Friends of Alewife Reservation (FAR)
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Mystic River Watershed Association comments on NOI
Footnotes in the original have been inserted as parenthetical comments. MYSTIC RIVER WATERSHED ASSOCIATION 20 ACADEMY STREET, SUITE 203 ARLINGTON, MA 02476 PHONE: 781-316-3438 o FAX: 781-641-2103 o WEBSITE: WWW.MYSTICRIVER.ORG February 9, 2004 Cambridge Conservation Commission Re: NOI, CambridgePark Area Drainage Project Dear Conservation Commission members: The Mystic River Watershed Association (MyRWA) submits the following comments on the city of Cambridge Department of Public Works Notice of Intent for the CambridgePark Area Drainage Project, comprising a stormwater wetland and compensatory wetland in Little River. MyRWA is a grassroots organization dedicated to the protection and restoration of the Mystic River, its tributaries, and the related natural resources throughout the watershed's 21 communities. Little River, where the proposed project is located, is a tributary to this system. This letter first makes some overall comments on the context in which this project takes place, followed by some specific comments. MyRWA has been working actively with the City of Cambridge and the Towns of Belmont and Arlington to address the serious flooding and water quality problems of the Alewife Brook area. We applaud the initiatives taken by the City of Cambridge to improve the water quality by making major investments in sewer separation and other means of reducing CSOs in the area. The construction of a stormwater wetland has the potential to enhance the water resources and wildlife values of the Alewife Reservation and downstream areas. Because the area is already experiencing flooding problems, any proposal which has the potential to increase flooding must be thoroughly evaluated to ensure that it will not have a significant impact, as any unanticipated increases will have serious repercussions. Current efforts by the Tricommunity Flooding Workgroup to understand the causes of flooding, along with research by Steve Kaiser and other area residents, suggest that:
While more work is needed to develop solutions to the severe flooding problems, it is already evident that any additional contribution to stormwater flows cannot be tolerated in this region. The same is true of efforts to improve water quality. CSO reduction is badly needed. However, since it this project will directly impact a rare public urban wild with important wildlife values that is under the stewardship of the Department of Conservation and Recreation, and is a direct tributary to the Mystic River system, it also deserves particularly careful evaluation. This project must be analyzed in the context of other proposals for the area that may have adverse impacts on flooding and water quality. These include the proposed residential development of the Belmont Uplands, which will abut the proposed stormwater wetland. Developments for other nearby sites, such as the Martignetti property, have yet to be proposed in detail. However, the potential combined impact of development in the Alewife area is significant. An attachment to this letter lists the projects we are aware of at this time. Design of the proposed stormwater system should anticipate these developments. In addition, we hope that Cambridge will join with other municipalities to consider the interrelated water quality and flooding impacts of all of these projects in combination, and consistently incorporate these concerns in their development and land use planning. This letter provides comments and suggestions that we hope will help the Conservation Commission in its evaluation of this project. Representatives from MyRWA have not be able to attend all of the Conservation Commission meetings where this project has been discussed, and we are aware that some of our concerns may have already been addressed. To the extent that the issues discussed in this letter have not been adequately addressed, we urge the Commission to ask for additional information. This will ensure a thorough and critical assessment of the soundness of the project's design and prospects for ameliorating the water quality and flooding problems in the Alewife area. As with all other projects in this area, we urge Cambridge to apply higher standards for mitigation of environmental impacts than it might require less sensitive areas. Concerns and Recommendations: Our comments relate to the complexity of the proposed project, the modeling used and the assumptions used in that modeling upon which the success of the project rests. While the constructed wetland technology holds much promise, problems in design could be very difficult and expensive to remedy. Hence, it is particularly important that the project design be based on a solid technical foundation. Our specific concerns and recommendations are as follows: Consistency with the Massachusetts Guidelines The Massachusetts Inland Wetland Replication Guidelines {Massachusetts Executive Office of Environmental Affairs, Massachusetts Inland WetGuidance No. BRP/DWM/WetG02-2; Effective Date: March 1, 2002) are not referenced in the NOI. We recommend that the applicant discuss the consistency of the proposed project with those guidelines (attached). Complexity of the Hydraulic and Hydrologic Analyses The NOI demonstrates that the drainage from the area (including CAM004) that the stormwater wetland will be receiving is quite complex. The wetland will store 10.3 acre feet of water. An advanced modeling software, Hydroworks, was used to predict storage and peak flows that need to be attenuated by the proposed wetland. The model evaluated 270 nodes and links. The model for the CAM004 drainage system was calibrated using flow and rainfall monitoring data from November 19, 2001 through January 2, 2002 and April 11, 2002 through May 23, 2002. The analysis resulted in modeled depths of flow that were generally lower, and modeled velocities that were higher, than actual observations. Discussions with the field crews revealed that sediment in the sewers is a problem. Adjustments were therefore made in the model. As a result, the monitoring events and model calibration used to design the wetland reflect existing conditions. MyRWA applauds the City's consultants for making this comparison to validate the modeling. However, we are concerned that, as a result of City efforts to clean the system (cleaning the sediment traps, more sweep cleaning etc.), a larger volume of water could flow into the proposed stormwater wetland than predicted, as the drainage system adjusts to new flow patterns. In addition, the anticipated attenuation of pollutants based on the 1-month storm should also receive careful evaluation, particularly in light of the winter conditions that may prevent attenuation from occurring at the time when it is most needed. This could have a significant negative impact on pollutant loads downstream. More generally, due to the complexity of the modeling, overall cost of this project, and documented flooding problems in this watershed, MyRWA recommends that a quality assurance and quality control review be performed on the Hydroworks model by an outside firm or consultant. Hydrology of the Stormwater Wetland: Water Budget The water budget is the main hydrological procedure used to evaluate wetland designs. The water budget was not explicitly evaluated for stormwater wetland or the compensatory wetland, however. MyRWA recommends that an outside consultant review the water budget and hydrologic and hydraulic analyses, if this has not already been done, to verify the many assumptions and calculations on which the analyses are based. Providing explicit water budget calculations, as spelled out in "Calculating a Wetland Water Budget" (Rolband,M.S. and N. Fomchenko. Calculating a Wetland Water Budget. Wetland Studies and Solutions Inc.) (attached), would be very useful in evaluating this project. In particular, the Hydrologic and Hydraulic Analysis (Section 2) does not present the following components of the stormwater wetland water budget:
We recommend a sensitivity analysis on the water budget for the stormwater wetland, particularly on the amount and velocity of runoff or inflow into the stormwater wetland after sediment is removed from CAM004 system (CAM 004 Area Hydrologic and Hydraulic Report Final Draft April 2003 City of Cambridge Department of Public Works). The stormwater wetland may be wetter than expected. This outcome has been observed with some of the compensatory wetlands that were designed for Route 3 North Connector project conducted by MassHighway. One of the created shrub, scrub and shallow aquatic-bed wetlands has become an open water habitat for waterfowl, which was not intended and not part of original design. The assumptions about rainfall levels are also of concern. The NOI does not state which NRCS Type III 24- hour rainfall distribution was used (either TR-20 or TP-40). Currently, the Massachusetts DEP requires the use of rainfall data published in the National Weather Bureau's Technical Paper 40 (TP-40). Recent research indicates that the precipitation amounts for various frequency storms are greater than those reported in TP-40. The document was published in 1961, and therefore does not reflect the last 40 years of precipitation data. A more recent study of precipitation patterns was published by the Cornell University Northeast Regional Climate Center in 1993, in "Atlas of Precipitation Extremes for the Northeastern United States and Southeastern Canada." In eastern Massachusetts, the more recent Cornell Study precipitation values show up to a 29 percent increase in rainfall amounts for each frequency. In general, the greatest disparity between values occurs with the more extreme storms, which are those most likely to cause flooding. Although the use of TP-40 is specified by the Massachusetts Wetlands Protection Act, more stringent design storms may be used under a local bylaw or ordinance. For example, the Town of Holliston requires the use of the Cornell data in hydrologic and hydraulic calculations (Town of Holliston, Wetlands Administration Bylaw Regulations, Approved September 4, 2001. http://www.townofholliston.us/ConCom/WABR.html). Regarding Existing Conditions (Section 3.2), Massachusetts is currently updating the FEMA Maps. Have the most up-to-date FEMA data been used in designing the stormwater wetland? If recent flood level data are not available, MyRWA recommends a sensitivity analysis based on data compiled by Steve Kaiser. Finally, Section 3.2.3 (page 3-12) notes that the depth of fill ranges from 2 -20 feet. Will the fill be removed and replaced with clean floodplain soils to construct the wetland? Fill may contain hazardous substances that may leach from fill when placed in an anaerobic wetland soils environment. Invasive and Noxious Species Control of invasive species needs more attention in the NOI discussion of Consistency with the Alewife Brook Master Plan (Section 3.1.3)). ACOE states that "projects should avoid introducing or increasing the risk of invasion by unwanted plants or animals (such as zebra mussels). Soils disturbed by projects are very susceptible to invasion by undesirable species. Be particularly alert to the risk of invasion on exposed mineral soils." It is our understanding that soils will be re-used for the construction of the wetland. Japanese knotweed was identified as a "dominant plant" in the project area and it grows in moist wet areas. Re-used soils will contain a seed base of invasive plants. The plan should identify regulatory and ecological constraints that may influence the design of a plan to control invasive plants and animals by biological, mechanical, or chemical measures. For example, if the state requires a permit for use of herbicide, this may constrain attempts to control an invasive plant species. If there are no constraints, the NOI should note that. The operations and maintenance plan should describe a strategy to recognize, respond to, and control any invasion of the mitigation site by Common Reed (Phragmites australis) and Purple Loosestrife (Lythrum salicaria), as well as Japanese knotweed. Any other species (Rhamnus spp., Elaeagnus spp., Rosa multiflora, etc.) identified as a problem at the site should also have a control plan. Controls might be mechanical (pulling, mowing, or excavating on-site), chemical (herbicides), or biological (planting fast-growing trees and shrubs for shading or releasing herbivorous insects). Operations and Maintenance The constructed stormwater wetland technology has not had a long track record in this region or climatic regime. Where it has been used, it has met with mixed results. The document "Performance of Stormwater Ponds and Wetlands in Winter" (Technical Note 16, Watershed Protection Techniques 1(2): 64-68.) (attached) describes some of the problems associated with stormwater quality and quantity control by wetlands during the winter months, when they ice over and are biologically inactive at a time when the highest flows and pollutant loads due to snowmelt are likely to occur. We recommend that the proponent provide examples of the performance of stormwater wetlands created in a northern urban setting like Cambridge, Massachusetts. O&M of constructed stormwater wetlands may be costly. The National Research Council's Wetland Mitigation Guidelines advises avoiding over-engineered structures in designing a wetland. They suggest the following: Design the system for minimal maintenance. Set initial conditions and let the system develop. Natural systems should be planned to accommodate biological systems. The system of plants, animals, microbes, substrate, and water flows should be developed for self-maintenance and self-design. Whenever possible, avoid manipulating wetland processes using approaches that require continual maintenance. Avoid hydraulic control structures and other engineered structures that are vulnerable to chronic failure and require maintenance and replacement. If necessary to design in structures, such as to prevent erosion until the wetland has developed soil stability, do so using natural features, such as large woody debris. The use of over-engineered structures and maintenance intensive plans for mitigation must include a comprehensive remedial plan and financial assurances along with a non-wasting endowment to insure that proper maintenance occurs. The NOI describes three engineering structures as part of the proposed design -- a box culvert, a sediment forebay, and an emergency spillway and weir at the outflow. The O&M plans should include all steps needed to maintain the performance of these structures. It is important that the O&M policies and procedures that will be the subject of an MOU between the DPW and DCR be provided in full to the Conservation Commission, to ensure that all necessary measures will be taken, in perpetuity, by the DPW or its consultants to maintain the design performance of the wetland and drainage system. Specifically, MyRWA recommends that the MOU include monitoring and actions to:
Conclusion The very substantial cost of this project and its potential benefits and hazards demand investment in the early stages to ensure that it rests on a solid technical foundation. MyRWA urges that the Conservation Commission takes this opportunity to ensure that a high level of technical review and quality assurance is provided. We support the extensive work Cambridge is doing to address the difficult water quality and flooding problems that plague the Alewife area, and we look forward to the benefits that will result from this and future CSO separation projects. Thank you for the opportunity to comment on this important issue. Please feel free to contact me with any questions or comments. Sincerely, Nancy Hammett PHONE: 781-316-3438 FAX: 781-641-2103 nbsp; WEBSITE: WWW.MYSTICRIVER.ORG |