Water is a dynamic resource. We see it as both a treasure and a burden, depending on whether it is flowing in a magnificent waterfall or into our basements.
These details are part of the hydrologic cycle, which is made up of processes that transfer water between the atmosphere, oceans, freshwater bodies, soil, and underground aquifers. Rainfall—more generally, precipitation—is one of the steps in this cycle. What happens next is hugely dependent on the landscape.
Rainfall becomes runoff when it cannot soak into the surface, either because the surface is impermeable or because rain is falling quicker than it can infiltrate the ground. Runoff can occur anywhere—from forests to cities—during heavy rainfall. However, runoff poses a number of unique problems in cities and other developed landscapes.
Why does landscape make a difference? How does development affect the water cycle? Are the effects negative?
Replacing forests with buildings and pavement greatly increases the area of impermeable surfaces, and in turn, greatly increases the quantity of runoff. Whereas runoff in forests is impeded by bumpy terrain (rocks, trees, plants), smooth pavement surfaces allow water to flow rapidly. The result is lots of fast-moving water.
Forests that grow in the climates of western Washington are reasonably well equipped to endure floods with little damage. By contrast, floods in homes, stores, and offices cause significant damage that is very costly to repair. Developed areas are characterized both by impermeable surfaces that create rapid runoff and by highly concentrated property assets. As a result, both the risk of flooding and the damages from flooding are greater in city landscapes.
The high risk and costs of floods in cities have led flood prevention, or stormwater management, to become a collective public interest. The term stormwater describes rainwater that runs off surfaces like roads, roofs, and parking lots in developed areas. The traditional approach to stormwater management was to construct storm drains that direct water through underground storm pipes into local waterways. Though these help to keep water from damaging property, over time people became more aware that there are a number of disadvantages to relying exclusively on storm drains for stormwater control. These disadvantages include:
1. Expensive to expand: If it were simple, all storm pipes would be constructed large enough to carry the runoff of the biggest storm imaginable. Storm pipes are planned with the best tools available, but there are still a number of variables that pose challenges. New areas are developed, which could increase the volume of water flowing into storm pipes. More data is collected and new scientific techniques are developed, which could show that previous predictions underestimated stormwater volumes. Whatever the cause, sometimes cities find that the existing storm pipes do not have enough capacity for the volume of stormwater during heavy rainfall. Projects to expand capacity are very costly. Storm pipes are buried under roads and other structures, which have to be disturbed and replaced if larger pipes need to be installed.
2. Polluted water: Our roads are not only impermeable but also covered in heavy metals and chemicals that are toxic to wildlife. Over dry periods, continuing traffic of vehicles and people along roads will cause pollutants to build up. When it subsequently rains, water runoff carries these toxic substances into storm drains or waterways. This water does not go through any filtration or treatment system; instead, it flows directly from storm pipes into local bodies of water. Automobiles pollute roads with drips and spills of gasoline, oil, and other fluids, deposits of exhaust particles, and wear of brake, tire, and pavement materials. Studies have found high concentrations of cadmium and lead in runoff from streets and vehicle service areas, as well as high concentrations of nickel in runoff from parking lots. Oregon Environmental Council, 2007R. Pitt & M. Lalor, 2000
In addition to roads and automobiles, developed areas have a number of other common sources of pollutants. High levels of zinc have been found in runoff from roofs. Fertilizers and pesticides used on landscaped areas can contaminate runoff with chromium, nickel, and organic toxicants. Wood used for decks, playgrounds, fences, utility poles, and other outdoor structures is typically treated with stains and preservatives, which can leach into rainwater, contaminating runoff with copper, chromium, arsenic, and organic toxicants. [R. Pitt & M. Lalor, 2000
3. Out of sight, out of mind: While storm drains do perform a function, they can also have the effect of sweeping stormwater problems ‘under the rug’ and out of the community’s consciousness. Water is transported to an unseen location, leaving residents with the illusion of a problem solved… they don’t know there is a problem and they can’t contribute to a solution. they don’t understand the work that goes into the solution.
Why does it matter that people may remain unaware of stormwater problems? (a) if they don’t know the problem, they may be adding to it (b) if
The Bremerton area has 15 CSO outfalls. [WA Department of Ecology]