Monday’s Bankfull Flows

Posted on: December 4th, 2020 by Tim Koch

Mon­day Novem­ber 30th, 2020 was a rainy day in the Ashokan water­shed. A home rain gauge in Boiceville mea­sured approx­i­mate­ly 4 inch­es over the course of the day. 

In response to the sig­nif­i­cant pre­cip­i­ta­tion the Beaver Kill, Lit­tle Beaver Kill, Bushkill, and Eso­pus Creek at Cold Brook reached bank­full dis­charge. Bank­full dis­charge is the stream flow that com­plete­ly fills the chan­nel in a geo­mor­phi­cal­ly sta­ble stream. Any flow that exceeds bank­full will put water onto the adja­cent floodplain. 

Cross sec­tion of a geo­mor­phi­cal­ly sta­ble stream where the entire chan­nel is filled dur­ing a bank­full flow.

Streams that have berms or lev­ees, are incised, or oth­er­wise unsta­ble do not have such a clear rela­tion­ship between bank­full dis­charge and chan­nel geometry.

In the North­east, a bank­full or greater flow hap­pens once every 1.5 years, on aver­age. How­ev­er, “on aver­age” means that some years see mul­ti­ple bank­full events while oth­ers have none. Mon­day’s event was the sec­ond time in 2020 that the Lit­tle Beaver Kill has equaled or exceed­ed its bank­full dis­charge of 909 cubic feet per sec­ond (cfs).

2020 Hydro­graph of the Lit­tle Beaver Kill. From USGS.

Bank­full flow events are impor­tant because over time, these flows move more sed­i­ment than any oth­er dis­charge, larg­er or small­er. This is because bank­full flows hap­pen reg­u­lar­ly, every 1.5 years on aver­age, as opposed to big floods that move a lot of sed­i­ment but are more infrequent. 

Due to the geo­mor­phic impor­tance of bank­full dis­charge events, the AWSMP reg­u­lar­ly vis­its stream restora­tion sites, cul­vert replace­ment projects, and oth­er stream reach­es fol­low­ing bank­full events to take pho­tographs and mon­i­tor any changes observed in the channel. 

AWSMP staff from the Ulster Coun­ty Soil & Water Con­ser­va­tion Dis­trict inspect a restora­tion site on Wood­land Creek fol­low­ing a bank­full flow in Novem­ber 2019. Pho­to by Tim Koch.

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