25 October 2004
STORMWATER MANAGEMENT USING THE HYDROLOGIC CYCLE APPROACH
INTRODUCTION. Development affects the natural hydrologic cycle as
shown in Figures 4-1 and 4-2. The hydrologic cycle consists of the following processes:
A hydrologic budget describes the amounts of water flowing into and out of an
area along different paths over some discrete unit of time (daily, monthly, annually).
Grading, the construction of buildings, and the laying of pavement typically affect the
hydrologic budget by decreasing rates of infiltration, evaporation, transpiration and
subsurface flow, reducing the availability of natural storage, and increasing runoff. In a
natural condition such as a forest, it may take 25 to 50 mm (one to two inches) of rainfall
to generate runoff. In the developed condition, even very small amounts of rainfall can
generate runoff because of soil compaction and connected impervious areas. The
result is a general increase in the volume and velocity of runoff. This, in turn, increases
the amount of pollution that is carried into receiving waters and amplifies the generation
of sediment and suspended solids resulting from bank erosion.
DESIGN INPUTS. Both LID and conventional stormwater management
techniques attempt to control rates of runoff using accepted methods of hydrologic and
hydraulic analysis. The particular site characteristics that are considered will depend on
the nature of the project. Land use, soil type, slope, vegetative cover, size of drainage
area and available storage are typical site characteristics that affect the generation of
runoff. The roughness, slope and geometry of stream channels are key characteristics
that affect their ability to convey water.
Figure 4-1. Natural Hydrologic Cycle
Source: McCuen, 1998.