Site investigation for micro hydro plant

Site investigation decides whether the available flow and head can be developed safely and cheaply.

Investigations

1. Reconnaissance: study of maps and a field walk to find possible intake, canal, forebay and powerhouse positions, and the demand (load) of the nearby village.
2. Head measurement: the gross head between the intake water level and the tailrace level.
3. Flow measurement: dry-season (minimum) and mean flow, from direct measurement or hydrological estimates.
4. Geology and soil: stable slopes, with no landslides or leaking ground along the canal and penstock; foundations for the powerhouse and anchor blocks.
5. Layout and cost: length of canal and penstock, access, flood level at the powerhouse, distance to the load.
6. Environment and water rights: impact on downstream users and fish, and on irrigation and drinking water needs.

Head measurement methods

• Water-filled tube (pipe) and pressure gauge: a long flexible tube filled with water is laid down the slope; the gauge reading at the lower end gives the head, $H = p/(\rho g)$. For long drops, readings are taken in steps and added.
• Spirit level and measuring stick: a level is used to measure the height difference in successive steps along the slope, and the steps are added. It is cheap and quite accurate for short drops.
• Abney level (clinometer): the slope angle $\theta$ is measured along a measured length $L$, and the height of each step is $L\sin\theta$. It is quick and suits long, steep slopes.
• Altimeter or GPS: readings at the top and bottom give an approximate height difference, with an error of a few metres, so it is used only in the first survey.
• Dumpy level / theodolite: gives the most accurate result in the detailed survey.

The net head is the gross head minus friction and bend losses in the penstock (typically 5–10 % of the gross head).