Understanding Streamflow Measurement: A Guide for Stormwater Designers
Welcome back, stormwater designers! Today we're diving into Part 10 of our hydrology education series with a focus on streamflow—a key concept in understanding water movement and managing stormwater systems effectively.
If you're just joining us, be sure to check out our earlier videos covering everything from rain gauges to the hydrologic cycle. Each video builds on the last to give you a comprehensive foundation in hydrology.
📘 Free Resource: Hydrology Terms Guide
Before we get started, don’t forget to download our Hydrology Terms Guide. This free five- to six-page resource includes definitions for basic and advanced hydrologic terms. It’s an excellent reference for industry professionals or anyone looking to boost their understanding. Find the download link in the video description.
💧 What Is Streamflow?
In the streamflow phase of the hydrologic cycle, water from a catchment area typically becomes concentrated in a single channel—think of it as runoff from rain being funneled into a storm sewer and then into a stream.
Once this water enters a defined channel, it becomes measurable. This allows hydrologists to record the flow rate (usually in cubic feet per second) and water level in the channel. This information is critical for designing downstream systems and understanding watershed behavior.
🧪 How Do We Measure Streamflow?
Streamflow is measured by establishing a relationship between the flow rate and the water level (also called stage) of a stream. Here's how it's typically done:
Small Streams
Use a weir to measure head (water level).
Convert head measurements into flow rates using formulas.
Large Streams
Weirs aren't practical.
Use current meters to directly measure discharge (flow rate).
Record stage at the same time to build a stage-discharge relationship.
These measurements are then used to develop what's known as a rating curve.
📊 What Is a Rating Curve?
A rating curve is a graph that plots measured discharge versus stage. It often uses logarithmic formulas and is heavily dependent on the geometry of the stream.
However, stream conditions change over time. Erosion or sediment buildup can alter the geometry, which in turn changes the rating curve. To maintain accuracy, consistent current meter measurements in a fixed position are necessary to continually update the curve.
📈 Sample Rating Curve
Here’s an example of what a streamflow rating curve might look like:
X-axis: Stage (in meters)
Y-axis: Flow (in cubic meters per second)
A plotted curve shows how flow increases with water level. This curve likely originates from long-term data collection using a current meter stationed in the stream.
⚠️ Considerations in Rating Curve Accuracy
Several external factors can influence the accuracy of your rating curve:
Backwater Effects: Water flowing back upstream from another source can increase the stage but not the discharge, skewing results.
Flat Channel Slopes: Small variations in slope can impact the flow and must be monitored using a slope record near the rating station.
💡 Plotting a Rating Curve in Excel: A Quick How-To
To create your own rating curve, you can use Excel:
Input your stage (ft) and discharge (cfs) values.
Highlight the data and insert a scatter plot.
Set stage on the X-axis and flow on the Y-axis.
Adjust axis bounds to better shape the curve.
For more precise analysis, use a logarithmic scale (base 10) on both axes.
This graphical representation allows for better understanding of flow dynamics and assists in deriving predictive equations.
✅ Final Thoughts
Understanding and measuring streamflow is essential for effective hydrologic analysis and stormwater design. In this video, we covered the basics of streamflow, how it’s measured, and how to plot a rating curve.
Stay tuned for our next video, where we'll dive deeper into how to derive equations from a rating curve and apply them in real-world hydrology projects.
👍 Like and Subscribe to stay updated with our hydrology education series. Thanks for watching, and we’ll see you next time!
