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) On-site and end-of-pipe stormwater management; wet-pond design (10 marks)

On-site systems (source controls) intercept runoff close to where it is generated — rain gardens, permeable pavement, rooftop/lot-level infiltration, rain barrels — reducing the volume and rate of runoff that ever enters the piped system, and thereby reducing peak flow and erosive energy delivered downstream at the source rather than after the fact. End-of-pipe systems (conveyance/regional controls) accept the runoff the piped network has already collected from a larger tributary area and detain or treat it just before discharge to the receiving watercourse — a regional dry pond, wet pond or constructed wetland at the outfall — attenuating the combined peak from many upstream lots/streets into a single controlled release that the downstream channel can convey without exceeding its erosive or flooding threshold. Used together, on-site controls reduce the volume/rate reaching the pipe network in the first place, while the end-of-pipe facility manages what remains, giving redundancy and treating both frequent small storms (best handled by distributed on-site controls) and the rarer large design storm (best handled by a sized regional facility).

Two key design principles for a wet pond: (1) a permanent pool (dead storage) sized for water-quality treatment — the permanent pool must provide enough detention time (commonly 24–48 hours-equivalent, expressed via a treatment-volume criterion) for suspended sediment and adsorbed pollutants to settle before the next storm displaces the pool; and (2) separate, additional live storage above the permanent pool for peak-flow attenuation, released through a controlled (typically orifice or weir) outlet sized so the post-development peak discharge for the design storm(s) does not exceed the pre-development peak. Two objectives of a wet pond: (1) water-quality improvement via settling and biological uptake within the permanent pool, and (2) peak-flow attenuation/erosion control, protecting the downstream channel from the higher peak rates and erosive energy that uncontrolled post-development runoff would produce.

(ii) Three methods of runoff control (10 marks)

1. Detention/retention ponds (on-site or regional). Advantage: effective, well-understood peak-attenuation for a wide range of storm sizes, and can be combined with water-quality treatment (wet ponds) in one facility. Disadvantage: requires significant land area and a permanent maintenance commitment (sediment removal, outlet structure upkeep, vector/safety management), which is often unavailable or costly in dense infill development.
2. Infiltration practices (infiltration trenches, permeable pavement, soakaways). Advantage: reduces runoff volume (not just peak rate), helping to preserve pre-development groundwater recharge and baseflow, with a relatively small surface footprint. Disadvantage: performance is highly sensitive to native soil infiltration rate and depth to water table/bedrock, and the facility can clog or fail prematurely without regular maintenance and adequate pre-treatment of sediment-laden inflow.
3. Conveyance oversizing / in-line storage (oversized trunk pipe or storage tunnel with a flow-control outlet). Advantage: attenuates peak flow within the existing right-of-way/pipe corridor without requiring additional surface land. Disadvantage: low-velocity storage segments are prone to sediment deposition and require dedicated access/flushing provisions, and retrofitting an existing trunk sewer with a flow-control structure is often costly and disruptive.