US Customary
Rational Method
Q = C Β· I Β· A
Peak runoff in cfs. Enter NOAA IDF intensities below, choose a return period, then enter Tc to interpolate I automatically.
Rainfall Intensity β€” IDF Entry
Upload a NOAA IDF screenshot for reference, then enter intensities (in/hr) for each duration and return period.
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Drop NOAA screenshot or tap to browse
IDF ref
Return Period:
Enter the 10-min and 60-min intensities from your NOAA IDF table. Tc is interpolated on a log-log curve between these two points.
Land Use Grp AHigh Infil. Grp BModerate Grp CSlow Grp DVery Slow
Click any row to apply the midpoint Group B value to C below.
Peak Flow Q
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Sharp-Crested Rectangular Weir
Q = (2/3) Β· Cd Β· sqrt(2g) Β· L Β· H^1.5
Francis formula, US customary. g=32.174 ft/sΒ². L and H in feet, Q in cfs.
Flow Q
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Broad-Crested Weir
Q = 3.087 Β· Cd Β· L Β· H^1.5
US customary critical-flow form. Cd typically 0.84–0.87. L and H in feet, Q in cfs.
Flow Q
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Basin Drawdown β€” Stage-Storage Routing
dS/dt = Qin - Qout(stage)
Define a stage-storage curve and up to 4 outlet structures. The model time-steps through drawdown. US customary: acres, ft, inches, cfs.
Stage-Storage Curve
Stage = water surface elevation (ft). Enter at least 2 points. Always include Stage = 0 ft, Storage = 0 as the first row β€” this anchors the curve bottom so the routing can drain to your target stage correctly.
Storage in:
Stage (ft)
Storage (ac-ft)
Outlet Structures (up to 4)
Simulation Settings
Total Drawdown Time
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Routing Table β€” Every Time Step
Manning's Equation
Q = (1.486/n) Β· A Β· R^(2/3) Β· S^(1/2)
US customary open-channel flow. Select a channel shape, then enter dimensions in feet, slope in ft/ft. Q in cfs.
Flow Q
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Orifice Flow
Q = Cd Β· A Β· sqrt(2g Β· H)
Cd β‰ˆ 0.6 sharp-edged, 0.82 well-rounded. Dimensions in inches, H in ft, Q in cfs.
Flow Q
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Culvert Capacity
Inlet: Q=CdΒ·AΒ·sqrt(2gΒ·HW) | Outlet: Manning's energy
Calculates inlet- and outlet-controlled capacity. Governing (lower) Q is reported. Circular diameter in inches, box in ft, heads in ft.
Culvert Capacity Q (governing)
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Inlet & Catch Basin Capacity
Grate: Q=CdΒ·PΒ·d^1.5 / CdΒ·A·√(2gd)  |  Curb: Q=CwΒ·LΒ·d^1.5
Interception capacity for grate, curb-opening, combination, and slotted-drain inlets. Based on HEC-22 (FHWA). Gutter spread and depth computed from approach flow. US customary throughout.
Gutter / Approach Flow
Grate Parameters
Intercepted Flow Qi
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Basin Routing β€” Modified Puls Method
2S/Ξ”t + Oβ‚‚ = (I₁+Iβ‚‚) + (2S/Ξ”t βˆ’ O)₁
Routes an inflow hydrograph through a detention basin using the Modified Puls (storage-indication) method. Define a stage-storage curve, outlet structures, and paste or enter an inflow hydrograph. Outputs peak outflow, peak stage, and a routed hydrograph table.
Stage-Storage Curve
Always include Stage = 0 ft, Storage = 0 as the first row. Stage = water surface elevation (ft).
Storage in:
Stage (ft)
Storage (ac-ft)
Outlet Structures (up to 4 β€” same types as Basin Drawdown)
Inflow Hydrograph
Enter time (min) and flow (cfs) pairs. Paste two columns from a spreadsheet, or add rows manually. Times must be evenly spaced.
Time (min)
Inflow Q (cfs)
Or paste two columns (time, flow) from a spreadsheet:
Routing Settings
Peak Outflow
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Routed Hydrograph
Time of Concentration β€” Tc
Tc = Ξ£ t_sheet + Ξ£ t_shallow + Ξ£ t_channel + Ξ£ t_pipe
Builds Tc by summing travel times for any combination of flow segments. Add segments in order from upstream to outlet. Methods: TR-55 Sheet Flow, TR-55 Shallow Concentrated Flow, Open Channel (Manning's), and Pipe Flow.
Flow Path Segments (add in upstream β†’ downstream order)
Total Time of Concentration
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Segment Breakdown
TR-55 Manning's n β€” Sheet Flow
Surface Description n
Street & Gutter Flow Capacity
Q = (1.486/n) Β· S_x^(5/3) Β· S_L^(1/2) Β· T^(8/3) / (Ku Β· ...)
Computes gutter spread T and ponding depth d for a given flow, or solves for Q given spread. Based on HEC-22 (FHWA 3rd Ed.) for uniform cross-slope gutters. US customary: ft, ft/ft, cfs.
Road Geometry
Spread Width T
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HEC-22 Design Guidelines
Max Spread T
Local roads: T ≀ Β½ lane width (typically ≀ 6 ft) for minor storms
Collectors: T ≀ driving lane (12 ft) for 10-yr; no curb overtopping 100-yr
Arterials: T ≀ shoulder + bike lane for 10-yr; no median flooding 100-yr
Typical Cross-Slopes Sx
Pavement: 0.015–0.040 ft/ft (2% typical)
Depressed gutter: Sw = 0.083 ft/ft (1 in/ft) typical
Min longitudinal slope: 0.003 ft/ft to maintain self-cleaning
Triangular (V-Notch) Weir
Q = (8/15) · Cd · √(2g) · tan(θ/2) · H^(5/2)
V-notch weir for low-flow measurement. US customary: H in feet, Q in cfs. ΞΈ is the full included notch angle. Cd β‰ˆ 0.611 for a sharp-edged 90Β° notch.
For a 90Β° V-notch: Q = 2.50Β·CdΒ·H^2.5. Accurate for H between 0.2–2.0 ft. Keep H/P > 0.1 and H < P (P = crest height above channel floor).
Flow Rate Q
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Common V-Notch Cd Values
Angle Cd (sharp-edged) Notes
22.5Β°0.616Low-flow precision
30Β°0.614
45Β°0.613
60Β°0.612
90Β°0.611Most common; Francis eq.
120Β°0.608Higher flow range
Trapezoidal & Cipolletti Weir
Q = (2/3)·Cd·√(2g)·L·H^(3/2) + (8/15)·Cd·√(2g)·tan(θ/2)·H^(5/2)
Combines a rectangular base with triangular end sections. The Cipolletti weir is a special case with side slopes of 1H:4V (14.04Β°) that compensates for end contractions, allowing the rectangular formula to apply directly.
Cipolletti: z = 0.25 (1H:4V). Q = 3.367Β·CdΒ·LΒ·H^1.5 in US customary. For general trapezoidal, the triangular side contributions are added separately.
Flow Rate Q
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