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Stormwater Management Basics 1
Our Freshwater Supply
Though Earth is sometimes called “the water planet”, due to the apparently abundant oceans covering the majority of its surface, the freshwater on which all living creatures depend is actually only a remarkably small percentage of the total of all the water on the planet.
More than 97.4% of all of Earth’s water is highly saline, or salty, and thus unusable by many organisms, for highly saline water is toxic to most living creatures. Some protists and fish have developed adaptive tolerance to salinity, while certain sea birds and marine iguanas have specialized means of collecting and expelling excess salt. Saltwater makes up all of the planet’s oceans and seas, as well as all saline (or brackish) groundwater within soils and rock (predominately throughout coastal areas and river mouths). Of the remaining 2.5% of the total that is freshwater — that is, water that has relatively low concentration of salt or other dissolved solids — almost 1.8% is trapped in ice caps, glaciers, ground ice, permafrost and permanent snow cover across the globe. That leaves a mere 0.8% of the entire planetary freshwater supply to support all life on Earth. Most of that supply is continually held within the soils and rocks of Earth as groundwater, with only the merest residual trace (just 0.008% of the total!) available to us on the surface, in lakes, ponds, swamps, rivers and streams. Still more sobering, it is estimated that the unpolluted freshwater supply of Earth is in fact only 0.003% of the total water on the planet.
Throughout all of mankind’s history, we have perpetually recycled that same miniscule amount of readily available freshwater — from rainfall and snowmelt, through crops and livestock and drinking water, through washing and bathing, through carrying away our wastes, to evaporation back to the clouds, to drainage back to surface waters, and to percolation down through soil and rock to groundwater. This recycling is called the water cycle (or the hydrologic cycle). See a more detailed description under The Water Cycle below.
From a scientific viewpoint, freshwater systems can be categorized as lotic systems of running water (streams, rivers), lentic systems of standing waters (lakes, ponds, swamps, bogs), and groundwater, flowing through rocks and soils and forming aquifers. Aquifers are underground layers of permeable or porous rock, gravel, sand or silt bearing water, which may therefrom be relatively easily or readily obtained by wells. Aquifers that generally lie closest to the ground surface, and that have no ‘capping’ or confining impermeable layer atop them, are considered unconfined aquifers. Aquifers that may lie deeper, beneath a confining impermeable layer, and which may thus be under greater pressure, are considered confined aquifers. In addition to being generally more accessible (often by being far closer to the ground surface), unconfined aquifers generally also provide greater hydraulic conductivity (i.e. greater and more ready and effective water productivity) than confined aquifers.
Zones of soil, sand and rock containing groundwater are termed saturated (or phreatic), while the substantially drier (though still moist) materials above remain unsaturated. The unsaturated zone is also known as the zone of aeration, within which oxygen transfer by microorganisms, plants and chemical reaction can more readily occur, or the vadose zone. The depth and configuration of the unsaturated zone is important both to the propagation of plant and animal life and to the filtration of water-borne pollutants that would otherwise descend into groundwater. The recharge of an aquifer, or the restoration of its water supply, also occurs through the unsaturated zone.
The line of interface between unsaturated earth and a saturated aquifer or groundwater supply below is called the water table. Unlike a true table, however, the water table in any given area may slope or rise or fall substantially and even be interrupted entirely, depending on the local topography and the varied configurations of layers and intermixing of subsurface materials. In general, it will tend to follow the same contouring as the surface above it. In certain areas, there may also be smaller, localized aquifers that lie above the line of the predominant water table, perched atop impermeable materials, and that are thus known as perched aquifers. Though not often targeted for wells or water supply, due to their limited size and sporadic locations, such perched aquifers may feed surface or artesian (meaning sources that flow freely due to high natural water pressure) springs or wells.
The Water Cycle
Our planet’s water cycle affects all living organisms and is embodied in Earth’s ongoing and ever-changing climate. The prime driver of the water cycle is the streaming energy of the Sun, which heats water in the planetary oceans, seas, lakes, rivers and streams, causing it to evaporate into the local atmosphere. When heated by the Sun, a portion of the ice and snow cover in polar regions and on mountaintops also sublimates directly into atmospheric water vapor. Rainwater that has previously fallen on rocks, soil and vegetation, along with surface snowmelt water, in part returns to the atmosphere through soil evaporation and plant transpiration (sometimes collectively termed evapotranspiration). All water vapor accumulating by these means is effectively clean freshwater, for it has been distilled by the Sun’s heat.
As air currents both cool and warm gather and drive accumulating water vapor about the atmosphere, clouds form, and storms develop, until eventually precipitation falls back to the Earth’s surface as rain, sleet and snow. A portion of such precipitation falls back atop mountains, glaciers and ice caps, only to remain frozen there for many years, if not millennia.
But a great percentage of that precipitation falls onto oceans and land forms, humans and animals, plants and soils, buildings and pavements. That precipitation may infiltrate or percolate (or soak) into porous, permeable or pervious ground to become groundwater, or it may become surface runoff, racing across the ground’s surface toward a further destination. It may also collect as surface water in a lake, pond, swamp, bog or wetland. Groundwater in turn may leach from porous soils and rocks at exposure to the surface, such as at ponds, cliff faces, rocky escarpments, and the heads of natural springs, to once again become surface water or surface runoff. The process by which such groundwater emerges at the ground’s surface is sometimes referred to as daylighting.
Groundwater can either remain at or near the ground’s exposed surface, replenishing stream flows, lakes and oceans, or delve much deeper underground to restore aquifers and underground streams. The interchange between groundwater and surface water is typically greatest where the two come into broadest, most frequent and most consistent contact, such as at flood plains, river valleys, swamps and wetlands. The region of such water interchange, called the hyporheic zone, is crucial to the processes of water filtration, microorganism growth, plant diversity and fish spawning. In terms of maintaining the overall environmental health of a particular biome, therefore, the hyporheic zone is deserving of particular study and care, and perhaps even occasional human intervention.
On undeveloped land areas, excess water that cannot immediately infiltrate the soil becomes surface runoff. It then finds its way, by way of gravity and topography, to ditches, depressions, valleys, and then eventually to ponds, lakes, streams, rivers and the oceans. On developed land areas that are densely overlain by impervious surfaces that resist water infiltration — such as pavements, building roofs and other man-made constructions — surface runoff must most often be channeled, collected and transported by man-made devices to an eventual storm sewer, reservoir, outlet or outfall. Since the greatest contributors to surface runoff are often heavy snow or rain storms, such accumulated surface runoff is, for engineering, design and regulatory purposes, typically collectively called stormwater.
The expanding development of natural landscapes into man-made landscapes significantly alters stormwater flows. First, the addition of impervious surfaces of pavements and roofs prevents the slow and steady infiltration of water into the ground, keeping greater quantities at the surface, needing somewhere to go. Second, the concomitant reduction in plantings and woods leaves less refuge for water that would otherwise lie upon foliage or be absorbed by plant roots and tissues. Third, the very stormwater systems and devices we create — smooth and sinuous and optimized to collect and direct flow — accelerate large quantities of stormwater from one area to another, increasing the scouring effect of erosive runoff and the potential impact of flooding downstream. Fourth, all of that rapidly moving water traveling across our ground surfaces collect increased amounts of waste, pollutants, plant material, debris, and other substances that can no longer be trapped by intervening plants or filtered by ground, and must therefore be cleansed or treated away. Expanding development thereby hijacks the natural water cycle, turning it into quite a different and more complex cycle that must be managed.
As we collect, transport, store, filter, treat and dispose of such stormwater, we undertake stormwater management. Our stormwater systems ideally return the water at such quantity and quality as to make it readily available and useful to us once again as a freshwater source. Eventually, a significant amount of surface water, as well as any water that passes through a stormwater management system, is once again exposed to the heat of the Sun, and begins yet another journey through the endless water cycle.
The collection, transport, storage, filtration and eventual disposition of stormwater embodies the field of stormwater management. Meanwhile, all of the sophisticated disciplinary efforts essential to creating the best and most effective stormwater systems falls to the fields of stormwater management design and planning.
Far earlier in man’s history, stormwater management was likely quite a simple exercise. All our ancestors had to do was to drink and bathe in the nearest clear mountain stream, and perhaps transport modest amounts of water to sprinkle atop sprouting crops. They could rely on nature’s perpetual water cycle all about them to do the rest. But as mankind has spread around the globe, increasing its burgeoning demand for ever-greater supplies of freshwater while paving a growing fraction of the planetary surface, effective stormwater management has become a daunting challenge. Today, roughly one-sixth of the world’s population has inadequate access to enough clean water to sustain life and health. And, as our technology, and material and water consumption advances continue apace, we will be demanding still greater amounts of fresh, clean water. Effective stormwater management is now essential. See more below under Goals of Stormwater Management.
For most of today’s large developments in urbanizing areas — shopping centers, office parks, golf courses, industrial facilities, university campuses, residential complexes — the design and planning of stormwater management systems is the responsibility of a cadre of appropriately trained professionals working in concert. These professionals usually rise from the related ranks of architects, civil engineers, landscape architects, structural engineers, utility designers, soils analysts, hydrologists, and stormwater system designers and regulators. In an increasing number of cases, the design and planning of an effective stormwater management system are crucial keys to a development’s feasibility.
Stormwater systems have significant and direct bearing on such matters as the quality and quantity of freshwater supplies, water availability and consumption, transmission of wastes, prevention of flooding and erosion, pollution control, human health, and the overall viability of our environment. It is appropriate, therefore, that the design, planning, construction, maintenance and continuing operation of stormwater management systems comply with strict Federal, state and local regulations, such as those of the United States Environmental Protection Agency (USEPA), The United States Army Corps of Engineers (USACE), state and regional agencies, and local stormwater management districts, boards of health, planning and zoning boards, building officials, and so on. See Stormwater Management Regulation below for further discussion.
The components and devices of effective stormwater management systems are also varied and numerous. These include everything from wetlands to grading to swales to ditches to culverts to catch basins to drain tiles to sewer pipes to retention and/or detention ponds to filtration devices to outlet structures, and so on. See Stormwater Management System Components below for a brief overview of the most common elements. But, to begin to fully understand stormwater management, we must first understand the surface of the Earth.
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