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HomeMy WebLinkAboutDrainage Reports - 10/02/1980No Text DRAINAGE REPORT For COACHLIGHT PLAZA P.U.D. INTRODUCTION. Engineering Professionals, Inc. was retained by Evergreen Park 2 and Hvolboll-Johnson, owners of property in and/or near the Evergreen Park development in northern Fort Collins, to prepare a master drainage plan for the Evergreen Park area. Coachlight Plaza P.U.D., being a part of Evergreen Park, is included in the master drainage plan. Therefore, the developers of Coachlight have also retained Engineering Professionals, Inc. to assure that the grading and drainage plan for Coachlight is in conformance with the master drainage plan for Evergreen Park. The master drainage plan for Evergreen Park Drainage Basin is approximately 30 days away from completion. However, a drainage improvements plan has been formulated and the hydrology and hydraulics of the plan have been analyzed to the extent that the plan is known to be realistic. From this point on, only minor deviations, which will not affect Coachlight Plaza, will be.made to the master plan before it is finalized. BASIN DESCRIPTION The Evergreen Park Drainage Basin is generally located in the western half of Section 1, Township 7 North, Range 69 West and the southern half of Section 26, Township 8 North, Range 69 West, and it consists of approximately 380 acres. Figure 1, included herein, shows the general outline of the drainage basin. The Eaton Ditch traverses the basin along the northerly boundary and the Lake Canal traverses along the southerly boundary of the basin. Although no detailed study of the Eaton Ditch has been made by this office, general investigations indicate that no overflow of the ditch due to storm runoff has occurred between •-1- North College Avenue and North Lemay Avenue. Storm runoff intercepted by the ditch west of North College Avenue would appear to spill at the location where Dry Creek intercepts the Eaton Ditch. Runoff generally flows north to south through the basin. Ground slopes range from 2% in the northern -most portions of the drainage basin to 0.2% in the southerly portions. Historically the basin was agricultural, but it is currently being developed as a combination of single family residential, multi -family residential, commercial, and light industrial. The Dry.Creek flood plain encompasses the southerly portion of the Evergreen Park Drainage Basin as indicated by the final report prepared by Gingery Associates, Inc. for the City of Fort Collins. However, Coachlight Plaza is. not in the 500 year flood 'plain as indicated by the report. PROCEDURE The Colorado Urban Hydrograph Procedure (CUHP) was used to develop hydrology for the drainage basin. The basic design criteria used was that the 2 year peak rate of runoff, assuming no development, would be'the maximum rate of discharge during a 100 year storm, assuming full development. Detention facilities were sized to assure that this criteria would be met. Routing procedures illustrated in the U.S. Bureau of Reclamation's "Design of Small Dams" were used to size said facilities. A system of open channels and culverts was planned to connect the detention facilities and to direct runoff around the various developments, including Coachlight Plaza., to assure no offsite exposure to the 100 year, developed design storm. HYDROLOGY AND HYDRAULICS Pond No. 3 serves as the detention facility for Coachlight Plaza. and subareas 3, 6, and 7 which affect Coachlight Plaza Therefore, only the hydrology and hydraulics through Pond No. 3 j will be discussed in detail. -2- l Subarea No. 7 currently drains into the Eaton Ditch. However, f policy currently being developed between the City of Fort Collins and the Larimer and Weld Ditch Company, owners of the Eaton Ditch, dictate that once the subarea begins to develop, no storm runoff, either historical or developed, will be allowed to discharge into the ditch. (With said policy, further assurance is given that the Eaton Ditch will not spill in the future in the vicinity of Coachlight Plaza.) Therefore, it was assumed that runoff would be transported below the ditch, through a culvert at the 2 year, undeveloped peak rate. The runoff would then be carried by open channel into Pond No. 3 as indicated on Figure 2. Area 60 Acres 100 Year, Developed Peak Rate of Runoff 211 c.f.s. 2 Year; Undeveloped Peak Rate of Runoff 35 c.f.s. Required Detention Volume 9.8 Ac.-Ft. Surface Area (Assumed 3.Ft. Depth) 3.5 Ac. 1 Subarea No. 6 currently drains to a low area in the southeasterly corner of the subarea bounded by Willox Lane and the Eaton Ditch. A 25" X 16" corrugated metal pipe arch currently carries the storm runoff under`Willox Lane.and to the south side of the entrance to Sundance Hills. The runoff is then discharged and allowed to flow overland. For purposes of sizing a detention facility, it was assumed that the 25" X 16" corrugated metal pipe arch would be replaced by a 45" X 29" horizontal elliptical reinforced concrete pipe. The 45" X 29" HERCP was chosen because it provided the maximum carrying capacity within the limited height restrictions available. An open channel will be sized to transport runoff from the 45" X 29" HERCP down the east side of Blue Spruce Drive into Pond No. 3 as indicated in Figure 2. The open.channel will be trapezoidal with a 12 ft. bottom width, 3:1 side slopes, and a minimum depth of 2 ft. It will be constructed at a slope of 0.25%. -3- Area 38 Acres 100 Year, Developed Peak Rate of Runoff 151 c.f.s. 2 Year, Undeveloped Peak Rate of Runoff 38 c.f.s. Detention Outflow Rate 80 c.f.s. Required Detention Volume 3.0 Ac.-Ft.. Surface Area (Assumed 1.5 Ft. Depth) 3.9 Acres Subarea No. 3 currently drains in a southerly direction. Storm runoff from the minor. storms is directed east along Bristlecone Drive and south along Blue Spruce Drive. A storm sewer system in Blue Spruce Drive and Bristlecone Drive intercepts the minor storm runoff and directs it easterly into Pond No. 3. However, the system is not large enough to intercept the major storm runoff. Therefore, an open channel is proposed along the south side of Bristlecone Drive to direct the 100 year storm runoff into Pond No. 3. The 100 year, developed storm runoff from the area in Sundance.Hills and the Sundance. Hills, Second Filing will be carried in an open channel along the southeasterly boundary of Coachlight Plaza. into Pond No. 3. The open channel will be trapezoidal with.a 12 ft. bottom width, 3:1 side slopes, and a minimum depth.of 2 ft. It will be ':constructed at a slope of 0.25%. As indicated on the grading plan for Coachlight, storm runoff, major and minor, will be transported overland to either the south between buildings 11 and 13 and directly into Pond No. 3 or to the northeast between buildings 7 and 9 into the drainage channel for Sundance Hills. Curb cuts with grates over the sidewalk will be constructed at said locations to transport the minor storm runoff. The curb cuts shall be a minimum of 6 ft. wide. The major storm runoff will be allowed to overtop the curb at said locations. For the drainageway between buildings 11 and 13, the developed 100 year and 2 year peak rates of runoff are 18.5 c.f.s. and 5.6 c.f.s. respectively. For the drainageway between buildings 7 and 9, the developed 100 year and 2 year peak rates of runoff are 17.0 c.f.s. and 5.1 c.f.s. respectively. The overall.parameters for area 3 are as follows: . -4- Area 80 Acres 100 Year, Developed Peak Rate of Runoff 321 c.f.s. 2 Year,Undeveloped Peak Rate of Runoff 68 c.f.s. 100 Year'.Peak Inflow (Areas 3,6 & 7 441 c.f.s. Contributing) Detention Outflow Rate 99 c.f.s. Required Detention Volume 20.0 Ac.-Ft. Surface Area (Assuming 4 Ft. Depth) 5.4 Ac. Maximum Water Surface Elevation 4966.0 RECOMMENDATIONS Storm drainage improvements discussed herein and shown on Figure 2 should generally be constructed as indicated. Final hydrology and hydraulics should be completed for the master plan as well as final sizing of facilities. Once the final report is approved, construction drawings should be prepared and storm drainage facilities should be constructed in an order of sequence as indicated in the final report. Coachlight Plaza P.U.D. should be constructed in accordance with the grading and drainage plan included in the submittal package. Minimum finished floor elevations should be at or above .elevation 4968.60. Although minor deviations in the hydrology and hydraulics could occur before the overall Evergreen Park Master Drainage Plan is completed, Coachlight Plaza P.U.D. will suffer no exposure to the 100 year offsite design storm. -5- ly'i (� ! +'� _ :�• 1 —5050 M- Renyor)�Corner 325 10 t .'�p.o •o Ste` 01*u. ;Ili ° OJ ell I l• .G - ( fig J e 4 • ee v 1 `t2 ee ° n g. 26 p r {i1 fS a473g ° O 5076 .. .. . •• '\./ p '� ; , II1I� ram\ __ q j O .\.o.Ell• tr: . A I t I i� �'`� if - Cours ^� t ; �.l \ • y. ` 1 \`�J ° i O •/�1 Ohm a 011, -500�e � �'a O �la,e• 7 I . m ✓ _avail �� Ya tl.... � � ice.. `s!� 1.�� � �4t: � . Z r..•:. CI�; J7. , 6.1 �aa.` ��i. !. �<.. J ssa.: o a _ Qocr 9 . , - a 5A. _...... . •C In Setting V Basin _ _ •• 9 1t1VEft: r- _:000 p .� �-- \I \ 'h, �• 'i'`ayEs �. � \ OLEO I�pe 7Rrve � �H. _...� sSo; •-.' 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