“Auxiliary Spillway” events at the Truesdale Lake Dam

This is documentation of the five heavy rainfall over-topping events of our dam to inject into the Truesdale Lake website if you wish. I’ve copied Janet as well because her reports are a part of the observations. Hurricane FLOYD occurred before she and I started reporting precipitation totals on CoCoRaHS. If you find your 1999 photo album, please add FLOYD photos when you put this on the website. Note that our “auxiliary spillway” is placed in quotes because of its usage rather than its proper design. (Compiled by Scot Evans, Feb 2024)

Over-topping of the dam — with water flowing over the road’s lowest elevation at the western third of the dam, which serves as our unauthorized and improperly designed “auxiliary spillway” — has occurred five times in the last twenty four years. 

• 1st event occurred 9/17/99 measured 13″ (NYC & Westchester) to 17” (Danbury) rain over 24 hours. (HURRICANE FLOYD plus aftereffects of Hurricane Dennis dissipation a week before) (no close local measurements, rainfall reports varied from 10″-17″ depending on the town/city)
• 2nd event occurred 8/28/11 with 5.94” rain over 27 hours. (HURRICANE IRENE)
• 3rd event occurred 9/2/21 with 5.55” rain over 21 hours. (HURRICANE IDA)
• 4th event occurred 9/30/23 with 5.79” rain over two spans of 58 hours and 37 hours. (HURRICANE OPHELIA and its remnants)
• 5th event occurred 1/10/24 with 4.12” rain over 18 hours with ground saturation and flooding exacerbated by a rapidly melted two-inch thick ice- and snow-pack. (Unnamed storm)

The operating siphons (two 12” & one 6”) prevented water from overtopping or from extensive flow over the road on two occasions. They also served to reduce the high volume impoundment along the south embankment. The specifics of these events (most recent is listed first) with precipitation reports inserted (which are used by the NWS and NOAA), as well as three photos at the end, are as follows:

4.12” rain over an 18-hour all-rain event from 1/9 at 1155 EST to 1/10 at 0545 EST. Strong south wind-driven heavy rain behind the warm front on the east side of massive steep-gradient Low Pressure Cell with widespread flooding. Rainfall occurred over a 2” snowpack causing widespread flooding as snowpack and frozen ground quickly melted. Cold front passage occurred at 0400 EST. Lake level rose 33 inches and briefly flowed over the road’s low point on the western third of the dam. 

• 1/10/2024 FIFTH over-topping of the dam:

             Local map showing four precipitation reports for this storm. 

Textual precipitation report for Truesdale Lake Station NY-WC-22 South Salem 0.8 N.

Note: The drawdown siphon status changed as the support rack collapsed and the west 12” siphon broke at the fill-Tee, separating upstream of the down tube junction. But, this is the second time this drawdown season that the siphons proved to be beneficial in protecting the dam from excessive water pressure impoundment and extensive auxiliary spillway overtopping.

• 9/30/2023 FOURTH over-topping of the dam:

Tropical Storm OPHELIA stalled between 9/23 to 9/26 and delivered 1.44” rain over 58 hours, causing ground saturation before its remnants from 9/28 to 9/30 dumped an additional 4.35” rain over 37 hours. This quickly raised the lake level two feet above the summer weir boards and over the road at the dam. The combined total for both storms related to the week-long duration of Tropical Storm Ophelia & remnants was 5.79” rain.

              Local map showing four precipitation reports for this storm. 

• 9/2/2021 THIRD over-topping of the dam:

9/1/21 Tropical depression IDA rain began 0530 EDT. Intermittent heavy to deluging rain resumed 1740 to 0220 EDT. Total rainfall for this 21-hour storm was 5.55 inches. Lake level rose rapidly and began flowing heavily over the secondary spillway for one day, submerging the lake walls and overtopping lawns. This storm set another rainfall record in NYC’s Central Park at 3.15” in one hour between 2051 and 2151 EDT. See https://truesdalelake.com/2021/09/hurricane-ida-post-storm-report/

• 8/28/2011 SECOND over-topping of the dam:

Hurricane IRENE struck 8/27/2011 and rain began at 10 am and dumped 5.07” by 8/28 at 7 am, and an additional 0.87” with rain ending at 1 pm. Total storm duration was 27 hours delivering 5.94” rainfall. Water flowed over the dam’s low point of the road in the western third of the embankment for two days.

Janet Andersen is the President of The Three-Lakes Council and the Twin Lakes Village Station Keeper for NY-WC-6 South Salem 2.1 NW reporting the 5.94″ rainfall total for Hurricane IRENE.

• 9/17/1999 FIRST over-topping of the dam:

Hurricane FLOYD struck this area 9/17/1999 and was a 100-year event, meaning a 1% chance of a flood or storm like this happening in any year. The heaviest rainfall occurred in our area ranging from 8 to 12 inches with peak wind gusts of 54 MPH at Stewart International Airport, and worsened by a pressure gradient between the extreme low pressure center of the storm over us (980 mb) and the high pressure cell over the Ohio Valley. Lake residents were afraid that the road over our dam would wash out as water flowed over the entire western two-thirds of the dam for about two days.

The hurricane’s track with color-coded dots indicating downgrades of intensity:

The satellite vapor loop showing the extremely water-laden concentration approaching our area.

This section of road over a dam in nearby Briarcliff Manor was washed out by FLOYD.

(photo above taken 9/19 by photographer Michael Raphael)

Rainfall accumulation showing records set in North Carolina and again intensifying over NYC, Westchester, and Fairfield Counties (click to enlarge). (News account with local rainfall totals noted)

Photos of lake level when water was flowing over the top of the dam’s lowest elevation (western third):

All lake walls were submerged (see two clay planters and attached dock ramp above) and …

… lawns were submerged up to chairs and tree bases.

The swollen Boway Stream – the main inflow from Pumping Station Swamp beneath Ridgefield’s “West Mountain.”

Notable exceptions: On 12/18/23, an unnamed storm delivered 4.12” rain total over 24 hours and caused the lake to rise up to the top of the dam and produced a puddle at the road’s lowest point but did not flow over the road. This strong coastal low from FL combined with trough from the west and delivered 3.65” rain on 12/17, rain began 1400 EST + .47” rain on 12/18, rain ended 1400 EST. Lake rose (in a 9:1 ratio over a saturated watershed with siphons flowing and the 6” siphon completely submerged) from 17” BSR (below spillway ramp) to 20” ASR (above spillway ramp) — a total of 37” rise after 4.12” rainfall. This was the first event this winter season where the three operating siphons proved to be beneficial in protecting the dam from excessive water pressure impoundment and secondary spillway overtopping.

Three big storms didn’t reach the top of the dam: 7/16/23 A strong low pressure cell dumped 4.17”rain over 13 hours from 0120 to 1420 EDT. Lake level rose to within 12” of top of the dam. A five-day storm from 4/28 – 5/2/23 total rainfall of 4.37″ caused lake level to rise to within 7” of top of the dam. 9/5/2022 the first drought-busting atmospheric river dumped 5.38” rain and our report was cited as the NYS record that day by Sam Champion on ABC News. The dry land of the watershed soaked up much of it, so overtopping of the dam did not occur. 

Scot Evans, Siphon Monitor, Weather Observer & Station Keeper for Truesdale Lake’s NY-WC-22 South Salem 0.8 N