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HomeMy WebLinkAboutGrading Plans - 08/12/2021Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: Dan Mogen Date: 08/12/2021 HIGHLAND �_"____ . =�.'T �L=P.VICES Memorandum T0: Kyle and Brandy Heustis FROM: Adele McKenna, EI Jason Claeys, PE DATE: 8/12/21 PROJECT: Heustis Residence — 2209 Moffett Dr HDS 106 N0: 21-1046-00 SUBJECT: Erosion Control This memorandum has been prepared to describe the drainage impacts of the proposed changes and provides impervious area calculations and comparisons for both the existing and ultimate conditions on the property. This project is located at 2209 Moffett Dr in Fort Collins and proposes the demolition of the existing house and the construction of a new single-family residence while protecting the existing outbuildings in place. The existing building proposed to be removed will not be removed immediately. The property owners are living in the existing house while their new residence is being built. As part of the requirements to receive their certificate of occupancy, the existing house will be removed. For the purposes of this analysis, we are reviewing the impacts of the ultimate condition with the existing house removed and the new residence completed. Impervious are calculations and estimated runoff rates specific to 2209 Moffett Dr are attached for reference. The rational method was utilized to estimate the peak runoff rates. The property is approximately 4.7± acres and is currently 5.0% impervious. With the removal of the existing house and construction ofthe new residence and driveway, it is estimated thatthe propertywill be 5.8% impervious, an increase of 1,832 sqft of impervious area (roof, drive/walk, gravel areas). The associated 100-yr runoff rates are 11.19 cfs under the existing conditions and 11.45 cfs under the proposed condition, a 0.26 cfs increase. The outfall for this site is an existing 15" culvert crossing Moffett Drive. To evaluate the overall impacts of the increase in impervious area, the overall basin that drains to the 15" culvert under Moffett Dr was reviewed. LiDAR contours obtained from the City along with aerial imagery were referenced to determine the approximate limits and impervious area of this basin. The basin is approximately 16.8± acres and 15.2% impervious. With the proposed improvements associated with 2209 Moffett Dr, the overall impervious area is estimated to be 15.4%. The existing and proposed 100-yr runoff rate is 48.03 cfs and 48.27 cfs, respectively. There is an estimated increase of 0.24 cfs in the overall basin, a 0.5% increase. In our opinion, this increase is considered negligible in reference to the overall basin runoff rates. Since all impervious paved areas drain to pervious area (either sheet flow or roadside swales), paved areas are assumed to be 90% impervious since the runoff has an opportunity to infiltrate prior to leaving the analyzed basins. Highland Development Services, Inc. 6355 Fairgrounds Avenue, Suite 100 � Windsor, CO 80550 � 970.674.7550 Heustis Residence 8/12/21 Page 2 of 2 The impervious percentage of 2209 Moffett is less than the assumed 30% imperviousness of the Spring Creek sub-basin 141 in the area master plan, therefore, this lot does not require detention. The disconnected impervious areas, on site swales, and roadside ditch provide sufficient water quality for this property. It is estimated that the area of disturbance is around 15,213±. A combination of rock socks and diversion ditches are proposed to manage erosion control during construction prior to establishment of vegetation. Reference the attached erosion control escrow calculation & proposed erosion control measures on the attached Grading & Erosion Control Plan. Based on this analysis of the existing and proposed conditions associated with the proposed improvements at 2209 Moffett Dr, and the overall basin draining to the existing 15" culvert under Moffett Drive, we don't believe that these improvements will have an adverse impact on the existing overall drainage conditions. 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T G � �� II CI U .� ._ d' � a-1 N II II 0 0 0 a a s � C V � +.?+ t6 _ � G � N � C tlD . � _ � � fD C � � C T �p C m II II � Q Q� ti` U� Z II o � � � W j: � a � � � H Z a m X X N U m U � 0 .� � 0 0 � � O N Erosion and Sediment Control Escrow/Security Calculation for The City of Fort Collins 7/8/2021 Project: Heustis Residence EROSION CONTROL BMPs Diversion Ditch Rock Sock Vehicle Tracking Control Pad Culvert Protection BMP Amount Estimated Units Quantity LF 135 $2.50 each 3 $150.00 each 1 $350.00 each 1 $150.00 Sub-Total: 1.5 x Sub-Total: Amount of security: Reseeding Amount Seeding AC 0.35 Disturbed Acres: 0.35 Unit Price $ $ $ $ $ $ $ $2,000.00 $ Sub-Total: $ 1.5 x Sub-Total: $ Amount to Re-seed: $ Miniumum Escrow Amount Minimum escrow amount: Residential $ Total Price 337.50 450.00 350.00 150.00 1,287.50 1, 931.25 1, 931.25 698.48 698.48 1,047.73 1, 047.73 1,500.00 Final Escrow Amount (Maximum between BMP, Reseeding and Minimum Escrow) Erosion Confro/ Escrow: $ 1,931.25 "Tlie arnount of jhe securin� `nust be brrsed on one nn�! one-half times tlae estimnte of t/:e cost ta install t/te approved mensures, or one mtd one-l�alf Pirrtes the cost to re-vesetnte dte dfsturbe�/ l�uid to dry land grasses based upon unit cost determi�zed by the Citp's Annual Revegetation antl StabiliZntion Bid, whichever is grea/er. 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