water in Bull Run rose above its normal channel and spilled onto the floodplain at Manassas Battlefield, May 2014
The shape of a river channel reflects the pattern of the waterflows than run down through that channel, as well as the rock through which the water has carved its path. The width and depth of stream channels is determined by the amount of water carried by the stream after a 2-year flood (the rainstorms with a 50% chance of occuring). Floods occur when the volume of the river channel is not sufficient to contain the runoff from bigger storms.
A river will carve a channel with clearly-defined banks on each side, in most cases. During occasional floods, the water level will rise to fill the channel. Newspapers may describe the river as running "full to the banks," but the river does not reach flood stage until it rises higher than the river banks and water spills over onto the adjacent land.
When the floodwaters spread out on the floodplain adjacent to the stream channel, the water slows down and deposits some of its sediment. Annual floods renourished the floodplains on the Nile River, and the sediment enriches the soil on Virginia's floodplains as well. Native Americans located their corn fields on floodplains. After the Euopeans arrived, early colonists were also dependent upon annual farm crops, and floodplains were settled before less-productive uplands or hillsides.
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. Pour two glasses, slowly, and all can be swallowed without spilling. Now pour two glasses of water at once, or pour from one glass much faster, and a flood will cascade down your shirt.
When the river level starts to drop, perhaps after the surge of water from an intense thunderstorm has passed, the river is described as having "crested."
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 44 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/structures to fall into the river) or rise higher (causing floods when waters spill over the riverbanks).
modern infrastructure, such as this Route 29 bridge over Bull Run, is designed to withstand large floods that have a 1% chance of occurring each year ("100 year" floods)
Even if a channel is unchanged by storm-induced erosion, 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 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.
in 2010, a driver ignored "Down Drown, Turn Around" advice, and the swift water rescue team went to work in Fairfax County
Source: Federal Emergency Management Agemcy (FEMA), My Time in the Water: Flood Safety Lessons Learned
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.
In many areas of the United States, the spring thaw brings on an annual flood as the winter snows melt. When the water flows over the riverbanks, it encounters obstructions - grass, trees, fences, even barns and houses.
The obstructions slow the flow of the water, and some of the silt carried in the floodwaters settles out along the edge of the river. The deposited silt fills in the low places near the river edge. In the resulting flat floodplains, the soil is rich in organic material. Floodplains are desirable farmland - the soil is fertile, enriched by nutrients eroded from upstream, while seeds/plants on the flat land are less likely to be washed away by normal rainstorms.
Along the Mississippi River, the deposition of silt along the riverbanks has raised them higher after the floods. These natural levees have been enhanced by man-made walls of earth constructed on either side of the Mississippi, so the annual spring runoff can be directed downstream to the Gulf of Mexico and the farmers can get their equipment into their fields without waiting for the floodwaters to drain away naturally.
In 1993, one of the intermittent changes in the pattern of rainfall created a major flood along the Mississippi River, especially downstream of St. Louis. The Federal government, tired of repetitive payments for "natural" disasters, responded by acquiring farmland that was regularly flooded, and even moving entire towns out of the floodplain.
The concept of "strategic retreat" rather than building higher levees could be applied to the 10,000 miles of coastal shoreline in Virginia. Sea-level rise is a slow-moving flood; rising water levels occur over decades rather than hours, but the impact is comparable to a flood.
height of historic floods at Great Falls Park on the Potomac River
Great Falls disappeared under a muddy Potomac River, in high water after a May 2014 rainstorm
Bull Run at Manassas Battlefield, May 2014