A hydrograph is a graph or plot that shows the rate of water flow in relation to time, given a specific point or cross section. These graphs are often used to evaluate stormwater runoff on a particular site considering a
development project.

A natural landscape with no development or impervious surfaces will have high levels of rainfall abstraction and produce less runoff due to the vegetation and infiltration capacity of the soils, which produces a gradually sloped hydrograph (1). In this scenario, rainwater will meet multiple obstacles while flowing towards a stream in the form of rainfall interception by vegetation,
transpiration by plants, evaporation from land surfaces, infiltration into soils, and ponding of water in surface depressions.

When the natural landscape is altered by development, trees and other vegetation are replaced by impervious surfaces such as roofs, driveways, gutters, and parking lots. These impervious areas curtail the landscape’s
ability to filter and infiltrate water, and results in higher peak flows and greater runoff volumes as seen in
hydrograph (2). Stripping the landscape’s ability to
naturally manage stormwater results in increased
erosion, sedimentation, and nutrients
entering our waterways.

So how can we prevent this from happening? Low
Impact Development (LID) is a design strategy that strives to mimic the pre-existing site hydrology using a variety of best management practices (BMPs). If a site planner knows what the hydrograph for a site looked
like prior to development, they can amend their site plan to accommodate increased runoff onsite using practices such as rain gardens and bio-swales. This will produce hydrograph (3).

What if my home is already built? LID can be used in redevelopment or retrofit
applications as well. For
previously developed sites, best management practices can be
installed within the confines of
existing site conditions. Simple practices, such as
disconnecting a downspout and redirecting runoff into a lawn or garden, will have positive impacts on a site’s hydrograph and help prevent pollutants from entering your local stream.

At the end of the day, little adjustments can help solve large problems and understanding your hydrograph is one step towards a cleaner Bay.

By Bryan Hofmann, FOR Programs Manager

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