Stormwater management aims to reduce surface runoff from rain and snowmelt by promoting infiltration and replenishing groundwater. It is important in urban areas with many impervious surfaces to prevent flooding and pollution. Traditional methods only moved water away, while modern approaches try to restore natural water cycles through retention basins, infiltration basins, bioswales, green roofs, and low impact development techniques. Calculating surface runoff estimates how much water will flow over an area based on its size, average rainfall, and surface type to help design effective drainage systems.

1. STORM WATER
MANAGEMENT
Ar.M.Tariq
AR-2014

2. When it rains, often more water hits the earth than can be absorbed
by the ground. The excess water, called surface runoff, drifts into
sewers or drains.
Surface runoff can erode sediment or pollute the water supply by
picking up contaminants before reaching the drain.

4. DEFINITION
Stormwater management is the effort to reduce runoff of rainwater or
melted snow into streets, lawns and other sites and the improvement
of water quality, according to the United States Environmental
Protection Agency (EPA).
Stormwater management means to manage surface runoff.
It can be applied in rural areas (e.g. to harvest precipitation water),
but is essential in urban areas where run-off cannot infiltrate because
the surfaces are impermeable.

5. Costs depend on technology and the size of the systems. Planning,
implementation and operation and maintenance require expert
knowledge.
Stormwater management is essential to prevent erosion of
agricultural land and flooding of inhabited urban or rural areas. Both
cases can cause severe damages and contamination of the
environment if sanitation facilities are flooded. This results in high
costs and notably massive suffering for the local communities.

6. Traditional stormwater management was mainly to drain high peak
flows away. Unfortunately, this only dislocates high water loads.
Modern approaches aim to rebuild the natural water cycle, i.e. to
store runoff water (e.g. retention basins) for a certain time, to
recharge ground water (e.g. infiltration basins) and to use the
collected water for irrigation or household supply.

7. STORMWATER SOLUTIONS
Reducing the amount of impervious surface cover will greatly reduce
the amount of runoff and may even slow the flow of water off a site.
Water collection systems can be used to collect water off the roof to
be used as bathing and washing water as well as for irrigation.
Bio-swales can be used to slow and filter water before moving to
streams.

8. BIOSWALES
Bioswales are landscape elements designed to concentrate or remove
debris and pollution out of surface runoff water.
They consist of a swaled drainage course with gently sloped sides
(less than 6%) and filled with vegetation, compost
Swales are used to convey water to plants, rain gardens, or storm
drains, while allowing the water to infiltrate into the soil and be
soaked up by the vegetation, therefore reducing flooding .

10. GREEN ROOFS
As opposed to traditional metal or asphalt roofs, green roofs absorb,
store and then evapotranspire precipitation. Through this process,
they act as a stormwater management system and reduce overall
runoff, preventing sewer overflow.

11. LOW IMPACT
DEVELOPMENT (LID)
The term low impact development (LID) refers to systems and
practices that use or mimic natural processes that result in the
infiltration, evapotranspiration or use of stormwater in order t
In stormwater management, LID systems generally increase
infiltration of rainwater runoff to minimize the overland flow volumes,
recharge groundwater systems, and improve the water quality before
it reaches a SWM facility or open watercourse.o protect water quality
and associated aquatic habitat.

12. RETENTION BASIN
A retention basin, sometimes called a wet pond, wet detention
basin or stormwater management pond, is an artificial lake with
vegetation around the perimeter, and includes a permanent pool of
water in its design.

13. CALCULATEING SURFACE RUNOFF
Designer must estimate runoff when building drainage systems in
order to minimize these problems.
Surface runoff is water, from rain, snowmelt, or other sources, that
flows over the land surface, and is a major component of
the water cycle. Runoff that occurs on surfaces before reaching a
channel is also called overland flow.
A land area which produces runoff draining to a common point is
called a watershed.

14. Measure the length and width of the site for which you want to
calculate surface runoff. Note the length and width in feet. Calculate
the area of the site in square feet by multiplying the length times the
width.
Multiply the area of your site by the average rainfall in feet in order to
get the volume of rainfall on the site.
Calculate the runoff by multiplying the volume by the surface
percentage. If the surface is hard (i.e. asphalt or concrete), the
surface percentage is 100 percent. If the surface is grassy, the
surface percentage is 60 percent.

15. Practice using an example. Suppose you are calculating run-off in a
grassy garden measuring 24 inches by 32 inches where the average
rainfall is 8 inches per year.
The area of the garden is therefore 2 feet x 2.67 feet or 5.34 square
feet.
Convert average rainfall from inches to feet: 8 inches/12 inches per
foot = 0.67 feet.
Multiply the average rainfall by the area of the site: 5.34 square feet x
0.67 feet = 3.578 cubic feet. Grassy areas have a runoff percentage
of 60 percent; 3.015 cubic feet x 0.60 = 2.147 cubic feet.