Question 2 of 6: Hydrology Terminology

Question 2: Hydrology Terminology (20 marks)

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.

a. Infiltration. The process by which water at the ground surface enters the soil and moves downward through the unsaturated zone. Its rate is governed by soil texture, structure, antecedent moisture and surface cover, and it typically starts high (dry soil, steep matric-potential gradient) and declines toward a roughly constant equilibrium rate as the soil wets up and surface sealing/compaction reduces intake capacity — the basis of the Horton, Green–Ampt and SCS-CN infiltration models used in Question 4.

b. 100-yr flood flow. The peak discharge that has a 1% probability of being equalled or exceeded in any given year (its average recurrence interval, from a long-term frequency analysis, is 100 years). It is not "a flood that happens once every 100 years" in a deterministic sense — two 100-yr floods can occur in consecutive years — and it is the standard design/regulatory benchmark for floodplain mapping and major hydraulic structures in Canada.

c. Effective precipitation. The portion of total precipitation that becomes direct surface runoff (i.e. reaches the stream as storm flow) rather than being lost to infiltration, depression storage or evapotranspiration; also called rainfall excess. It is the quantity actually routed through a unit hydrograph to generate the storm hydrograph, and it is what Questions 4b and 5a solve for.

d. Unit Hydrograph. The direct-runoff hydrograph resulting from one unit depth (e.g. 1 mm or 1 in) of effective precipitation, generated uniformly over the watershed at a constant rate for a specified duration. Because watershed response is treated as linear and time-invariant, unit hydrographs of different durations can be combined (via convolution) to synthesize the runoff hydrograph for any storm of arbitrary intensity pattern.

e. Flow duration curve. A cumulative-frequency plot of streamflow, ranking all discharge observations (typically daily) from highest to lowest and plotting each against the percentage of time that flow is equalled or exceeded. It characterizes the full range of flow behaviour at a gauge — the steep left end shows flood flashiness, the flat right end shows baseflow sustainability — and is widely used for hydropower, water-supply reliability and low-flow (e.g. $Q_{7,10}$) design.

f. Flood routing. The procedure for predicting how a flood hydrograph changes shape (attenuates and lags) as it travels through a river reach or a storage element such as a reservoir or detention pond, by applying mass continuity ($dS/dt = I - O$) together with a storage–outflow relationship. Question 5's pond-attenuation problem is a simplified reservoir-routing application of this concept.

g. Water Budget. An accounting of all water entering, leaving and stored within a defined system (watershed, aquifer, reservoir) over a chosen time period, expressed as $P = ET + Q + \Delta S \pm$ (deep percolation/groundwater exchange), where $P$ is precipitation, $ET$ evapotranspiration, $Q$ runoff and $\Delta S$ change in storage. It is the fundamental conservation-of-mass check underlying every hydrologic model.

h. Level Pool Method. A simplified reservoir/detention-pond routing technique (also called the storage-indication method) that assumes the water surface in the storage element remains horizontal (level) at every instant, so storage and outflow are each single-valued functions of pool elevation only. Continuity is solved by combining the elevation–storage and elevation–discharge (e.g. weir) curves into a single storage–indication curve, avoiding the need to track spatial variation in the pool.

i. Plotting position. An empirical formula (e.g. Weibull, $p=m/(n+1)$) that assigns an exceedance probability to each ranked observation $m$ of $n$ years of record in a frequency analysis, allowing the data to be plotted against a theoretical distribution on probability paper. It provides the graphical/tabular check for goodness-of-fit used in Question 3b.

j. IDF curve. An Intensity–Duration–Frequency curve (or family of curves) giving the design rainfall intensity as a function of storm duration for a fixed return period, derived by frequency analysis of historical rainfall records at a station. It is the standard input for the Rational Method and for reading design intensities at any storm duration, as used directly in Question 1.