush hold-and-infiltrate, then controlled-overflow" device rather than an all-or-nothing bypass.
