What erosion pins measure
Erosion pins are one of the most straightforward field methods for quantifying bank surface change over time. Each pin is a metal rod — typically steel rebar or galvanized wire — driven horizontally into a bank face perpendicular to the flow direction. At each reading, the length of exposed pin is measured. An increase in exposed length indicates net erosion at that point; a decrease (or pin burial) indicates net deposition.
Pins do not capture the mechanism of erosion — they record only the net change in bank surface position at the pin location. To interpret what is driving observed change (fluvial scour, freeze-thaw crumbling, mass wasting), field observations made during or immediately after significant flow events are needed alongside the pin records.
Installation procedure
Erosion pin arrays are typically installed in vertical transects on the bank face, with pins placed at fixed vertical intervals — commonly 25 cm, 50 cm, or at specific positions relative to the bank morphology (upper bank, mid bank, bank toe). Multiple transects are installed across a monitored reach, allowing spatial variation in erosion rates to be captured.
Pin specifications
Pins used in documented Canadian conservation authority monitoring programs are typically 6–10 mm diameter steel rod, driven to a depth of 30–40 cm into the bank face, with an initial exposed length of 10–15 cm recorded at installation. The pin must be long enough to remain in place through a period of significant erosion, but not so long that it creates turbulent flow interference at the bank face.
Flagging and documentation
Each pin is flagged at installation with a numbered label tied to the exposed end. The position of each pin relative to a fixed benchmark (survey stake or GPS coordinate) is recorded so that the array can be located precisely at each return visit. Photographs taken at installation and at each subsequent reading provide visual context for interpreting pin changes.
Pins can be dislodged by ice scour, beavers, or equipment during maintenance work. Arrays should be checked after major flow events and damaged pins noted in the record. Readings from dislodged or missing pins must be flagged as invalid in the dataset.
Measurement intervals
The timing of pin readings significantly affects what the data captures. Annual readings taken after the spring freshet period capture cumulative erosion from the highest-discharge events of the year, which is often the dominant erosion period in Canadian creek systems. However, single annual readings mask the timing and source of erosion — a spring reading after a major ice jam event cannot be distinguished from a spring reading after a gradual melt without interim records.
| Reading interval | Captures | Typical use |
|---|---|---|
| Annual (post-freshet) | Cumulative annual change | Long-term trend monitoring |
| Semi-annual (fall + spring) | Winter period vs. growing season erosion | Vegetation effectiveness assessment |
| Event-based (after significant flow) | Single event erosion magnitude | Process-based research |
Comparing vegetated and unvegetated sections
Erosion pin monitoring designed to assess the effect of riparian buffer planting requires paired arrays: one set in the planted section and one set in a comparable unplanted or minimally vegetated control section of the same bank. The two sections should have similar bank height, substrate, aspect, and exposure to stream energy.
Without a control section, it is not possible to separate the effect of riparian vegetation from year-to-year variation in flow magnitude or changes in channel geometry. Monitoring programs that include unpaired planted sections only can describe trends in erosion rates over time but cannot attribute changes specifically to vegetation establishment.
Interpreting pin data in the context of buffer planting
A well-designed erosion pin dataset collected over multiple years in paired sections allows for the following types of interpretation:
- Whether net erosion rates in planted sections are lower than in control sections across the monitoring period.
- Whether differences between planted and control sections increase over time as root systems mature.
- Whether erosion is concentrated at the bank toe (suggesting fluvial scour) or distributed across the bank face (suggesting surface processes like frost action or desiccation cracking).
- Whether specific events — spring freshet, summer thunderstorms — account for a disproportionate share of annual erosion at the site.
Interpreting these patterns requires combining pin records with flow data from a nearby gauge station and with field observations made during and after significant events. Flow data for many Canadian creek systems is publicly available through the Water Survey of Canada national hydrometric database.
Data recording and storage
Erosion pin data should be recorded in a consistent format that includes pin ID, date of reading, exposed length in millimetres, and any field notes (pin condition, bank surface condition, recent weather or flow events). Data recorded in the field on paper should be transferred to a digital record promptly to avoid transcription errors.
Records should be maintained through multiple monitoring cycles without modification to historical entries. Corrections should be noted with a date and reason rather than overwriting the original record.
Limitations of the method
Erosion pins capture change at point locations. Spatial interpolation between pins carries uncertainty, particularly if bank morphology is irregular or if erosion is concentrated in localized areas (for example, adjacent to obstructions or at root ball locations). Pin readings do not capture the volume of material eroded — only the change in bank surface position at the pin location.
In fine silty-clay banks, pins may move slightly through frost heave during winter periods, introducing error in spring readings. This effect can be partially identified by comparing readings from multiple pins at the same height on the bank face.