Municipal Stormwater Ponds - Sidewalks and Stormwater
Municipal Runoff
As new residential or commercial developments are built, like roads, parking lots, sidewalks, and subdivisions, the natural landscape is no longer able to absorb water as it normally would. Rapid urbanization means that large portions of soil are converted into impermeable surfaces as grass is pulled away, trees are removed, and porous earth is covered with concrete and pavement. A city block covered by rooftops and asphalt generates an average of 5x as much runoff than the same area would if covered by forest. This makes sense, when you consider that a raindrop falling onto a patch of forest would be absorbed by thirsty roots, soaked into natural aquifers, or evaporated after being caught on leaves or other sticky plant surfaces.
Conversely, this also means that when rain falls on your neighborhood, or on a busy street in London, each individual raindrop will have a lot further to travel before finding a surface it can soak into. Precipitation over these surfaces will gather in much larger quantities, moving a lot faster, sometimes resulting in severe flooding. The stronger the flow of this water grows, the more it will dislodge exposed soil or loose particles. As this runoff picks up more and more solids, its ability to erode away at its environment increases, fueling its own abrasive abilities. With enough time, this increase in water and erosion can result in scouring of natural channels or stream banks, fallen trees from destabilized soil, and noticeably turbid or muddy waters.
Transporting Stormwater
Stormwater is often caught via curbside gutters and carried through storm sewer systems into a nearby receiving stream or water system. These underground drains transport the water through narrow, straight paths, allowing the water to gather even more speed and erosional power. Sometimes, stormwater is caught alongside sewage and treated in a municipal wastewater plant before being discharged back into the watershed. However, most of the time, stormwater is directly released into the receiving body of water. Water discharging from the end of the stormwater drainage system is often flowing at extreme rates, especially after a severe storm. The strength of the blast will batter and smother aquatic vegetation and animal life as it dislodges clay and other sediments from the river floor. In addition, water that flows over sun-warmed pavement and metal will be warmed in turn. This sudden increase in temperature from heated stormwater run-off dispersing into the waterway can cause oxygen levels in the water to plummet. This suffocates local critters and aquatic greenery and encourages algae growth (which only contributes to and compounds upon the anaerobic conditions).
Contaminants
As snow melts or rain falls on highways, cars, rooftops and streets, these slopes and avenues built for human use become canals for runoff, allowing surface water to pick-up all kinds of chemicals and other debris along the way. This can include detergent and fertilizer, trash like cigarette butts or single-use plastics, viruses or chemicals from pet waste, motor and cooking oil, and debris fallen from buildings and trees after a severe storm. Analysis into waterways located downstream of major urban run-off sources have found chemicals and metals like phosphorus, ammonia, nitrate and nitrite, copper, lead, and zinc. On average, an urban watershed produces around 5 to 20x as much phosphorus per unit per year than a rural or less-developed region of the same size and conditions. Water-borne pathogens can also make their way into polluted run-off. Some waterways downstream of major urban centers and where they release their stormwater have been found to include bacteria that’s harmful to humans, including those that cause E.Coli, pneumonia, sepsis, giardia, cryptosporidiosis, and more.
These pollutants can have widespread and exponential consequences further downstream. Dumping the used oil from even a single oil change can impact up to one million gallons of freshwater. According to the EPA, US households dump around 193 million gallons of used oil each year, or about 91% of the amount of oil lost in the 2010 Deepwater Horizon oil spill. According to the 1996 Water Quality Inventory, urban runoff is the primary source of pollutants leading to water quality impairment along the nation’s beaches, affecting nearly 47% of all impaired miles of shoreline.
How Reservoirs Help
Water found in dammed valley or bank-side retention reservoirs is often already taking part in the first step of the water-treatment process. While resting in the reservoir, bacteria, algae, and other microorganisms utilize many toxins and chemicals in their growing process, releasing the processed pollutants via waste. Time for the water to settle also encourages sedimentation, decreasing the overall turbidity of the water. Less turbidity means more sunlight is allowed to permeate the water, encouraging plant life, and improving the overall health of the water. Reducing sediments also lessens the amount of work that needs to be done later in the treatment process.
After the water’s retention time (the period it’s allowed to rest in the reservoir), it is then released to a water treatment plant. From there, it’s filtered, disinfected, and made ready for human consumption through a combination of biological, chemical, and physical processes. The water is then supplied to commercial and residential pipelines through water mains, where it’s distributed and released through faucets, hoses, showerheads, fire hydrants, and sprinklers.
