In an S-curve derived by a unit hydrograph U of D-h duration
A 4 hour unit hydrograph of a basin with area of 1728 km2 can be approximated as a triangle with base period of 48 hours. The peak ordinate of hydrograph is
An instantaneous unit hydrograph is a direct runoff hydrograph
A finite unit hydrograph is indicated as the duration D → 0. The limiting case of a unit hydrograph of zero duration is known as Instantaneous Unit Hydrograph (IUH).
The shape of the recession limb of a hydrograph depends on
The climatic factors control the rising iimb and the recession limb is independent of storm characteristics, being determined by catchment characteristics only.
Instantaneous unit hydrograph is a hydrograph of
In Synder’s method of synthetic unit hydrograph development, basin lag is taken as
The most important characteristic of a basin affecting a hydrograph due to a given storm is basin lag. Basin lag is the time difference between the centroids of the input (rainfall excess) and the output (surface runoff).
The ordinate of the IUH of a catchment at any time t is the
If a 4-hour unit hydrograph of a certain basin has a peak ordinate of 80 m3/s, the peak ordinate of a 2-hour unit hydrograph for the same basin will be
With the reduction of unit hydrograph duration, 1 cm excess rainfall will occur in reduced period. So peak ordinate of UH will increase and time base will decrease. Thus the peak ordinate of 2 hr UH will be greater than 80 m3/s.
The point of inflection on the recession limb of a hydrograph represents
The starting point of the recession limb, i.e., the point of inflection represents the condition of maximum storage.
A flood-hydrograph due to an isolated storm consists of
The hydrograph is the response of a given catchment to a rainfall implicit. It consists of flow in all the three phases of runoff, viz. surface runoff, interflow and base flow.