WWHM-SWMM Tech Specs

Continuous Simulation Modeling

WWHM-SWMM uses continuous simulation to accurately model the generation of runoff from rainfall and the routing of runoff through user-defined conveyance systems.

As we’ve seen time and time again in major flood disasters, single-event modeling simply doesn’t work.

Using a single hypothetical storm event to design a stormwater system or mitigation facility doesn’t provide the stormwater engineer with any real information on how the stormwater system will work in real-world conditions with real-world rainfall.

This is a HUGE problem.

EPA HSPF continuous simulation hydrology solves this problem by modeling the entire hydrologic cycle for multiple years at an hourly or smaller time step.

Routing runoff through a conveyance system can be done using either HSPF or SWMM or both interactively. WWHM-SWMM includes both EPA HSPF and EPA SWMM together with long-term historic rainfall and evaporation data stored in a database directly accessed by WWHM-SWMM.

How It Works

You simply input the drainage area soils, vegetation, and land slope in a visual interactive Windows interface.

Then select one or multiple generic and commercial stormwater storage and water quality facilities (including LID practices) available in WWHM-SWMM and route the runoff to a downstream point of compliance to meet NPDES MS4 permit requirements.

Most WWHM-SWMM conveyance and storage elements also includes an auto-sizing feature so you can easily select the optimal size for the hydromodification or water quality facility to meet NPDES MS4 requirements.

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Tech Specs for LID Elements in WWHM-SWMM

Green Roof

The WWHM-SWMM Green Roof element includes state-of-the-science algorithms that correctly and accurately model green roof evapotranspiration and discharge via roof drains.

Rainfall on a green roof must go somewhere. Green roof runoff doesn't just disappear.

The green roof water either goes up into the atmosphere through evapotranspiration or down a roof drain into a stormwater collection system.

The WWHM-SWMM Green Roof element models actual evapotranspiration based on daily potential evapotranspiration (PET), taking into account the depth of the soil layer on the roof and the type of green roof vegetation specified.

How It Works.

Simply input the green roof surface area, the maximum distance to the nearest roof drain (including where the outlet drains to), the roof slope, the thickness of the soil layer, and the type of vegetation on the roof.

WWHM-SWMM then computes the runoff and evapotranspiration from the green roof using HSPF continuous simulation hydrology.

Impervious Runoff Dispersion

The WWHM-SWMM Lateral Flow Basin elements includes state-of-the-science algorithms that correctly and accurately model the dispersion of impervious runoff on an adjacent pervious surface.

Impervious runoff dispersion (also known as impervious runoff disconnection and lateral flow dispersion) offers unique opportunities to reduce impervious surface runoff by allowing it to sheet flow across an adjacent pervious surface, such as a lawn area.

Impervious runoff flowing across the pervious landscape with some of the impervious runoff infiltrating into the soil and some becoming evapotranspiration.

How It Works.

WWHM-SWMM uses two elements in series to model this lateral movement of water draining from the impervious area to the pervious.

  1. The Lateral Flow Impervious Area element represents the impervious surface area, whether it is a roof, parking lot, or road surface.
  2. The Lateral Flow Soil Basin represents the adjacent pervious area. Select the soil, vegetation, and land slope that is representative of this pervious area.

WWHM-SWMM computes the runoff and evaporation from the impervious surface area and then spreads the impervious runoff over the adjacent pervious area soil and vegetation using HSPF continuous simulation hydrology.

The reduced total runoff can then be routed to a downstream conveyance system or storage to be in compliance with NPDES MS4 stormwater hydromodification/ flow control and water quality requirements.

Permeable Pavement

The WWHM-SWMM Permeable Pavement element includes state-of-the-science algorithms that correctly and accurately model the movement of water through permeable/porous pavement.

Permeable/porous pavement offers unique opportunities to reduce or eliminate runoff from what would otherwise be an impervious surface.

Water drains through the permeable pavement into a gravel sublayer/subgrade that provides storage and discharge through either an underdrain and/or infiltration to the native soil.

The WWHM-SWMM Permeable Pavement element models this vertical movement of water draining into the gravel subgrade, its storage, and discharge.

WWHM-SWMM computes the runoff and evaporation from the permeable pavement and the gravel subgrade using HSPF continuous simulation hydrology.

How It Works.

You simply input the physical dimensions of the permeable pavement, the outlet configuration (including whether or not there is an underdrain and/or infiltration to the native soil), thickness and porosity of the pavement and gravel subgrade, and the bottom slope.

If the bottom slope is greater than 2%, include an effective volume factor to account for the placement of dams in the gravel layer to prevent water flowing to and ponding at the bottom of the slope.

The WWHM-SWMM Permeable Pavement element also includes the option for inflow from adjacent impervious or pervious surfaces and ponding on the permeable pavement, if allowed.

Bioretention/Rain Garden

The WWHM-SWMM Bioretention element includes state-of-the-science algorithms that correctly and accurately model the vertical movement of water through the bioretention engineered soil layers.

The modeling of bioretention hydrologic performance is not the same as modeling a stormwater pond or other stormwater storage facility.

Unlike in a pond or a gravel trench, stormwater does not enter a rain garden and immediately flow to the bottom and fill the facility from the bottom up.

Water enters the rain garden and flows down from layer to layer to the bottom.

The WWHM-SWMM Bioretention element models this vertical movement with the water entering the facility at the top surface layer.

The water then infiltrates into the top layer of engineered soil mix based on the modified Green-Ampt equation.

The water moves down through the topsoil layer at a calculated rate, determined by Darcy's and Van Genuchten's equations.

Each soil layer has a separate set of calculations based on its individual set of soil properties.

How It Works.

Simply input the physical dimensions of the rain garden, the outlet configuration (including whether or not there is an underdrain and/or infiltration to the native soil), and engineered soil mix and thickness for up to three bioretention soil layers.

The bioretention element includes specific soil mixes you can choose from.

Or you can input your own.

The WWHM-SWMM Bioretention element also includes an auto-sizing feature so you can easily select the optimal bioretention size to meet NPDES MS4 requirements.

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