How to Read a River: Assessing Head and Flow Before You Commit

Before you spend a penny on a turbine, a penstock, or a planning consultant, you need two numbers: head and flow. These figures decide whether a site is viable or a non-starter. Here is a practical walkthrough for measuring both on a potential UK mini-hydro site.

Why Head and Flow Come First

Head is the vertical drop between the intake and the turbine. Flow is the volume of water passing a point each second. Multiply them with gravity and an efficiency figure, and you get a power estimate. Get either number wrong and your business case collapses.

In the UK, small sites often succeed or fail on surprisingly modest differences. A drop of 2 metres with 100 litres per second is a different proposition to 20 metres with 10 litres per second, even though the raw product looks similar. Measure both before you commit.

Measuring Head Accurately

Head is not just the height of a waterfall. It is the vertical distance between the water surface at the intake and the water surface at the outfall, minus losses in the channel or pipe. Use a surveyor's level, a laser level, or a water-filled hose and pressure gauge for a low-cost check.

Walk the entire route. Note bends, drops, and any section where the water could be diverted. A rough drop measured with a clinometer is useful, but a precise figure from a level survey is what lenders and the Environment Agency will expect. Record everything in a table like this:

Measuring Flow Without a Weir

Flow is the harder number. The most accurate method is a temporary sharp-crested weir, but that needs permission and construction. For an initial assessment, use the velocity-area method: measure the cross-section of the channel, then time a float over a known distance.

"A float test is crude, but it tells you within 20 percent whether the site is worth a second visit."

Repeat the test at least three times. Average the results. If the stream is shallow, use a bucket and stopwatch where water spills over a small ledge. Always note the date and recent rainfall, because UK flows swing wildly between summer and winter.

1. Choose a straight, uniform stretch at least 10 metres long.
2. Measure width and depth at three points to get the cross-sectional area.
3. Time an orange peel or wooden float over the 10 metres.
4. Multiply area by velocity, then apply a correction factor of about 0.8.

What to Record and What to Ignore

Record everything: date, time, weather, water colour, debris, and any signs of abstraction or fish passage. Ignore glossy manufacturer claims about "typical" flow. Your site is unique. Also ignore any head figure that comes from a map contour alone.

Once you have both numbers, compare them against the realistic minimum for a micro-hydro scheme, often around 1 kW for a domestic off-grid setup. If your site falls short, stop. If it passes, you have earned the right to spend money on a proper feasibility study.

Next Steps Before You Commit

With head and flow measured, you can estimate power. Then check abstraction licences, grid connection costs, and environmental constraints. A few hours with a level and a stopwatch can save you thousands.

Treat this walkthrough as a filter, not a final design. It is the cheapest test you will ever run on a river, and the most important one.