Precipitation, which may be in a solid or liquid state
is derived from the atmosphere. It results from the condensation of moisture in
the atmosphere due to the cooling of a parcel of air. The most common cause of
cooling is dynamic or adiabatic lifting of the air. This occurs when a given
portion of air is made to rise with the consequent cooling and possible
condensation into very small cloud droplets. When they are of sufficient size
to overcome the air resistance, the resultant is precipitation. Rainfall
remains at the land surface as depression storage and either evaporates,
infiltrates or is discharged as overland flow (USDOT, 2002)
However, of these, the principal
concern in highway design, is the surface runoff.
The most important step
commencing the hydraulic design of a highway drainage structure regardless of
its size or cost is the determination of the maximum runoff that the highway
drainage structure is anticipated to carry or control (AASHTO, 1999).
Catchment Runoff Generation
Precipitation is the most essential process for the
generation of runoff at a catchment scale. The distribution of precipitation
varies spatially and temporally in nature. Precipitation can be in the form of
snow, hail, dew, rain and rime. In this study precipitation is considered in
the form of rain only (USDOT, 2002).
When a storm occurs, a portion of
rainfall infiltrates into the ground and some portion may evaporate. The rest
flows as a thin sheet of water over the land surface which is termed as overland
flow. If there is a relatively impermeable stratum in the subsoil, the
infiltrating water moves laterally in the surface soil and joins the stream
flow, which is termed as underflow (subsurface flow) or interflow. If there is
no impeding layer in the subsoil the infiltrating water percolates into the
ground as deep seepage and builds up the ground water table (GWT or phreatic
surface). The ground water may also contribute to the stream flow, if the GWT
is higher than the water surface level of the stream, creating a hydraulic
gradient towards the stream. Low soil permeability favours overland flow. While
all the three types of flow contribute to the stream flow, it is the overland
flow, which reaches first the stream channel, the interflow being slower reaches
after a few hours and the ground water flow being the slowest reaches the
stream channel after some days. The term direct runoff is used to include the
overland flow and the interflow (Raghunath,
Rainfall travels in a catchment in different directions.
Due to vegetation, part of rainfall is intercepted by vegetation canopy.
Interception is known as a loss function to catchment runoff depending on
vegetation type and density. The rest of rainfall moves down the vegetation as
stem flow, drip off the leaves, or directly falls to the ground as throughfall
Rainfall remains on the land surface as depression
storage and either evaporates, infiltrates or is discharged as overland flow.
Based on the time delay between the precipitation and
the runoff, the runoff is categorized in to two categories; as (1) Direct
runoff, and (2) Base flow.
Direct runoff: it is that
part of runoff which enters the stream immediately after the rain fall. It
includes surface runoff, prompt interflow and rainfall on the surface of the
stream. In case of snow melt the resulting flow entering the stream is also a