Storm Calc Suite

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US Customary

Rational Method

Q = C · I · A

Peak runoff in cfs. Enter NOAA IDF intensities below, choose a return period, then enter Tc to interpolate I automatically.

Rainfall Intensity — IDF Entry

Upload a NOAA IDF screenshot for reference, then enter intensities (in/hr) for each duration and return period.

Upload a NOAA IDF screenshot for reference:

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Drop NOAA screenshot or tap to browse

Return Period:

10 min in/hr

60 min in/hr

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.

Tc min

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.

C 0–1

I in/hr

A acres

Peak Flow Q

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Sharp-Crested Rectangular Weir

Q = (2/3) · Cd · sqrt(2g) · L · H^1.5

Francis formula, US customary. g=32.174 ft/s². L and H in feet, Q in cfs.

Crest Length L ft

Head H ft

Flow Q

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Broad-Crested Weir

Q = 3.087 · Cd · L · H^1.5

US customary critical-flow form. Cd typically 0.84–0.87. L and H in feet, Q in cfs.

Crest Length L ft

Head H ft

Flow Q

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Sump (Low Point) Grate Inlet

Weir: Q = Cw·P·d^1.5 | Orifice: Q = Cd·A·√(2g·d)

Computes capacity of a grate inlet in a sump condition (no bypass — all flow must enter). Evaluates both weir and orifice equations per HEC-22 and reports which governs. Use for low points, parking lot drains, and ponded areas. US customary throughout.

Grate Type

Grate Length L ft

Grate Width W ft

Orifice Coeff Cd

Weir Coeff Cw default 3.0

Ponding Depth d ft

Depth of water above the grate

Clogging Factor 0–1

HEC-22 recommends 0.5 (50%) for design

Solve For

Governing Capacity Q

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HEC-22 Sump Transition Criteria

Weir controls when d < 0.4 ft — Q = Cw · P · d^1.5
Orifice controls when d > 1.4 ft — Q = Cd · A · √(2g·d)
Transition zone 0.4 ≤ d ≤ 1.4 ft — interpolated between weir and orifice
Perimeter P = 2(L+W). Effective area Ae = L·W·(1-clog). Clogging only affects orifice regime. HEC-22 recommends 50% clogging for design.

Basin Drawdown — 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 — this anchors the curve bottom so the routing can drain to your target stage correctly.

Storage in:

Stage (ft)

Storage (ft³)

Outlet Structures (up to 4)

Simulation Settings

Initial Stage ft

Final Stage ft

Continuous Inflow cfs

Routing Time Step min

Internal calc step. Smaller = more accurate.

Table Display Interval min

How often to show a row in the output table.

Total Drawdown Time

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Routing Table — Every Time Step

Manning's Equation

Q = (1.486/n) · A · R^(2/3) · S^(1/2)

US customary open-channel flow. Select a channel shape, then enter dimensions in feet, slope in ft/ft. Q in cfs.

Channel Section Type

Surface / Channel — n

n (override)

Bottom Width b ft

Water Depth y ft

Side Slope z H:1V

Slope S ft/ft

Flow Q

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Orifice Flow

Q = Cd · A · sqrt(2g · H)

Cd ≈ 0.6 sharp-edged, 0.82 well-rounded. Dimensions in inches, H in ft, Q in cfs.

Shape

Diameter in

Head H ft

Flow Q

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Culvert Capacity

Inlet: Q=Cd·A·sqrt(2g·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.

Shape

Diameter in

Number of Barrels

Length ft

Headwater HW ft

Tailwater TW ft

Slope S ft/ft

Material — n

Manning's n

Inlet Type — Ke

Ke (inlet loss)

Culvert Capacity Q (governing)

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Inlet & Catch Basin Capacity

Grate: Q=Cd·P·d^1.5 / Cd·A·√(2gd) | Curb: Q=Cw·L·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.

Inlet Type

Gutter / Approach Flow

Gutter Flow Q cfs

Cross-Slope Sx ft/ft

Longitudinal Slope SL ft/ft

Gutter Manning's n

n (override)

Grate Parameters

Grate Type — Cd

Grate Length L ft

Grate Width W ft

Depth at Grate d ft (0=auto)

Clogging Factor 0–1

Intercepted Flow Qi

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Basin Routing — Modified Puls Method

2S/Δt + O₂ = (I₁+I₂) + (2S/Δt − 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 — 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

Initial Stage ft

Time Step Δt min

Peak Outflow

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Routed Hydrograph

Time of Concentration — Tc

Tc = Σ t_sheet + Σ t_shallow + Σ t_channel + Σ 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 → downstream order)

Total Time of Concentration

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Segment Breakdown

TR-55 Manning's n — Sheet Flow

Surface Description	n

Street & Gutter Flow Capacity

Q = (1.486/n) · S_x^(5/3) · S_L^(1/2) · T^(8/3) / (Ku · ...)

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.

Gutter Section Type

Manning's n — Gutter/Pavement

n (override)

Road Geometry

Cross-Slope S_x ft/ft

Longitudinal Slope S_L ft/ft

Road Width (1 side) ft

Curb Height in

Gutter Flow Q cfs

Spread Width T

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HEC-22 Design Guidelines

Max Spread T
Local roads: T ≤ ½ 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. θ is the full included notch angle. Cd ≈ 0.611 for a sharp-edged 90° notch.

Notch Angle Preset

Notch Angle θ degrees (full)

Discharge Coeff Cd

Head over Notch H ft

For a 90° V-notch: Q = 2.50·Cd·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
22.5°	0.616	Low-flow precision
30°	0.614
45°	0.613
60°	0.612
90°	0.611	Most common; Francis eq.
120°	0.608	Higher 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 (14.04°) that compensates for end contractions, allowing the rectangular formula to apply directly.

Weir Type

Crest Length L ft

Head H ft

Discharge Coeff Cd

Cipolletti: z = 0.25 (1H:4V). Q = 3.367·Cd·L·H^1.5 in US customary. For general trapezoidal, the triangular side contributions are added separately.

Flow Rate Q

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