Trinkus Engineering Review Feb 3. 2013 | Comments (0)
September 3, 2012
Ms. Helen Garten
4 River Lane
Westport, Connecticut 06880
RE: 5 River Lane
Proposed three lot subdivision
At your request, I have performed a third party engineering review of the above referenced project. This review is based upon the following plans and documents which were submitted to the Town of Westport Flood and Erosion Control Board. Copies of the maps and reports were obtained from the Town of Westport.
1. Subdivision Map by Richard W. Plain, LS (March 12, 2012)
2. Septic Tank System Plan by HKA Associates, Sheet 2 (8/14/12)
3. Erosion & Sedimentation Plan by HKA Associates, Sheet 3 (7/10/12)
4. Details by HKA Associates, Sheet 4 (May 14, 2012)
5. Predevelopment Watershed by HKA Associates (7/17/12)
6. Development Impact on Watershed by HKA Associates (7/17/12)
7. Zero Runoff Computations, SCS Method by HKA Associates (August 6, 2012)
At the current time, there is a single family residence with pool found on the site. The residence is served by a driveway from River Lane. It is proposed to create two new building lots in addition to the existing residence. One of these lots will front on Westport Road and the other will front on River Lane.
The focus of this engineering review will be on stormwater management and erosion & sediment control, compliance with Town of Westport standards as well as the 2002 CT DEP Guideline for Soil Erosion and Sediment Control as well as the 2004 CT DEP Storm Water Quality Manual. I have the following comments for your information.
- Contour lines are only label outside the property boundaries making very difficult to determine the existing elevations in the vicinity of proposed improvements.
- Due to the scale of the plans, it is very difficult to read and understand many of the proposed design features, in particular, those related to the stormwater management system.
- The grading shown on the northwest side of the house on Parcel A does not conform to the CT Building Code. As shown the grade pitches toward the house which is not in compliance with this requirement.
- Retaining walls are proposed for the driveway on Parcel A and appear to be approximately 8’ in height. Based upon the provided details, it is not clear how shallow groundwater will reach the “Bagged Stones” and enter the weep hole. Since the weep holes will discharge onto the driveway surface, winter icing will likely occur and create a safety issue. This is a concern on Parcel A and Parcel B.
- Based upon the information shown on Sheet 3, it is not possible to determine the heights of all of the retaining walls proposed for this project.
- While I am not a structural engineer, it does not appear that the footings for the proposed retaining wall, especially those greater than 6’ in height will be sufficient to counter the weight of the soil above the wall. It is recommended that a qualified structural engineer review the designs of the retaining walls to ensure that they are adequate for the anticipated soil loading.
- The two proposed driveways do not appear to provide adequate access for emergency vehicles due to the horizontal alignment of the driveway nor do they provide sufficient room to maneuver these vehicles at the house site itself due to the large extent of retaining walls.
- The calculation of the Water Quality Volume (per 2004 Manual) is not correct and is not in compliance with the 2004 DEP Manual.
- There is no calculation of the Groundwater Recharge Volume (per 2004 Manual).\
- As this site is appears to be within 500’ of a tidal wetland, the Runoff Capture Volume must be provided and is not under the current proposal.
- Underground galleries are proposed to contain the runoff from the newly created impervious cover on each lot. On Parcel A, 294 lf (scaled) of 2’ x 4’ galleries are proposed in an area north of the existing wetland area. There is a discrepancy with the stormwater report which calls out 304 lf of galleries for this lot. Which was is correct?
- There is only one test pit in the vicinity of the proposed gallery system (north end). As the gallery rows extend to the south, the soil conditions are not known at this location. As test pit, A4 calls out mottling at 42”, I have a concern whether the required vertical separation to shallow groundwater of 3’ from bottom of the gallery from the 2004 Stormwater Quality Manual is met.
- Based upon the close proximity of the delineated wetland boundary and lack of adequate soil tests, it is questionable whether sufficient suitable soil exists for the infiltration galleries.
- Additionally, portions of the lower three rows will be located above grade, yet no finished contours are proposed to reflect this condition. What type of fill material will be placed here and how will this affect the functionality of this gallery system?
- No percolation tests were performed in the area of the gallery system on Parcel A. As the system was modeled assuming an exfiltration rate of 3” per hour, what is this value based upon?
- While percolation tests are commonly used to obtain infiltration rate, they are not the recommended test by the 2004 DEP Manual. Percolation tests measure both horizontal and vertical movement of water from the bottom of the hole. For infiltration systems, only the vertical infiltration rate is used, so a percolation test may over-estimate the actual infiltration rate. Double ring infiltration tests are the appropriate methodology.
- On Parcel C, there are two sets of underground galleries. The first set containing 5 rows at 24’ each (120’ in total) is located under the driveway and adjacent slope area. Test pit A6, shown just south of the driveway shows mottling at 48” and the bottom of the galleries are to be set at elevation 84’. This will place the bottom of the galleries either 2’ below existing grade or 4’ below existing grade. In either case, the required 3’ vertical separation from the bottom of the gallery to shallow groundwater will not be provided.
- There is no test pit at the northern end of the galleries, nor are there any percolation tests in the vicinity of the driveway gallery system.
- An additional 128 lf (scaled) of galleries are proposed just below the existing residence. There is only one test pit located off the southeastern end of the galleries with no percolation test in the area. As this system is located in an existing of prior man-made improvements (pool and patio), the singular test pit is not adequate).
- As the galleries are located across approximately 6’ of slope, some of these galleries will be deep under the soil, while other portions will be located out of the ground. Similar to Parcel A, fill will be required for some of the galleries, but there is no information as to the type and depth of fill for this gallery system.
- For Parcel C, two gallery systems are proposed. The first is located in the eastern portion of the lot containing 110 lf of gallery. Based upon the elevations provided, portions of the entire system will be located above existing grade. No finish contours are provided nor are there any fill specifications provided.
- There is inadequate soil testing in the vicinity of the eastern gallery system. One test pit is located above the proposed gallery system, but there are no test pits below the system. There are also no percolation tests in the vicinity of this gallery system.
- The second gallery system is located under the proposed driveway and contains 110 galleries (per plan notation). There are no soil tests in the vicinity of this gallery system, so it is impossible to ascertain whether this system is in compliance with the 2004 DEP Manual and will function at all.
- Additionally, this system under the driveway will be located in fill which increases the concern whether it will function appropriately.
- Identical to Item #8 above, an assumed exfiltration rate is used for the modeling of the system with no test results to support this rate.
- In the routing analysis for the two gallery systems on Parcel C, the two systems are combined. This is an invalid analysis as each system handles runoff from a separate watershed area and it is incorrect to treat them as a singular system.
- On Parcel B, the same situation exists for the modeling of the two separate stormwater gallery system as was done on Parcel C.
- There are no overflow provisions on any of the gallery systems to pass storm events greater than the 25-yr design storm. If the design engineer is assuming that the gallery systems will fully infiltrate these larger storm events, no evidence has been provided to document this position.
Water Quality Issues:
- It has not been demonstrated that the stormwater management system consisting of standard catch basins with 24” deep sumps and the infiltration galleries will provide adequate water quality treatment to reduce pollutant loads from in non-point source runoff. Catch basins with 24” sumps are considered a “secondary practice” according to the DEP 2004 Manual as they are not very effective at addressing water quality issues.
- It has not been demonstrated that the required water quality volume is being “contained and treated” within the stormwater management treatment system. It is not enough to simply provide storage for the water quality volume and assume that adequate treatment of the runoff will be provided.
- While the CT DEEP only requires that Total Suspended Solids (TSS) in post-development runoff is reduced by 80%, there are other common pollutants in non-point source runoff which should be evaluated. In this particular case, nutrients, both phosphorous and nitrogen are a concern due to the presence of the tidal section of the Saugatuck River to the east and downgradient from the subdivision parcel. Reduction of nitrogen is especially important in a tidal environment as excess nitrogen can kill tidal wetland grasses in marsh areas.
- Metals and hydrocarbons are also not addressed by the current stormwater management system.
Erosion and Sediment Control:
- The erosion narrative is not in compliance with the form and content of the 2002 DEP Guidelines.
- There are no soil stockpiles shown on any of the three lots.
- A perimeter siltation fence barrier is shown on each of the three building. Portions of the proposed siltation fence are shown being installed perpendicular to the contours which is inappropriate.
- The legend on Sheet 3 states that the erosion control line is “silt fence and row of hay bales”. How are these two barriers to be installed? There is no detail for the hay bale installation.
- While a detail is provided for the anti-tracking pad on Sheet 3, no construction entrances are shown on the site plan.
Based upon a thorough review of the submitted plans and reports, the current plans for this property are not in compliance with the 2004 CT DEP Storm Water Quality Manual and the 2002 CT DEP Guidelines for Soil Erosion and Sediment Control. While the plan purports to be in compliance with the Town of Westport stormwater requirements, there are issues as noted above which do lead to a conclusion that the proposed stormwater management system is not in compliance with the Westport requirements.
A copy of my professional qualifications is attached for your information.
Please contact me if you have any questions concerning the information found in this letter.
Very Truly Yours,
Trinkaus Engineering, LLC
Steven D. Trinkaus, PE, CPESC, CPSWQ