SWMM 5.2.2 Code for LID Function trenchFluxRates

This code is a function called "trenchFluxRates" that calculates flux rates from the layers of an infiltration trench LID. It takes in two input arrays, "x" and "f", which represent the vector of storage levels and the vector of flux rates, respectively. The function uses several intermediate variables, including "availVolume" and "maxRate". It also makes use of several properties of the storage layer, such as "storageThickness" and "storageVoidFrac".

The code first retrieves the moisture levels from the input vector "x" and converts them to volumes. It then calculates the ET rates and nominal storage inflow, as well as the exfiltration rate out of the storage layer. The code also checks for underdrain flow and limits the exfiltration rate and underdrain flow by available volume. The storage inflow is limited to not exceed the storage layer capacity. Finally, the function finds the net fluxes for each layer and stores them in the output array "f".

A table of the variables used in this function and their descriptions is as follows:

Variable Name	Description
x[SURF], x[STOR]	Input vector of storage levels
f[SURF], f[STOR], f[SOIL]	Output vector of flux rates
surfaceDepth	Moisture level variable for the surface layer
storageDepth	Moisture level variable for the storage layer
availVolume	Intermediate variable for available volume
maxRate	Intermediate variable for maximum rate
storageThickness	Property of the storage layer representing its thickness
storageVoidFrac	Property of the storage layer representing its void fraction
SurfaceVolume	Converted surface moisture level to volume
SoilVolume	Converted soil moisture level to volume
StorageVolume	Converted storage moisture level to volume
SurfaceInflow	Rate of inflow to the surface layer
SurfaceEvap	Rate of evaporation from the surface layer
SurfaceOutflow	Rate of outflow from the surface layer
StorageInflow	Rate of inflow to the storage layer
StorageEvap	Rate of evaporation from the storage layer
StorageExfil	Rate of exfiltration from the storage layer
StorageDrain	Rate of underdrain flow from the storage layer
Tstep	Time step
theLidProc	Pointer to LID process data

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

//

//  Purpose: computes flux rates from the layers of an infiltration trench LID.

//  Input:   x = vector of storage levels

//  Output:  f = vector of flux rates

//

{

    // Moisture level variables

    double surfaceDepth;

    double storageDepth;

    // Intermediate variables

    double availVolume;

    double maxRate;

    // Storage layer properties

    double storageThickness = theLidProc->storage.thickness;

    double storageVoidFrac = theLidProc->storage.voidFrac;

    //... retrieve moisture levels from input vector

    surfaceDepth = x[SURF];

    storageDepth = x[STOR];

    //... convert moisture levels to volumes

    SurfaceVolume = surfaceDepth * theLidProc->surface.voidFrac;

    SoilVolume = 0.0;

    StorageVolume = storageDepth * storageVoidFrac;

    //... get ET rates

    availVolume = (storageThickness - storageDepth) * storageVoidFrac;

    getEvapRates(SurfaceVolume, 0.0, 0.0, StorageVolume, 1.0);

    //... no storage evap if surface ponded

    if ( surfaceDepth > 0.0 ) StorageEvap = 0.0;

    //... nominal storage inflow

    StorageInflow = SurfaceInflow + SurfaceVolume / Tstep;

    //... exfiltration rate out of storage layer

   StorageExfil = getStorageExfilRate();

    //... underdrain flow rate

    StorageDrain = 0.0;

    if ( theLidProc->drain.coeff > 0.0 )

    {

        StorageDrain = getStorageDrainRate(storageDepth, 0.0, 0.0, surfaceDepth);

    }

    //... limit storage exfiltration by available storage volume

    maxRate = StorageInflow - StorageEvap + storageDepth*storageVoidFrac/Tstep;

    StorageExfil = MIN(StorageExfil, maxRate);

    StorageExfil = MAX(StorageExfil, 0.0);

    //... limit underdrain flow by volume above drain offset

    if ( StorageDrain > 0.0 )

    {

        maxRate = -StorageExfil - StorageEvap;

        if (storageDepth >= storageThickness ) maxRate += StorageInflow;

        if ( theLidProc->drain.offset <= storageDepth )

        {

            maxRate += (storageDepth - theLidProc->drain.offset) *

                       storageVoidFrac/Tstep;

        }

        maxRate = MAX(maxRate, 0.0);

        StorageDrain = MIN(StorageDrain, maxRate);

    }

    //... limit storage inflow to not exceed storage layer capacity

    maxRate = (storageThickness - storageDepth)*storageVoidFrac/Tstep +

              StorageExfil + StorageEvap + StorageDrain;

    StorageInflow = MIN(StorageInflow, maxRate);

    //... equate surface infil to storage inflow

    SurfaceInfil = StorageInflow;

    //... find surface outflow rate

    SurfaceOutflow = getSurfaceOutflowRate(surfaceDepth);

    // ... find net fluxes for each layer

    f[SURF] = SurfaceInflow - SurfaceEvap - StorageInflow - SurfaceOutflow /

              theLidProc->surface.voidFrac;;

    f[STOR] = (StorageInflow - StorageEvap - StorageExfil - StorageDrain) /

              theLidProc->storage.voidFrac;

    f[SOIL] = 0.0;

}