Question 6 of 7: Urban Stormwater Wet Ponds and IDF-Based Rational Method Design

Problem 6: Urban Stormwater Wet Ponds and IDF-Based Rational Method Design (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) Wetpond Design for Quality Control — Cold-Water Fishery Protection

A stormwater wetpond designed primarily for quality control ahead of a sensitive cold-water fishery is sized around the water-quality volume (WQV) — typically the runoff volume from a small, frequent "first-flush" storm (e.g., the 90th-percentile storm, commonly on the order of 25–40 mm of rainfall depending on jurisdiction) that captures the bulk of the annual pollutant load, since most solids and associated contaminants wash off in the early, smaller, more frequent events rather than the rare large storm. The pond maintains a permanent pool at least equal to the WQV so that incoming storm inflow displaces standing water gradually, giving particulates (and particulate-bound nutrients/metals) enough quiescent residence time (commonly a target of 24–48 hours mean hydraulic residence time) to settle rather than pass straight through.

Because the receiving water supports a cold-water fishery, two additional design features are essential beyond a generic wetpond: (1) thermal control — a shallow, sun-exposed permanent pool warms stormwater in summer, so the outlet is designed to draw from near the pond bottom (a submerged/inverted outlet or extended detention with a low-level withdrawal) rather than skimming warm surface water, and shading/vegetated buffers around the pond further limit solar heating; and (2) extended detention plus a forebay — a small sediment forebay at the inlet captures coarse solids before they reach (and disturb) the main pool, and extended detention of the WQV (draining over 24+ hours through a well-vegetated littoral shelf) promotes biological uptake of dissolved nutrients and organics in addition to physical settling of solids, which a purely physical (quantity-only) detention pond would not provide. Quantity control (peak attenuation of the 2- to 100-year design storms) is layered on top as additional temporary storage above the permanent pool, released through a multi-stage outlet structure so that the 1:1 pre/post development peak-flow match required to protect the downstream channel does not compromise the water-quality function below.

(ii) Rational Method 50-Year Peak Runoff — Catchments A1 and A2

Given.

Find. $Q_1$, $Q_2$ (and the combined peak $Q$) for the 50-year event.

Approach. Use the metric Rational Formula $Q=\dfrac{CiA}{360}$ ($Q$ in m³/s, $i$ in mm/h, $A$ in ha), applied to each sub-catchment at its OWN time of concentration — the design duration for the Rational Method is always $t=t_c$, since that is the shortest storm duration over which the entire catchment area is simultaneously contributing runoff to the outlet, giving the highest $i\times A$ product for that catchment.

1. Read the 50-year intensities from the IDF chart. At $t_c=60$ min: $i_1\approx68.4$ mm/h. At $t_c=70$ min: $i_2\approx61.9$ mm/h (both on the Tr = 50-year curve).
2. Peak runoff, catchment A1. $$Q_1=\frac{C_1\,i_1\,A_1}{360}=\frac{0.7\times68\times30}{360}=\boxed{3.99\ \text{m}^3/\text{s}}.$$
3. Peak runoff, catchment A2. $$Q_2=\frac{C_2\,i_2\,A_2}{360}=\frac{0.8\times62\times40}{360}=\boxed{5.50\ \text{m}^3/\text{s}}.$$
4. Combined peak at the common outlet. If A1 and A2 drain to the same point, the simple sum $$Q=Q_1+Q_2=\boxed{9.49\ \text{m}^3/\text{s}}$$ is a reasonable (slightly conservative) estimate here since the two $t_c$ values are close (60 vs. 70 min); a more rigorous composite analysis would route each sub-area's hydrograph to the common point and add ordinates in time rather than simply summing the two independently-computed peaks.