Definition: The three
dimensional, largely hidden, dense systems of urban water networks that include
a potentially important network of buried headwater streams give rise to a
highly connected network in which groundwater (slow) flows are interspersed
with faster groundwater and surface water flows which resemble a karst
hydrologic system.
Explanation: Urban landscapes offer streams (often
buried in densely developed areas) and a dense surface drainage system for
stormwater (e.g., gutters, swales, etc.), but these are also part of a buried,
hidden water network which includes sanitary sewers, storm drains, and potable
water. These interact with streams in
very complex ways that result in both positive and negative water flows between
the engineered, groundwater and natural stream and groundwater systems.
Example: The Baltimore
stormwater drainage pipe in the headwater portion of the Watershed 263
catchment exhibits constant baseflow, even in dry weather, which is similar in
magnitude to a natural forested catchment of similar drainage area.
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Figure 1: Underground engineered
network pipe types. Left: Storm drains also
carry baseflows from groundwater seeping into pipe joints. Right: Water supply pipes are under pressure
and lose large quantities of their flow (up to 30%), increasing groundwater
flows. Bottom: Sanitary sewers can both contribute to and take water from
groundwater and streams depending on their condition and hydraulic position in
the landscape. They can also contribute sewage directly through engineered
CSOs, and overflows and shunts to storm drains.
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Figure 2: Baseflow in the ultra urban
residential Baltimore Street headwater storm drain in Watershed 263. Left and Lower Right: Baseflow in dry
weather in the small headwater storm drain.
Upper right: Manhole access point on Baltimore street; with downtown
Baltimore in background.
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Why Important: These complex
interactions between natural and human water systems greatly increase
connectivity and impact stream ecosystem structure and function in profound ways. Examples include the contributions of deicing
salt to long term increasing stream chloride trends (over many decades), flows
of water between streams and buried sanitary sewer interceptors (and subsequent
bacterial pollution), and rising water tables because of leaking potable and
sanitary sewage waters.
Contributed by BES Co-PI Kenneth T. Belt
