Loading...
HomeMy WebLinkAbout06-29-09 Gay Hill Road Preliminary CommentsPreliminary Engineering Review Comments Burlake Rd. LLC (Previously Gay Hill Rd. Subdivision) June 29, 2009 General Plan Format / Content 1. The plans have inconsistent tittles blocks. Cover sheet and some drawings are titled Gay Hill Road Subdivision. Others are titled Burlake Rd LLC. The Gay Hill Road sheets appear to be from the previous application and some details are not consistent with current design. Some examples are noted in the comments below. 2. Plans should show existing and proposed tree lines and clearing limits. 3. Easements should be clearly identified on the grading and drainage plans. Also, the plans are not consistent as to whether easements are required/proposed over open space areas. 4. All areas of riprap shown on the plan should be drawn to scale consistent with the dimensions provided on the construction details. Of specific concern are the areas of riprap which appear to extend out of the designated drainage easements and in at least one case into the wetland. 5. Many of the lots have building drains (presumably footing drains) directed into proposed catch basins. It should be confirmed that the drains have adequate slope to drain and that they tie into the catch basin above the maximum headwater elevation for the 10 year design storm. If this a concern with such connections, backflow preventers can be incorporated to avoid any potential impact to the house from the road drainage system in a flooding condition. 6. We have not yet identified all of the test hole and standpipe locations on the plan and would want to review this information more thoroughly. A note in the table as to the location (Lot #), or a separate location map would be helpful. We did note that it appears there were no test holes or standpipes in the area of the pond forebay near Lot 19 or the large pond adjacent. Drainage Design and Calculations 1. Stormwater ponds should be identified by number on the plans so they can be correlated to the details and drainage calculations. 2. Existing tree lines and clearing limit lines should be shown on the pre-development and post-development maps. Composite curve numbers can not be reviewed without this information. 3. Some of the time of concentration paths which are outside of the limits of construction change from pre-development to post-development (ex. E-1(pre) & P-27(post), E-2(pre) & E-2(post), E-3(pre) & P-36(post), etc.). In addition all time of concentration paths should be shown on the plans; Some are not shown (ex. E-2(post), P-36(post), etc.). All discrepancies (including those not listed in the above noted examples) should be resolved. 4. The composite curve number and time of concentration calculations for the post-development catch basin drainage areas are provided in the Appendix but the composite curve number calculations and time of concentration calculations for the post-development drainage areas utilized in the Hydraflow Hydrographs models. 5. The hydrological analysis which yields pre and post development peak runoff rates should include a separate post development drainage area map which provides delineated drainage areas to the design points consistent with the pre development design points; and drainage areas which are consistent with those utilized in the Hydraflow Hydrograph models. The post development drainage area map provided for inlet structures does not provide the required information. 6. Many of the post-development sub-areas delineated on the post-development drainage area map appear to be inconsistent with the calculations provided in the Appendix. For example, area P-6 includes a large amount of open space on the drainage map however the calculations appear to only account for the small paved portion of the drainage area. All post-development sub-areas should be reexamined for accuracy between the areas delineated and the calculations. 7. We cannot verify that buildings are being fully accounted for as impervious area in the composite curve number calculations. For example, post-development area postdevelopment area E-2 (to Videll property) and P-33 (to Souza property) both have three houses and three driveways, however the impervious area for E-2(post) is 0.3 acres, whereas to the P-33(post) is only 0.15 acres. 9. The Stage/Storage Table for Pond No. 2 should be reexamined. The plans show a lowspot on the southwest end of the berm below elevation 336, however the calculations account for storage up to elevation 337.50, with the 100-year water surface elevation being 336.28. 10. We question why catch basin CB25 has two outlet pipes, one to the forebay of the pond and one to the main section of the pond. We would recommend removing the outlet pipe into the main section of the pond as the water is likely to short circuit the storage and flow out of the pond outlet pipe directly across from the aforementioned pipe. 11. Per the Table B-1 in Appendix 6.B of the 2000 ConnDOT Drainage Manual, the 10year 24-hour rainfall for New London County should be 5.0 inches (calculations use 5.5 inches), all other 24-hour rainfall rates utilized in the calculations appear consistent with the ConnDOT Drainage Manual. 12. Calculations should be provided for the output values in column 2 (Time to Inlet), column 5 (AC Entering Catch Basin), column 12 (Average Velocity), and column 14 (Headwater). The values stated in column 2 and 5 are not consistent with the values summarized in the Appendix for each catch basin drainage area. Stormwater Depressions / Rain Gardens As described in the Hydrologic and Drainage Study, the proposed stormwater management concept is partially based on mitigating the increase in post development stormwater runoff from proposed roof areas with small stormwater depressions on each lot. These are also indicated as rain gardens at various points on the plans and in the report. We have the following comments concerning these rain gardens: 1. The detail on the Plan Sheet 24 (which appears to be from the previous application) indicates the depressions are to be a minimum 4’ x 4’ and should contain the capacity equivalent to 1” of rain over the impervious outlets. Although the Drainage Study indicates that the depressions will promote infiltration, the design does not provide design details or calculations to support this. If the basins were to fill and retain water, they would not be effective for prolonged or repeated storms or for attenuation of post development runoff other than the small isolated storm events. 2. The plans and detail are not consistent as to whether the rain gardens will receive inflow from roof drains or footing drains or both. The detail specifically references sump pump outlet pipes. 3. No details are provided as to how the rain gardens will be planted. 4. The rain gardens are generally located beyond the limits of the proposed locations of silt fence shown on the erosion control plan, which would seem to indicate that additional clearing and site disturbance will be required. 5. Typically, rain gardens require specific construction and proper maintenance in order to work effectively. How will construction and maintenance requirements be enforced? We question who will be responsible for the maintenance of the rain gardens which are not located on the property of the associated building (ex. Lot 11 and Lot 34). 6. Several of the rain gardens for the lots on the west side of Monahan Drive, including Lot 2, Lot 4, Lot 5 and Lot 6 are located immediately upgradient of the road. If they were to fill and overtop, the outflow will cause a concentrated flow over the curb onto the road. Proposed Well Locations We assume that proposed well locations will be reviewed by the Uncas Health District, however we did note the following concerns with regard to the locations shown on the current plans: 1. Per the Department of Public Health Public Health Code 2000, Section 19-13-B51d, no well should be located within twenty-five feet of a drain carrying surface water or of a foundation drain. As such the wells on the following lots are within 25’ of a rain garden or a drainage pipe and should be reexamined: Lot 1, Lot 3, Lot 12, Lot 13, Lot 16, Lot 17, Lot 18, Lot 19, Lot 23, Lot 25, Lot 30, and Lot 40. 2. Per the Department of Public Health Public Health Code 2000, Section 19-13-B51d, wells should be located at a relatively high point on the premises and should be protected against surface wash. Wells on the following lots are located within swales and should be reexamined: Lot 1, Lot 7, Lot 21, and Lot 40. 3. Per the 2004 Connecticut Stormwater Quality Manual, Section 11-P3, infiltration practices (i.e. rain gardens) should be located at least 75 feet from drinking water supply wells. In addition, infiltration practices should be located at least 100 feet upgradient from a building foundation and at least 25 feet downgradient of a building foundation. Details 1. Drawing Sheet 22 of 24 shows two details for temporary sediment basins. It should be indicated to what basins these details apply. The Standpipe Outlet Detail includes elevations which do not correlate to any of the basins on the plan. 2. The Typical Section Through Stormwater Detention/Retention Basin on Drawing Sheet 23 appears to be a generic detail. It should be indicated to which pond or ponds this applies. 3. The Typical Cross Section on Sheet 17A of 24 appears to be specific to Pond #1 and should be indicated as such. Also, the detail above on this sheet is labeled Proposed Stormwater Detention Basin, but appears to be a swale detail. 4. The Chain Link Fence and Gate Detail on Sheet 21 of 24 should indicate where on the project it is proposed. Also on that sheet, we question if the project will really include concrete driveways as shown on the Concrete Driveway Ramp Cross Section. 6. The construction details for the level spreaders on Sheet 18 of 24 indicate which pond they are associated with, however the ponds are not labeled on the plans. Miscellaneous 1. Lot 12 and Lot 13 each have a roof drain outlet to a rain garden and two drain outlets connected to catch basins. It is not clear why multiple drains are provided. 2. The swale upgradient of Monahan Drive is located on Lot #1 and property to be conveyed to Molloy. Since the this swale protects the road but is on private property, it would seem that easements should be provided to the Town of Montville for purposes of maintenance and to ensure the property owners do not modify or place obstructions in the swale. 3. A maintenance access drive should be provided to the northwest forebay associated with the pond near Lot 19. 4. Storm drainage outlet piping extends from Glen Close Drive off-road through multiple lots. Maintenance access will be a concern due to the length and location of the system. Also, it is not clear why easements are provided over lots17 and 18, as there appears to be no portion of the drainage system extending on those lots. 5. Pond # 1 has a stone berm through the center of the basin. If the purpose of the berm is to extend the flow path and/or create a forebay, it should be extended such that it creates a barrier between the inlet and outlet pipe which are located very close. 6. Guiderail is called out on the plan between Monahan road and Pond #1, but the location or extent is not shown on that plan sheet. 7. An additional easement should be provided on Lot 10 in favor of Lot 11 for the shared driveway as well as the sanitary sewer. 8. The proposed emergency access should be reviewed by the Director of Public Works and the Fire Marshall / Emergency Services Representative. 9. The proposed construction sequence should include separate items for construction of the temporary sediment basins and permanent ponds. Also, it is recommended that in phase I, the upgradient swale and any work on Gay Hill Road be completed before the road construction begins. 10. The A.D. and K values on the Road Profiles are not correct.