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Asset Management of Green Stormwater Infrastructure in Orange County CA through an Open Source Tool

Orange County, CA

Orange County, CA

A study in South Orange County CA on asset management of green stormwater infrastructure led to the development of an open source web-based tool called OC Stormwater Tools, which standardized data collection methods, asset information, and enabled the modeling of pollutant load reduction benefits.

Topics Covered

Stormwater Management
Open Data
Water Quality
Asset Management
Process Improvement


Initial: 300 Thousand USD


General Fund/Existing Public Funds

Project Status

Operational since 2018

Gov Champion

Orange County Public Works

Problem Addressed

Stormwater runoff from urbanized areas in South Orange County, CA is regulated by the municipal separate storm sewer system (MS4) permit issued by the San Diego Water Board.

The permit requires demonstrated progress in achieving water quality improvement and reducing pollutant loads from the MS4.

Eleven municipalities of South Orange County were subject to the permit. They recognized a need to standardize data collection methods, asset information, and pollutant load reduction benefits. The County was looking for ways to:

1. Assess the effectiveness of implemented strategies/best management practices (BMPs);

2. Identify problematic catchments that can be prioritized for future BMPs/strategies; and

3. Assess progress as a result of reducing runoff and pollutant loading over time.

Solutions Used

The municipalities collectively developed a Water Quality Improvement Plan (WQIP) for South Orange County.

The plan contained a special study with these four elements:

1. Development of a system to inventory relevant GSI and stormwater quality BMPs and develop delineations to these assets.

2. Development of supporting datasets and resources to help define drainage assets, regional watershed delineations, and watershed characteristics.

3. Development of a model of pollutant loading and BMP performance to estimate the runoff and load reduction benefits of BMPs.

4. Application of this system for one or more high priority watershed to inventory assets and quantify benefits.

The outcome of this effort was the development of the OC Stormwater Tools web platform. It includes modules and workflows to support municipalities in performing asset inventories, modeling pollutant loading and GSI performance. Additionally, this effort has included an extensive field and desktop mobilization to populate inventories.

This project leveraged existing open source software developed for the Lahontan Regional Water Quality Control Board and the Nevada Department of Environmental Protection for the Lake Tahoe Region (Lake Tahoe Info Stormwater Tools). Building on this open source code allowed the initial release to occur with less time and cost, allowing user input to start earlier. The project added considerable functionality to the code. By keeping these efforts in the open source domain, the project’s intent was to foster inter-agency cooperation to pool resources and share costs.

The project took an agile approach involving a cycle of incremental releases, user testing, feedback, and prioritization of next steps. The long-term vision helped guide overall efforts, but frequent user feedback helped guide the prioritization of specific features and functionality. Beginning in February 2018 (approximately three months after the study started), early adopters began using this system and providing incremental feedback.

Wherever possible, the study elected to connect to existing managed datasets rather than duplicating these datasets into the tool (data federation). Maintaining a single system of record for each type of data (where possible) will help with the long-term sustainability of the system. OC Public Works/OC Survey played an important role in publishing or adapting centralized datasets and developing geoprocessing services to allow the tool to connect to these endpoints rather than replicating these datasets.

Open source code that was developed as part of the previous and current efforts is available at: https://github.com/sitkatech/neptune (OC Stormwater Tools) and https://github.com/geosyntec/nereid (water quality modeling engine supporting OC Stormwater Tools). This software can be freely redistributed and modified under the GNU Affero General Public License as published by the Free Software Foundation.



Development of an Inventory Module which allows GSI and other structural stormwater BMPS to be inventoried, and their inspection and maintenance to be tracked in a standardized way.


Development of a Trash Module which enables municipalities to calculate the performance of trash capture structural controls and assets, and tracking of on-land visual trash assessments.


Development of a Modeling Module designed to provide results (water quality and quantity) for inventoried assets based on asset type, location, drainage area, and modeling attributes.


Development of delineation tools which provide flexible options for either importing delineations (tributary areas) from other sources or defining them within the platform.

Lessons Learned


Establishing a system of record and workflows associated with developing and maintaining a regional stormwater network GIS dataset has been critical to this effort.

Something Unique

OC Public Works/OC Survey maintains a digital elevation model (DEM) with 1-meter resolution and using this dataset, they developed a geoprocessing service to perform automated surface delineations. This service uses flow direction and flow accumulation algorithms. The inlet features in the layer are “burned” into the DEM to produce flow sinks. When a user queries a point, the service returns the area that drains to the point of interest and excludes area that flows into a different inlet.

Who Should Consider

Anyone looking to standardize and centralize the inventorying, maintenance tracking, and assessment of benefits of green stormwater infrastructure in an open source, flexible asset management platform.

Last Updated

Mar 30th, 2021
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