Although
many resources are available to aid in the design and construction of most
structural stormwater best management practices (BMPs), few guides exist
pertaining to their operation and maintenance. Historically, it seems as though
a “build ’em and walk” approach has been commonplace. However, increasing focus
upon mass balances, numeric goal setting, and total maximum daily loads (TMDLs)
now requires that much more emphasis be placed upon BMP operation and
maintenance for permitting and reporting requirements—for example, for the
National Pollutant Discharge Elimination System (NPDES) municipal separate storm
sewer system (MS4) permit program, and as a part of stormwater pollution
prevention plan (SWPPP) reporting.
Typically, we
think of structural stormwater BMP
operation for optimizing (1) the
removal of pollutants and (2) the reduction of runoff volumes/rates via the
management of stormwater networks or treatment trains. BMP
maintenance is the purposeful
management of a BMP to maintain a desired level of performance and efficiency.
Maintenance consists of short-term (routine or more frequent), long-term
(non-routine or less frequent), and major (rare) actions (Figure 1).
Stormwater BMPs
have a lifecycle from their creation (design and construction) through operative
stages (functional or not) that is largely dictated by operation and maintenance
(O&M) actions. As maintenance involves a significant amount of resources
(personnel, equipment, materials, sediment disposal expense, etc.), the more we
learn about BMP operation, the more likely we are to maintain optimal
performance and improve cost efficiencies. The purpose of this article is to
advance short- and long-term maintenance considerations to develop more
realistic O&M plans. To do this, we have used a combination of a national
literature search for maintenance costs coupled with a detailed municipal public
works survey.
Minnesota BMP Maintenance
Survey
The statewide survey
of Minnesota Municipal Public Works managers to define maintenance needs and
guidelines was conducted by the University of Minnesota and partly funded by the
Minnesota Pollution Control Agency. Previously, the University of Minnesota
produced a manual, Assessment
and Maintenance of Stormwater Best Management Practices, which includes source reduction and four levels
of assessment (from visual to state-of-the-art monitoring). The manual is
available online at www.pca.state.mn.us/water/stormwater/stormwater-research.html
or
wrc.umn.edu/outreach/stormwater/bmpassessment/index.html.
The specific goals
of the survey were to identify and inventory stormwater BMP maintenance in
Minnesota. Survey questionnaires focusing on the following questions were sent
to 106 cities; we received 27 responses, for a slightly higher than 25% response
rate.
- How many BMPs are in your
watershed?
- How often are your BMPs
inspected?
- What is the average
staff-hours spent per routine inspection/maintenance?
- How complex is the
maintenance of your BMPs?
- Which factors most frequently
cause the performance deterioration of your BMPs?
- What are the costs for
non-routine maintenance activities?
We attempted to
make the survey as simple as possible, requesting information for typical
response ranges of common BMPs. Although the number of respondents was
relatively low, we believe that the results will help refine operation and
maintenance needs.
Inspection
Frequency and Staff-Hours.
The required frequency of stormwater BMP maintenance actions and the associated
required staff-hours are two key parameters that are necessary to reasonably
budget and schedule inspection and maintenance. Frequency and staff-hours vary
according to BMP design, climate conditions, accessibility of the BMP, and
maintenance strategies of the BMP operators. As part of the survey, cities were
asked to provide information regarding their frequency of routine maintenance
actions for various kinds of BMPs.
As shown in
Figure 2, the majority (84%) of cities conduct routine maintenance actions once
a year or less. Inspection frequency varies significantly due to BMP
accessibility and use of proactive or reactive response strategies. “We are
working on and around the site every business day, and so inspections happen
informally every day,” said Julie Vigness-Pint, BMP grant coordinator at
Ramsey-Washington Metro Watershed District, MN. Jim Hafner, the stormwater manager
for the city of Blaine, MN, voiced a more typical response strategy. “Our city
has not had a proactive program for BMP maintenance. It has generally occurred
when problems have popped up,” he said. These differences in maintenance
strategies and the execution of maintenance plans have undoubtedly led to a wide
variation in the type and frequency of maintenance
actions.
The required
maintenance staff-hours depend on the size of the BMP, design configurations,
the maintenance strategy, and the complexity of the maintenance action. Figure 3
shows staff-hours spent on routine maintenance actions. Note that values range
from 1 to 4 hours except for wetlands and rain gardens, which typically require
more dedicated levels of effort. Staff-hours for wetlands and rain gardens were
noted to vary more widely according to the intensity of the maintenance
effort.
Maintenance
Complexity. To assess
maintenance complexity, a scale from 1 to 4 was used, with 1 being minimal
complexity (i.e., stormwater professional or consultant is seldom needed) and 4
being the most complex (i.e., stormwater professional or consultant is always
needed). The complexity of maintenance that BMP operators actually experience
depends on the intensity of maintenance efforts; the desired performance level
of the BMP; and the maintenance crew’s training, experience, and skills. For
most BMP categories, as shown in Table 1, a majority of the responses indicated
that maintenance was minimal or simple. For wetlands and porous pavements,
however, approximately 50% of the responses indicated that maintenance efforts
were moderate or complicated.
Respondents were
also prompted from a defined list of possible items on the factors that most
frequently caused deterioration in the performance of their BMPs. Litter and
debris accumulation and sediment buildup were identified most frequently (Figure
4). Pipe clogging was listed frequently for wet ponds, dry ponds, and wetlands,
while invasive vegetation was a specific concern for constructed wetlands and
rain gardens. For infiltration basins and trenches, high groundwater levels were
identified more frequently than all other issues except for sediment buildup and
litter and debris.
Maintenance
Costs. The cost of
maintenance actions may vary significantly based on location and the maintenance
strategy (proactive versus reactive) implemented. Both the size and design of a
stormwater BMP influence the required maintenance frequency and associated
costs. However, due to the difficulty in collecting detailed as-built
configurations of BMP facilities, size and design were not considered in this
survey. Based on survey results, removal of accumulated sediment incurred the
largest cost for all BMP categories. For sedimentation practices such as wet
ponds, dry ponds, and underground sedimentation devices, inlet and outlet
structure maintenance was the second largest cost.
Figures 5 and 6
show the cost of maintenance as a function of the time interval between
maintenance actions for sediment removal and inlet/outlet structural maintenance
in wet ponds and dry ponds, respectively. The scatter in the data is likely
because the data originated from BMPs of varying sizes and designs. Despite the
scatter, it appears that any trend line for sediment removal in wet ponds would
have a negative intercept, which would mean that there is an overall cost
advantage in more frequent dredging of ponds. Ideally, the maintenance interval
should be optimized to minimize overall lifecycle maintenance costs without
losing the desired BMP performance. To accomplish this, frequent inspection is
needed so that potential problems are addressed before the BMP performance
deteriorates to unacceptable levels. For example, because sediment buildup
occurs frequently (Figure 4) and has a large per-event cost to remove (Figure
5), obtaining accurate estimates of sediment buildup is necessary if budgets and
maintenance plans are to be optimally developed. The next section addresses this
aspect of sediment management in wet ponds.
Sediment Management in Wet
Ponds
We will focus on wet ponds, the most widely used
stormwater BMP in Minnesota. As discussed earlier, sediment buildup frequently
causes deterioration of BMP performance, and sediment removal has the largest
per-event maintenance cost. Therefore, to predict the frequency and cost of
sediment removal, it is important to accurately estimate sediment buildup
rates.
A wet pond
typically operates as a continuous-flow sedimentation tank during a storm event,
using settling to remove solids. As with sedimentation tanks, the hydraulic
loading (i.e., flow divided by surface area) is an important parameter in
determining wet pond pollutant-removal efficiency. Storage capacity is another
key component, as declining storage capacity can also reduce the hydraulic
residence time and, hence, decrease removal efficiency (Hafner and Panzer 2005)
and subject accumulated sediments to washout and downstream transport. Finally,
pollutants such as phosphorus can be chemically released from the sediment under
anaerobic conditions. Therefore, accumulated sediment should be removed before
it significantly degrades the performance of the BMP. One guideline has
recommended that sediment be excavated when 25% of the permanent pool volume has
been lost (Haubner et al. 2001). By following a predetermined maintenance
schedule, sediment depth can be monitored to determine when excavation is
required. As an alternative approach, although not as site-specific, sediment
buildup can be estimated using the methodology defined in the
sidebar.
Overall O&M Cost
Estimation
Because actual
O&M cost data was insufficient, publications from across the United States
that contained cost estimates of typical maintenance schedules for common
stormwater BMP categories were evaluated. Cost estimates of lifecycle
maintenance expenses were developed for dry ponds, wet ponds, constructed
wetlands, bioinfiltration filters (rain gardens), sand filters, and
grassed/vegetative swales. All costs were adjusted by means of regional cost
adjustment factors (USEPA 1999) to reflect 2005 costs in
Minnesota.
Lifecycle
Maintenance. The USEPA (1999)
published annual stormwater BMP O&M costs, as a percentage of total
construction cost ranging from less than 1% for dry ponds to a maximum of 20%
for infiltration trenches. The more recent analyses (Landphair et al. 2000,
Wossink and Hunt 2003) have expanded this range. We assembled cost estimates
from the literature to create Figures 7a through 7f (Weiss et al. 2005), which
are plotted on a log-log scale and contain the equation for the best-fit line.
Using known total construction costs, the annual budget for selected stormwater
BMP maintenance can be estimated using the equation for the best-fit line
(dashed lines) with 67% confidence intervals (solid
lines).
Operation and
maintenance costs are a substantial portion of lifecycle stormwater BMP costs.
Figure 7 indicates that annual predicted O&M costs are on the order of 10%
of total construction costs for a stormwater BMP that costs $10,000 (2005 year),
and on the order of 5% for a stormwater BMP that costs $100,000 (2005 year). This means that the
O&M costs of the stormwater BMP will roughly equal the construction cost (in
constant dollars) after 10 years and 20 years, respectively. We can also see
that the trend of predicted annual O&M cost, as a percentage of the
construction cost, decreases with increasing total construction cost for all
BMPs. The trend is slight for sand filters (Figure 7e) but, as shown in Figure
7, an economy of scale exists for O&M costs: more expensive (and presumably
larger) BMPs require a smaller fraction of the original construction cost for
annual operation and maintenance.
Conclusions
Many MS4 communities
are struggling to define stormwater BMP operations and maintain needs without
available guidelines. As a first step to rectify the lack of guidance, we
surveyed Minnesota cities to quantify the frequency of inspection, level of
effort needed for routine maintenance, BMP deterioration factors, and
maintenance complexity.
Our evaluation of
these factors was used to estimate annual maintenance costs of common BMPs for
lifecycle operations. Sediment removal and inlet/out structure maintenance were
found to incur the largest maintenance costs for most BMPs considered. To
continue this effort in Minnesota, an expert council is being formed to
establish cooperative efforts related to compilation of maintenance data in
standardized formats.
The lifecycle O&M costs are also shown to be a
substantial portion of total costs and should be taken seriously. As a general
rule of thumb, O&M costs of the stormwater BMP will roughly equal the
construction cost (in constant dollars) after 10 years for a $10,000
installation and 20 years for a $100,000 installation (2005 dollars).