How Much Water Flows During a Storm?

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If a low-lying area near a river near you usually gets about 50 inches of rain a year, you might think "Well, that is about 1 inch per week, so that won't cause any flooding". But, nature doesn't think the same way, and often a large percentage of a year's precipitation can fall in a major storm, in a single day. Your river might not react much to a 1 inch rain, but things might be much different if 10 inches of rain falls in one day. Read on to investigate storm flows in rivers.

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How Much Water Flows During a Storm?

Much of the total streamflow occurs in a short period of time

As is typical of small urban streams, Peachtree Creek in Atlanta, Ga., USA, rises quickly and exhibits a large increase in streamflow when a major rainstorm hits the watershed. Thus, during a storm, many times more water can flow in a few hours as flows in a few days of base flow (generally periods of minimal/near minimal streamflow when no precipitation has fallen for a while). The pie charts below show that a large amount of the total streamflow occurring during a year can occur in just a few days. In 2001, for example, the 10 highest days of daily mean streamflow accounted for 36 percent of the total streamflow for the year.

Pie charts showing how significant percentages of a year's streamflow at a river can occur in a day-week-month.

Comparison of streamflow before and during the flood of Dec. 24, 2002, Peachtree Creek at Atlanta, Georgia. Data shows the huge extent that so much more water can fall during a big storm in a short time. The instantaneous streamflow at 10:00 was about 154 times greater than it was at midnight. Almost 50,000 gallons of water per second was flowing during the peak streamflow period.

 

Icon of rain

 

Have ever wondered how many gallons of water falls during a storm?
Use our interactive rainfall calculator to find out!

 

 

Case study: The storm of December 24, 2002

On Dec. 24, 2002, about two inches of rainfall fell in the Peachtree Creek watershed. This provides a good example to describe streamflow characteristics during a storm since the rain fell for only a few hours on that day and Peachtree Creek was at base-flow conditions before the rain started.

The chart below shows rainfall, in inches, during each 15-minute increment on Dec. 24th and the continuous measure of streamflow, in cubic feet per second (ft3/s).

Bar/line chart showing streamflow and rainfall for Dec 24, 2002 at Peachtree Creek, Atlanta, Georgia.

On Dec. 24, 2002, about two inches of rainfall fell in the Peachtree Creek watershed. This provides a good example to describe streamflow characteristics during a storm since the rain fell for only a few hours on that day and Peachtree Creek was at base-flow conditions before the rain started.

A lot more water flows during a storm than during base-flow conditions

Comparison of base flow and flooding along Peachtree Creek at Atlanta, Ga.

What a typical flood on Peachtree Creek looks like is shown below in a "before and after" picture from the homeowner's now 10-foot high entryway. The flood picture (on the right) was taken on May 6, 2003 in the late afternoon when stream stage was about 17 feet. Peak stage that day occurred at 7:30 PM EST in the evening, when the stream stage reached 17.77 feet with a corresponding instantaneous streamflow of 6,960 cubic feet per second (cfs). Alternatively, base flow at Peachtree Creek (left picture) is around 2. 5 feet, with a streamflow of about 25 cfs. (Credit: Howard Perlman, USGS)

This picture is a good example of how much more water is flows during high-water event than during baseflow conditions. You might be surprised at just how much more water is flowing. As the table of streamflows during the storm of Dec. 24, 2002 shows, streamflow during a flood can be well over 100 times more than during base flow.

Comparison of streamflow before and during the flood of Dec. 24, 2002

  Instantaneous streamflow  
Time Stream stage, in feet Cubic feet per second Gallons per second Streamflow, in gallons, during 15-minute interval
Midnight 2.81 43 322 289,000
10:00 17.33 6,630 49,600 44,600,000

The instantaneous streamflow at 10:00 was about 154 times greater than it was at midnight. Almost 50,000 gallons of water per second was flowing during the peak streamflow period.

It is possible to estimate the total amount of water that flowed during Dec. 24, 2002, and compare it to a day when the streamflows are at baseflow conditions. Using the rating curve for Dec. 24th, on a day when stream stage stays at 2.81 feet, an estimated 27,800,000 gallons of water will flow by the Peachtree Creek gaging station. Using mean streamflows for each 15-minute period during the storm of Dec. 24, an estimated 4,290,000,000 gallons flowed by. That would be about 154 times more water than during a day of base flow.

 

Peachtree Creek is typical of urban streams

The chart of the Dec. 24th storm above illustrates a number of typical patterns significant to how small urban streams react to heavy rainfall.

Peachtree Creek rises quickly when a heavy storm hits
The watersheds of urban streams often have a lot of impervious areas, such as roads, parking lots, and development. In natural watersheds, more precipitation seeps into the ground, but impervious surfaces prevent this. Water falling on impervious surfaces runs off into storm sewers that empty into streams. Thus, a lot of runoff enters Peachtree Creek within minutes of a storm.

Streamflow quickly returns to baseflow conditions
As the chart shows, Peachtree Creek streamflow had fallen back to near-baseflow conditions within hours of the end of the storm. In natural conditions, precipitation that seeped into the ground would be gradually released into the streambank at a much slower rate.

Streamflow rises faster than it falls
The storm on Dec. 24th brought a lot of rainfall in a short period of time. The intensity and amount of rainfall caused a lot of runoff to drain into Peachtree Creek very quickly.

Streamflow continues to rise after rain stops
The Peachtree Creek at Northside Drive measurement site is at a location that is a collection point for the runoff in the 86.8 square miles of watershed upstream in the Peachtree Creek watershed. It can take time for runoff miles upstream to make its way past the site.

The rainfall/streamflow pattern may be different for different storms
Since the streamflow at Peachtree Creek reflects precipitation patterns in the complete Peachtree Creek watershed, rainfall that occurs only miles upstream can affect streamflow at the site. Streamflow can rise at the gaging site without any rainfall occurring at the site.

Base flows are lower than they used to be
Water seeping into the stream beds from the surrounding ground is responsible for the water flowing in streams when no rainfall has occurred in a while (base flow). As the amount of impervious surfaces increase in a watershed, the amount of water that infiltrates into the ground when it rains decreases, thus, there is less water in the surrounding ground to supply water for base flow.

 

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