Question 6 of 7: Stormwater Ponds and IDF-Based Peak Runoff Design

Problem 6: Stormwater Ponds and IDF-Based Peak Runoff 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) Purpose and Design Basis of Stormwater Dry Ponds

A dry pond temporarily detains stormwater runoff during a storm and releases it slowly afterward, standing empty between events. Its purpose in protecting downstream receiving waters is peak-rate attenuation: by storing the rising limb of the inflow hydrograph and metering the outflow through a small controlled orifice, the pond reduces the peak discharge delivered downstream to (or below) a target rate — typically the pre-development peak — preventing the erosive/scouring peak velocities and channel instability that an unattenuated, fully-developed peak would cause. The key design basis is storage–indication (level-pool) routing: the pond's stage–storage and stage–discharge relationships are combined with the inflow hydrograph to size the storage volume needed so the routed peak outflow meets the target rate for the design storm, with an emergency spillway provided for storms beyond the design event.

(ii) Wet Pond Design for a Downstream Fish Hatchery

(1) Permanent-pool volume sized to the target settling velocity. Under Hazen's ideal-settling theory, a particle is fully removed if the pond's surface overflow rate ($Q_{25}/A_{surface}$) is less than the particle's settling velocity; sizing the permanent pool's surface area (and hence its detention time) so that the 25-year design inflow's overflow rate stays below the settling velocity of the fine sediment fraction ensures suspended solids have time to settle before the pond discharges, even during the design storm. (2) An inlet forebay with a submerged (not surface-skimming) outlet. A separate forebay cell at the inlet traps coarse sediment and dissipates inflow turbulence before water reaches the main pool, and drawing the outlet from a mid-depth submerged riser (rather than the surface) avoids re-entraining floatables and avoids drawing from the turbulent, high-turbidity zone that forms near the inlet during a large storm — both features keep effluent turbidity low specifically during the high-flow event that would otherwise resuspend settled solids.

(iii) Composite Rational Method — Catchments A1 and A2

Given. Two contributing sub-areas draining to a common outlet, each with its own time of concentration to that outlet:

Find. The 100-year peak design discharge, in m³/min, for the combined catchment.

Approach. Because A1 and A2 have different times of concentration to the common outlet, the critical storm duration is not obvious a priori: evaluate the peak discharge at each candidate duration (each sub-area's own $t_c$), assuming the area whose $t_c$ has not yet been reached contributes only the fraction of its area proportional to elapsed time (a linear time-area assumption), and take the larger resulting $Q$ as governing.

1. Read/interpolate the 100-year intensity. Fitting $i=a\,t^{-b}$ to the four chart points and evaluating at 90 and 105 min: $$i_{90} \approx 2.50\ \text{in/hr} = 63.5\ \text{mm/hr}, \qquad i_{105} \approx 2.25\ \text{in/hr} = 57.2\ \text{mm/hr}.$$
2. Candidate duration $t=90$ min (A1 fully contributing; A2 contributes a $90/105$ fraction of its area). $$C_{eff}A_{eff} = C_1A_1 + C_2A_2\left(\frac{90}{105}\right) = (0.6)(30) + (0.7)(40)(0.857) = 42.0\ \text{ha (effective)},$$ $$Q_{90} = \frac{i_{90}\,C_{eff}A_{eff}}{360} = \frac{(63.5)(42.0)}{360} = 7.41\ \text{m}^3/\text{s} = \boxed{444.5\ \text{m}^3/\text{min}}.$$
3. Candidate duration $t=105$ min (both areas fully contributing). $$C_wA_{tot} = C_1A_1 + C_2A_2 = 18.0+28.0 = 46.0\ \text{ha}, \qquad Q_{105} = \frac{(57.2)(46.0)}{360} = 7.31\ \text{m}^3/\text{s} = 438.4\ \text{m}^3/\text{min}.$$
4. Governing case. $Q_{90} = 444.5\ \text{m}^3/\text{min} > Q_{105} = 438.4\ \text{m}^3/\text{min}$: the shorter, more intense storm governs, because the higher intensity at 90 min outweighs A2's not-yet-fully-contributing area. Design peak runoff $\boxed{\approx 444.5\ \text{m}^3/\text{min}}$.