I-5 Ship Canal SW Facility

Overview

This analysis compares monitored (Manning equation) and modeled stormwater discharge flows at the I-5 Ship Canal SW Facility for both WSDOT and City of Seattle drainage systems. Flows are capped at the Target Flow Rate (TFR) for analysis. Rainfall data from rain gauge RG33 is included for context.

Analysis Parameters

Target Flow Rates & Baseflow

WSDOT Target Flow Rate (TFR):	3 cfs	WSDOT Baseflow:	0.094 cfs
COS Target Flow Rate (TFR):	3 cfs	COS Baseflow:	0.119 cfs

Baseflow Method: 95th percentile flow during dry periods (6+ hours without rainfall) — conservative estimate
Note: All flows are capped at the TFR for volume analysis.

BASEFLOW (0 cfs - 0.09 cfs WSDOT / 0.12 cfs COS)

PRIMARY (Baseflow - TFR)

Key Findings

WSDOT (I-5) — Capped at 3 cfs

Metric	Monitored	MGSFlood Modeled	Difference
Total Volume (capped)	37,806,019 gal	40,574,486 gal	-2,768,467 gal (-6.8%)
Baseflow (max dry weather)	0.094 cfs

City of Seattle - U District — Capped at 3 cfs

Metric	Monitored	MGSFlood Modeled	Difference
Total Volume (capped)	22,444,753 gal	37,048,586 gal	-14,603,834 gal (-39.4%)
Baseflow (max dry weather)	0.119 cfs

Storm Distribution & Recurrence Intervals

Storm Distribution — Monitoring Period

Return intervals based on NOAA Atlas 14 precipitation frequency estimates for Seattle, WA (24-hour duration)

Storm Size	Precip Range (in)	Count
0.15 - 0.24”	0.15 - 0.24	10
0.25 - 0.49”	0.25 - 0.49	6
0.50 - 0.99”	0.50 - 0.99	2
1.00 - 1.66”	1.00 - 1.66	Not observed
— NOAA Atlas 14 Return Intervals —
1-year	1.67 - 1.99	Not observed
2-year	2.00 - 2.46	Not observed
5-year	2.47 - 2.87	Not observed
10-year	≥ 2.88	Not observed

Total Storms: 18 distinct storm events identified (≥ 0.15”)
Source: NOAA Atlas 14, Volume 2, Version 3.0 — Seattle, WA vicinity

WSDOT (I-5): Monitored vs Modeled Flow

BASEFLOW (0 - 0.09 cfs) | PRIMARY (0.09 - 3 cfs)

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City of Seattle - U District: Monitored vs Modeled Flow

BASEFLOW (0 - 0.12 cfs) | PRIMARY (0.12 - 3 cfs)