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Assign Manning's roughness

Source: R/roughness.R
roughness.Rd

Produces a Manning's roughness coefficient (\(n\)) for use by the routing stage, by one of three methods. Roughness is the single most sensitive hydraulic parameter, so the function makes the choice explicit and always lets the user override it.

Usage

roughness(
  x = NULL,
  method = c("constant", "landcover", "ndvi"),
  value = 0.035,
  table = floodflow_lc_roughness,
  n_min = 0.02,
  n_max = 0.12,
  data = NULL
)

Arguments

x

A flood_project, or a data input directly. For method = "constant" the data input is ignored. For method = "landcover" it is a character/factor vector of classes or a land-cover SpatRaster. For method = "ndvi" it is a numeric vector or an NDVI SpatRaster.

method

One of "constant", "landcover" or "ndvi".

value

Manning's \(n\) for method = "constant". Default 0.035 (natural channel).

table

Named numeric vector mapping land-cover classes to \(n\) for method = "landcover". Defaults to floodflow_lc_roughness.

n_min, n_max

Roughness bounds for method = "ndvi".

data

Optional explicit data input when x is a flood_project; overrides looking in the project. Land-cover classes or NDVI values, as a vector or SpatRaster.

Value

If x is a flood_project, the same object with its roughness slot populated by a list of class flood_roughness. Otherwise the flood_roughness list directly, with elements method, n (the resulting roughness: a scalar, numeric vector, or SpatRaster), and summary (min, mean and max of \(n\)).

Details

"constant"

A single \(n\) applied everywhere. Simplest and fully reproducible.

"landcover"

Look up \(n\) from land-cover classes using a table (the built-in floodflow_lc_roughness by default, or a user-supplied named vector).

"ndvi"

Derive \(n\) from NDVI with a monotonic empirical function, so remotely-sensed vegetation density sets roughness.

The function operates on plain vectors and, when terra is installed, on raster inputs (SpatRaster). Raster handling is optional: if the input is a raster and terra is not available, the function stops with an informative message.

References

Manning, R. (1891). On the flow of water in open channels and pipes. Transactions of the Institution of Civil Engineers of Ireland, 20, 161-207.

Chow, V. T. (1959). Open-Channel Hydraulics. McGraw-Hill, New York.

See also

floodflow_lc_roughness for the default lookup table.

Examples

# Constant roughness
roughness(method = "constant", value = 0.03)
#> <flood_roughness>
#>   method: constant
#>   type:   constant
#>   Manning n: min=0.030 mean=0.030 max=0.030

# From land-cover classes
cls <- c("urban", "cropland", "forest", "water")
roughness(cls, method = "landcover")$n
#> [1] 0.015 0.040 0.100 0.030

# From NDVI values
roughness(c(0, 0.3, 0.6, 0.9), method = "ndvi")$n
#> [1] 0.02 0.05 0.08 0.11