Loading...
HomeMy WebLinkAboutLAURIE SUBDIVISION PUD - PRELIMINARY - 44-89D - REPORTS - DRAINAGE REPORTREVISED DRAINAGE & EROSION REPORT Submitted for Preliminary Approval of LAURIE SUBDIVISION SECOND FILING 6.4 ACRES LOCATED IN THE NE 1/4, S3, T 6 N, R 69 W CITY OF FORT COLLINS, COUNTY OF LARIMER STATE OF COLORADO Prepared for Dr. William Musslewhite 5001 S. Shields Street Fort Collins, CO 80526 August 5, 1991 Jack Johnson Company 1910 Prospector Avenue Suite 200 Park City, Utah 84060 (801) 645-9000 TABLE OF CONTENTS Table of Contents . . . . . . . . . . . . . . . . . . . . . . i DRAINAGE Scope and References . . . . . . . . . . . . . . . . . . . . . 1 Applewood Estates Irrigation Ditch . . . . . . . . . . . 1 CulvertSizing . . . . . . . . . . . . . . . . . . . . . . . . 1 Conclusions and Recommendations . . . . . . . . . . . . . . . 2 Calculations . . . . . . . . . . . . . . . . . . . . . . . . . 2 EROSION Erosion Hazard . . . . . . . . . . . . . . . . . . . . . . . 3 Erosion Control During Construction . . . . . . . . . . . . . 3 Revegetation Methodology . . . . . . . . . . . . . . . . . . . 4 i 0 SCOPE and REFERENCES This report references the Preliminary Drainage Report already completed by Foundation Engineering, Loveland Co., and submitted to the City of Fort Collins. A copy of the aforementioned report is attached. Amendments to this report are in response to Planning Department letter of July 17, 1991 which comments on the necessary revisions to be made for preliminary approval. A revegetation methodology is also outlined. This preliminary drainage report and revegetation methodology along with the Erosion Control and Grading Plan and General Landscape Plan identify the erosion control methods to be used in establishing and maintaining a natural open space within the project site. APPLEWOOD ESTATES IRRIGATION DITCH The Applewood Estates irrigation ditch will be piped through the project and under Wooded Creek Court. Flow rates provided by the Applewood Estates Irrigation Company indicated a maximum flow rate of 2.5 cfs. As shown on the grading plan the slope of the 18 inch pipe is 1.5%, and with outlet control the pipe has a flow capacity of approximately 4.5 cfs thus providing adequate capacity. CULVERT SIZING CULVERT #1 Culvert #1 has flows following the ravine which are supplied by Offsite Area "C" and Onsite area 113". These flows plus the irrigation overflow are calculated at 42.86 cfs for a 100 year storm as shown on page 8. A 48" diameter culvert is recommended. 1 0 With an available 10' HW (before the flow goes over the private drive) the maximum capacity of the culvert is approximately 185 cfs. This largely oversized pipe is used to minimize velocity and to reduce chances of any plugging. If plugging does occur, the private road has been designed with a low point at the ravine, allowing flows to pass over the road back into the ravine. At the time of final construction drawings, a stilling pond will be identified at the outlet of this culvert. CULVERT #2 Culvert #2 has flows which are supplied by offsite Area "B" and Onsite area 112". The flows are calculated at 7.87 cfs for a 100 year storm as shown on page 8. An 18" diameter culvert is recommended. It has a flow capacity of 12 cfs using inlet control. CONCLUSIONS and RECOMMENDATIONS 1. Final design and construction are to be performed in accordance with City of Fort Collins Storm Drainage Design Criteria and Construction Standards. 2. Onsite detention of stormwater is not warranted. CALCULATIONS Rational Factors "C" Existing Conditions 0.20 = offsite irrigated 0.25 = Offsite steep non -irrigated 0.25 = Onsite irrigated areas 0.30 = Onsite steep non -irrigated 2 0 Post Developed Conditions 0.45 = Estate Residential See pages 7 and 8 for Hydrology Calculations. See page 9 for culvert calculations. EROSION HAZARD The greatest erosion potential is within the existing ravine through the center of the project. The area which has the most active erosion will be stabilized by the placement of fill for the road across the ravine at this location. Prior to construction, erosion will be minimized along the ravine by pre -placement of straw bales near the ravine and the smaller drainage to the east. (See Erosion Control and Grading Plan) After construction is complete, additional erosion along the ravine will be controlled by riprap, rock and revegetation. (See Revegetation Methodology below) Other new possible erosion areas will be exposed by the cuts and fills of the private road which will be controlled by strategic placement of bales until revegetation and permanent erosion control takes place. EROSION CONTROL DURING CONSTRUCTION As shown on the Erosion Control and Grading Plan straw bales will be placed to minimize erosion during construction. The 48" culvert placed in the ravine should be constructed during a low flow period to reduce erosion. Riprap will also be placed as shown on the Erosion Control and Grading Plan to reduce erosion at both the inlet and outlet of both the Culvert #1 and the Culbert #2. All construction will conform to The City of Fort Collins Erosion Control Reference Manual. 3 REVEGETATION METHODOLOGY 1. General The area of the site that will be disturbed by the construction of the private drive and by other utility construction, i.e., culverts, will be revegetated. (See the General Landscape Plan for the extent of this area.) This revegetation will mitigate the visual impact of the disturbed areas and will provide erosion control. The revegetated plantings will have a natural appearance by being placed in tree and shrub groupings scattered randomly throughout the disturbed area. Grasses will be utilized to cover the ground plane. This revegetation process will use native plant material to fit with the existing plant material on the site. In addition to erosion control though plantings of native plant material, soil tackifiers will be used in the grass seeding process to halt wind and water erosion. Also, erosion control blankets will be used as necessary on steep slopes to ensure establishment of grass seeding. 2. Process Earthwork In the initial cutting of the road grade, all topsoil will be stripped and stockpiled to be used in the final grading as an over layer for planting. Placement of topsoil will be laid at a minimum depth of 4 inches. Soil Stabilization Method Soil Tackifier Soil tackifier should be placed on disturbed areas with the hydroseed grass mixture as recommended by the tackifier manufacturer. 4 Erosion control Blankets Erosion control blankets should be used where required and installed as recommended by the manufacturer. Bank Stabilization The stream bank is to be stabilized at points of inlet and outlet of culverts and other existing areas requiring stabilization. This stabilization is to be accomplished by placing boulders of varying sizes in unstable areas and by placing riprap at the inlet and outlet points of culverts. Extent and amount of riprap and boulder placement to be determined at the time of final design. Revegetation Plant Relocation Existing plant material identified as being in areas to be disturbed are to be tagged. Of the trees and shrubs tagged, those which are of a size and condition suited for transplanting are to be transplanted to a location identified on site by the project landscape architect in coordination with City Forester. Trees too large to transplant will be placed on site in the open space as deadfall for wildlife habitat. Proposed Trees and Shrubs Existing plant material will be enhanced by proposed tree and shrub plantings. The proposed plant material to be used is identified on the general landscape plan. The proposed plant material will be native to the site and located in such a manner to create a natural appearance. 5 Grasses All areas disturbed by construction shall be seeded by a grass mixture as recommended by the City of Fort Collins. The seed mixture shall be as specified on the General Landscape Plan. The method of seed planting shall be the Hydroseed Method. 3. Maintenance The maintenance of the areas of revegetation will be performed by the contractor during construction and until accepted as complete by the owner and City officials. The contractor is to guarantee all planting for the period of one year after substantial completion. The contractor whenever notified by the owner of City official shall immediately place in satisfactory condition in every instance any of such guaranteed work. After substantial completion and acceptance, maintenance of the revegetated area shall be by the homeowner's association. N.