Bankfull Flows on Friday

Posted on: August 20th, 2018 by Tim Koch

On the evening of Friday, August 17th, 2018 something very important happened in the world of stream science and management. The Esopus Creek, Birch Creek, and Woodland Creek reached “bankfull discharge”.

Bankfull discharge is the amount of water flowing in the stream when the channel is completely filled to the brim, measured in cubic feet per second (cfs). At bankfull, all the water is still contained within the channel. Once bankfull has been exceeded, water spills out onto the floodplain and the stream is technically flooding. The photograph below shows the Stony Clove Creek at near bankfull flow. The  undeveloped floodplain visible on the left is accessible to the stream and ready to accept any flood waters in excess of bankfull.

(a)bkf2(b)Stony Clove headwater close to bankfull

(a) Stream cross section showing bankfull stage, (b). the headwaters of Stony Clove Creek flowing at ~800 cfs, approaching bankfull

In the humid northeast, most streams will see a bankfull flow event every 1.5 years on average. The last time we saw a bankfull flow in the Ashokan watershed was October 2017.

If any flow over bankfull means that the stream is flooding, then we can expect our rivers and streams to flood at least once every year and half. This is natural, stable stream behavior and highlights the importance of floodplain management.

Bankfull discharge is critical to stream managers because over time it is the flow that moves the largest amount of sediment, effectively shaping and maintaining the channel form. Due to the importance of bankfull discharge for channel maintenance, some of the research projects that we fund at AWSMP have triggers that are activated following bankfull flow events.

Below is a chart that compares the stream flows observed on Friday the 17th (far right) to bankfull discharge. The Q1.5 column is an estimated bankfull discharge based on statistics. This is a useful estimate for bankfull when field derived values are not available. The most accurate way to determine bankfull discharge is to read the stream. A well-trained eye can locate bankfull indicators in the field such as depositional features or changes in bank slope, vegetation, and particle size. By using these indicators as a guide to measure the dimensions of the stream channel, highly accurate bankfull discharge values can be calculated (middle column).

 

USGS Gage Station

Q1.5 (cfs) Calibrated Qbkf (cfs)

Qmax 8/17/18 (cfs)

Esopus Creek at Allaben

2,524 2,772 2,950

Esopus Creek at Coldbrook

10,343

7,069

12,000

Birch Creek at Big Indian 287 331

588

Woodland Creek above Phoenicia 1,650

1,720

 

Q1.5 = statistically calculated flow that occurs roughly every 1.5 years, often used a substitute for field-calibrated bankfull discharge

Calibrated Qbkf = field calibrated bankfull discharge

Qmax = instantaneous peak flow measurement from Friday, August 17th, 2018

 

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