Viii lid technology: case studies and watershed restoration
Figure 7 – Livestock in stream and riparian area (NCD)
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- Stormwater Basin Retrofits
- New Forebay Basin Retrofit
- Ex. Stream Discharge Figure 8 – Small Basin Retrofit (Trinkaus Engineering, LLC)
- Pollutant Renovation Analysis
- 2’ Micro-pool 4’ Micro-pool High Marsh Area Low Marsh Area Low Flow Path
- Figure 9 – Large Basin Retrofit (Trinkaus Engineering, LLC) Table 2 – Results of Simple Method and Treatment System Evaluation
- Table 3 – Results of Simple Method and Treatment System Evaluation
- 257.3 20.4 205.6 42.1
Figure 7 – Livestock in stream and riparian area (NCD) After evaluating all of the potential sources of pollutant loading in the Northfield Brook Watershed, it was determined that the failing stormwater basins in the Highwood Estates development were the largest single source of increased sediment & pollutant loading to Northfield Lake. The other potential sources were not ignored in this process. The unstable stream crossing was replaced with a bridge. The owners of the agricultural uses, particularly those with livestock uses were provided recommendations to prevent livestock from reaching the stream and riparian areas as well as funding sources from USDA. Stormwater Basin Retrofits NCD applied for and received 319 Funds to retain a consultant to design the retrofits for Highwood Estates basins. While the town owned the land surrounding both basins, the focus of the retrofits was to work within the existing footprint of the basins to affect a practical solution, yet minimize the potential cost of implementing the retrofit for the Town of Thomaston. The first step was to inspect the basins in the field to observe the conditions in person. The smaller basin was completely non-functional with any runoff quickly entering and leaving the basin, neither detention or water quality treatment was being provided. A survey with topographic information was obtained to provide the necessary base information for design purposes. After the survey was done, it was time to analyze the contributing watershed areas in order to design the retrofits. Small Basin Existing Conditions. The small basin has a 24.5 acre watershed area consisting of residential roads and ½ acre building lots. The basin consists of a small, elliptical footprint with the outlet structure located at the north end of the basin. Runoff is directed to the northeast corner of the basin by a riprap swale which conveys the runoff from the road drainage system. Due to the proximity of the inlet and outlet to each other, the runoff has cut a direct path between the two points, resulting in most of the basin not being used. The peak rate of runoff for a 2-year storm was calculated by the HydroCAD model. Approximately 30.09 cfs is directed to the basin during this storm event. In addition, the Water Quality Volume (WQV) as found in the CT DEP 2004 Stormwater Quality Manual (Manual) was determined for the watershed. A total of 35,278 cubic feet of storage volume would need to be provided for the small basin to achieve this goal. Retrofit Design Due to site constraints, the retrofit options were limited for this basin. First, a well-defined depressed forebay was created above the existing basin. The forebay provides 2,568 cubic feet of storage volume (7.3% of the WQV, the goal is to have 10%). The riprap swale was redirected to direct runoff into the east end of the forebay with the outlet being located at the western end. The forebay is slightly over four feet in depth. This is important as to minimize the resuspension of fine sediments in the forebay during subsequent runoff events. The basin itself was excavated to provide a single, deep pool feature six feet in depth. A vegetated, aquatic shelf was created along the perimeter of the deep pool. The single outlet pipe was replaced with a staged orifice outlet design to provide a slight reduction of the peak rate of runoff in the basin above the permanent pool. The peak rate of runoff for the 2-year event will be reduced from 30.09 cfs to 28.87 cfs. The regraded basin and new forebay provide a total of 5,278 cubic feet of fixed volume for water quality purposes. This is approximately 15% of the calculated WQV, but is the maximum available based upon site limitations. The features of the basin retrofit are shown in Figure 8. Large Basin Existing Conditions The large basin has a 28.32 acre watershed area consisting of residential roads and ½ acre building lots. The basin is approximately circular in shape. The inlet swale enters the basin in the northeast portion, while the outlet structure is located at the southeastern end. Similar to the small basin, runoff short circuits the storage area of the basin and makes a quick line in and out. The outlet control structure consists of a square 18” x 18” opening which does not provide any measure of rate reduction. The peak rate of runoff for a 2-year storm was calculated. Approximately 30.66 cfs is directed to the basin during this storm event. In addition, the Water Quality Volume (CT DEP 2004 Stormwater Quality Manual) was determined for the watershed. A total of 41,810 cubic feet of storage volume would need to be provided for the large basin. 
New Forebay Basin Retrofit  
Redirected Riprap Swale 
Ex. Stream Discharge Figure 8 – Small Basin Retrofit (Trinkaus Engineering, LLC) Retrofit Design There is more space available for this basin retrofit. A large, separate forebay was constructed above and north of the existing basin. This forebay is six feet in depth and provides a fixed storage volume of 5,285 cubic feet. This is approximately 12.6% of the required WQV which is more than the suggested 10% of the WQV for a forebay under the Manual. The flow from the existing riprap swale was directed into the forebay at the east, with the outlet from the forebay being on the western end. A new riprap swale will direct runoff from the forebay to the northwest corner of the basin. The larger basin size allowed for a more significant retrofit to be implemented compared to the smaller basin. A two (2) foot micro-pool was placed at the inlet of the new riprap swale. A second, deeper micro-pool was created just before the existing outlet control structure. A low flow path was created from the shallow micro-pool to the deeper one in a circuitous path. The majority of the basin bottom will be planted to create a shallow marsh environment. Two areas will be raised by 6” to create high marsh areas which will encourage a low, slow flow path for runoff within the basin as well as maximizing the contact time between stormwater and the vegetation. A total of 17,996 cubic feet of fixed volume is provided between the forebay and permanent pool in the basin, which is approximately 43% of the required WQV. The outlet structure was modified to create a staged orifice system. Due to the size of this basin, the 2-year peak rate of runoff will be reduced from 30.66 cfs to 5.00 cfs. This is a substantial reduction that will prevent the further erosion of the existing stream channel between the basin and Northfield Lake. The redesigned basin is shown in Figure 9. Pollutant Renovation Analysis The Simple Method was used to calculate the estimated pollutant loads from the contributing watershed area for each basin on an annual basis. The effectiveness of the stormwater management system for each basin was evaluated for removal of TSS, TP, TN, Zn, TPH and DIN. Removal efficiencies for the various treatment systems were taken from both University of New Hampshire Stormwater Center and the ASCE BMP Database. 
New Forebay 
2’ Micro-pool 4’ Micro-pool High Marsh Area Low Marsh Area Low Flow Path  
Figure 9 – Large Basin Retrofit (Trinkaus Engineering, LLC) Table 2 – Results of Simple Method and Treatment System Evaluation
It can be seen by the modeling results that sediment loads will be substantially reduced by these basin retrofits and thus bacteria concentrations will also be reduced due to their affinity to attach to sediment particles. Table 3 – Results of Simple Method and Treatment System Evaluation
Implementation As of the spring of 2012, the Town of Thomaston is soliciting bids from contractors to construct the basin retrofits by September. The goal is to have the retrofits completed prior to fall of 2012. Conclusion The retrofits of these two storm water basins will provide a measurable improvement to stormwater quality which will ultimately reach Northfield Lake. In addition, the cost of implementing these retrofits by the Town of Thomaston was minimized by working with the natural conditions to the maximum extent possible. The approaches and concepts used in these retrofits can easily apply to other stormwater basins to increase the benefits of old standard detention basins. References CT DEP (2004); “Stormwater Quality Manual” Hayden, Sean (June 2009); “Northfield Brook in Thomaston and Northfield, Connecticut Track Down Survey Report” Houle, James and Roseen, Robert (2009); “University of New Hampshire Stormwater Center 2009 Annual Report” Kitchell, A., Schueler, T. (2005); “Urban Subwatershed Restoration Manual No. 10: Unified Stream Assessment: A User’s Manual (Version 2.0) 1 This paper only concerns itself with conventional pollutants coming from the METRO system. The lake is also a listed Super-fund site with many sub-sites. These are being remediated under a different program. 2 The RTF might or might not meet the EPA CSO policy requirement to be at least as effective as primary sewage treatment. 3 No attempt has been made to bring all of the expenditures and estimated expenditures up to 2011 dollars. 4 This effort was also enhanced by US EPA’s giving more scrutiny under the National Environmental Protection Act before additional federal funds could be released for RTFs. 5 Ms. Mahoney has subsequently been re-elected, running without opposition. 6 The new plan would be a change from the previous court approved ACJ and therefore changes would need to be brought before the Court for approval. 7 Available at http://www.onondagalake.org/docs/ACJSTIPsigned16November2009.pdf. Detailed descriptions, technical plans, and related materials can be found on numerous websites, and all documents may be viewed, by appointment, at the ASLF office in Syracuse, NY. 8 See the Save the Rain website, http://www.savetherain.us. 9 As feasible state and federal owned parcels are also being evaluated for GI. 10 The Green Improvement Fund (GIF) program has been very successful. Details of it and the application can be seen on the website. See footnote 9. 11 Available at http://www.nrdc.org/water/pollution/rooftopsii/ 12 Most governmental units rely on consultants to do all the heavy lifting. These consultants are motivated by profit and these usually lead them to suggest similar solutions to all problems regardless of the nuanced local situations. These solutions are often relatively easy to design and construct with little consideration for long term operating costs, ecosystems, or public support. Regulators like the tried and true methods and so permitting and related issues are simpler as well. 13 Large scale projects, in theory, need to get full environmental review under either the National Environmental Policy Act or state equivalents depending on who is funding the project. Often, however, under the guise that pollution control is a benefit to the environment, little or no review and looking at alternatives is performed. 14 Federal, state, and local procurement policies are often very rigid and out of date regarding flexibility needed for many small projects. Higher cost thresholds and other changes should be considered in these procedures. Related issues also arise as there is a desire to employ local people in “green jobs programs,” but these well intended practices often cannot be met. 15 The paucity of new construction negates the eventual effectiveness in the MS4 program to alleviate stormwater issues. Whenever new or renovation construction trips the thresholds of that program, it is implemented and enforced here as well, we hope, across the nation. 16 See previous reference to the GIF program. In the future, the County will gradually decrease this incentive program from present 100% reimbursement to lower and lower amounts as the program matures and we are closer to our final 2018 goals. 17 In some cases, engineers, landscape design professionals, and even urban foresters still either disregard the use of native plants or are actually hostile. The many arguments pro and con will not be further spelled out here, but the need to consider urban ecological issues and even how the City fits within a regional ecosystem is very important and must be considered. 18 For more information, please visit http://savetherain.us/suburban-gip-announcement/ Download 3.33 Mb. Share with your friends:
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