18-Env-A2 Hydrology and Municipal Hydraulics Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 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 (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 ($P = R + ET + I + \Delta S$, precipitation partitioned into runoff, evapotranspiration, infiltration/deep percolation and change in storage) applies to both rain and snow, but the two inputs traverse it on very different timescales. Rainfall is abstracted nearly as it falls: interception by canopy, depression storage in surface micro-topography and infiltration into the soil are all satisfied within the storm's own duration (minutes to hours), after which the remaining excess becomes direct runoff following the storm's own intensity pattern — the watershed's response is essentially synchronous with the rainfall event.
Snow instead first accumulates as a distinct storage on the watershed — the snowpack itself is an explicit term the hydrologic equation must track, separate from soil moisture or channel storage. Little or no runoff is generated while the pack is sub-freezing and building "cold content"; melt only begins once the pack is isothermal at 0°C and its liquid-water-holding capacity is exceeded, and the melt rate is then governed by the energy balance (or a temperature-index proxy) rather than by the original snowfall's intensity. The result is that snowmelt runoff is decoupled in time from the precipitation that produced it — a winter's accumulated snowfall is released as runoff over the following weeks of the spring freshet, not within hours of falling, which is why watershed water-balance and flood-design studies must model the snowpack as its own delayed-release reservoir rather than treating snow like rain with a longer lag.
| Component | Function / importance |
|---|---|
| (a) IDF curves | Give the 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 $i$ used directly in the Rational Method (or a design hyetograph for hydrograph methods) — without an IDF curve there is no defensible design storm. |
| (b) Storm sewer | The closed, gravity-flow conduit network that collects runoff from inlets across the minor system and conveys it to an outfall or receiving water, sized by Manning's equation for the design (typically 2–10 year) storm. |
| (c) Storm infiltration ditch | An open, vegetated or gravel-filled trench that captures runoff and infiltrates it into the underlying soil rather than piping it away; it reduces both peak rate and total volume delivered downstream and provides a measure of water-quality treatment and groundwater recharge, complementing (or substituting for) piped conveyance. |
| Component | Function / importance |
|---|---|
| (a) Sanitary forcemain | A pressurized pipe that conveys sewage away from a pumping station, uphill or across a ridge where gravity grade is unavailable, until the alignment can resume gravity flow — without it, low-lying service areas could not be connected to the gravity collection network at all. |
| (b) Sanitary sewer | 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 dry-weather flow via a peaking factor (e.g. Harmon's formula). |
| (c) Sewage pumping station overflow | A relief structure that discharges flow in excess of the station's firm pumping/wet-well capacity during an abnormally high-flow event, protecting the station and the upstream collection system from surcharging and backup — at the cost of an intermittent untreated bypass, which is why firm capacity and overflow frequency are closely regulated design choices. |
