years that a stated intensity is equaled or exceeded. Thus, F equals (n + 1)/m where
n is the number of events and m is the order of magnitude of a particular event; t is
the duration of rainfall in minutes. The values expressed all present i in units of in/h.
A ¼ the tributary drainage area in hectares or acres or mi
2 . For ready conversion
factors, there are 640 ac/mi
2 . Also, if English units of area in acres are combined
with precipitation in in/h, the value of Q becomes ac-in/h and this is approximately equal to 1 ft
3 /s (cfs) (actual value, 1.002). If metric units of hectares are
used for area, the value Q is given in ha-cm/h, which can be converted since
ha-cm ¼ 100 m
3 . The intensity of the rainfall can also be determined by a
precipitation gauge and knowing the duration of precipitation. This will provide
the average intensity of rainfall during the time of precipitation.
Numerous empirical relationships based upon the rational method have been
proposed as shown in Fair et al. [9] and Gray [10]. As may be expected, the empirical
equations apply most validly to the specific drainage basin being studied by the
researchers who published their results.
The rational method is useful for mean storm runoff values over the period of time
of a storm. For the maximum flow, the time of precipitation used for the equation
should be the time of concentration, t c . To determine runoff at any particular time
during and after a storm occurrence, various values of time would have to be chosen
and the total flow integrated over a longer period of time.
In order to avoid the multiple calculations in determining storm flow by the
rational method, Sherman [11] has devised a system called the unit hydrograph. The
hydrograph applies only to a specific drainage basin and is derived from data
pertaining to runoff over a period of one complete storm event. Based on this
storm hydrograph, the anticipated flow from other storms in that same drainage
basin can be estimated using four basic assumptions. These are:
1. The effects of all physical effects of a basin are reflected in the shape of the storm
runoff hydrograph. That is, all runoff in that specific basin will have the same
characteristic shape of curve.
2. For a given duration of rainfall, the duration of runoff is essentially constant and
independent of the magnitude of the rainfall. This indicates that the curve is the
same length for the same time of precipitation.
3. The ratio of added runoff for a certain duration of rainfall is proportional to the
total volume of storm runoff. This means that an increased magnitude of precipitation over the same duration of rainfall will increase proportionately all portions
of the hydrograph curve.
4. Precipitation extending over several durations of time, with or without interruption, creates a hydrograph that is a composite of all the hydrographs from unit
hydrographs starting with the first duration of time of rainfall. The total runoff is a
sum of the individual runoffs based upon the unit hydrograph values.
The unit hydrograph is the hydrograph of direct runoff from any storm that
produces exactly 1.0 in of net rain [12]. The duration of rainfall varies as a function
of the size of the basin; for stormwater inlets it may be in the order to 10 min,
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
255
n is the number of events and m is the order of magnitude of a particular event; t is
the duration of rainfall in minutes. The values expressed all present i in units of in/h.
A ¼ the tributary drainage area in hectares or acres or mi
2 . For ready conversion
factors, there are 640 ac/mi
2 . Also, if English units of area in acres are combined
with precipitation in in/h, the value of Q becomes ac-in/h and this is approximately equal to 1 ft
3 /s (cfs) (actual value, 1.002). If metric units of hectares are
used for area, the value Q is given in ha-cm/h, which can be converted since
ha-cm ¼ 100 m
3 . The intensity of the rainfall can also be determined by a
precipitation gauge and knowing the duration of precipitation. This will provide
the average intensity of rainfall during the time of precipitation.
Numerous empirical relationships based upon the rational method have been
proposed as shown in Fair et al. [9] and Gray [10]. As may be expected, the empirical
equations apply most validly to the specific drainage basin being studied by the
researchers who published their results.
The rational method is useful for mean storm runoff values over the period of time
of a storm. For the maximum flow, the time of precipitation used for the equation
should be the time of concentration, t c . To determine runoff at any particular time
during and after a storm occurrence, various values of time would have to be chosen
and the total flow integrated over a longer period of time.
In order to avoid the multiple calculations in determining storm flow by the
rational method, Sherman [11] has devised a system called the unit hydrograph. The
hydrograph applies only to a specific drainage basin and is derived from data
pertaining to runoff over a period of one complete storm event. Based on this
storm hydrograph, the anticipated flow from other storms in that same drainage
basin can be estimated using four basic assumptions. These are:
1. The effects of all physical effects of a basin are reflected in the shape of the storm
runoff hydrograph. That is, all runoff in that specific basin will have the same
characteristic shape of curve.
2. For a given duration of rainfall, the duration of runoff is essentially constant and
independent of the magnitude of the rainfall. This indicates that the curve is the
same length for the same time of precipitation.
3. The ratio of added runoff for a certain duration of rainfall is proportional to the
total volume of storm runoff. This means that an increased magnitude of precipitation over the same duration of rainfall will increase proportionately all portions
of the hydrograph curve.
4. Precipitation extending over several durations of time, with or without interruption, creates a hydrograph that is a composite of all the hydrographs from unit
hydrographs starting with the first duration of time of rainfall. The total runoff is a
sum of the individual runoffs based upon the unit hydrograph values.
The unit hydrograph is the hydrograph of direct runoff from any storm that
produces exactly 1.0 in of net rain [12]. The duration of rainfall varies as a function
of the size of the basin; for stormwater inlets it may be in the order to 10 min,
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
255