Transcript الشريحة 1 - جامعة فلسطين

Hydrology
The Unit Hydrograph
The Unit Hydrograph
The Unit Hydrograph
The Unit Hydrograph
• The unit hydrograph is defined as a hydrograph of direct
runoff (excluding the baseflow) observed at the downstream
limit of a basin due to one unit of rainfall excess (net
precipitation) falling for a unit time tr
• The unit of excess precipitation is taken as I in. or 1 mm. The
unit of time for precipitation may be 1 day or less, but must
be less than the time of concentration
Assumptions
• The basis of the unit hydrograph concept
– A hydrograph reflects all of the combined physical
characteristics of the drainage basin (shape, size,
slope, soils) and that of the causative storm (pattern,
intensity, duration).
– The theory of superposition applies and linearity of
the relation is assumed.
– Variations in storm characteristics do, however, have
a significant effect on the shape of hydrographs. This
includes (a) rainfall duration, (b) intensity, and (c)
areal distribution in the basin.
DERIVATION OF UNIT HYDROGRAPH
Inverse Procedure
1. The hydrograph associated with a storm is plotted.
The baseflow is separated, thus obtaining the
direct runoff hydrograph (DRH).
2. The area under the DRH that represents the
volume of surface runoff is computed. This volume
of runoff is converted to a depth.
3. Each of the ordinates of the DRH is divided by
volume. The result is a unit hydrograph
DERIVATION OF UNIT HYDROGRAPH
Inverse Procedure
Example
• The hydrograph of Example 7.1 was produced by a storm of
12-hour duration considered to have uniform intensity over
the basin. determine the unit hydrograph.
Example
Example
Changing the Unit Hydrdgraph Duration
Lagging Method
Lagging
Method
Example 7.5
ESTIMATION OF STREAMFLOW FROM
UNIT HYDROGRAPH
• Given below is a hydrograph that resulted from
an isolated 2-hour duration storm of 1,5 in net
rainfall. Determine the streamflow hydrograph
from the storm sequence indicated. Assume that
the losses amount to 60% of the precipitation
Solution
• The base flow is separated from the hydrograph by the
recession curves method, the DRH thus obtained, the unit
hydrograph is derived using the inverse procedure . The
values are shown in columns 1 and 2, respectively, of Table
7.13
• The net storm (net rainfall) sequence is obtained, excluding
60% losses from the precipitation.
• The ordinates of the UH are multiplied by the successive
values of the net rainfall, each lagged by the effective
duration as shown in columns 3, 4, 5, and 6.
• The total of columns 3 through 6 results in the DRH. Adding
baseflow to this provides the streamflow hydrograph.