Flow duration curve and planning parameters

Flow duration curve

A flow duration curve (FDC) is a graph of river discharge against the percentage of time that discharge is equalled or exceeded. It is drawn by arranging the recorded flows in descending order and plotting each against $P = \frac{m}{N+1}\times 100\%$. It is used to find firm flow, design discharge (e.g. Q40, Q65) and annual energy.

Parameters influencing planning of hydropower projects

1. Hydrology: the amount and variation of river flow (FDC, mean monthly flows, floods, low flows) decides firm power, installed capacity, and spillway size. Long, reliable records are needed.
2. Topography (head): power depends on $Q \times H$. Steep rivers with a short distance between intake and powerhouse give high head cheaply. Valley shape decides dam type, reservoir area and layout.
3. Geology: the foundation of the dam, tunnel route and powerhouse must be strong and stable. Faults, weak rock, landslides and seismic zones raise cost and risk (very important in the young Himalaya).
4. Sediment: Himalayan rivers carry heavy sediment, which needs desanders and causes turbine wear and reservoir filling.
5. Power market and demand: size and type of plant (base load or peaking, run-of-river or storage) must match the load curve, demand growth and possibility of export.
6. Access and transmission: distance to roads and to the grid; long transmission lines add cost and losses.
7. Environmental and social factors: land acquisition, resettlement, environmental flow, fish, effect on downstream users; an EIA/IEE is required.
8. Economic and financial factors: project cost, interest rate, energy tariff, benefit–cost ratio, IRR and payback period.
9. Legal and institutional aspects: licences, water rights, government policy and royalty.
10. Construction materials and logistics: availability of sand, aggregate and cement near the site.
11. Multipurpose use: irrigation, flood control, water supply and navigation needs can change the reservoir operation.