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11.16 Storm Runoff and Flood Flows 387

Solution 2 (SI System):
1. From Table 11.10 for a frequency of 2 years.
5,230
i (mm/h) =
i + 30
2. The intensity is
5,230
i = = 104.6 mm/h
20 + 30

11.16 STORM RUNOFF AND FLOOD FLOWS

The flood flows descending the arterial system of river basins or collecting in the storm
drains or combined sewers of municipal drainage districts are derived from rains that fall
on the tributary watershed. The degree of their conversion into runoff is affected by many
factors, especially in the varied environment of urban communities. Component effects
and their relative importance must, therefore, be clearly recognized in the interpretation of
storm runoff or flood experience in relation to intense rainfalls.
Flows normally reach flood crest at a given point on a stream or within a drainage
scheme when runoff begins to pour in from distant parts of the tributary area. There are ex-
ceptions to this rule, but they are few. An important exception is a storm traveling upstream
or sweeping across a catchment area so rapidly that runoff from distant points does not
reach the point of concentration until long after the central storm has moved on.
Diminution of effective area or retardance of this kind is rarely taken into consideration in
American practice, but it should be in some circumstances. In a given storm the maximum
average rate of rainfall is always highest for the shortest time interval or duration.
Therefore, the shorter the elapsed time or time of concentration in which distant points are
tributary to the point of concentration, the larger the flows.
The time of concentration is shortest for small, broad, steep drainage areas with rap-
idly shedding surfaces. It is lengthened by dry soil, surface inequalities and indentations,
and vegetal cover, and by storage in water courses, on floodplains, and in reservoirs. In
short intense thunderstorms, peak urban flows often occur when only the impervious or
paved areas are shedding water. The volume of runoff from a given storm is reduced by in-
filtration, freezing, and storage; it is swelled by snow and ice melt, seepage from bank stor-
age, and release of water from impoundages either on purpose or by accident. Maximum
rates are obtained when storms move downstream at speeds that bring them to the point of
discharge in about the time of concentration, making it possible for the runoff from the
most intense rainfall to arrive at the point of discharge at nearly the same instant.
Among the ways devised for estimating storm runoff or flood flows for engineering
designs are the following:
1. Statistical analyses based on observed records of adequate length. Obviously these can
provide likely answers. Unfortunately, however, recorded information is seldom suffi-
ciently extensive to identify critical magnitudes directly from experience. Information
must be generalized to arrive at rational extrapolations for the frequency or recurrence
interval of design flows or for the magnitude of flows of design frequency.
2. Statistical augmentation of available information through cross-correlation with
recorded experience in one or more adjacent and similar basins for which more
years of information are available; through correlation between rainfall and runoff