Peak runoff control to PUB Code of Practice on Surface Water Drainage (COP-SWD) Cl. 7.1.5, sized with the PUB Technical Guide for On-site Stormwater Detention Tank Systems (TG-DT). Yellow cells are inputs.
| Description | Symbol | Sub-catchment 1 | Sub-catchment 2 | Sub-catchment 3 | Sub-catchment 4 | Reference |
|---|
| Total site area | Asite (ha) | Σ sub-catchments | |
| Time of concentration for the whole site | tc,site (min) | TG-DT Table 3.1.1 (auto by site area; editable). COP-SWD Cl. 7.1.4 | |
| 10-yr rainfall intensity at tc,site | i10 (mm/hr) | TG-DT Eq. 3.1.3: i10 = 8913/(tc+36) | |
| Maximum allowable peak discharge | Qallowable (m³/s) | TG-DT Eq. 3.1.5; COP-SWD Cl. 7.1.5 (C = 0.55, 10-yr storm) | |
| Check Σ Qtarget ≤ Qallowable | – | TG-DT Eq. 3.2.1 |
Site-tc auto values (TG-DT Table 3.1.1): 0.2–2 ha → 5 min; 2–6 ha → 10 min; 6–10 ha → 15 min; ≥10 ha → 15 min (a longer tc must be substantiated by hydraulic calculation). COP-SWD Cl. 7.1.5 applies to developments ≥ 0.2 ha.
| Description | Value | Reference |
|---|
| Description | Value | Reference |
|---|
Routing to TG-DT Eq. 3.4.5: I − Q = ΔV/Δt, so Vn = Vn−1 + Δt·(I − Q) and d = V/At. Inflow is the trapezoidal hydrograph of TG-DT Appendix C: it rises linearly to Qz at tc, stays at Qz to the end of rainfall tz, then recedes linearly to zero over tc. Inflow over each step is the average of the start and end values. Each pump runs while the water depth is at or above its start depth (no separate stop level). Results can differ slightly from a spreadsheet that uses another routing convention.
| Formula | Source |
|---|---|
| Q = C·i·A / 360 (Q m³/s, i mm/hr, A ha) | COP-SWD Cl. 7.1.1 Rational Formula; TG-DT Eq. 3.1.1 |
| Runoff coefficient: paved 1.00, urban 0.90, vegetated/pervious 0.45; weighted C allowed | COP-SWD Cl. 7.1.2 (as amended by Addendum No. 3) |
| Cpost = Σ(Ai·Ci) / ΣAi | TG-DT Eq. 3.1.2 |
| tc = to + td; overland time 5–15 min | COP-SWD Cl. 7.1.4; typical values TG-DT Table 3.1.1 |
| i10 = 8913 / (tc + 36) mm/hr | TG-DT Eq. 3.1.3 (10-yr fit to the IDF curves in COP-SWD Appendix 4, Cl. 7.1.3) |
| Qpost = Cpost·i10·A / 360 | TG-DT Eq. 3.1.4 |
| Qallowable = 0.55·i10·Asite / 360; 10-yr storms of all durations up to 4 h | COP-SWD Cl. 7.1.5; TG-DT Eq. 3.1.5 |
| Σ Qtarget ≤ Qallowable | TG-DT Eq. 3.2.1 |
| Choice of detention system and design method | TG-DT Table 3.3.1; COP-SWD Appendix 6 (General Requirements for Detention Tank) |
| Vt = Qinflow(tc+tx) − ½(2tc+tx)Qtarget | TG-DT Eq. 3.4.1 (Modified Rational Method) |
| K1 = 8913·Cpost·A/6; K2 = 8913·Ctarget·A/12; K3 = tc+36; tx,max = −K3 + √(36·K1·K3/K2) | TG-DT Eq. 3.4.2 |
| tx,limit = (Cpost − Ctarget)(tc+36)/Ctarget; tx,critical = min(tx,max, tx,limit) | TG-DT Eq. 3.4.3 and Section 3.4.1.2 |
| Vt = K1(tc+tx)/(K3+tx) − K2(2tc+tx)/K3 (m³) | TG-DT Eq. 3.4.4 |
| I − Q = ΔV/Δt (Δt ≤ 1 min) | TG-DT Eq. 3.4.5; Appendix C (Template C) |
| tz = tx,critical + tc; iz = 8913/(tz+36); Qz = Cpost·iz·A/360 | TG-DT Appendix C, Step 2 |
| Qo = Co·Ao·√(2·g·Ho), g = 9.81 m/s², Ho = max head to centre of orifice | TG-DT Eq. 3.5.1 |
| At = Vt / dt | TG-DT Appendices B and C (detention system configuration) |
This calculator is a design aid. The Qualified Person / PE (Civil) remains responsible for the design and must check every input and result against the current editions of the documents above before submission. Under COP-SWD Cl. 7.1.5 the peak runoff calculations must be endorsed by a PE (Civil).