Flood Risk Project Saratoga County, New York, Hydrology Meeting
April 29, 2021 Agenda
Recap/Refresh Hydrology Path Forward Analysis Review
1 Project Recap
Two Discovery Projects • Hudson-Hoosic Watershed ▸ . Meetings held in October 2012 . Discovery project completed in April 2014 • Sacandaga Watershed . Meetings held in July 2018 . Discovery project completed in March 2019 Current Saratoga Study Progress • Kickoff meeting: Held virtually due to COVID19 – May 6, 2020 ▸ • Engineering models notification to communities: August 14, 2020 • Field survey: Spring 2020 – Fall 2022 • Hydrologic analysis: June 2020 – Present
2 Leveraged Data Recap
Flood Risk Review meetings held in November 2016 ▸ Detailed – 142 miles • Anthony Kill ▸ • Ballston Creek and Lake • Drummond Creek • Dwaas Kill • Fish Creek • Hudson River • Kayaderosseras Creek • Mohawk River • Round and Saratoga Lake Approximate – 70 miles
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3 Project Scope
First time digital maps
▸ Flooding sources analyzed • Detailed riverine studies (AE Zone) – 10 ▸ streams, 56 miles • Detailed lake studies (AE) – 1 lake, 34 miles • Approximate (A) studies – multiple streams, 359 miles 30 updated communities
▸ 197 map panels ▸ Review meetings • Hydrology Meeting ▸ • Hydraulics Meeting • Flood Risk Review Meeting
4 Hydrologic Analysis Methods
Typical Methods FEMA utilizes HEC-HMS Model
▸ • Statistical Gage Analyses • Regression Analyses • Rainfall Runoff Modeling Gage/Regression are based on availability stream gage data ▸ Rainfall-Runoff physical modeling chosen due to limited gage data ▸ • Leverage existing HEC-HMS models from Hudson-Hoosic Watershed studies including Mohawk River Discharges developed for
▸ . 10%, 4%, 2%, 1%, 1%+, 1%-, 0.2% . Inputs for hydraulic analyses
5 Gage Analysis • Statistically analyze measured flows at gages Hydrology – Gage Analysis
Drainage Period of Number of Gage Type USGS Gage No. Description Area (sq. Record Records mi) Sacandaga River At Stewarts 1325000 1055 1908-2020 113 Bridge Near Hadley NY 1318500 Hudson River At Hadley NY 563.9 1913-2020 100 Discharge Glowegee Creek At West Milton 1330000 26 1949-2020 45 NY Kayaderosseras Creek Near 1330500 84.2 1927-1998 70 West Milton NY Great Sacandaga Lake At 1323500 1044 1930-2021 92* Conklingville NY Stage Saratoga Lake At State Highway 1330884 217 2014-2021 8* 9P NY *Number of years of daily data. Peak yearly data not provided for stage gages Flow gage analysis performed Viable gage = minimum in support of rainfall-runoff 10 years current record ▸ ▸ model validation and regression Bulletin 17C analysis methodology Stage gage analysis performed ▸ with HEC-SSP ▸
7 Hydrology – Gage Analysis
8 Regression Analysis • USGS StreamStats Discharges • Relationships between peak flows and watershed characteristics • Regional Regression Equations • Gage Weighting • Urban Regression Equations Hydrology – Regression Analysis
Regression Analysis = Saratoga (362 miles)
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10 Regional Regression Equations and Analysis
USGS New York regression equation: SIR 2006-5112 ▸
Study area falls within USGS NY regression ▸ Region 1
USGS StreamStats v5.02 p7
▸ Primary method for Zone A streams ▸
11 Summary of Regression Equations
USGS NYS Hydrologic Region 1 = 2310 . + 1 . . + 1 . + 80 . = 4580 0.968 + 1 −0.184 1.241 + 1 −0.482 + 80 −1.549 𝑄𝑄10 𝐴𝐴 𝑆𝑆𝑆𝑆 𝑃𝑃 𝐿𝐿𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹 = 7030 0.965 + 1 −0.192 1.167 + 1 −0.500 + 80 −1.582 𝑄𝑄25 𝐴𝐴 𝑆𝑆𝑆𝑆 𝑃𝑃 𝐿𝐿𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹 = 10300 0 963. + 1−0 197. 1 131. + 1−0 511. + 80−1 610. 𝑄𝑄50 𝐴𝐴 𝑆𝑆𝑆𝑆 𝑃𝑃 𝐿𝐿𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹 = 22000 0.962 + 1 −0.202 1.106 + 1 −0.520 + 80 −1.638 𝑄𝑄100 𝐴𝐴 𝑆𝑆𝑆𝑆 𝑃𝑃 𝐿𝐿𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹 where, 0 959 −0 210 1 067 −0 539 −1 704 500 Qx = 𝑄𝑄peak flow for x-year𝐴𝐴 storm event𝑆𝑆𝑆𝑆 (cubic feet per𝑃𝑃 second)𝐿𝐿𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹 A = drainage area (square miles) ST = basin storage (percentage of total drainage area) P = mean annual precipitation (inches) LAG = basin lag factor, Calculated as L / [( SL_UP + 1) ( SL_LO + 1)] ^ 0.5 . L = Main-channel stream length, in miles. Longest distance from gaging station or site of interest to top of basin . SL_LO = Slope of lower half of main channel (feet/mile) . SL_UP = Slope of upper half of main channel (feet/mile) FOR = Basin forested area (% total area)
12 Summary of Gage Weighting Streams with Regression Discharges
Gaged Sites
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• QT(w) = weighed peak discharge at gage location • QT(g) = peak discharge at gage site • N = number of years of annual peak-discharge record
• QT(r) = regional regression estimate discharge • E = average equivalent years of record for regression equation
13 Summary of Gage Weighting Streams with Regression Discharges
Ungaged Sites on Gaged Streams • Done at locations with a drainage area of 50% to 150% of the gage drainage area. ▸
• QT(U)w = weighted estimate of discharge for recurrence interval 2 • Au = drainage area of ungaged site (mi ) 2 • Ag = drainage area of gage site (mi ) • ∆A = |Ag-Au| • QT(U)r = peak-flow estimate for recurrence interval at ungaged site • QT(U)g = peak-flow estimate for recurrence interval at ungaged site by adjusting for the effect of the difference in drainage area between the streamflow-gaging station and the ungaged site • b = exponent from the appropriate drainage-area-only equation (table 3, NY SIR 2006)
14 Urban Adjustment Three Parameter Equations
Base regression equations not applicable to urban areas ▸ Peak flows adjusted for basins with >15% urban land use (from ▸ NLCD layer) Affected Detailed Reaches: ▸ o Snook Kill Affected Approximate Reaches: where, UQx = urban-adjusted peak flow for x-year storm event Gordon Creek, Stony ▸ o (cubic feet per second) Creek, Tributary to Spring A = drainage area (square miles) Run, and several BDF = basin development factor, calculated using the unnamed streams methods described in Sauer and Others (Sauer, 1983) * USGS WSP 2207 (1983) RQx = regression peak flow for x-year storm event (cubic feet per second) 15 Urban Adjustment Factor Basins
16 Stage Frequency Analysis • Estimate frequency lake levels using gage data • Using annual peak lake levels and fitting statistical distribution to estimate lake level frequency Great Sacandaga Lake at Conklingville, NY (USGS 01323500)
The Great Sacandaga Lake - 39.0 sq. mi - DA 1,044 sq. mi ▸ Water stage recorder is in NGVD 1928; NAVD 1988 = ▸ NGVD 1928 - 0.88 feet.
In 2000, an agreement known as “Offer of Settlement” by ▸ the HRBRRD manages current lake stages
Last 18 years were used for statistical analysis ▸
18 Water Year Peak Stages – Great Sacandaga Lake
USGS 01323500 Great Sacandaga Lake Daily and Peak Water Surface Elevations 780
775
770
765
760
ft ft (NAVD88) 755
750
745
740 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Water Year (October - September)
Daily Values Water Year Peak Stages The 2000 Offer of Settlement started operation in 2002 and is maintained by the Hudson River – Black River Regulating District (HRBRRD)
19 Stage Frequency Analysis – Great Sacandaga Lake
Stage Frequency Analysis Percent Confidence Limits Event Year Lake Level (ft was done using HEC-SSP Chance (ft NAVD88) Equivalent NAVD88) ▸ Weibull Statistical Exceedance 1% + 1% - Distribution was used for 0.2% 500 775.82 777.13 774.86 ▸ 0.5% 200 775.15 776.34 774.27 analysis 1% 100 774.60 775.69 773.79 According to the 2000 “Offer 2% 50 774.01 774.99 773.26 of Settlement” any stage 5% 20 773.13 773.96 772.48 ▸ higher than 773 feet (NAVD 10% 10 772.35 773.07 771.78 88) is considered minor 20% 5 771.43 772.03 770.93 flooding (+2 feet above 50% 2 769.71 770.17 769.24 spillway crest)
20 Rainfall-Runoff Analysis • Creation of hydrologic models to calculate flows at outlet, node or subbasin • Various inputs required • Typically used for detailed studies Hydrology – Rainfall-Runoff Modeling
Total 8 streams of 31.2 miles studied
▸ Streams located within Fish Creek watershed: • Kayaderosseras Creek ▸ • South branch Kayaderosseras Creek • Mud Creek • Gordon Creek • Glowegee Creek • Spring Run Streams located in Anthony Kill watershed: • Cooley Kill ▸ • Plum Brook Previously developed watershed models were refined to include sufficient discharge points. ▸
22 Hydrology – Rainfall-Runoff Modeling
Fish Creek/ Anthony Kill ▸ watersheds (31.2 stream miles)
23 Rainfall-Runoff Methodology
HEC-HMS 4.0 was used instead of current version 4.8 – 2017 study’s HMS models used version 4.0. ▸ Rainfall Depths: NOAA Atlas 14 Precipitation Frequency Data Server, 24-hour duration. ▸ Frequency Storm temporal distribution
▸ Loss Methodology: SCS Curve Number (TR-55), with average antecedent runoff condition ▸ Hydrograph Methodology: SCS Unit Hydrograph
▸ Lag Time (60% of Time of Concentration) Channel▸ Routing: Muskingum-Cunge using 8-point cross-sections ▸
24 Rainfall-Runoff Methodology
Fish Creek HEC-HMS Model Anthony Kill HEC-HMS Model • 6 studied streams • 2 studied streams • Added 36 new subbasins • Added 2 new subbasins
25 Rainfall-Runoff Modeling Rainfall Depths-Atlas 14
NOAA Atlas 14 Rainfall Data: The same ▸ frequency storm data developed by RAMPP study in 2017 was used for current scoped study Rainfall runoff analysis using HEC HMS
26 Rainfall-Runoff Modeling SCS Curve Numbers
Updated existing HEC-HMS models.
▸ SCS CNs developed by the RAMPP study were used for majority of subbasins
▸ 2016 NLCD Land Use data and SSURGO hydrologic soil dataset were used to estimate CNs for additional subbasins where needed ▸
Fish Creek watershed - Less than 5% CN change was made to 6 new Subbasins;
▸ Anthony kill watershed - No CN change was made.
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27 Rainfall-Runoff Modeling Time of Concentration (Tc) / Lag Time
Longest flow path = longest time that a drop of water would take to travel through a watershed ▸ Developed from 10-meter Digital Elevation Model (DEM) data and HEC- GeoHMS extensions ▸ Flow paths split into different types:
▸ • Sheet flow maximum = 100 ft • Shallow concentrated flow: from end of sheet flow segment to visual open channel • Channel flow: begins at end of shallow concentrated flow segment and ends at sub-basin outlet
Lag times = 60% of Time of Concentration
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28 Longest Flow Path Example
29 Model Validation / Results
• Check computed flows against results that one would expect from nearby gages Rainfall-Runoff Modeling Model Validation
Model verification for Fish Creek Watershed –Current study 2021
31 Rainfall-Runoff Modeling Model Validation
Model verification for Fish creek Watershed –Current study HECHMS 2021
32 Rainfall-Runoff Modeling Model Validation
Model verification for Anthony Kill Watershed –Current study HECHMS 2021
33 Rainfall-Runoff Modeling Model Validation
Comparison of Q100 vs DA for Detailed reaches with effective & LP-III
34 Rainfall-Runoff Modeling Comparison to Effective Flows
Study results found to be: • Consistent with Flood Insurance ▸ Study (FIS) flows • Consistent with gage analysis flows • Compare well with regression analysis
35 Saratoga County Next Steps
Field reconnaissance
▸Hydraulic analysis ▸ • Hydraulic modeling/report/submittal • Hydraulic analysis webinar Floodplain Mapping
▸Flood Risk Review meeting ▸ • Comment period for communities
36 Project Timeline towards Preliminary Issuance
Flood Risk Review Meeting Saratoga County We are Here Spring 2022
Preliminary Map Hydrology Hydraulics Floodplain Products Issued Presentation Presentation Mapping Fall 2022* April 2021 Fall 2021 Winter 2021
*Current timeline could be impacted by Flood Risk Review or Preliminary Map Comments
Graphic Above Not to Scale
37 Contacts
FEMA Project Monitor STARR II Project Manager
▸ • Shudipto Rahman ▸ • David Sutley, PE • 202-702-4273 • 303-951-0612 • [email protected] • [email protected] FEMA Outreach Coordinator STARR II Regional Support Center
▸ • Stephanie Gootman ▸ • Rosemary Bolich • 202-802-3137 • 646-490-3848 • [email protected] • [email protected]
38 Questions? Comments?
Thank you!
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