In brownhr/excess: Tools for Cross-Section Hydrological Data

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
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Calculating the cross-sectional area of a stream

For this guide, we'll take two approaches for calculating the area: one for non-georeferenced data (i.e., just a table with your measurements, as seen in testxs), and one for georeferenced data (made by sf).

Non-georeferenced data

Suppose you have a dataset like that which exists in testxs:

library(excess)
library(dplyr)
library(ggplot2)
library(units)

head(testxs)

You'll notice that the data has linear units listen in [ft]. This comes from the units package. Units of measurement aren't necessary -- the functions work fine without them -- but it's definitely good practice to ensure you're calculating accurate results!

Anyways, we wish to calculate the cross-sectional area of the stream, and we have both the "x coordinate" (TAPE) and the "z-coordinate" (InvertRod). The Bankful column represents the bankful height, which is the "reference" for the depth measurement.

To calculate the cross-sectional area, excess uses the Trapezoidal Rule:

testxs$depth_baseline <- pmin(testxs$InvertRod, testxs$Bankful) - testxs$Bankful

testxs %>% 
  ggplot(aes(x = TAPE, y = depth_baseline)) + 
  geom_path() + 
  geom_point() + 
  # geom_vline(aes(xintercept = TAPE), linetype = "dashed") + 
  geom_area(fill = "blue",
            alpha = 0.25) + 
  geom_segment(aes(x = TAPE, xend = lead(TAPE), 
                   y = pmax(depth_baseline, lead(depth_baseline)), 
                   yend = pmax(depth_baseline, lead(depth_baseline))),
               color = "blue",
               alpha = 0.5,
               linetype = "dashed",
               size = 1) + 
  geom_segment(aes(x = TAPE, xend = TAPE,
                   y = 0, yend = depth_baseline),
               color = "blue",
               alpha = 0.5,
               linetype = "dashed",
               size = 1) + 
  labs(x = "Tape",
       y = "Baseline Depth") + 
  theme_light()