Reduce Barriers to Downstream-Most Per Flow Path

Given a point table on an FWA stream network, remove any points that have another point from the same table downstream of them on the same upstream flow path. The result is the minimal set of points needed to define access blocking per reach — equivalent to bcfishpass's "non-minimal removal" step.

Usage

frs_barriers_minimal(
  conn,
  from,
  to = "working.barriers_minimal",
  tolerance = 1,
  overwrite = TRUE
)

Arguments

conn

A DBI::DBIConnection object (from frs_db_conn()).

from

Character. Source table (schema-qualified) with barrier points. Must contain blue_line_key, downstream_route_measure, wscode_ltree, and localcode_ltree columns. Enrich with ltree columns via frs_col_join() if needed.

to

Character. Destination table for minimal barriers. Default "working.barriers_minimal".

tolerance

Numeric. Tolerance in metres when comparing positions on the same reach. Default 1 — two points within 1 m on the same blue_line_key are treated as coincident and both are kept. Matches bcfishpass convention; prevents near-coincident gradient barriers (different gradient classes at the same vertex) from cancelling each other out.

overwrite

Logical. If TRUE (default), drop to before creating.

Value

conn invisibly, for pipe chaining.

Details

On a full watershed group this typically reduces ~27,000 raw gradient barriers to ~700 downstream-most per reach. Once the downstream-most barrier on a path is present, any barrier upstream of it is redundant for access-blocking purposes.

Examples

if (FALSE) { # \dontrun{
conn <- frs_db_conn()

# Typical pipeline: detect gradient barriers, enrich with ltree cols,
# then reduce to the minimal set for segmentation.
frs_break_find(conn,
  table     = "whse_basemapping.fwa_stream_networks_sp",
  to        = "working.barriers_raw",
  attribute = "gradient",
  classes   = c("15" = 0.15, "20" = 0.20, "25" = 0.25, "30" = 0.30))

frs_col_join(conn, "working.barriers_raw",
  from = "whse_basemapping.fwa_stream_networks_sp",
  cols = c("wscode_ltree", "localcode_ltree"),
  by   = "blue_line_key")

n_before <- DBI::dbGetQuery(conn,
  "SELECT count(*) FROM working.barriers_raw")[[1]]

frs_barriers_minimal(conn,
  from = "working.barriers_raw",
  to   = "working.barriers_minimal")

n_after <- DBI::dbGetQuery(conn,
  "SELECT count(*) FROM working.barriers_minimal")[[1]]

message("Reduced ", n_before, " -> ", n_after, " barriers (",
        round(100 * (1 - n_after / n_before)), "% removed)")

DBI::dbDisconnect(conn)
} # }