Posts Tagged ‘sediment’

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

What is a Stream Feature Inventory (SFI)?

Posted on: July 7th, 2020 by Tim Koch

Hold on tight for a bit of reverse engineering:

The Ashokan Water­shed Stream Man­age­ment Pro­gram (AWSMP) is a col­lab­o­ra­tion between Cor­nell Coop­er­a­tive Exten­sion of Ulster Coun­ty, the Ulster Coun­ty Soil & Water Con­ser­va­tion Dis­trict, and the New York City Depart­ment of Envi­ron­men­tal Protection.

All of the AWSM­P’s stream man­age­ment activ­i­ties are under­tak­en in coor­di­na­tion with a local Stake­hold­er Coun­cil. The Stake­hold­er Coun­cil uses rec­om­men­da­tions from Stream Man­age­ment Plans to guide their deci­sion mak­ing. Man­age­ment plans con­tain a com­pre­hen­sive review of stream char­ac­ter­is­tics, data, maps, and rec­om­mend­ed man­age­ment strategies.

The large amount of data and obser­va­tions required to write a man­age­ment plan for a stream come from a Stream Fea­ture Inven­to­ry (SFI). This is where the rub­ber meets the road, or, where the wad­ing boots meet the stream bed.

Dur­ing a SFI, AWSMP staff from the Ulster Coun­ty Soil & Water Con­ser­va­tion Dis­trict walk a stream from top to bot­tom, col­lect­ing data on erod­ing stream banks, log­jams, and infra­struc­ture. These data are then ana­lyzed and ulti­mate­ly used to write a stream man­age­ment plan.

Join AWSMP Stream Edu­ca­tor Tim Koch as he joins the assess­ment crew on a SFI of the Elk Bushkill Creek in the Town of Shan­dak­en. This SFI is part of a larg­er effort by AWSMP to assess mul­ti­ple head­wa­ter trib­u­taries of the Eso­pus Creek, includ­ing McKin­ley Hol­low Creek and Lit­tle Peck Hol­low Creek. These trib­u­taries may be con­tribut­ing exces­sive sed­i­ment loads to the upper Eso­pus  Creek in the Oliv­erea val­ley. Excess sed­i­ment sup­ply leads to aggra­da­tion, or sed­i­ment “fill­ing in” the stream, which can sub­se­quent­ly trig­ger bank ero­sion and raise flood ele­va­tions.  SFI’s of the Eso­pus Creek head­wa­ters may help to locate and pri­or­i­tize restora­tion project sites aimed at reduc­ing the sed­i­ment sup­ply reach­ing the valley.

Stay tuned in the com­ing months for a SFI report on the Eso­pus Creek Head­wa­ters and for a new stream man­age­ment plan for the Lit­tle Beaver Kill in the Town of Woodstock.

 

Share