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 a Stormwater Dry Pond

A dry pond temporarily detains stormwater runoff during a storm and releases it slowly afterward through a small controlled outlet, standing empty between events. Its purpose in controlling downstream erosion is peak-rate attenuation: by storing the rising limb of the inflow hydrograph and metering the outflow, 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 that an unattenuated developed-condition peak would otherwise cause in the receiving channel. The key design basis is storage–indication (level-pool) routing: the pond's stage–storage and stage–discharge relationships are combined with the design inflow hydrograph to size the storage volume needed so the routed peak outflow meets the downstream target rate, with an emergency spillway sized for storms beyond the design event.

(ii) Wet Pond Maintenance and Operations (25-Year Design Life)

Three important recurring maintenance/operational activities keep a wet pond's effluent total suspended solids (TS) consistently low over its 25-year design life. (1) Periodic sediment (forebay) removal. Accumulated sediment in the inlet forebay and main pool progressively reduces the pond's effective permanent-pool volume and detention time; scheduled dredging/dewatering (typically every 5–15 years, or sooner if a bathymetric survey shows the forebay has filled) restores the design detention time and settling performance. (2) Vegetation and buffer management. Mowing/maintaining the littoral (aquatic bench) and buffer vegetation controls resuspension from wave action and wildlife, maintains the intended flow path length (preventing short-circuiting), and prevents woody growth from compromising the embankment or outlet structure. (3) Inlet/outlet structure inspection and debris removal. Regular inspection of the inlet energy dissipator, the submerged outlet riser/orifice and the emergency spillway for clogging, erosion or structural damage ensures the pond continues to route flow through its designed detention path (rather than short-circuiting or bypassing) and prevents an uncontrolled release of previously-settled solids during a subsequent storm.

(iii) Composite Rational Method — Catchments A1 and A2

Given. Two catchment areas, each with its own runoff coefficient and time of concentration:

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

Approach. Read the $T=100$ intensity off the IDF chart at each area's own time of concentration, convert to mm/hr, then apply $Q=CiA/360$ to each area and sum for the combined peak.

1. Read the 100-year intensities. Tracing the printed $T=100$ curve directly: $i_{70}=3.02$ in/hr at A1's $t_c=70$ min, $i_{80}=2.78$ in/hr at A2's $t_c=80$ min. Converting: $$i_{A1}= 3.02\times25.4 = \boxed{76.8\ \text{mm/hr}}, \qquad i_{A2}=2.78\times25.4=\boxed{70.6\ \text{mm/hr}}.$$
2. Peak discharge, catchment A1. $$Q_1 = \frac{C_1i_{A1}A_1}{360} = \frac{(0.8)(76.8)(30)}{360} = 5.12\ \text{m}^3/\text{s} = \boxed{307\ \text{m}^3/\text{min}}.$$
3. Peak discharge, catchment A2. $$Q_2 = \frac{C_2i_{A2}A_2}{360} = \frac{(0.6)(70.6)(40)}{360} = 4.71\ \text{m}^3/\text{s} = \boxed{283\ \text{m}^3/\text{min}}.$$
4. Combined design peak. With each area's own routing time already reflected in its own $t_c$, the combined 100-year peak at the common downstream point is $$Q_{tot}=Q_1+Q_2 = 5.12+4.71 = 9.83\ \text{m}^3/\text{s} = \boxed{589.6\ \text{m}^3/\text{min}}.$$