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 hydrologic equation ($P = R + ET + I + \Delta S$) governs both forms of precipitation, but the key difference is timing. Rainfall is abstracted essentially within the storm's own duration — interception, depression storage and infiltration are all satisfied inside minutes to hours, so the runoff hydrograph tracks the rainfall hyetograph almost synchronously, and peak-runoff prediction for a rain event reduces to routing a known input intensity through the watershed's time of concentration. Snow, by contrast, is first stored on the watershed as its own explicit reservoir (the snowpack); no significant runoff occurs while the pack is sub-freezing and accumulating "cold content," and melt begins only once the pack is isothermal at 0°C and its liquid-holding capacity is exceeded, with the melt rate then set by an energy balance or temperature-index relation rather than by the original snowfall rate. Peak-runoff prediction from snowmelt therefore requires modelling the pack as a delayed-release storage driven by air temperature/radiation, decoupled in time from when the precipitation actually fell — a spring freshet peak may occur weeks after the snow that produced it.
| Component | Function / importance |
|---|---|
| (a) IDF curves | Give design rainfall intensity as a function of storm duration and return period; entering the curve at the catchment's time of concentration and a chosen design frequency supplies the intensity used directly in the Rational Method (or a design hyetograph for hydrograph methods) — without an IDF curve there is no defensible design storm. |
| (b) Combined storm sewer | A single pipe network that conveys both stormwater runoff and sanitary sewage together to treatment (an older, pre-separation practice); it is economical to build once but must be sized for the much larger wet-weather flow and, once its capacity is exceeded, discharges a diluted mix of stormwater and untreated sewage (a combined sewer overflow) directly to the receiving water. |
| (c) Stormwater pumping station | Lifts stormwater from a low-lying collection area (below the receiving water's flood or high-tide elevation, or too deep to drain by gravity to an outfall) up to a point where it can discharge by gravity; sized for the peak design flow with enough wet-well storage and pump staging to avoid excessive cycling. |
| Component | Function / importance |
|---|---|
| (a) Sanitary sewers | The gravity-flow pipe network that collects and conveys only sanitary (domestic/industrial) wastewater — kept hydraulically separate from stormwater in a separated system — to the treatment plant, sized for peak flow via a peaking factor (e.g. Harmon's formula) applied to average dry-weather flow. |
| (b) Sanitary drop structures | A manhole structure that carries the incoming pipe invert down to the outgoing pipe invert through an internal or external drop pipe rather than a steep open channel through the manhole; it lets a sewer negotiate a steep grade change (e.g. down a hillside) while keeping velocities inside downstream reaches within the permissible range and protecting maintenance personnel from a high-velocity cascade inside the structure. |
| (c) Sewage pumping stations | Lift sanitary flow from a low-lying service area, or across a ridge where gravity grade cannot be maintained, into a forcemain or a higher gravity reach; firm pumping capacity, wet-well storage and standby power are all sized to prevent surcharge/backup during the peak design flow and during a single-pump-out-of-service condition. |
