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.

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.

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 for open channels and pipes. Determine the depth at which specific energy is minimum and Froude number equals 1. Supports circular, rectangular, trapezoidal, and triangular sections.

Enter circular culvert diameter, slope, and roughness to estimate full-flow capacity with Manning's equation and review inlet- versus outlet-control guidance from FHWA HDS-5.

Calculate outlet control hydraulics for culverts including headwater depth, outlet velocity, critical depth, normal depth, and head loss breakdown. Uses FHWA HDS-5 methodology for circular and box culverts.

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.

Enter annual peak flows to estimate return-period magnitudes with Log-Pearson Type III following USGS Bulletin 17C, optionally alongside a separate Gumbel analysis.

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.

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 channel geometry, slope, roughness, and depth to calculate discharge and velocity with Manning's equation for rectangular, trapezoidal, triangular, or parabolic sections.

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

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.

Calculate normal depth for open channels and pipes using Manning's equation. Iteratively solve for the depth at which uniform flow occurs. Supports circular, rectangular, trapezoidal, and triangular sections.

Design riprap aprons and outlet protection structures for culverts using FHWA HEC-14 methodology. Calculate D50 stone size, blanket thickness, and apron dimensions.

Calculate outlet velocities from pipes and culverts to assess erosion potential. Determine scour depth, scour hole dimensions, and protection requirements using FHWA HEC-14 methodology.

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

Enter watershed and release criteria to estimate preliminary detention storage with the Modified Rational Method and size orifice and weir outlets.

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

Generate SCS/NRCS 24-hour rainfall distributions (Type I, IA, II, III) for design storms. Creates hyetographs showing rainfall intensity over time per TR-55 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.

Calculate riprap stone sizing using Isbash, USACE, HEC-11, or Maynord methods. Determine D50 median stone size, gradation requirements, and blanket thickness for channel protection.

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

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

Enter rainfall and a selected or area-weighted composite CN to calculate direct runoff depth, retention, and initial abstraction with the SCS/NRCS Curve Number method.

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.

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 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.

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.

Enter drainage area and applicable basin characteristics to get an educational, screening-level peak-flow estimate from generalized regional equations; use state-specific USGS publications or StreamStats for design.

Calculate vegetated swale hydraulics including normal depth, velocity, and capacity using Manning's equation with variable roughness based on vegetation retardance.