Question 3 of 7

Problem 3 (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.

(i) The natural hydrologic cycle — five components, five interactions (8 marks)

The five key components are the atmosphere/cloud layer (moisture storage and transport), the vegetated land surface (interception and the source of surface runoff), the unsaturated (soil) zone, the groundwater aquifer (saturated zone) and the river/receiving water body that ultimately collects the basin's discharge. The five main interactions linking them are:

1. Precipitation — moisture condenses in the atmosphere and falls to the land surface as rain or snow, the primary input to the terrestrial part of the cycle.
2. Evapotranspiration — combined direct evaporation from soil/water surfaces and transpiration through vegetation returns moisture from the land surface back to the atmosphere, closing the loop without the water ever reaching a stream.
3. Surface runoff (overland flow) — precipitation in excess of the infiltration capacity and depression storage flows overland to the nearest channel, the dominant fast pathway that produces a storm hydrograph's peak.
4. Infiltration — water entering the soil surface percolates through the unsaturated zone, replenishing soil moisture and, where it exceeds field capacity, continuing downward as recharge.
5. Groundwater discharge (baseflow) — recharge that reaches the water table moves slowly through the aquifer and eventually discharges to the river, sustaining streamflow between storms and through dry periods when surface runoff has ceased.

(ii) IDF analysis and the hydrologic cycle in 'major' system design (6 marks)

'Major' stormwater system components (overland flow routes, road rights-of-way used as an emergency conveyance, open channels and detention facilities) are explicitly designed for the rare, large storm — typically the 100-year event — that exceeds the capacity of the buried 'minor' pipe network. IDF analysis supplies the design rainfall intensity for that rare event at the catchment's actual time of concentration, but that intensity only becomes a design discharge once it is filtered through the hydrologic-cycle processes that determine how much of it becomes runoff: antecedent soil moisture and infiltration capacity (already-saturated ground during a major storm produces a much higher runoff coefficient than the same storm on dry ground), the shape and time-lag of the resulting overland-flow hydrograph, and any snowmelt contribution that can coincide with a spring rainfall event. The major system is therefore sized using the 100-year IDF intensity combined with a conservatively high runoff coefficient (reflecting saturated antecedent conditions), so that when the minor system surcharges, the major system's overland routes and channels can still safely convey the excess to the receiving water without flooding structures.

(iii) The hydrologic cycle and 'minor' system components (6 marks)

Two important linkages are: (1) infiltration and inflow (I/I) into the piped minor system — the same infiltration process that recharges groundwater in the natural cycle also finds its way into aging, cracked sanitary and combined sewer pipes below the water table, adding a base "clear water" load that must be accounted for in minor-system capacity alongside the sanitary or storm design flow; and (2) time of concentration and the minor system's design return period — the minor (pipe) system is sized using a shorter-return-period, higher-frequency IDF curve (commonly 2–10 year) at the catchment's time of concentration, which is itself a hydrologic-cycle quantity governed by overland-flow travel time, so the same rainfall-runoff processes that generate the major system's flood peak also directly set the minor system's day-to-day design discharge, just evaluated at a more frequent, smaller storm.