2011-00005 - new structure - Laserfiche WebLink

Home Browse Search

Kathy Raymond From: James H. Parker[JimP@advsur.com] Sent: Wednesday, February 16, 2011 4:18 PM To: Mike Sharratt Cc: Info(Group Email) Subject: Revised survey for your review, our job 100495 Attachments: 100495 EXWATERSHEDS.pdf; 100495 PROP WATERSHEDS Rev 02 16 11.pdf; 100495 PROPOSED Rev 02 16 11.pdf I attach the revised survey, "100495 PROPOSED Rev 02 1611"for your review. Below are some comments on issues brought up by the city and their consultant. 1. The capacity of the 12 inch PVC emergency overflow pipe at a grade of.71�o is 4.0 cfs based on a Manning's coefficient of 0.010 and Kutter's formula. 2. The 100 year storm event would produce a peak flow of 3.37 cfs from the site to the watershed tributary to the infiltration area after the project. That represents an increase of 0.73 cfs for that watershed in the 100 year event before the project. The rational formula parameters and calculations are shown on the attached "100495 PROPOSED WATERSHEDS Rev 02 0611" drawing and the attached "100495 EXWATERSHEDS" drawing. There is an 0.075 acre area on the neighbor to the northeast that drains into the infiltration area and an area of 0.106 acres on the neighbor to the southwest that flows into the infiltration area and those areas would contribute about an additional 20%by area and would increase the peak flow from the entire area tributary to the infiltration area to about 4.0 cfs. 3. If the emergency overflow were to be partially or completely plugged up,water would rise until it flowed across the neighbor's driveway to the southwest. When the water reached the neighbor's garage floor elevation of 933.7 where damage could begin to occur,the flow across the driveway would be about 0.3 feet deep at its peak and the triangular shaped flow cross section would be about 56 feet wide or 8.4 square feet and would represent an 18 cfs flow, based on a Manning's n value of 0.16 for asphalt and a cross slope of 1%. Note that the driveway is above its summer level by about 0.2 feet due to frost heave as is evidenced by the garage floor, apron elevations. 4. The 12 inch emergency overflow is therefore capable of handling the entire 100 year event flow from the area tributary to the infiltration area. And even if it is plugged the second emergency overflow across the neighbor's driveway would more than handle the flow before damage to the neighbor's home occurred. 5. We have shown significant trees on both neighboring properties in the areas where grading is proposed and beyond and don't notice anywhere that the grading would impact the neighbor's trees to a significant extent. 6. We have moved the outlet of the pipe closer to the lake,and eliminated the retaining walls, but we still have a flat area with rip rap to dissipate energy and while the 1.5 to 1 slopes proposed down to the pipe are on the steep side,the rip rap should prevent erosion. Because this is an emergency overflow, not much water will ever flow through it. �,�CEIVED FEB � 7 2011 C�-f1(OF ORONO 2/17/2011