Guam Stormwater Management Manual and Calculation Tool
Overview
When it rains on Guam, runoff from roofs, roads and construction sites carries sediment, nutrients and other pollutants into our streams, the Northern Guam Lens Aquifer and the coral reefs that surround the island. The Guam Stormwater Management Manual sets the island’s design standards for controlling that runoff, both while a site is under construction and for the life of the finished project.
Guam’s standard is the CNMI and Guam Stormwater Management Manual, Volumes 1 and 2 (October 2006), prepared for Guam EPA and the CNMI Division of Environmental Quality. Guam formally adopted it under Executive Order 2012-02, signed January 26, 2012. It replaced older references, including the U.S. Army Corps of Engineers’ Guam Storm Drainage Manual (1980), the CNMI Stormwater Control Handbook (1989) and Guam EPA’s Erosion and Sediment Control Manual (2000).
Who uses this manual
The manual applies to non-transportation construction: private development, subdivisions, and Government of Guam projects, including those of autonomous agencies. Public roadway and highway projects, and work that ties into a public right-of-way, follow the companion Guam Transportation Stormwater Drainage Manual (Department of Public Works, 2010), adopted under the same executive order.
Engineers, architects, site planners, contractors and plan reviewers use it to prepare and check erosion and sediment control plans and permanent stormwater designs submitted for clearing, grading and building permits.
What’s in the manual
-
Volume 1: stormwater treatment standards, acceptable best management practices (BMPs), and construction and post-construction requirements.
-
Volume 2: detailed design information, sizing methods and hydrologic tools.
-
Stormwater Design Specifications: a supplement that adapts several designs to island conditions, such as recharge through limestone.
2024 Manual Update and Calculation Tool
In 2024 the U.S. Army Corps of Engineers, working with BSP, Guam EPA and DPW, developed the Guam Stormwater Manual Update and the Storm Drainage Runoff Calculation Tool. Use the Update together with the Tool, which works from Guam rainfall data and covers:
-
TR-55 peak runoff (graphical method)
-
Runoff volumes using manual rainfall
-
Simplified infiltration basin design (Guam EPA method)
-
Swale design
-
Bio-remediation measures
-
Sand filter design
A recorded training session on the Tool, first held for the Guam Contractors Association and design community on May 14, 2025, is available under Calculation Tool Training Video.
Key design criteria at a glance
For quick reference only. Always design from the full manual and the 2024 Update.
|
Criterion |
Requirement |
|
Groundwater recharge |
1.5 in of rainfall in limestone areas; soil-group recharge rates in volcanic areas. No infiltration of hotspot runoff without full pretreatment |
|
Water quality volume |
0.8 in (80% rule) for moderate-quality waters; 1.5 in (90% rule) for high-quality waters and hotspots |
|
Channel protection |
24-hour extended detention of the post development 1-year, 24-hour storm |
|
Overbank flood control |
Hold the 25-year, 24-hour peak discharge to |
|
Construction sediment control |
Sediment traps sized for at least 1.5 in of rainfall, or basins of at least 5,500 cu ft per acre |
Channel protection and overbank flood control are waived for sites that discharge directly to coastal waters, reservoirs, rivers or streams.
How the manual is enforced
The manual is a design standard. It becomes a requirement through permits: Guam EPA reviews erosion and sediment control plans under the Guam Soil Erosion and Sediment Control Regulations, DPW issues clearing, grading and building permits, and sites disturbing one acre or more also need coverage under U.S. EPA’s Construction General Permit. See Regulatory and legal map below.
Questions
Guam EPA Water Pollution Control Program: (671) 588-4751 Bureau of Statistics and Plans, Guam Coastal Management Program: (671) 472-4201/2/3
Where the manual applies
The 2006 manual governs every non-road project on Guam; road and right-of-way work uses the 2010 DPW manual, and both feed the same Guam EPA and DPW permits.
|
Project type |
Governing design manual |
Local permits / reviews |
Federal coverage |
|
Private development, subdivisions, commercial and |
2006 CNMI & Guam Stormwater Management Manual + 2024 Update |
DPW clearing & grading permit (21 GCA §66202.1) with Guam EPA review of the erosion control plan (22 GAR Ch. 10); DPW building permit |
EPA Construction General Permit if 1 acre or more is disturbed |
|
Government of Guam and autonomous agency projects |
2006 manual + 2024 Update (EO 2012-02) |
Same as above |
Same as above |
|
Public highways, roads, and local or federal-aid |
2010 Guam Transportation Stormwater Drainage Manual (DPW) |
DPW review; Guam EPA erosion and sediment control review |
CGP if 1 acre or more; FHWA requirements on federal-aid projects |
|
Private work in or connecting to a public right-of-way or |
2010 Transportation manual for the ROW portion; 2006 manual for the site |
DPW right-of-way and drainage review |
CGP if 1 acre or more |
|
Projects with federal permits, funding or activities |
2006 manual referenced as Guam’s enforceable nonpoint standard |
GCMP federal consistency review (BSP) |
CZMA consistency; CWA §401 water quality certification by Guam EPA |
Sites that discharge directly to coastal waters, reservoirs, rivers or streams are exempt from the channel protection and overbank flood criteria, but not from recharge or water quality treatment.
Regulatory and legal map
Regulatory and legal map · federal law to project permits
The manual carries no regulation of its own: EO 2012-02 adopts it, and it reaches a project through Guam EPA, DPW, U.S. EPA and GCMP permit reviews.
Legal instruments reference
The manual has no regulation of its own: it binds through EO 2012-02 and through the permits and rules below, which is why Guam EPA has proposed adopting it by regulation.
|
Level |
Instrument |
Administered |
How it |
|
Federal statute |
Clean Water Act §402 (NPDES) |
U.S. EPA Region 9 |
Guam has no delegated NPDES program, so EPA issues all permits on Guam, including the 2022 Construction General Permit (modified April 8, 2025) for sites disturbing 1 acre or more |
|
Federal statute |
Clean Water Act §401 |
Guam EPA |
Guam EPA certifies that federal permits such as the CGP meet Guam Water Quality Standards |
|
Federal statute |
Coastal Zone Management Act; 15 CFR Part 930 |
BSP – Guam Coastal Management Program |
Federal actions with coastal effects must be consistent with GCMP enforceable policies, including stormwater controls |
|
Federal statute |
CZARA §6217 (Coastal Nonpoint Pollution Control Program) |
Guam EPA and BSP-GCMP, approved by NOAA and EPA |
Guam’s program was fully approved in 2007 and relies on the erosion control regulations and BMP manuals |
|
Organic law |
Organic Act of Guam |
Governor |
Authority cited for EO 2012-02 |
|
Guam statute |
10 GCA Ch. 47, Water Pollution Control Act |
Guam EPA |
Enabling authority for 22 GAR Ch. 10 and Ch. 45 |
|
Guam statute |
21 GCA Ch. 66, §66202.1, Clearing and Grading Permit |
DPW, with Guam EPA review |
Permit through which erosion and sediment control plans are reviewed against the manual |
|
Guam regulation |
22 GAR Div. II Ch. 10, Guam Soil Erosion and Sediment |
Guam EPA |
Requires erosion and sediment control plans and BMPs; the manual’s 11 construction standards implement it |
|
Guam regulation |
22 GAR Div. IX Ch. 45, Soil Erosion and Sedimentation |
Guam EPA |
Companion soil erosion rules; a 2010 draft proposed renaming it the Guam Erosion Control and Stormwater Management Regulations and referencing the manual |
|
Guam regulation |
22 GAR Div. II Ch. 5, Guam Water Quality Standards |
Guam EPA |
Sets the water classes that decide the 80% vs 90% water quality rule |
|
Guam regulation |
18 GAR Ch. 7, Territorial Seashore Protection Commission |
Seashore Protection Commission |
Applies to development in the seashore reserve |
|
Executive order |
EO 2005-35 (Oct 25, 2005) |
Governor |
Interim adoption of the CNMI & Guam criteria for all |
|
Executive order |
EO 2009-13 |
Governor |
Stayed the 2005 adoption for roads pending a DPW manual |
|
Executive order |
EO |
Governor; DPW and Guam EPA implement |
Adopts the 2006 manual for non transportation and private construction, and the 2010 DPW manual for transportation projects |
|
Executive order |
EO 2004-04, Watershed Planning Committee |
BSP and member agencies |
Directs agencies to plan development consistent with the erosion control regulations and §6217(g) guidance |
Below is the Guam Stormwater Management Manual Update that should be used in conjunction with the Storm Drainage Calculation Tool that can be downloaded below
Storm Drainage Runoff Calculation Tool
NRCS Web Soil Survey Map
Contact Us
- P.O. Box 2950 Hagatna, Guam 96932
- (671) 472-4201/2/3
management
- Lola Leon Guerrero - Director - lolalg@bsp.guam.gov
- Matthew Santos - Dep. Director - matthew.santos@bsp.guam.gov
find us
- 513 W Marine Dr. Hagåtña, Guam 96910