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.
In priority sequence of their direct control over surface runoff generation from a rural (largely pervious) watershed: (1) Precipitation (intensity and duration) — the primary driver, since no precipitation input means no runoff regardless of the other processes. (2) Infiltration capacity of the soil (governed by soil type, antecedent moisture, and land cover) — controls what fraction of the precipitation input becomes surface runoff versus subsurface flow, and is the process most sensitive to management on a rural watershed. (3) Interception and depression storage / vegetative land cover — the abstraction that must be satisfied before overland flow begins at all, particularly significant on a lower-imperviousness rural watershed where canopy and surface storage remove a comparatively larger share of small events than on an urbanized catchment.
The Rational Method is only appropriate for small catchments (rule of thumb roughly under 80–200 ha, or time of concentration under about 30 minutes), so a large urban watershed discharging to a trunk sewer and then a river requires a hydrograph method rather than a single peak-$Q$ formula. The procedure: delineate sub-catchments tributary to the trunk; develop a design storm hyetograph (an IDF-based Chicago or SCS Type II distribution) at the governing return period; apply a unit-hydrograph or kinematic-wave/hydrologic-routing model to each sub-catchment to generate its runoff hydrograph; then route and combine the sub-catchment hydrographs — accounting for the travel time/lag along the trunk sewer itself — to the discharge point, giving both the design peak flow (for sizing the trunk) and the full hydrograph shape (needed to check the receiving river's capacity and any backwater influence on the outfall).
Junction inlet chamber. Collects the incoming surface sewer flow at the top of the structure and directs it into the vertical shaft, providing the transition from the gently sloped approach sewer to the near-vertical drop — it also allows access for inspection/maintenance at the point where flow direction changes most sharply.
Vertical shaft (adit). Conveys the flow down the full elevation difference between the surface sewers and the deep main tunnel in a controlled, contained manner, dissipating the drop's kinetic energy over the shaft's length rather than allowing an open uncontrolled fall — this protects the downstream tunnel invert and joints from the impact and cavitation damage that an unmanaged vertical drop would otherwise cause.
Deaeration chamber. Positioned where the shaft meets the main tunnel, it allows the air entrained by the falling water (and any air displaced ahead of the flow) to separate out and vent (via the vent shaft) before the flow enters the enclosed tunnel, preventing air-pocket surges, geysering at downstream vents, and pressurization of the tunnel crown that entrained air would otherwise cause.
