Posts Tagged ‘floodplain’

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.

Share

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

 

Share