Peak runoff Q = C·i·A using the Rational Method. Composite catchment with up to 6 weighted sub-areas, time of concentration (Kirpich / overland flow), IDF curve interpolation, Manning pipe sizing, and hydraulic gradient line (HGL) check.
Free preview: Q, Tc, C, DN. Sign in as Pro or Civil to unlock pipe sizing table, HGL analysis, and step-by-step calculation.
The Rational Method (Q = CiA) estimates peak stormwater runoff as the product of the runoff coefficient C, design rainfall intensity i (mm/hr), and catchment area A (ha), divided by 360 to convert to m3/s. It is widely used for urban drainage design up to about 300 ha (EN 1991-1-3, ASTM D2321).
FrameAI uses the Kirpich equation by default: Tc (min) = 0.01947 * L^0.77 * S^(-0.385), where L is the overland flow length in meters and S is the slope (m/m). For flat terrain or complex catchments, switch to the overland-flow kinematic wave: Tc = 0.007 * L^0.8 * S^(-0.4). Both are empirical best-estimate methods.
When a catchment contains multiple surface types (e.g. 60% asphalt, 40% lawn), the composite C is the area-weighted average: C_w = SUM(A_i * C_i) / SUM(A_i). This reflects that impervious areas produce much more runoff than pervious areas. FrameAI supports up to 6 sub-areas per calculation.
Manning's equation for full flow in a circular pipe: Q = (1/n) * A * R^(2/3) * S^(1/2), where A = pi*D^2/4 and R = D/4. FrameAI tests standard DN sizes (150-900 mm) and selects the smallest DN that carries the design Q at full flow with velocity between 0.6 m/s (self-cleansing) and 3.0 m/s (erosion limit).
The Hydraulic Gradient Line (HGL) is the total energy grade line in the pipe. At the inlet, HGL = invert elevation + h_elbow (90 deg bend, K=0.9) + h_friction (S*L). The pipe crown must be above the HGL to avoid pressurised flow and surcharging. FrameAI checks crown >= HGL and flags PASS/FAIL.
FrameAI requires at least 3 user-supplied IDF points (duration, intensity pairs) to define the design storm intensity-duration relationship. You can use regional IDF tables or standard curves (e.g. API 5.1, EUR 147) to populate these fields. The tool supports any IDF data format — just enter 3+ data points.