Herrera Environmental Consultants

Overview

This page presents monitored flows at the 120th facility in part of the South Thornton Natural Drainage System (NDS) Project. This facility is comprised of four HPBSM bioretention cells with a shared underdrain along N 120th St (Meridian Ave N to 1st Ave NE) that treat runoff and drain to two UIC wells. Flow that exceeds the wells’ capacity returns to the storm drain system. Rainfall from SPU gauge RG-33 is shown for context.

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 cells #C-2 and #C-3. The combined underdrain flows to MH #45-1, where it is diverted to UIC #45-1. When UIC #45-1 is at capacity, bypass flows into the underdrain of #C-4, combining treated flow from #C-4 and draining to UIC #46-1. When #46-1 is at capacity, bypass flows then drain to the existing storm drain.

Monitoring Stations (QAPP Tables 4 & 11)

Station ID Alias Location Measurement Notes
120-UIC-IN IN BRC #C-1 (inlet) 6-in H-flume (Figure 10) Influent at the most upstream inlet (BRC #C-1)
120-UIC-UND UND MH #45-1 (inlet) 12-in Thel-mar weir (Figure 11) Treated flow from cells BRC #C-1 through #C-3, drains to UIC #45-1.
120-UIC-BP1 BP1 MH #45-1 (outlet) 6-in rated pipe (Figure 11) Bypass from UIC #45-1
120-UIC-BP2 BP2 MH #45-3 (inlet) 12-in rated pipe (Figure 12) Bypass from UIC #46-1
RG-33 Rainfall SPU Gauge (vicinity) Tipping bucket, 5-min

BP2’s monitoring enclosure sits by MH #45-2; flow is measured exiting MH #45-3 (Figure 12). Contributing basins total ~6 ac (1.5 ac effective impervious area).

Station Map

Orange strips: 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: IN → UND/BP1 → BP2.

Site Figures

Figure 4. HPBSM Bioretention and UIC Project Location
Figure 4. HPBSM Bioretention and UIC Project Location — monitoring stations, BRCs #C-1 through #C-4, UIC wells #45-1 and #46-1, and contributing monitoring basins.
Figure 10. Plan View of 120-UIC Monitoring Locations and Components (Part 1)
Figure 10. Plan View of 120-UIC Monitoring Locations (Part 1) — 120-UIC-IN with 6” H-flume and influent sample intake at BRC #C-1; red arrows indicate flow direction.
Figure 11. Plan View of 120-UIC Monitoring Locations and Components (Part 2)
Figure 11. Plan View of 120-UIC Monitoring Locations (Part 2) — 120-UIC-UND (12” Thel-mar, effluent sample intake) and 120-UIC-BP1 (6” rated pipe) at MH #45-1 / UIC #45-1; total system bypass monitored downstream.
Figure 12. Plan View of 120-UIC Monitoring Locations and Components (Part 3)
Figure 12. Plan View of 120-UIC Monitoring Locations (Part 3) — 120-UIC-BP2 (UIC bypass, 12” rated pipe) with monitoring enclosure near MH #45-2; flows measured as they exit MH #45-3, the downstream-most point of the 120-UIC system. Red arrows indicate flow direction.

Analysis Parameters

UIC Target Flow Rates

UIC #45-1: 30 gpm UIC #46-1: 45 gpm

5-minute data | IN period of record begins October 28, 2025 (sensor replacement at BRC #C-1)

Key Findings

Period-of-Record Volumes

Gauge Total Volume Notes
IN (Influent, BRC #C-1) 53.4 kgal Partial record (from 10/28/2025); excludes downstream curb-cut inflow
UND (Underdrain to UIC #45-1) 166.2 kgal Cells in series, BRC #C-1 through #C-3
BP1 (Bypass, UIC #45-1) 128.7 kgal Corrected rating; RPD vs UND = 25.4%
BP2 (Bypass, UIC #46-1) 352.7 kgal Includes BRC #C-4 contribution entering between BP1 and BP2

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.

Loading volumes…

Generated: 2026-07-22 08:28:08.871856
Data Source: 20260506 corrected dataset (Herrera) | Period: May 14, 2025 - April 08, 2026
Rainfall: SPU RG-33 | Interval: 5 minutes