Comparing LID and Stream Restoration
Finding cost-effective ways to reduce sediment and nutrient loads
The goal of improving the health of the Chesapeake Bay has long driven efforts to find more efficient, cost-effective methods for managing stormwater in the mid-Atlantic states. Over time, the prevailing paradigm has shifted from controlling peak flows to reducing runoff volumes so that they match a target condition, typically by means of low-impact-development (LID) concepts that encourage infiltration and disconnect impervious areas from waterways. Although retrofitting existing development to include such measures can reduce runoff and improve water quality, this approach may prove complex and expensive in urbanized areas. Therefore, the sources of pollutants must be considered while cost-effective strategies for reducing inputs of pollutants into Chesapeake Bay and its tributaries are formulated. In certain cases in which retrofits would be costly, stream restoration may turn out to provide a more cost-effective approach for reducing loadings of such pollutants as sediment and nutrients.
Planning for the Chesapeake Bay TMDL
On December 29, 2010, USEPA finalized the Chesapeake Bay total maximum daily load (TMDL), an ambitious regulatory effort to reduce loadings of total nitrogen (TN), total phosphorus (TP), and sediment to the bay and its tributaries. Under the TMDL, Virginia, the District of Columbia, and the five other states—Delaware, Maryland, New York, Pennsylvania, and West Virginia—located within the bay’s vast 64,000-square-mile watershed must implement numerous measures by 2025 to ensure reductions of the three pollutants considered the greatest threats to the health of the bay. All told, the TMDL mandates that the jurisdictions together reduce discharges of the three pollutants by the following amounts: TN by 25%, TP by 24%, and total suspended solids (TSS) by 20%, compared to 2009 levels as estimated by EPA.
Fairfax County, VA, is located within the watershed of the Potomac River, the second-largest tributary of the Chesapeake Bay. Therefore, the Chesapeake Bay TMDL will require the county to build on previous efforts to reduce pollutants to the Potomac River. Although Virginia and the other jurisdictions are still in the process of determining how such cuts will be made at the local level, for Fairfax County the impact is clear: It can expect to have to devote even greater technical and financial resources to significantly reduce the amounts of nitrogen, phosphorous, and sediment entering its waterways from point and nonpoint sources alike. These resources are in addition to ongoing measures for the 19 established TMDLs as well as the requirements for 56 new TMDLs to be developed for impaired water bodies in Fairfax County. Examples of these impairments include excessive bacteria levels and issues surrounding degraded benthic macroinvertebrate communities.
Putting the LID on the Problem
The most current stormwater management approach in the mid-Atlantic states is based on the principle of reducing runoff volumes to match a target condition, as opposed to the old paradigm of controlling peak flows. Volume reduction is to be achieved by retaining water onsite and releasing it to natural pathways, primarily to interflow. LID techniques that encourage the disconnection of impervious areas from waterways and involve distributed microcontrols that facilitate infiltration are well suited to this approach. This new paradigm is more effective in addressing pollutant loads than the old one. Because the pollutant load is a function of the volume of runoff, reducing runoff volume directly reduces pollutant loads to Chesapeake Bay tributaries.
Some stormwater practitioners challenge, often on the basis of regional issues, the applicability of LID technology compared to approaches that rely on detention ponds. The argument often rests on the misconception that LID involves only infiltration and no detention. More fundamentally, this misconception can be traced to confusion between LID principles and techniques. It is easy to find situations in which LID techniques like bioretention and green roofs do not work well because of site-specific conditions related to soil or climate. However, all agree that the fundamental LID principle of reducing the magnitude of the problem (i.e., the amount of runoff) is a sound approach that results in environmental and economic benefits. Questions about how much reduction is necessary and how to achieve it have sparked yet another debate and a flurry of active academic research, especially in light of the limited knowledge about the cumulative impact on receiving aquatic ecosystems from deploying stormwater controls watershed-wide.
In part, this imperfect knowledge has created an aura around LID, fueling a movement to “put the ‘LID’ on everything.” This movement is often based on simplistic performance requirements derived without adequate understanding of site-specific watershed dynamics, climate, regulations, and socioeconomic factors. For example, runoff reduction by means of LID retrofits faces significant challenges in urbanized areas. The following examples represent commonly found conditions that may complicate implementation and greatly
- reduced infiltration in urban soils,
- utility conflicts,
- limited opportunities for deployment of controls because of private property ownership,
- inconsistencies between LID principles and land development codes and ordinances,
- prohibitions on spending public funds for installations on private
- uncertainty about long-term performance because of concerns regarding adequate maintenance of controls on private property.
LID principles are a sound guide for stormwater management, provided that they are applied in concert with realistic and achievable goals for watershed restoration. In urban settings, more often than not it is infeasible or impossible to return watersheds to a pristine state. However, it is possible to formulate watershed restoration plans leading to functioning ecosystems that provide a suitable level of ecological and societal services. To define this level of service, one must consider the watershed-specific factors previously mentioned and their relation to larger regional issues. To achieve this level of service, flexibility is needed to foster application of a varied array of tools, the outcomes of which support the restoration goals.
Although LID-based stormwater retrofits are one of these tools, they should not be considered a panacea for all stormwater ailments. As reported by the National Research Council (2009), 42% of urban lands in the United States will be redeveloped by the year 2030. This means that stormwater professionals have a second chance to do things right when it comes to stormwater management, and LID is a crucial part of the solution. In the meantime, we need to look at other tools to address short-term challenges.
Planning for Watershed Health in Fairfax County
Fairfax County’s population is expected to grow more than 37% over the next 20 years. Like many jurisdictions nationwide facing these levels of growth, the county is addressing issues of regulatory compliance and watershed health by infusing a holistic approach into its stormwater management program. The mission of this program is to preserve and restore the natural environment and aquatic resources, consistent with the environmental agenda adopted by the Fairfax County Board of Supervisors in June 2004. In addition, the county must also comply with local, state, and federal laws and mandates, principally county ordinances and policies, the federal Clean Water Act, and Virginia’s Chesapeake Bay Initiatives. Under the Virginia Pollutant Discharge Elimination System, the county holds an individual municipal separate storm sewer system (MS4) permit that outlines multiple requirements to comply with the Clean Water Act. One of these requirements is the development of watershed management plans for all watersheds in the county. Additionally, the plans are intended as a guide for the county’s efforts to fulfill its commitment to the Chesapeake Bay 2000 Agreement to restore the ecological health of the bay. These management plans will also provide a platform for addressing the requirements of the Chesapeake Bay TMDL for reducing TN, TP, and TSS in county waterways.
Fairfax County completed its first set of watershed plans in the 1970s. Land-use changes, population growth, new regulations, and advances in stormwater management drove the county’s initiative to update the plans to account for these changes and provide more targeted strategies for
addressing watershed health. These original plans provided for a 20-year horizon and were effectively outdated by the late 1990s. A second watershed planning effort began in 2003, and plans have since been completed for all 30 watersheds in the county. Each watershed plan includes a prioritized 25-year list of proposed capital improvement projects, in addition to nonstructural programs and projects. The plans promote the use of innovative practices in stormwater management such as stormwater facility retrofits, LID techniques, and stream restoration, among others. To maximize the effectiveness of these plans, community engagement and involvement from diverse interests were emphasized during the development process.
The watershed planning effort encompasses the following goals:
- Improve and maintain watershed functions, including
water quality, habitat, and hydrology.
- Protect human health, safety, and property by reducing stormwater impacts.
- Involve stakeholders in the protection, maintenance, and restoration of county watersheds.
To implement these goals, the watershed management planning process consists of the following steps:
- review and synthesis of previous studies and data
- public involvement to gain input, provide education, and build community support
- evaluation of current watershed conditions and stormwater modeling for present and ultimate development conditions
- development of nonstructural and structural watershed improvement projects
- development of preliminary cost estimates, cost/benefit analyses, and prioritization of capital projects
- adoption of the final watershed management plan by the board of supervisors
Since its inception in 2003, the watershed management planning process has been supported by the board of supervisors. In fiscal year 2006, the board dedicated $0.01 per $100 of assessed value from the county’s real estate tax revenue toward the overall stormwater management program. This revenue supported the development and implementation of watershed plans and eventually evolved into the adoption of a stormwater service district starting in fiscal year 2010. The board recently approved increasing the dedicated amount to a penny and a half for fiscal year 2011.
Atkins developed the plans for the Pohick Creek and Lower Occoquan River watersheds within Fairfax County. As part of this effort, Atkins prepared an evaluation for the county summarizing the estimated costs and potential benefits of conducting a variety of stormwater controls, including LID imperviousness retrofits and stream restoration, throughout the Pohick Creek watershed. This article summarizes the findings of this evaluation, which show that the unit costs for removal of sediment and nutrients through stream restoration are much less than those for imperviousness retrofits. Although stream restoration alone may not be able to achieve all of the reductions needed, it represents a cost-effective option, especially for sediment.
Figure 1. Map of Fairfax County showing location of the Pohick Creek watershed
|Figure 2. Distribution of imperviousness in the Pohick Creek watershed
Tracking the Pollutant Sources
The strategy to reduce pollutant loads to the Chesapeake Bay must consider the sources of the pollutants. Pohick Creek offers a useful case study. Located in the central southern portion of Fairfax County (Figure 1), Pohick Creek drains a 36.5-square-mile watershed, almost all of which is located within the county. According to the 2001 dataset from the National Land Cover Database, approximately 40% of the watershed is forested, while roughly 25% of the watershed comprises low-density residential development. Because the watershed is considered largely built out, significant new development is not expected. However, redevelopment of the existing housing stock often results in larger houses and greater imperviousness.
In terms of total area, the Pohick Creek watershed consists of 23,249 acres, of which nearly 24% is impervious area (Figure 2). Meanwhile, most of the impervious area is considered directly connected, meaning that it discharges directly to a receiving water body without treatment provided by a stormwater control. A large portion of the pervious land is likely to have reduced infiltration rates because of compaction typical of urban areas. This imperviousness distribution is common to many watersheds in the Chesapeake Bay.
Between 2001 and 2002, the county conducted a physical stream assessment to diagnose the condition of all of its streams. Figure 3 summarizes the results for the Pohick Creek watershed and shows that 85% of the stream segments are in fair, poor, or very poor physical condition. The study also indicated that all of the 269,671 feet of stream channel surveyed in the Pohick Creek watershed show moderate to severe erosion. An example of one of the severely eroding streams in the county in shown in Figure 4.
Modeling results indicate that Pohick Creek receives on an annual basis more than 8,000 tons of TSS, nearly two-thirds of which result from instream erosion (Table 1). Meanwhile, the creek is estimated to receive nearly 123,000 pounds per year of TN and more than 21,000 pounds per year of TP. For both nutrients, the majority originates from upland sources.
Nutrient pollutant load estimates from instream erosion are based on an approximate nutrient content per unit weight of soil. Typically, these nutrient contents are based on agricultural data. However, these estimates do not consider the pollutant load associated with vegetation that falls in the stream as a result of sloughing of the streambanks (Figure 5). Therefore, the total nutrient load from instream erosion may be greatly underestimated. Nonetheless, Table 1 shows that nutrient contributions from this source are not insignificant. On a subwatershed basis, the fraction of TN from instream erosion ranges from 1% to 12% of the total load, while TP ranges from 2% to 25%. As for TSS, the fraction associated with instream erosion on a subwatershed basis ranges from 22% to 78%.
|Figure 3. Physical condition of the streams in the Pohick Creek watershed
|Photo: Fairfax County Department of Public Works and Environmental Services
Figure 4. An example of severe streambank erosion in Fairfax County
Evaluating Approaches Head to Head
The magnitude and origin of these loads suggest that a strategy for controlling pollutants from instream sources must consider stream restoration throughout the watershed, particularly in reference to TSS. This statement does not imply that upland stormwater retrofits should not be implemented, but rather that watershed-wide stream restoration should have a more prominent role in management plans, specifically to abate sediment loads.
The improvements identified in the Pohick Creek watershed management plan comprise nearly 200 projects, including stream restoration, daylighting of piped streams, pond retrofits, outfall improvements, bioswales, bioretention areas, pervious pavement, street sweeping, obstruction and dumpsite removal, and buffer restorations. All of these projects were estimated to yield an 18% reduction in the annual TSS load, a 3% reduction in the annual TN load, and a 5% reduction in the annual TP load, at a cost of $94 million. However, further reductions will be needed to meet obligations related to the county’s MS4 permit and the Chesapeake Bay TMDL. The following sections analyze the extent to which these additional reductions may be obtained by means of imperviousness retrofits or stream restoration.
Imperviousness Retrofits. The term imperviousness retrofit is used here generically to refer to a suite of technologies that seek to mitigate the untoward effects associated with directly connected impervious areas. Typically, such areas were developed without any stormwater controls. Examples could include a parking lot retrofit in which bioretention facilities are installed to collect runoff, or the replacement of part of the lot’s existing impervious surface with pervious pavement. A common performance requirement is to design these facilities to capture and treat the runoff generated by the first inch of rainfall for every storm. Imperviousness retrofits typically are more difficult to install in roads and residential areas, primarily because of utility conflicts. Relocation of these utilities or working around them increases project costs.
In the Washington, D.C., metropolitan area, retrofitting 1 acre of imperviousness using bioretention costs between $125,000 and $200,000, according to estimates developed by Montgomery County, MD. Montgomery County used these values to estimate the cost of implementing its MS4 permit. On the basis of these estimates, the cost to retrofit 80% of the 4,514 acres of directly connected impervious area, or 3,611 acres, in the Pohick Creek watershed would range from $451 million to $722 million. Table 2 summarizes the performance and estimated costs associated with this type of restoration.
|Photo: Fairfax County Department of Public Works and Environmental Services
Figure 5. Streambank erosion deposits biomass in the stream channel.
The assumed average removals used in Table 2 are consistent with bioretention facilities that are designed for removing phosphorus and nitrogen. Typically, phosphorus is removed through sorption, and nitrogen is removed by means of denitrification in an anaerobic environment created by an internal water storage zone within the bioretention facility (McNett et al. 2010, Li and Davis 2009).
Stream Restoration. Stream restoration is assumed to provide a stable channel conforming to the existing hydrologic regime, such that the streambanks are not sloughing and the streambed is in a state of dynamic equilibrium involving water, sediment, and vegetation. Because such a channel is presumed to behave similarly to a natural stream, some net sediment transport will occur.
The effectiveness of stream restoration for pollutant removal has not been fully ascertained. However, EPA’s Chesapeake Bay Program issued a guidance document in 2005 indicating how it would credit jurisdictions for reducing pollutant loads to the bay and its tidal rivers by means of stream restoration. In the guidance, the Chesapeake Bay Program included removal-efficiency rates based on research involving one stream restoration project along Spring Branch in Baltimore County, MD (USEPA 2005). The research included monitoring data from one year before and three years after construction of the Spring Branch project. For every linear foot of stream restored, the EPA guidance stipulates removals of 2.55 pounds per year of TSS, 0.02 pound per year of TN, and 0.0035 pound per year of TP.
Although the available literature is unclear regarding the source of the influent concentration, it appears that the removal takes place through hyporheic processes acting on the streamflow as it passes through the restored reach. However, EPA’s removal estimates do not appear to account for any effects associated with preventing streambank erosion. In addition to TSS, such erosion delivers to the stream the phosphorus and nitrogen content in the soil. As an aside, it should be noted that not all nutrients are created equal. For example, a pound of TP from the effluent of a wastewater treatment plant is not the same as one from a storm drain discharging into a stream, or a pound in the form of soil and plant material from an eroding streambank. The reason is that they have very different fractions of bioavailable nutrients, which is what matters when it comes to impairments in receiving water bodies such as the Chesapeake Bay. A similar situation exists for nitrogen. However, this article uses TP and TN because these are the constituents addressed by the Chesapeake Bay TMDL.
A study is currently underway to improve the scientific understanding of the pollutant-removal benefits of stream restoration. The study—which involves Big Spring Run, near Lancaster PA—is being conducted by a consortium that includes EPA, the US Geological Survey, and several universities. Results from the study are not expected to be published for another two years. Nevertheless, under the presumption that a stable channel is attained, it follows that a significant portion of the pollutants will be eliminated.
In the mid-Atlantic states, the cost of stream restoration ranges from $200 to $800 per linear foot, with the unit cost tending to the lower end of the range for projects involving long reaches. Assuming that 80% of the total of 269,671 feet of eroding streams in the Pohick Creek watershed, or 215,737 feet, are restored, the cost would range between $43 million and $173 million. Table 3 summarizes the performance of stream restoration within the Pohick Creek watershed, using the removals assumed by EPA’s Chesapeake Bay Program. The removed loads identified in Table 3 would hardly justify conducting stream restoration projects for the sake of protecting the Chesapeake Bay. Compared with imperviousness retrofits, there is virtually no cost advantage.
However, a different picture emerges if stream restoration is assumed to reduce bank erosion. EPA’s model known as the Spreadsheet Tool for Estimating Pollutant Load (STEPL) was used in the watershed management plan to estimate the nutrient content of upland contributions to sediment loads. For the Pohick Creek watershed, the values used are 1.60 pounds of TN and 0.62 pound of TP per ton of soils. A nutrient correction factor was applied to account for the soil type, because nutrients tend to attach more to small particles. Sandy and silty soils are predominant in the watershed. For sandy soils, the factor is 0.85, while for silty soils the factor is 1.0. These nutrient contents and factors were used to estimate the nutrient content in sediment caused in streambank erosion.
Stream restoration does not eliminate sediment transport completely. Even pristine streams deliver a sediment load to receiving water bodies downstream. In addition, localized bank failures may occur, representing a source of sediments. The analysis presented in Table 4 assumes conservatively that stream restoration reduces streambank erosion and the associated sediment and nutrient inputs by 70%.
If this rationale holds, stream restoration reduces pollutant loads both by preventing streambank erosion and treating the water in the stream channel by the processes documented in the Spring Branch study. Therefore, the actual removal rates of stream restoration should be better represented by the sum of the removals in the previous two tables, which is summarized in Table 5.
|Figure 6. Comparison of the range of unit costs for pollutant removal for imperviousness retrofits and stream restoration
Figure 6 compares the unit costs for imperviousness retrofits and stream restoration, suggesting that the latter approach should take precedence as far as maximizing the impact of watershed restoration funds. As an example that illustrates this cost efficiency, the unit costs in Table 5 were used to evaluate a scenario that treats 80% of the total directly connected impervious area in the Pohick Creek watershed. As mentioned earlier, the cost of efforts related to imperviousness retrofits ranges between $451 million and $722 million. Table 2 indicates that imperviousness retrofits can reduce total sediment loads by 2,019 tons per year, which is 24% of the total load.
A comparison with stream restoration can be made by estimating removals per linear foot using the values in Table 5. For example, the removal rate for TSS is equal to the total removed load of 3,231 tons per year divided by the restored length of 215,737 feet, which equals 0.015 ton per year per foot. Therefore, to compare with imperviousness retrofits, a removal of 2,019 tons per year can be accomplished by restoring 134,810 feet of stream—that is, 50% of the eroding stream channels—at a cost of between $27 million and $108 million.
Beyond the projects identified in the 25-year project implementation program for the Pohick Creek watershed management plan, imperviousness retrofits and stream restoration are the only options left to further reduce the input of sediment and nutrients from the watershed to the Chesapeake Bay. The analysis above suggests that stream restoration can play a major role in watershed improvements to meet the requirements of the Chesapeake Bay TMDL. The unit costs for removing sediment and nutrients are much less than those of imperviousness retrofits. Although stream restoration alone may not be able to achieve all of the reductions needed, the practice represents a cost-effective option, especially for sediment. Nutrient removal is not as cost effective, but even if it is a smaller fraction, the unit cost of removal is much lower than that for imperviousness retrofits. Furthermore, stream restoration can confer additional benefits, such as aquatic ecosystem restoration and reconnection of streams with their floodplains.
Based on this analysis, it appears that local governments seeking to effect the necessary watershed improvements required by the Chesapeake Bay TMDL may want to consider a greater role for stream restoration on a watershed-wide basis within significantly urbanized areas. Imperviousness retrofits based on LID techniques ideally should be implemented in connection with redevelopment projects to reduce the cost of implementation. Greenfield development should follow LID principles. Finally, it should also be emphasized that there is great need for applied research to increase the body of knowledge on the benefits of stream restoration.
Author's Bio: Shannon Curtis is a senior ecologist for the Fairfax County Department of Public Works and Environmental Services, Stormwater Planning Division, Watershed Planning and Assessment Branch.
Author's Bio: Daniel Medina, Ph.D., P.E., D.WRE, CFM, is an associate vice president in the Beltsville, MD, office of Atkins North America Inc. (formerly PBS&J).