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.
The natural hydrologic cycle is the continuous circulation of water between the ocean, atmosphere and land: evaporation from open water lifts moisture into the atmosphere, which condenses and returns as precipitation; on land, precipitation is partitioned between surface runoff, infiltration, and evapotranspiration (including transpiration by vegetation), and both surface runoff and infiltrated water (as groundwater flow) eventually return to the ocean, closing the loop (Fig. 2).
Three key components/processes that control how much of a major storm's rainfall becomes surface runoff are: (1) infiltration capacity of the soil — a low-permeability or already-saturated soil forces rainfall in excess of its infiltration rate to run off (infiltration-excess/Hortonian runoff), so soil type and antecedent wetness directly set the runoff fraction; (2) land cover and interception — vegetation canopy and litter intercept and temporarily store rainfall (delaying and reducing the volume that reaches the ground at all), while impervious urban cover removes infiltration capacity entirely, so the same storm produces very different runoff on a forested versus paved catchment; and (3) catchment slope and drainage density — steep slopes and a dense natural or piped drainage network shorten the time water spends on the land surface before reaching a channel, concentrating the runoff into a sharper, higher flood peak than the same rainfall volume would produce on a flat, poorly-drained catchment.
Assumption: the two storms strike the same catchment within a few hours, i.e. well inside the soil's natural drainage/drying time, so the second storm begins with elevated antecedent moisture left by the first. Two ways antecedent soil moisture changes the runoff response are: (1) reduced available storage capacity — the soil's remaining storage deficit at the start of the second storm is smaller (or zero, if the first storm brought the soil to field capacity), so a larger fraction of the second storm's rainfall runs off immediately instead of infiltrating; using a simple storage-based model, if $S$ is the soil's total available storage and $S_1$ was already used by storm 1, the second storm's initial abstraction is reduced to $S-S_1$ before runoff begins, versus the full $S$ for a "dry" catchment. (2) reduced infiltration capacity itself — infiltration capacity (e.g. in the Horton model, $f = f_c + (f_0-f_c)e^{-kt}$) decays toward its lower, saturated-soil limit $f_c$ as moisture content rises, so the second storm starts closer to $f_c$ rather than the higher initial rate $f_0$ a dry soil would offer, meaning infiltration-excess runoff begins sooner and at a higher rate throughout the second event. Both effects point the same way: for an equal-magnitude second storm, the runoff coefficient (and therefore the peak and volume of runoff) is higher than it was for the first storm.
Two causes of sediment transport to a downstream river from an urbanized area are: (1) increased runoff velocity and volume from impervious surfaces — paving and roofing eliminate infiltration and shorten flow paths, so a much larger fraction of rainfall reaches channels quickly and at higher velocity, and higher-velocity flow both erodes exposed soil (construction sites, unprotected banks, unpaved shoulders) and has more capacity to carry the eroded sediment; and (2) construction/land-disturbance activity — grading, excavation and vegetation removal during urban development expose bare soil directly to rainfall-splash and sheet erosion, so sediment yield from an actively-developing catchment can be an order of magnitude higher than from the same area in a stable, vegetated condition.
Two ways to reduce that sediment transport are: (1) erosion-and-sediment control (ESC) at the source — silt fences, sediment traps/basins, mulching and staged/temporary vegetation on active construction sites intercept sediment before it reaches a watercourse; and (2) stormwater quantity/quality control facilities downstream (extended-detention or wet ponds, bioretention) that slow runoff velocity and provide settling time, both reducing the erosive energy delivered to the receiving channel and trapping sediment already mobilized before it reaches the river.
