Drainage Calculators

Enter surface slope, allowable ponding, and drain capacity to calculate area-drain spacing using TR-55 sheet-flow and HEC-22 methods.

Calculate optimal channel dimensions for maximum hydraulic efficiency. Design channels with minimum excavation for a given discharge using rectangular, trapezoidal, or triangular sections.

Design bioretention facilities with underdrain systems for sites with poor native soil infiltration. Calculate required area, storage volumes, underdrain capacity, and infiltration split using ASCE and Virginia DEQ methodology.

Calculate HEC-18 long-term, live-bed or clear-water contraction, pier, and abutment scour components, optional armoring checks, and resulting design scour elevations.

Enter design inflow and inlet geometry to estimate HEC-22 weir/orifice inlet capacity, then screen outlet-pipe capacity with Manning's equation and preliminary basin and sump dimensions.

Design channel linings using permissible velocity or tractive force methods. Evaluate grass, riprap, concrete, and other lining adequacy for erosion protection.

Calculate head losses in channel transitions including contractions and expansions. Supports abrupt, gradual, warped, and wedge geometries with standard loss coefficients.

Calculate cistern sizing for rainwater harvesting. Determines collection potential, storage requirements, and annual water balance using ASABE S597.1 and ARCSA guidelines.

Select land cover or enter mixed land-use areas to look up Rational Method C values and calculate an area-weighted composite runoff coefficient.

Calculate flow in compound channels with multiple subsections using the divided channel method. Analyze main channel with floodplains, composite roughness, and velocity distribution.

Calculate critical depth, critical velocity, and minimum specific energy for circular, rectangular, trapezoidal, and triangular sections, where the Froude number equals 1.

Apply the FHWA culvert inspection decision process to condition ratings, performance problems, access constraints, and site triggers to screen maintenance, repair, lining, or replacement actions.

Calculate FHWA HDS-5 outlet-control headwater for circular and box culverts from tailwater, entrance, friction, and exit losses, with outlet velocity plus critical- and normal-depth results; check inlet control separately.

Screen circular culvert barrel size and full-flow capacity with Manning's equation, then continue to the separate FHWA HDS-5 outlet-control workflow; this tool explains inlet control but does not calculate inlet-control headwater.

Estimate preliminary detention pond storage with the Modified Rational Method and size orifice or weir outlets from watershed and allowable-release criteria; use level-pool routing for final design.

Enter roof area and rainfall intensity to select round or rectangular downspout sizes from IPC sizing tables.

Enter pipe size, material, design flow, and drainage type to calculate hydraulic fall or full-flow capacity and screen eligible foul-drain conditions against Approved Document H Table 6.

Calculate dry well sizing for stormwater infiltration. Determines storage volume, infiltration capacity, and drawdown time using ASCE MOP 77 methodology.

Design stilling basins, impact basins, and energy dissipation structures using FHWA HEC-14 and USBR methodology. Calculate basin dimensions, sequent depth, and energy dissipation.

Calculate foundation drain sizing for basement and crawlspace waterproofing. Determines pipe capacity, groundwater inflow, and sump pump requirements using HEC-22 methodology.

Enter trench, aggregate, pipe, and soil parameters to calculate French drain storage, infiltration, and pipe capacity using Darcy's law and Manning's equation.

Calculate Froude number and classify flow regime for open channels. Determine if flow is subcritical, critical, or supercritical. Includes wave celerity and specific energy calculations.

Calculate water surface profiles using the Direct Step Method. Analyze backwater and drawdown curves with automatic profile classification (M1, M2, S1, etc.) for open channels.

Calculate green roof stormwater retention and detention performance. Determine water holding capacity, runoff reduction, peak flow delay, and structural loads using FLL guidelines and ASCE methodology.

Enter gutter shape, dimensions, slope, and roughness to calculate flow capacity, velocity, hydraulic radius, and Froude number with Manning's equation.

Use the FHWA HEC-21 horizontal-grade equations to calculate maximum inlet spacing, minimum inlet perimeter, design discharge, contributing area, and curb depth for flat or near-flat roadway decks.

Calculate runoff hydrographs using unit hydrograph convolution. Applies the principle of superposition to combine rainfall increments with unit hydrograph response per NRCS NEH Part 630.

Design infiltration trenches for stormwater management. Calculate required trench dimensions, storage volume, infiltration capacity, and drawdown time using EPA and ASCE methodology.

Enter gutter flow and grate, curb-opening, or combination-inlet geometry to calculate HEC-22 interception efficiency, captured flow, and bypass flow.

Enter an inflow hydrograph plus stage-storage and stage-discharge data to route a level pool with the Modified Puls method and return the outflow hydrograph, peak attenuation, maximum stage, and storage.

Enter circular pipe diameter, slope, roughness, and flow depth to calculate full- or partial-flow discharge, velocity, and hydraulic properties with Manning's equation.

Analyze HEC-22 median and ditch drop inlets on grade or in sag locations, including trapezoidal-channel hydraulics, grate interception, bypass, clogging, and downstream channel-block height.

Enter runoff coefficient, drainage area, time of concentration, IDF data, and an allowable release rate to estimate peak discharge and preliminary detention storage across storm durations with the Modified Rational Method.

Enter an inflow hydrograph, time step, and Muskingum K and X parameters to calculate the routed outflow hydrograph, peak attenuation, and translation time.

Solve Manning's equation iteratively for uniform-flow normal depth in circular, rectangular, trapezoidal, and triangular sections, with velocity and flow-regime results.

Calculate discharge, velocity, hydraulic properties, or normal depth with Manning's equation for rectangular, trapezoidal, triangular, parabolic, and surveyed station/elevation channel shapes.

Calculate pipe and culvert outlet velocity plus cohesionless-soil scour depth, width, length, and volume with FHWA HEC-14 Equation 5.1; use the separate outlet-protection tool for riprap design.

Calculate permeable pavement system design including base course depth, storage volume, and infiltration capacity using ASCE and EPA methodology.

Calculate rain garden sizing for stormwater management. Determines required area, storage volumes, and infiltration capacity using ASCE and EPA methodology.

Enter runoff coefficient, rainfall intensity, drainage area, and return period to calculate Rational Method peak discharge (Q = CiA), adjusted C, and a transparent calculation breakdown.

Size a riprap apron for a circular culvert with FHWA HEC-14 Equations 10.4 and 10.5, selecting the next adequate riprap class and its apron length, end width, and blanket thickness.

Screen FHWA HEC-23 D50, nominal class, thickness, and application geometry for channels, bridges, overtopping, bottomless culverts, and wave attack, with bounded granular and geotextile filter checks.

Solve HEC-22 uniform or depressed-composite roadway gutter flow in either direction, returning spread, curb depth, flow, velocity, and the fraction carried in the depressed section.

Analyze Wolman, riprap, or calibrated-photo particle measurements to calculate D-percentiles, a percent-finer curve, sediment class, and compliance with project gradation envelopes.

Enter roof area and rainfall intensity to select primary drains, overflow drains, and scuppers from IPC sizing tables.

Generate SCS triangular unit hydrographs for simplified runoff analysis. Calculates peak discharge, time to peak, and runoff volume per TR-55 methodology.

Enter drainage area, runoff depth, and timing parameters to calculate peak discharge and generate an SCS/NRCS dimensionless unit hydrograph.

Generate SCS/NRCS 24-hour Type I, IA, II, or III design-storm distributions and tabulate cumulative rainfall, incremental depth, and rainfall intensity as a hyetograph.

Calculate direct runoff depth, retention, and initial abstraction with the SCS/NRCS runoff Curve Number method, or area-weight multiple land-cover and soil-group subareas into a composite CN.

Estimate preliminary detention storage with either a Modified Rational peak-flow approximation or one selected quick method (Baker, FAA, or simplified) using that method's inputs.

Enter design flow, slope, and pipe material to screen standard 8- to 36-inch RCP or HDPE storm-drain and stormwater-pipe sizes with Manning's equation and velocity guidance in US customary units.

Calculate sump pump capacity and pit sizing for basement drainage. Determines required pump capacity, total dynamic head, cycle frequency, and verifies motor protection requirements.

Fit basic Log-Pearson Type III or Gumbel distributions to complete positive systematic annual peaks for screening; EMA, MGBT, historical/censored data, and zero-flow handling are not included.

Calculate tailwater depth at culvert outlets with support for multiple channel types (rectangular, trapezoidal, triangular, natural). Analyze normal depth, critical depth, Froude number, flow regime, and outlet submergence. Optional rating curve generation.

Enter watershed length, slope, cover, and method-specific parameters to calculate time of concentration with Kirpich, FAA, NRCS Lag, Kerby-Hathaway, or TR-55 segmental methods.

Calculate trench drain capacity using Manning's equation. Determines flow capacity, velocity, and grate interception efficiency for linear drainage systems.

Design underground detention systems for stormwater management. Calculates storage volume, outlet sizing, and drawdown time per ASCE MOP 77 and FHWA HEC-22 methodology.

Calculate vegetated swale capacity, normal depth, velocity, freeboard, and Froude number with Manning's equation and vegetation-retardance roughness.

Calculate discharge or solve for head for rectangular, Cipolletti, four V-notch, broad-crested, and irregular weirs using the FHWA Hydraulic Toolbox coefficients and irregular-profile workflow.