By Kevin Smith, AICP and Zach Magdol, PE

Introduction

As planners, we often talk about creating a “Vision for the Future” for the regions and communities we serve. That vision may include elements of future land use, urban design, growth policies, housing, social justice, economic development, and so on.

Inherent within the planning process is the need to plan for the infrastructure necessary to achieve that vision. The planning process typically includes provisions for transportation infrastructure. Other infrastructure, including stormwater management, can be overlooked. However, much of the work that planners perform has a direct relationship with stormwater management: influencing the amount of runoff and where the major runoff corridors exist.

In this article we discuss several of the most important aspects of stormwater management along with two project-specific examples of stormwater master plans.

Stormwater Management – Just a Good Idea, or Required?

The short answer is: both. Stormwater management is a combination of the need for protection of life and property against the threat of local and regional flooding, and meeting regulatory requirements intended to protect the environment. For example, municipalities enact ordinances requiring mitigation for the increased quantity of runoff generated by development, installation of infrastructure to convey stormwater safely, and the identification and preservation of drainage corridors.

The federal Clean Water Act (CWA), through the development of the National Pollutant Discharge Elimination System (NPDES), established the regulatory framework to improve surface water quality by requiring the reduction of pollutants that can be transported by stormwater into a storm drainage system and, ultimately, into a receiving water such as a lake, stream, or river. Municipal separate storm sewer systems (MS4s), construction activities, and industrial activities are all regulated under NPDES requirements.

The conversion of land from a previously undisturbed condition to a level of urbanization can affect the runoff characteristics. However, as communities grow and develop with increasing amounts of impervious area, the need to consider the effects on the environment become more important.

Public entities are placing a greater emphasis on creating master plans to address the health and safety and regulatory requirements associated with the management of stormwater, as well as the ongoing financial considerations that result from the construction and maintenance of storm drainage systems. The planning profession is well-suited to add value to these planning efforts.

City of Sioux Falls, South Dakota - Stormwater Best Management Practices (BMP) Master Plan Update

Sioux Falls is located in the southeastern corner of South Dakota and has a population of approximately 190,000 residents. Urban development is planned to expand southwest of the City. Having a stormwater master plan in place will be beneficial as development progresses so the City, landowners, and potential developers understand how stormwater can be effectively managed. The project area is included in the City’s southwest growth area and lies within two separate undeveloped watersheds outside existing city limits.

The intent of the Best Management Practices (BMP) Master Plan update was to provide clear recommendations to the City regarding the management of stormwater runoff. This would be accomplished by identifying existing drainage patterns and developing conceptual layouts for regional conveyance along with detention systems to mitigate the impacts of growth.

During the initial phases of the project, City Planning and Development Services staff provided future land use projections, aerial imagery, and contour information that was used to develop a stormwater model for the area. A series of conceptual regional stormwater facilities, arterial roadway crossing designs, and associated cost estimates were included in the final document. Figure 1 depicts the current land uses within the project area.

Figure 1: Current land uses within the project area

While the study was underway, City Planning and Development Services staff were presented with a proposal for a private development within the study area. Through collaboration with the Public Works staff and the consultant, the City was able to determine that the land uses and general pattern for the proposed development would not be compatible with planned drainage improvements in the area.

Interdepartmental coordination between Planning and Public Works throughout the study resulted in a consistent approach both departments can follow when considering future development proposals. Additionally, both departments have gained a better understanding of the factors that need to be considered in this environmentally-sensitive area of Sioux Falls.

Figure 2 illustrates the locations for new drainage improvements and a corresponding table of capital improvement costs.

Figure 2: Locations for new drainage improvements and a corresponding table of capital improvement costs

Key outcomes of the master plan include:

Stormwater modeling that acknowledges all impacts and multiple scenarios.
Careful interdepartmental review of development plans.
Development decisions based on cost of services.
Unified stance from Planning and Public Works on land use decisions involving stormwater impacts.
Prioritized infrastructure investments.

City of Kalispell, Montana - West Study Area Storm Drainage

Kalispell is located within the Flathead Valley of northwestern Montana. In 2008, the City of Kalispell updated their Stormwater Facility Plan that identified short- and long-term stormwater management needs and recommendations throughout the community. This included a series of improvements in two study areas as shown in Figure 3. In 2018, the City acknowledged the need to update the original study for the following reasons:

The initial concepts developed in 2008 provided the City with the general conceptual framework for each planning location. However, because the 2008 plan was a high-level planning document, additional detail was needed for the City to move forward with design;
The 2008 plan was outdated, and several recent site-specific development details had been established since that plan was developed; and
Development pressure in and around the two study areas have increased with the recent uptick in the economy, the anticipated development of the West Side Sanitary Sewer Interceptor, and the planned North Town Commercial Center.

Figure 4: Maximum allowable impervious area for future development

The primary goal of the project was to provide clear recommendations to the City for managing stormwater runoff in the West and North Study Areas by:

Evaluating stormwater management and drainage alternatives based on criteria developed in coordination with City staff that meet current city, state, and Federal standards;
Providing a recommended improvement option that will allow the City to quickly move into preliminary design; and
Identifying permitting, land acquisition, and easement challenges that might arise during the design phase.

One of the key assumptions in developing the list of recommendations was the amount of future impervious area that will occur within the City’s Annexation Policy Boundary.

Figure 5: Maximum Allowable Impervious Area for Future Development (North)

The future percent of impervious area was calculated following a three-step method, which is briefly described below:

Future land use for the undeveloped areas within the study boundary was determined by using the City’s anticipated land use classification GIS layer. The land use classification layer provided a general overview of the type of development that could occur within the study area (i.e., urban residential, high-density residential, commercial, etc.).
Adjacent fully developed areas with the same land use designations were delineated to determine impervious versus pervious area utilizing GIS.
A comparison between the GIS delineations and the City of Kalispell’s Zoning Ordinance (July 19, 2010) percent impervious recommendations was reviewed by the project working group. Final percent impervious values were agreed upon and used in the analysis.

Existing land uses for both Study Areas and the contributing watersheds that drain through them were determined based on 2017 aerial photography. Most of the West Study Area consists of agricultural lands, range, and forest. Future land use was only defined within the Annexation Policy Boundary for both Study Areas. Future land use within the West Study Area will be classified as predominantly urban residential whereas the future land use within the North Study Area will be classified as a higher density (urban mixed-use).

Key Outcomes of the Study:

A recommendation that developers be held to the maximum impervious area percentages shown in Figure 4 and Figure 5.
Development of a GIS-based hydrologic and hydraulic model that quickly allows the evaluation of alternative conceptual layouts for regional conveyance and detention systems.
Recommended policies and standards for developers to incorporate into developer-installed on-site stormwater facilities.

Conclusion

Successful stormwater management requires a truly comprehensive planning approach as illustrated by the examples from Sioux Falls and Kalispell. In both case studies presented in this article, collaboration between Planning and Public Works staff resulted in the identification of a series of drainage improvements, corresponding land use strategies, and a better overall acknowledgement that stormwater management and land use planning can achieve common goals.

Kevin Smith, AICP is the Operations Manager for the Sioux Falls, SD office of Advanced Engineering and Environmental Services, Inc. (AE2S). Kevin has over 20 years of municipal Planning and Public Works experience and assists communities with planning for their infrastructure and growth needs. AE2S is a specialized civil/environmental consulting engineering firm that provides professional services and a unique brand of extreme client service to clients in the Upper Midwest and Rocky Mountain Region.

Zach Magdol, PE is a Project Engineer in the Lehi, UT office of AE2S and contributed to this article.

Stormwater Management and Master Planning - Why it Matters