HomeMy WebLinkAboutHazen HS_Falling Head Test Results Memo_260223
255 S. King Street, Suite 800, Seattle, WA 98104 | 206.426.2600 | JACOBSONENGINEERS.COM
February 23, 2026
City of Renton Development Engineering
1055 S Grady Way
Renton, WA 98057
Attn: Michael Sippo
RE: Hazen High School Improvements (C24005771)
StormTrap Detention Facility – Falling Head Test
Dear Michael:
The StormTrap Detention Facility on the west side of the Baseball Field at Hazen High School (1101
Hoquiam Ave NE, Renton, WA), is required to have a falling head test performed for water tightness of
the vault according to StormTrap Supplemental Installation Note #6 on Civil Sheet C202 as follows:
According to the StormTrap vault manufacturer, a Level 3 water tightness test is required per notes #5 &
#6 on StormTrap manufacturer detail sheet 1.1 on Civil Sheet C316. See Figure 1 below:
FIGURE 1 – STORMTRAP DETAIL
255 S. King Street, Suite 800, Seattle, WA 98104 | 206.426.2600 | JACOBSONENGINEERS.COM
To perform the water tightness/falling head test, the contractor shall plug ALL inlet and outlet pipes,
then fill to the highest connection invert, which is just the one inlet pipe we have at IE = 452.00 and then
after a 24 hour period, the contractor shall measure the water level, then calculate to ensure there is
less than 15% of volume loss.
The vault has a total storage capacity of 52,632.52 cubic feet (cf) at 10-ft of total depth. This equates to
5,263.25 cf per foot of depth. If contractor fills vault to elevation 452.00, that equates to 8.5-ft of water
depth or 44,737.63 cf. For 15% of acceptable loss over a 24 hour period, the water level can only drop
by approximately 1.27-ft or 1’-3” max.
Falling Head Test Results
The contractor, Forma Construction, proceeded to fill up the vault with water, and at approximately 9
am on 2/2/26, the water level was filled up to elevation 452.00. The contractor then measured the
water level at roughly 9:58 am on 2/3/26 (24 hour period) with results noted below:
Tape Measure Reading taken at 9:00 am on 2/2/26 read 50-inches from structure to top of water
(baseline)
Tape Measure Reading taken at 9:58 am on 2/3/26 read 53-inches from structure to top of water
The results indicate a 3-inch drop in water level in a 24-hour period.
Falling Head Test Review & Conclusion
The measurements taken above coincide with approximate elevation of outlet pipe at 452.00, and with
only a 3-inch drop over a 24 hour time period, this meets the requirements of the falling head test.
With water up to the elevation of pipe, per calculations above, the drop in water level to meet the
requirements for the test can be up to a maximum of 1’-3”.
We, the Civil Engineer of Record for the project, therefore finds the results to be acceptable and
therefore the StormTrap Detention Vault passes the falling head test for water tightness.
Please feel free to reach out to me if there are any questions regarding the Falling Head Test at
(206) 293-9134 or sascha@jacobsonengineers.com. I would be happy to have any further discussions or
provide additional information.
Regards,
Sascha Eastman, P.E., Senior Project Manager