Maintenance Costs of Stormwater Control Measures (SCMs) in North Carolina

Abstract

With increasing urbanization the need for stormwater control measures (SCMs) to meet federal and state regulations grows. Maintenance is necessary and typically required by state and local regulations to ensure SCMs remain functional and continue to mitigate the impacts of urbanization. In addition to functional drivers, SCMs are maintained for aesthetic appeal. Previous research has quantified maintenance costs for SCMs in North Carolina but (1) this study is dated, (2) there remains a lack of data regarding costs for routine, preventative/proactive, and restorative maintenance, and (3) the relationship between a SCM’s location within a parcel and its maintenance frequency has not been evaluated. North Carolina State University (NCSU) partnered with the Stormwater Consortium (SC) of the Water Resources Research Institute of the University of North Carolina (WRRI) to answer these questions. While maintaining the interviewees’ anonymity, NCSU interviewed (1) representatives from the WRRI-SC communities, (2) private contractors who have been certified through the NCSU Stormwater BMP Inspection & Maintenance workshop series, and (3) staff from two state entities. A standard set of questions was developed, including (but not limited to): how many practices are maintained, the characteristics of the maintained practices (e.g., age), the location of the maintained practices with respect to the parcel, the types of maintenance tasks needed for a particular SCM, the frequency of and time spent on each maintenance task, and the costs associated with each task. Quantifying maintenance costs for SCMs was difficult; factors such as maintenance access and SCM location greatly impact costs. Additionally, record keeping prevented maintenance costs from easily being identified. The investigators found private contractors or multiple municipal departments usually maintain municipal SCMs. Despite limited participation in the study, routine and restorative maintenance costs were quantified for the following SCMs: wet ponds, stormwater wetlands, dry ponds, bioretention cells, sand filters, level spreader-filter strips, rainwater harvesting systems, permeable inter-locking concrete pavers (PICP), Contech StormFilters®, ADS BayFiltersTM, and Contech Filterra systems®. Missing data for infiltration systems, disconnected impervious surfaces (DIS), treatment swales, and other proprietary systems could be the result of a lack of participation from companies who maintain these practices or fewer implementations of these SCMs for stormwater management limited maintenance understanding. Certain tasks such as replacing trees for DeepRoot Silva Cells® or mowing treatment swales and infiltration basins can be estimated using data reported for other SCMs. Interviews with private contractors revealed average annual routine maintenance costs of SCMs with mowing in 2018 ranged from $9,440 per ac of SCM surface area to $16,875 per ac of SCM surface area. Mowing grass was a significant maintenance factor and refers to mowing the landscape surrounding (or inside) the SCM. Factors such as mowing and maintaining vegetative cover increase costs, and these tasks usually occur on a monthly basis. Maintenance tasks include those set forth by state regulations. We learned that a SCM’s location did not impact the level of service (LOS) provided. Interviewees stressed a high LOS is provided to avoid hazardous site conditions, improve water quality, and reflect the interviewee’s dedication to their profession. There were differences between routine, proactive, and restorative maintenance costs. However, restorative maintenance costs tend to be much higher than routine and proactive costs, and it is more economical to routinely maintain a practice rather than waiting to maintain a SCM once it loses functionality. Information from this study will be disseminated through state and national conferences, a journal publication, NCSU workshops, and an Excel based tool.