South Thornton Natural Drainage System — 120th St Site
UIC Flow Monitoring Analysis
October 2025 - April 2026
DRAFT — PROVISIONAL DATA: All data presented on this page are provisional and subject to revision.
Overview
This page presents monitored stormwater flows at the newly constructed bioretention cells (BRCs) and UICs along North 120th Street between Meridian Ave North and 1st Avenue Northeast in Seattle, WA. The series of BRCs and UICs (the facility) was constructed in 2025 as part of the South Thornton Natural Drainage System (NDS) Project. The facility is comprised of four HPBSM bioretention cells with a shared underdrain along N 120th St that treat runoff and then drain to two UIC wells. Flows that exceed the wells’ capacity return to the storm drain system. Rainfall from SPU gauge RG-33 is shown for context; the rain gauge is located 3.9 miles to the SE of the facility. Elements of the South Thornton NDS Quality Assurance Project Plan (QAPP) are presented on this page.
How Water Moves Through the System
Runoff enters BRC #C-1 via a 6-inch H-flume at the most upstream location. BRC #C-1 through #C-3 cells are linked by a single connected underdrain, with additional runoff entering BRC #C-2 and #C-3 from surface curb cuts which are not monitored. The combined underdrain of BRC #C-1, #C-2, and #C-3 flows to MH #45-1, where it is diverted to UIC #45-1. When UIC #45-1 is at capacity (or when it is offline), UIC bypass flows into the underdrain of downstream BRC #C-4. This BRC also has unmonitored surface inlets. Comingled UIC #45-1 bypass and underdrain flow from BRC #C-4 then is routed to UIC #46-1. When UIC #46-1 is at capacity (or when it is offline), bypass flows drain to the existing storm drain.
The data presented herein are from 10/28/25 to 4/7/26 and represent a period when both UIC #45-1 and UIC #46-1 were offline (UIC valve closed) and thus zero infiltration was occurring in the UICs.
The objective of this page is to clarify the existing plumbing of this complicated treatment train and to develop estimates of the flows expected to enter the UICs when they are brought online.
Monitoring Stations (QAPP Tables 4 & 11)
| Station ID | Alias | Location | Measurement | Contributing Area | Notes |
|---|---|---|---|---|---|
| 120-UIC-IN | IN | BRC #C-1 (west curb cub inlet) | 6-in H-flume (Figure 10) | 0.4–2.4 ac | Influent at the most upstream inlet (BRC #C-1) |
| 120-UIC-UND | UND | MH #45-1 (inflowing pipe from west) | 12-in Thel-mar weir (Figure 11) | 4.6 ac | Treated flow from cells BRC #C-1 through #C-3, drains to UIC #45-1. |
| 120-UIC-BP1 | BP1 | MH #45-1 (outflowing pipe to east) | 6-in rated pipe (Figure 11) | 4.6 ac | Bypass from UIC #45-1 |
| 120-UIC-BP2 | BP2 | MH #45-3 (inflowing pipe to north) | 12-in rated pipe (Figure 12) | 6.1 ac | Bypass from UIC #46-1 |
| RG-33 | Rainfall | SPU Gauge (vicinity) | Tipping bucket, 5-min | — | Rain gauge located 3.9 miles SE of the facility |
Contributing basins: BRC #C-1/#C-2 (combined) 3.0 ac, #C-3 1.6 ac, #C-4 1.5 ac.
Station Map
Orange polygons: bioretention cells BRC #C-1–#C-4 (approximate). Purple: UIC wells #45-1 and #46-1 (positions per plan-view stationing). Green: monitoring stations (QAPP Table 4 coordinates). Flow runs west to east.
Site Figures
Interactive Time Series: All Gauges
Drag on a chart or use the range selector to zoom — the volume boxes below each chart update to the visible window, so you can isolate a single storm and read its volumes directly. Double-click to reset.
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Note: Bypass 2 (BP2) flows are estimated by using a scaling factor of contributing area.
Generated: 2026-07-22 14:53:50.228519
Data Source: 20260722
dataset (Herrera) | Period: October 28, 2025 - April 08, 2026
Rainfall: SPU RG-33 | Interval: 5 minutes