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Define unit hydrograph and its uses

Short answer

A unit hydrograph is the direct runoff hydrograph from 1 cm effective rainfall uniformly over a catchment in a specified duration. Uses include estimating flood hydrographs and peaks for design and forecasting, deriving synthetic UH for ungauged basins, studying land-use change impacts, and reservoir routing.

Define unit hydrographs and explain the uses of hydrograph.

Unit hydrograph

A unit hydrograph (UH) is the direct runoff hydrograph resulting from one unit (1 cm or 1 mm) of effective rainfall generated uniformly over the drainage basin at a uniform rate during a specified period (the unit duration DD, e.g. the 3-h UH).

It is based on the assumptions of linearity and time invariance, so hydrographs of other storms of the same duration are found by scaling and superposition.

 Q |      /\
   |     /  \     area = 1 cm x A
   |    /    \__
   |___/        \____
   +------------------- t
        |<--- Tb --->|

Uses of the hydrograph (unit hydrograph) in engineering

  1. Estimation of flood hydrograph and peak flow for any storm on the catchment, by multiplying the UH by the effective rainfall and adding baseflow.
  2. Design flood for dams, spillways, culverts, bridges and flood-control works, using the design storm.
  3. Flood forecasting and warning from observed rainfall.
  4. Synthetic UH (Snyder, SCS) for ungauged basins, derived from catchment properties.
  5. Studying the effect of land-use change (urbanization, deforestation) on runoff.
  6. Deriving UH of other durations by the S-curve method.
  7. Reservoir and flow routing studies which need inflow hydrographs.
  8. Water-resources planning, such as estimating storm runoff volume and time to peak.

✓ Answer checked by Claude Sonnet 5.5

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