Note the fast increase in streamflow of Deep Creek in Amelia County, after Tropical Storm Gaston
Source: USGS - Water Resources of Virginia
Floods occur when the volume of the river channel is not sufficient to contain the runoff. Try pouring one glass of water into your mouth carefully - if you pour slowly enough, you can swallow the water without spilling any down your chin. Now pour thre glasses of water at once, or pour from one glass a lot faster... and the flood will cascade down your shirt. That's what happened on August 30, 2004 in Richmond, when Tropical Storm Gaston dropped 12 inches of rain in one afternoon onto downtown Richmond. Shockoe Creek filled up and water overflowed the stream banks, covering the Shockoe Slip restaurant district up to the second story of some buildings.
Weather forecasters were surprised that the routine tropical storm traveling north from the Caribbean would stop and create such heavy rains in a small area. Normally, such storms dump just an inch or three on any one place before moving through the area, but Gaston kept on dropping water in the same local area.
Virginia residents are accustomed to about 40 inches of rain per year, and the vegetation reflects it. It takes only a few years for grass, shrubs, vines, and/or trees to create a green cover on Virginia's soils, thanks in large part to the rainfall nurturing new plants. The Atlantic Ocean and Chesapeake Bay moderate the Virginia climate, too. In contrast, the dry California desert still shows the scars of tank tracks from military training exercises during World War II, while the human-caused surface disturbances in Virginia are obscured within months in many cases.
Floods are becoming more common, independent of any change in the climate due to global warming, because our developments are modifying patterns of runoff and the shape of the river channels. When a river channel is narrowed, often in an urban area by filling in the marshes on the shoreline, the volume of the channel is decreased. The inevitable consequence, according to the laws of physics, is that the water must either flow faster (eroding the riverbanks and causing trees or structures to fall into the river) or rise higher (causing floods when waters spill over the riverbanks).
Even if the channel is unchanged, development can increase the probability of floods downstream. When forests and grasslands are replaced by impermeable surfaces, such as parking lots, roads, and buildings, the rainfall no longer seeps gradually into the ground. Instead, it rapidly flows to the edge of the pavement or roof. There it may have sufficient energy to wash away soil and vegetation and carve gullies, washing silt into streams and causing water pollution.
More importantly for flood control, the rapid flow of stormwater off the surface of the land creates a higher peak flow downstream. For example, look how a one-inch summer thunderstorm in a small stream valley would create a small rise in water level at a stream gauge downsteam. The storm would deposit an excess of water that could not be absorbed in the ground. In this example, assume that the excess normally would flow downstream over the course of 24 hours, and cause the water level at the stream gauge to rise to a peak of 6" above average flow four hours later.
Building a shopping center in that small stream valley would not change the total amount of water deposited by the same storm. However, covering the ground with impermeable surface would dramatically speed up the the time during which the excess rainfall washed downstream. Instead of taking 24 hours to drain, the excess water may race off the roofs and parking lots into gutters, and be gone soon after the rain stopped (except for a few puddles). The runoff would be faster and the peak water level at the stream gauge would be higher.
A "flashy" stream has an unusually rapid rise in water level and then a quick drop back to the height of the average flow. Over time, such streams will carve deeper channels, incising themselves into the floodplain and creating riverbanks that erode during every storm. Trees and paths along such riverbanks are vulnerable to erosion - look for an unusual number of trees tilting over streams in urban parks, and you're probably seeing signs of runoff that was accelerated by upstream development.
The normal water cycle in Virginia is not always normal. Water in Virginia can cause its own dramatic changes in the surface. The effects of Hurricane Agnes in 1972 are still visible in Nelson County, and the Madison County floods in 1995 substantially reshaped the stream channels in that part of Shenandoah National Park.
Virginians have filed over 21,000 flood insurance claims and received over $150 million in Federal flood payments.1
