SWMM5 landuse.c Summary

 Below is a concise explanation of how the landuse.c module in SWMM manages pollutant buildup and washoff for different land uses.

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1. Purpose of landuse.c

This module in SWMM handles how pollutants accumulate (buildup) over time on land surfaces (land uses) and how they are washed off by runoff. In particular, it:

1. Reads and stores user-supplied buildup parameters (functions and coefficients).
2. Reads and stores washoff parameters (functions and coefficients).
3. Computes how pollutant mass accumulates on a land surface over a given dry period, and how that mass is removed by runoff or street sweeping.
4. Calculates how some pollutants can produce co-pollutants during washoff.

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2. Key Functions

Reading Input

• landuse_readParams():

  • Reads basic land use parameters (like sweeping interval, sweeping removal fraction, initial days since last sweeping).
  • Stores them in the Landuse[j] structure.

• landuse_readPollutParams():

  • Reads pollutant-specific parameters (units, concentration in rain and groundwater, decay/growth rate, co-pollutant index, etc.).
  • Saves them to Pollut[].

• landuse_readBuildupParams():

  • Reads the buildup function type (e.g. power, exponential, saturation, external).
  • Reads function coefficients, normalizer (area or curb length), and sets the maximum days to build up (maxDays).

• landuse_readWashoffParams():

  • Reads the washoff function type (exponential, rating curve, or EMC).
  • Reads washoff coefficients, exponents, plus sweeping and BMP removal efficiencies.

Computing Initial Buildup

• landuse_getInitBuildup():
  • Assigns initial buildup for each pollutant on each land use in a subcatchment.
  • If a subcatchment has an initial buildup or an antecedent dry period, it uses the chosen buildup function to find the starting pollutant load.

Updating Buildup

• landuse_getBuildup():
  • Main routine to incrementally update pollutant buildup for a land use over a time interval.
  • Uses one of the recognized functions (power, exponential, saturation, or external time series).

Washoff Calculation

• landuse_getWashoffLoad():

  • Calculates how much pollutant mass (e.g., lbs or kg) is washed off during a time step, given:
    • The current buildup
    • The runoff rate
    • The washoff function (exponential, rating curve, or EMC)
  • Subtracts the load from the land use’s buildup.
  • Considers possible BMP removal fraction.

• landuse_getWashoffQual():

  • Returns the concentration of pollutant in washoff (mass per volume).
  • Called internally by landuse_getWashoffLoad().

• landuse_getCoPollutLoad():

  • If pollutant p has a co-pollutant (like TSS having a fraction that becomes attached phosphorus), it returns extra mass generated proportionally.
  • This is added into the overall washoff mass.

Internal Helpers

• landuse_getBuildupDays() and landuse_getBuildupMass():

  • Convert from “current buildup mass” to “equivalent # of buildup days,” and vice versa.

• landuse_getExternalBuildup():

  • For “EXTERNAL_BUILDUP” type, obtains a buildup rate from a time series over the time step.

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3. How Buildup and Washoff are Modeled

Buildup

SWMM supports several buildup function types:

1. POWER_BUILDUP

   $$\text{buildup}(t) = c_1 \, t^{\,c_2} \quad \text{(capped at } c_0 \text{)}$$

2. EXPON_BUILDUP

   $$\text{buildup}(t) = c_0 \left(1 - e^{-\, c_1 t}\right)$$

3. SATUR_BUILDUP

   $$\text{buildup}(t) = \frac{t \, c_0}{c_2 + t}$$

4. EXTERNAL_BUILDUP

   • A user-supplied time series that directly prescribes a buildup rate over time.

The user also selects a normalizer: either per unit area or per unit curb length.

Washoff

Several washoff functions are supported:

1. EXPON_WASHOFF

   $$W = \text{coeff} \times (\text{runoff})^{\,\text{expon}} \times \text{buildup}$$
   • With a typical form:
   $$\frac{d(\text{buildup})}{dt} = - k \cdot R^\beta \cdot \text{buildup}$$

   for some exponent $\beta$.

2. RATING_WASHOFF

   $$W = \text{coeff} \times (\text{runoff})^{(\text{expon})}$$

3. EMC_WASHOFF

   • A fixed concentration (Event Mean Concentration), so mass = $\text{Concentration} \times \text{runoff volume}$.

BMP Efficiency & Sweeping

• A fraction of the washoff mass can be removed by a BMP treatment, specified by bmpEffic.
• Street sweeping can reduce buildup or offset it periodically, governed by an interval, removal fraction, and last-swept day.

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4. Flow Through the Code

1. Read Land Use & Pollutant Buildup/Washoff
   • The user inputs a series of lines specifying the function type, coefficients, normalizer, and BMP/sweeping parameters.
2. Initialize
   • During subcatchment initialization, landuse_getInitBuildup() assigns initial buildup based on an optional initial mass or an antecedent dry day calculation.
3. During a Runoff Step
   • As time steps progress, landuse_getBuildup() is called to incrementally update buildup.
   • Then, landuse_getWashoffLoad() is called whenever runoff occurs to find how much mass leaves with the stormwater.
   • The rest remains as updated buildup on the surface.
   • BMP or co-pollutant processes are accounted for.
4. Output
   • The resulting washoff load is added to subcatchment runoff quality.
   • Buildup is updated for the next time step.

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5. Summary

• landuse.c organizes the parameters and calculations for buildup and washoff on each land use within a subcatchment.
• Multiple functional forms (power, exponential, saturation, or time-series-based) are supported for the accumulation of pollutants over dry days.
• Washoff uses exponential, rating curve, or EMC-based formulas to transform buildup mass into pollutant runoff concentration.
• Sweeping and BMP removal features further adjust the net washoff load.
• Co-pollutants allow a fraction of the washoff from one pollutant to create mass in another.