RIVER DRAINAGE BASINS

A RIVER SYSTEM ACTS LIKE A SYSTEM OF DOWNPIPES AND GUTTERING ON A HOUSE - IT ALLOWS THE MOVEMENT OF RAINWATER INTO THE SEA

Watershed (higher ridge of land, the boundary between one drainage basin and another) Source (the place where a river begins -a river system will have numerous sources, such as springs) Confluence (a place where two rivers meet) Tributary stream (a smaller river that flows into a larger river) A TYPICAL RIVER DRAINAGE BASIN. THIS IS THE CATCHMENT AREA FOR ALL THE RAIN THAT FALLS WITHIN THE WATERSHED

All rain falling within a drainage basin ends up in the river system. How does it get there?

OVERGROUND Water runs downhill over the ground towards a river or stream - SURFACE RUNOFF Raindrops might fall directly into a river or stream - CHANNEL FLOW

UNDERGROUND (uppermost level of saturated ground) Raindrops INFILTRATE soil and rock. The water then moves underground towards a stream or river

So, which route to the river is quicker - surface runoff or infiltration / groundwater flow? ANSWER: Rain travelling as surface runoff will get to rivers and streams much more quickly, reaching a river or stream with little delay,within a short time-span. Water that infiltrates and travels underground travels much more slowly, finally reaching a river or stream bit by bit over a much longer period of time. Which situation is a river more likely to be able to cope with, without flooding?

Peak rainfall Scenario 1 Peak discharge overland flow- high Scenario 2 Peak discharge groundwater flow - lower 1.After how many hours does rainfall peak? 2.What is peak river discharge in Scenario 1? (in cumecs) 3.What is peak river discharge in Scenario 2 (in cumecs) 4.What is the lag time in Scenario 1? 5.What is the lag time in Scenario 2? A FLOOD HYDROGRAPH

So: Surface runoff Shorter lag time Higher river peak discharge More likelihood of FLOODING

What conditions will make surface runoff more likely? The rainfall itself - very heavy, in big droplets (less likely to infiltrate) Water table already very high (little scope for infiltration if ground is already saturated) Steep slopes in drainage basin (water flows downhill rapidly -less time for infiltration to occur Non-permeable rock within drainage basin (water can’t infiltrate easily) Few trees (tree roots break the soil, providing little gaps for water to infiltrate). Trees then take up and store water from underground Urbanisation within the drainage basin - large areas concreted or tarmaced over, making surface non- permeable. (Storm drains are built in, of course, but these carry water very quickly to the river, making lag time even shorter)

Storm drains are connected to rivers and streams by an underground network of pipes, called the storm sewer system, which helps to prevent flooding from rain.

Hurricane Floyd In September 1999, Hurricane Floyd made landfall in eastern North Carolina, dropping nearly 17 inches (430 mm) of rain during the hours of its passage. Many residents weren’t aware of the flooding until the water came into their homes. Most localized flooding happened overnight, and the Tar River suffered the worst flooding, exceeding 500-year flood levels along its lower stretches

An additional 20+ inches of rain had fallen prior in the month from the two passes of Hurricane Dennis Damages in Pitt County alone were estimated at $1.6 billion (1999 USD, $1.87 billion 2006 USD).

Some residents in Greenville had to swim six feet underwater to reach the front doors of their homes and apartments. Due to the heavy flooding in downtown Greenville, the ECU Pirates were forced to relocate their football game against #9 Miami Hurricanes to NC State's field in Raleigh where they beat them

It is important that we have some understanding of drainage basin conditions and storm hydrographs (both natural and man made) if we are to understand what can be done to reduce future flood risks