Posts Tagged ‘flooding’

How Antecedent Moisture Conditions Impact Flooding

Posted on: August 4th, 2020 by Tim Koch

The amount of pre­cip­i­ta­tion that falls dur­ing a storm obvi­ous­ly has an impact on the flood dynam­ics of rivers and streams. When it rains a lot, rivers and streams can flood dra­mat­i­cal­ly. Flood­ing from Trop­i­cal Storm Irene in 2011 is an all too famil­iar exam­ple.

Flooding in Boiceville as a results of Tropical Storm Irene

Flood­ing in Boiceville as a results of Trop­i­cal Storm Irene

Anoth­er impor­tant but less well known influ­ence on flood­ing is the antecedent mois­ture con­di­tion.

To under­stand what antecedent mois­ture con­di­tion is and how it impacts floods we need to briefly dis­cuss the water bal­ance:

     P = RO + ET + ΔS

where,

     P = pre­cip­i­ta­tion,
     RO = runoff,
     ET = evap­o­tran­spi­ra­tion, and
     ΔS = change in ground­wa­ter or soil stor­age.

This gen­er­al­ized equa­tion is say­ing that all the water that falls as rain either (1) runs off the sur­face and becomes flow in a stream, (2) is evap­o­rat­ed or tran­spired (i.e., used by plants), or (3) is stored in the ground, often in the pore spaces between soil par­ti­cles.

Soil can be thought of as a giant sponge that can absorb large amounts of water. Antecedent mois­ture con­di­tion is how wet or dry that soil stor­age sponge is when it starts to rain.

If the soil stor­age sponge is already sat­u­rat­ed before the storm hits, only a small per­cent­age of the rain­fall can be absorbed, mean­ing a large por­tion of the rain­fall total will become runoff. For exam­ple, pri­or to TS Irene in 2011 the antecedent mois­ture con­di­tion was rel­a­tive­ly high, as can be seen in the stream gage hydro­graph at Allaben (below). The orange tri­an­gles rep­re­sent the aver­age flow for that day (approx 20–30cfs). In the week lead­ing up to Irene, flow in the Eso­pus Creek was well above aver­age (blue line, 100–200 cfs), indi­cat­ing that soil mois­ture lev­els were already high when the storm hit.

Hydrograph of Esopus Creek at Allaben prior to TS Irene in 2011.

Hydro­graph of Eso­pus Creek at Allaben pri­or to TS Irene in 2011.

Con­verse­ly, if the soil stor­age sponge is most­ly dry when the storm hits a larg­er per­cent­age of the pre­cip­i­ta­tion can poten­tial­ly be absorbed, or stored in the soil sponge rather that becom­ing runoff.  Less runoff can some­times mean less dra­mat­ic flood­ing.

Today, as we await the arrival of Trop­i­cal Storm Isa­ias, antecedent mois­ture con­di­tions are rel­a­tive­ly low, with flow in the Eso­pus at Allaben hov­er­ing near the approx­i­mate aver­age val­ue for ear­ly August (20–30 cfs), far less than what it was pri­or to Irene. There is more room for water in the sponge.

Antecedent moisture conditions prior to the arrival of TS Isaias.

Antecedent mois­ture con­di­tions pri­or to the arrival of TS Isa­ias.

This does not mean that flood­ing can’t hap­pen when antecedent mois­ture con­di­tions are low. Even with a dry soil stor­age sponge, the rate of pre­cip­i­ta­tion is also an incred­i­bly impor­tant com­po­nent of flood dynam­ics. If rain falls faster than it can infil­trate into the soil, water will run off regard­less of antecedent mois­ture con­di­tions, which can cause dam­ag­ing flash floods.

The soil stor­age sponge also has a lim­it­ed capac­i­ty and can become sat­u­rat­ed quick­ly.

Please refer to our recent post on the Flash Flood Watch issued for the Ashokan Water­shed for infor­ma­tion on how to pre­pare for a flood.

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Elevation and Floodproofing Workshop Advances Flood Mitigation

Posted on: April 12th, 2019 by Brent Gotsch
A workshop participant observes an engineered flood vent at the Elevation and Floodproofing Workshop held on March 26 and 27, 2019. Photo by Tim Koch.

A work­shop par­tic­i­pant observes an engi­neered flood vent at the Ele­va­tion and Flood­proof­ing Work­shop held on March 26 and 27, 2019. Pho­to by Tim Koch.

 

Poten­tial­ly thou­sands of struc­tures across the NYC West of Hud­son Water­shed are locat­ed with­in mapped FEMA flood­plains. Many are locat­ed in down­town ham­let areas and are vital to the local econ­o­my. More intense flood events and ris­ing flood insur­ance rates are threat­en­ing these struc­tures and the com­mu­ni­ties that rely on them for tax base, habi­ta­tion, eco­nom­ic activ­i­ty, and sense of place.

Prop­er­ty own­ers in flood zones are advised to reduce their flood risks and take action. A range of risk reduc­tion mea­sures are being test­ed and imple­ment­ed across the coun­try. The Ashokan Water­shed Stream Man­age­ment Pro­gram brought speak­ers with nation­al exper­tise to the region on March 26 and 27 to deliv­er a work­shop for local offi­cials to learn more about ele­va­tion and flood­proof­ing of struc­tures. The work­shop was held at the Emer­son Inn in Mount Trem­per and attend­ed by near­ly 50 build­ing depart­ment and oth­er offi­cials from Ulster, Greene, Sul­li­van, and Delaware coun­ties.

The work­shop fea­tured pre­sen­ters from Ducky John­son Home Ele­va­tions out of Hara­han, LA, and con­sul­tants recent­ly retired from the NYS Dept. of Envi­ron­men­tal Con­ser­va­tion and the U.S. Army Corps of Engi­neers.

“Every dol­lar spent on mit­i­ga­tion saves six dol­lars in recov­ery costs,” said Rod Scott of Ducky John­son. “Ele­va­tion and dry flood proof­ing are proven flood haz­ard mit­i­ga­tion tech­niques used to reduce flood risk and flood insur­ance pre­mi­ums,” he said.

In the 2018 hur­ri­cane sea­son alone, U.S. ter­ri­to­ries expe­ri­enced 15 storms and 8 hur­ri­canes respon­si­ble for $50 bil­lion in dam­age. In response to this “new nor­mal” of bil­lions in annu­al loss­es due to prop­er­ty dam­age, Con­gress has man­dat­ed flood insur­ance rate hikes for struc­tures with mort­gages in the FEMA flood­plain.

“Ele­vat­ing or flood­proof­ing struc­tures pro­vides a way for com­mu­ni­ties to keep their build­ing stock, and their tax base sta­ble while also decreas­ing flood insur­ance pre­mi­ums for the own­ers and less­en­ing their risk of flood-relat­ed dam­age,” said Brent Gotsch, Resource Edu­ca­tor for Cor­nell Coop­er­a­tive Exten­sion of Ulster Coun­ty and orga­niz­er of the work­shop. “With increas­ing pre­cip­i­ta­tion pat­terns and more dam­ag­ing flood events, it’s vital that com­mu­ni­ties con­sid­er using these meth­ods to adapt and become more resilient,” he added.

Elevation and Floodprooging Workshop participants view an elevated home in Mount Tremper, NY. Photo by Brent Gotsch

Ele­va­tion and Flood­proof­ing Work­shop par­tic­i­pants view an ele­vat­ed home in Mount Trem­per, NY. Pho­to by Brent Gotsch

 

Dur­ing the work­shop, local code offi­cials learned the dif­fer­ences between wet and dry flood­proof­ing and effec­tive ele­va­tion meth­ods for struc­tures. They learned how these prac­tices change flood insur­ance pre­mi­ums and how sim­ple mea­sures such as fill­ing-in a base­ment can reduce pre­mi­ums by hun­dreds or even thou­sands of dol­lars.

A bus tour showed par­tic­i­pants local exam­ples of struc­tures retro­fit­ted with ele­va­tion and flood­proof­ing mea­sures. At one prop­er­ty, water­tight shields were installed to pre­vent water from flow­ing into the liv­ing area. Anoth­er stop fea­tured a res­i­dence with engi­neered “smart vents” that allow water to safe­ly flow under­neath the structure’s first floor and equal­ize poten­tial­ly dan­ger­ous pres­sures that could buck­le the foun­da­tion.

At the end of the work­shop, local offi­cials left with increased knowl­edge about how to prop­er­ly retro­fit flood­prone struc­tures. Going for­ward, coun­ty part­ners plan to work with local munic­i­pal­i­ties to iden­ti­fy and access fund­ing for ele­va­tion and flood­proof­ing projects and min­i­mize costs to prop­er­ty own­ers.

Addi­tion­al pre­sen­ta­tions by the Catskill Water­shed Cor­po­ra­tion, the NYS Divi­sion of Home­land Secu­ri­ty and Emer­gency Ser­vices, and FEMA informed par­tic­i­pants about poten­tial fund­ing oppor­tu­ni­ties for ele­va­tion and flood­proof­ing projects. Pre­sen­ters walked through the appli­ca­tion process and gave advice on how to cre­ate a strong appli­ca­tion.

Fund­ing for the work­shop was pro­vid­ed by the New York City Depart­ment of Envi­ron­men­tal Pro­tec­tion.

A manager of a local bank branch shows Elevation and Floodproofing Workshop participants how they install the floodproofing barriers. Photo by Tim Koch.

The man­ag­er of a local bank branch shows Ele­va­tion and Flood­proof­ing Work­shop par­tic­i­pants how they install flood­proof­ing bar­ri­ers. Pho­to by Tim Koch.

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Public Comment Period Open for New Flood Risk Documents

Posted on: June 29th, 2018 by Brent Gotsch

The New York State Depart­ment of Envi­ron­men­tal Con­ser­va­tion (DEC) is now accept­ing pub­lic com­ment on two flood-risk man­age­ment doc­u­ments. The “State Flood Risk Man­age­ment Guid­ance” doc­u­ment and the “Guid­ance for Smart Growth Pub­lic Infra­struc­ture Assess­ment” doc­u­ment can be down­loaded and reviewed by vis­it­ing the NYSDEC web­page ded­i­cat­ed to the Com­mu­ni­ty Risk and Resilien­cy Act (CRRA). The dead­line for pub­lic com­ments is August 20. Com­ments should be sub­mit­ted by email to climatechange@dec.ny.gov and include “CRRA Com­ments” in the sub­ject line or by mail­ing writ­ten com­ments to DEC, Office of Cli­mate Change, 625 Broad­way, Albany, NY 12233–1030.

The guid­ance doc­u­ments describe how sea-lev­el rise and river­ine flood­ing pro­jec­tions adopt­ed by NYSDEC in 2017 should be incor­po­rat­ed into project design in spec­i­fied facil­i­ty-sit­ing, per­mit­ting, and fund­ing pro­grams. The CRRA Act seeks to address issues relat­ed to cli­mate change in New York State by adopt­ing offi­cial sea-lev­el rise pro­jec­tions; con­sid­er sea-lev­el rise, storm surge and flood­ing for appli­cants of cer­tain pro­grams; imple­ment smart growth pub­lic infra­struc­ture pol­i­cy; pro­vide guid­ance on nat­ur­al resilien­cy mea­sures; and devel­op mod­el local laws con­cern­ing cli­mate risk.

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Shandaken-Allaben Local Flood Analysis Final Public Meeting Scheduled for Dec. 18

Posted on: December 13th, 2017 by Brent Gotsch

The Shan­dak­en Area Flood Assess­ment and Reme­di­a­tion Ini­tia­tive (SAFARI), a com­mit­tee work­ing under the direc­tion of the Shan­dak­en Town Board, invites every­one to attend a pub­lic meet­ing to see the results of the Shan­dak­en-Allaben Local Flood Analy­sis (LFA). Through­out the year, SAFARI, the con­sult­ing firm Milone & MacB­room and oth­er part­ner agen­cies have been inves­ti­gat­ing flood­ing issues in the ham­lets of Shan­dak­en and Allaben and will present their find­ings to the pub­lic at this meet­ing. Mit­i­ga­tion rec­om­men­da­tions will also be pre­sent­ed.

The meet­ing will take place on Mon­day, Decem­ber 18 at the Shan­dak­en Town Hall at 6:30pm. Atten­dees will have an oppor­tu­ni­ty to ask ques­tions and to com­ment on the plan before a final draft is pre­sent­ed to the Town Board ear­ly next year.

A copy of the report can be viewed on the Town of Shan­dak­en web­site.  To learn more about the LFA process a video from the first pub­lic meet­ing is also avail­able on the Town web­site.

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Managing Flood Risk Workshop December 11

Posted on: December 4th, 2017 by Brent Gotsch

AWSMP and Cor­nell Coop­er­a­tive Exten­sion of Ulster Coun­ty (CCEUC) Resource Edu­ca­tor Brent Gotsch in con­junc­tion with CCEUC Live­stock Edu­ca­tor Jason Det­zel will be be offer­ing a work­shop on flood­plain man­age­ment for live­stock and agri­cul­tur­al pro­duc­ers, or any­one with a flood­plain on their prop­er­ty. The work­shop will occur on Mon­day, Decem­ber 11 and run from 6:00pm to 8:00pm at the Cor­nell Coop­er­a­tive Exten­sion of Ulster Coun­ty offices locat­ed at 232 Plaza Road in Kingston, NY.

It is free to attend but reg­is­tra­tion is required. Please con­tact Jason Det­zel at jbd222@cornell.edu or call 845–340-3990 x327 to reg­is­ter.

The work­shop will help flood­plain prop­er­ty own­ers bet­ter under­stand their risk of flood­ing and how to read and inter­pret FEMA Flood Insur­ance Rate Maps (FIRMs) and Flood Insur­ance Stud­ies (FISs). Addi­tion­al top­ics to be cov­ered include:

  • The dif­fer­ent types of flood zones
  • How to iden­ti­fy flood­way and flood fringe areas of a flood­plain
  • How to com­pute the base flood ele­va­tion of the flood­plain
  • What exact­ly is a “100-Year flood”
  • Com­put­er and elec­tron­ic tools to help iden­ti­fy flood haz­ard zones
  • Live­stock dis­as­ter infor­ma­tion

 

If time per­mits there will also be a brief dis­cus­sion on flood insur­ance and how that applies to struc­tures in a mapped flood­plain.

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Hurricane and Flood Preparedness

Posted on: September 7th, 2017 by Brent Gotsch

Hur­ri­cane Irma is like­ly to be one of the most pow­er­ful storms ever record­ed and is cur­rent­ly on track to make land­fall in the state of Flori­da this week­end. At this point it is unclear whether this storm will con­tin­ue on with the same strength or inten­si­ty and make its way to the North­east. Now is a good time to make prepa­ra­tions in case the storm does reach our water­shed. A good first step to pre­pare for poten­tial flood­ing is review AWSM­P’s Flood Pre­pared­ness Guide and guides from emer­gency man­age­ment agen­cies like FEMA.

It may also be use­ful to know if you are in a mapped flood haz­ard zone. You can do this by view­ing paper Flood Insur­ance Rate Maps at your local Town Hall (also avail­able for down­load from the FEMA Map Ser­vice Cen­ter) or by view­ing an inter­ac­tive ver­sion on the Nation­al Flood Haz­ard Lay­er or the Ulster Coun­ty Par­cel View­er. Now would also be a good time to stock up extra sup­plies of food, water, and med­i­cine in case there are dis­rup­tions in deliv­ery of such items. In addi­tion, the NY Exten­sion Dis­as­ter Edu­ca­tion Net­work (NY EDEN) has exten­sive infor­ma­tion on how to pre­pare for flood­ing, hur­ri­canes and oth­er emer­gen­cies. By being informed and pre­pared we can all be more resilient in the face of nat­ur­al dis­as­ters.

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