SWMM 5.2.2 Code for LID Function swaleFluxRates

This code defines a function called "swaleFluxRates" which computes the flux rates for a vegetative swale LID. The function takes in a vector of storage levels (x) and a vector of flux rates (f) as inputs and does not return any outputs. The function starts by initializing several variables such as depth, topWidth, botWidth, length, surfInflow, surfWidth, surfArea, flowArea, lidArea, hydRadius, slope, volume, dVdT, dStore, and xDepth.

The function first retrieves the state variable of the surface water depth from the work vector, and limits it to the thickness of the surface layer. It then calculates the depression storage depth and the bottom width of the swale using the top width and the side slope. It then calculates the length of the swale and the surface area of the current ponded depth.

The function then calculates the wet volume and effective depth, the surface inflow into the swale, the evaporation rate, the infiltration rate to the native soil, and the surface outflow. If the depth is below the depression storage, the surface outflow is set to 0.0, otherwise, the function modifies the flow area to remove the depression storage and calculates the hydraulic radius using the Manning equation.

Then the function calculates the net flux rate (dV/dt) in cubic feet per second and assigns it to the SurfaceInflow and SurfaceOutflow.

Here is a table summarizing the variables and their descriptions in the code:

Variable	Description
depth	depth of surface water in swale (ft)
topWidth	top width of full swale (ft)
botWidth	bottom width of swale (ft)
length	length of swale (ft)
surfInflow	inflow rate to swale (cfs)
surfWidth	top width at current water depth (ft)
surfArea	surface area of current water depth (ft^2)
flowArea	x-section flow area (ft^2)
lidArea	surface area of full swale (ft^2)
hydRadius	hydraulic radius for current depth (ft)
slope	slope of swale side wall (run/rise)
volume	swale volume at current water depth (ft^3)
dVdT	change in volume w.r.t. time (cfs)
dStore	depression storage depth (ft)
xDepth	depth above depression storage (ft)
theLidProc->surface.thickness	thickness of the surface layer
theLidUnit->fullWidth	full width of the swale
theLidProc->surface.sideSlope	side slope of the sw

Variable	Description
SurfaceInflow	inflow rate into the surface layer (cfs)
EvapRate	rate of evaporation from the surface layer (cfs)
SurfaceInfil	infiltration rate into the native soil from the surface layer (cfs)
SurfaceOutflow	outflow rate from the surface layer (cfs)
theLidProc->surface.alpha	coefficient used in the Manning equation
theLidProc->surface.voidFrac	void fraction of the swale

Note that this function is specific for swale LID, and it computes the flux rates based on the storage level and the LID properties. It calculates the depression storage, the bottom width, the length, the top width, the surface area, the flow area, the wet volume, the effective depth, the surface inflow, the evaporation rate, the infiltration rate, and the surface outflow.

void swaleFluxRates(double x[], double f[])

//

//  Purpose: computes flux rates from a vegetative swale LID.

//  Input:   x = vector of storage levels

//  Output:  f = vector of flux rates

//

{

    double depth;            // depth of surface water in swale (ft)

    double topWidth;         // top width of full swale (ft)

    double botWidth;         // bottom width of swale (ft)

    double length;           // length of swale (ft)

    double surfInflow;       // inflow rate to swale (cfs)

    double surfWidth;        // top width at current water depth (ft)

    double surfArea;         // surface area of current water depth (ft2)

    double flowArea;         // x-section flow area (ft2)

    double lidArea;          // surface area of full swale (ft2)

    double hydRadius;        // hydraulic radius for current depth (ft)

    double slope;            // slope of swale side wall (run/rise)

    double volume;           // swale volume at current water depth (ft3)

    double dVdT;             // change in volume w.r.t. time (cfs)

    double dStore;           // depression storage depth (ft)

    double xDepth;           // depth above depression storage (ft)

    //... retrieve state variable from work vector

    depth = x[SURF];

    depth = MIN(depth, theLidProc->surface.thickness);

    //... depression storage depth

    dStore = 0.0;

    //... get swale's bottom width

    //    (0.5 ft minimum to avoid numerical problems)

    slope = theLidProc->surface.sideSlope;

    topWidth = theLidUnit->fullWidth;

    topWidth = MAX(topWidth, 0.5);

    botWidth = topWidth - 2.0 * slope * theLidProc->surface.thickness;

    if ( botWidth < 0.5 )

    {

        botWidth = 0.5;

        slope = 0.5 * (topWidth - 0.5) / theLidProc->surface.thickness;

    }

    //... swale's length

    lidArea = theLidUnit->area;

    length = lidArea / topWidth;

    //... top width, surface area and flow area of current ponded depth

    surfWidth = botWidth + 2.0 * slope * depth;

    surfArea = length * surfWidth;

    flowArea = (depth * (botWidth + slope * depth)) *

               theLidProc->surface.voidFrac;

    //... wet volume and effective depth

    volume = length * flowArea;

    //... surface inflow into swale (cfs)

    surfInflow = SurfaceInflow * lidArea;

    //... ET rate in cfs

    SurfaceEvap = EvapRate * surfArea;

    SurfaceEvap = MIN(SurfaceEvap, volume/Tstep);

    //... infiltration rate to native soil in cfs

    StorageExfil = SurfaceInfil * surfArea;

    //... no surface outflow if depth below depression storage

    xDepth = depth - dStore;

    if ( xDepth <= ZERO ) SurfaceOutflow = 0.0;

    //... otherwise compute a surface outflow

    else

    {

        //... modify flow area to remove depression storage,

        flowArea -= (dStore * (botWidth + slope * dStore)) *

                     theLidProc->surface.voidFrac;

        if ( flowArea < ZERO ) SurfaceOutflow = 0.0;

        else

        {

            //... compute hydraulic radius

            botWidth = botWidth + 2.0 * dStore * slope;

            hydRadius = botWidth + 2.0 * xDepth * sqrt(1.0 + slope*slope);

            hydRadius = flowArea / hydRadius;

            //... use Manning Eqn. to find outflow rate in cfs

            SurfaceOutflow = theLidProc->surface.alpha * flowArea *

                             pow(hydRadius, 2./3.);

        }

    }

    //... net flux rate (dV/dt) in cfs

    dVdT = surfInflow - SurfaceEvap - StorageExfil - SurfaceOutflow;

    //... when full, any net positive inflow becomes spillage

    if ( depth == theLidProc->surface.thickness && dVdT > 0.0 )

    {

        SurfaceOutflow += dVdT;

        dVdT = 0.0;

    }

    //... convert flux rates to ft/s

    SurfaceEvap /= lidArea;

    StorageExfil /= lidArea;

    SurfaceOutflow /= lidArea;

    f[SURF] = dVdT / surfArea;

    f[SOIL] = 0.0;

    f[STOR] = 0.0;

    //... assign values to layer volumes

    SurfaceVolume = volume / lidArea;

    SoilVolume = 0.0;

    StorageVolume = 0.0;

}