Risk-Based Approach to Stormwater Asset Management

This featured blog by Alex Nice (Director of Water Resources) discusses our unique and innovative approach to Stormwater Asset Management.

Due to aging and expanding infrastructure, there is a growing need for municipalities and other organizations to strategically manage stormwater assets to maintain their level of service in both a cost-effective and predictive manner.  To address this need, organizations are looking to develop and implement stormwater asset management programs that are designed to reduce costly and reactive repairs and to optimize overall performance.  Effective stormwater asset management is also integral to community resiliency and quality of life.     

What are Stormwater Assets? 

Stormwater assets vary by organization and location and have been categorized different ways and using various nomenclature and buzzwords, many of which have overlapping and vaguely defined meanings. In general, stormwater assets can be divided into both engineered and non-engineered assets. Non-engineered assets include any naturally occurring water resource, such as streams or wetlands, that provides some stormwater-related value and benefit and thus should be managed (or protected) as an asset. Engineered assets include “grey” infrastructure such as pipes, open channels, culverts, manholes, inlets/catch basins, dams, and “green” or “blue-green” infrastructure, which includes stormwater control measures (SCMs), stormwater the best management practices (BMPs), low impact development (LID) practices, nature-based solutions (NBS), and many other names. Stormwater assets can also be categorized according to their function, such as conveyance systems (e.g., pipes and open channels), treatment systems (SCMs), and flood control systems (dams and stormwater detention systems); some assets such as restored streams or wet ponds could fall into multiple functional categories.  

Developing an Asset Management Plan/Program

A stormwater Asset Management Plan (AMP) or program may be developed systematically or opportunistically, depending on the available funding and political will. Important aspects of a stormwater AMP include:

• Establishing current state of assets through inventory and condition assessment
• Defining level of service (LOS) goals and estimating LOS for current assets
• Assessing the business risks for assets through probability of failure (POF) and consequence of failure (COF)
• Identifying and procuring funding for operation and maintenance and for capital improvement projects
• Establishing a sustainable long-term funding strategy to implement the AMP

Some key enablers for a successful asset management program are:

• Strong leadership and organizational alignment where everyone across the organization understands roles and responsibilities
• The transfer of knowledge to manage loss of information with change in staffing, and 
• Providing the appropriate technology, tools, and training to the team.

Business Risk Exposure

A risk-based approach for assessment of stormwater assets can involve calculation of Business Risk Exposure (BRE), which is essentially a measure of the asset’s potential for failure and criticality. The intent of estimating BRE for assets is to assist an organization in prioritizing work and the allocation of funds. The BRE for an asset is calculated as the product of the probability or Likelihood of Failure (LoF), Consequence of Failure (CoF), and redundancy, as follows:

For most stormwater assets, the redundancy factor will be zero.  The following figure depicts BRE graphically assuming LoF and CoF with scores from 1 to 5.  As shown in this example, the BRE would be a score from 1 to 25, with 1 having the lowest risk and 25 having the highest risk exposure (negligible to extreme).  

Likelihood of Failure (LoF)

For stormwater assets, LoF is typically a direct correlation to the asset’s condition assessment, which can be estimated through inspections or based on the asset’s age.  Some approaches use performance curves to represent the deterioration of an asset over time.  For inspections, a scoring system should be developed so that a consistent approach is used for all assets evaluated.  Scoring systems will be different for “grey” infrastructure and “green” or “blue-green” infrastructure.  For instance, condition assessment and scoring for stormwater pipes and conveyance systems may be derived from the Pipeline Assessment Certification Program (PACP) methods as developed by the National Association of Sewer Service Companies (NASSCO). While condition scoring for SCMs may incorporate more diverse considerations, such as health of plantings and adequacy of filter media.  Regardless, a consistent numerical scale should be used for all assets.  The images below show Gradient staff conducting inspections for both grey (left) and blue-green (right) stormwater assets. 

Consequence of Failure (CoF)

CoF is a measure of an asset’s criticality or the potential economic, environmental, or social (triple bottom line) impact if the asset fails. To develop a scoring system for CoF, various “triple bottom line” factors or subcategory ratings can be developed and weighted according to desired level of importance. As with the LoF and condition scoring, the scoring system for CoF will vary depending on the asset type.  For a stormwater pipe asset, economic subcategory ratings could be depth, size and location of the pipe, while environmental subcategory ratings could be proximity to water bodies or a hazardous site.  For an SCM, a social subcategory rating could be based on the educational or demonstration value.  After all “triple bottom line” subcategory ratings have been assigned for an asset, the CoF can be calculated as a composite weighted average of all assigned subcategory ratings, according to the following equation: 

Closing

While stormwater asset management is still very much a developing field, implementing a risk-based asset management approach as outlined here can help an organization to assess and prioritize assets and to be proactive with maintenance and repairs. Gradient staff has gained substantial experience supporting stormwater asset management programs in recent years, which has included conducting inspections and condition assessments, developing and implementing risk-based approaches, and preparing repair and restoration plans for stormwater assets.  As an example of our project experience, see our presentation about Stormwater Asset Management in the City of Henderson at this link.  Please don’t hesitate to reach out if you would like to discuss management of your organization’s stormwater assets.