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Home My WebLink About20012843.tiff ALLES AND 428 North 2 Street LaSalle, Colorado 80645 Telephone: 970-284-9562 ASSOCIATES, INC. Fax: 970-284-9564 July 6, 2001 Project No. Hoskins 0101 Weld County Public Works Weld County Engineering Department 1111 H Street P.O. Box 758 Greeley, Colorado 80632 Attention:. Ms. Diane Houghtaling Reference: Hoskins Recorded Exemption No. 2954 W Yz NE 1/4 Section 29, T 4N, R 68W, 6 P.M., Weld County, Colo. Subject: Peliminary Drainage Study of the Subdivision Dear Ms. Houghtaling, Enclosed is a drainage study of the above referenced subdivision with support documents. The guiding reference for this study has been Chapter 2 of the Field Engineering Manual "Estimating Run-off'published by the Soil Conservation Service of the United States Department of Agriculture. Rainfall intensity curves were also from SCS. The soil classifications found on site appeared to matched those in the soil survey of the area performed by SCS. The soil types are Kim Loam, Thedalund, Wiley-Colby Complex and Aquells in the creek bottom. The first three have fair to good permeability ratings. The average percolation rate from table 13 was used in the calculations as a conservative approach to sizing the ponds. Please call if any questions or any items I can clarify over the phone to save time in reviewing the study. Sincerely, George D. Paulsberg Colorado P.E. # 35359 Enclosures: Preliminary Drainage Study EXHIBIT 1 (77Y 2001-2843 July 6, 2001 Project No.: Hoskins 0101 DRAINAGE STUDY (Preliminary) Objective: Evaluate and calculate the quantity of rainfall run-off that will occur on the project property and provide for the containment of subject run-off that meets or exceeds the requirements of the Weld County Subdivision Ordinance No. 173-E and Planned Unit Development Ordinance No. 197. Location: Property is in the W1/2, NE1/4 Section 29, Township 4 N., Range 68 W., 6 Principal Meridian, Weld County, Colorado. Project Name: Hoskins - Recorded Exemption No. 2954 Reference: A). Soil Survey of Weld County, Colorado -Northern Part by United States Department of Agriculture, Soil Conservation Service. B). Soil Conservation Service Engineering Field Manual, Chapter 2.- Estimating run-off. Site Conditions: A mix of farm fields and rolling hills with low lands along the two creek drainages on the property. The general drainage flows along the NW corner of the property and along the east property line. Creek flows are generally north. Future Use & Conditions: The future use of the land will be for one home on each lot with a garage and possibly one or two small buildings as barns or sheds. The lawn of grass is estimated not to exceed one acre and the balance of the lots to remain in hay or natural vegetation. Design Parameters: The following items are fixed by others that apply to this project. A. 5 Yr. Storm frequency for general run-off calculations. B. 100 Yr. Storm frequency for quantity of water required to be retained on the property. C. Land use to be residential. D. Rainfall intensity curves furnished by the Soil Conservation Service. E. Constants, coefficients, charts and equations by Soil Conservation Service. Narrative: The procedure has been to perform a series of tasks in sequence to arrive at values for run-off and flow retainage requirements. Task one was to classify the soils found on the site in accordance with the soils conservation maps for the area. Page two Project No.: Hoskins 0101 Task two involved the identifying of areas contributing run-off onto and from within the property and into individual drainage pockets. Task three was the classification of future land uses to curve numbers for hydro logic soil groups and proportioning by weighed values for a combined CN for later use. Also, plotted drainage flow direction and length of flow on the contour map. Task four was the use of rainfall intensity curves, CN's, watershed slope and length to establish a time of concentration for each drainage basin. Task five applies these various items to determine the peak discharge and quantity needed for each retainage pond. Task six the estimation of percolation rates from the SCS manual for the retainage ponds. Task seven involved the trial sizing of each pond to match the percolation rate and quantity of water to percolate in a 24 hr. period. Task eight located the drainage ponds and ditching needed on the contour map to divert the water into the ponds. Task nine was the wrap-up of report writing, transmittal letters, copying of support documents and final drawings for agency review. Construction Recommendations: The procedure best suited to this site with reasonable construction effort and cost is to build retainage ponds within each drainage basin. Special ditching will be needed to route the surface run-off to these ponds. In sub basin areas the ponds will need to be as close as possible to the creeks to properly retain all the run-off leaving the site. The problem resulting is that the ponds will be within the designated flood way as shown on the Flood Insurance Rate Maps for that area. Moving the ponds outside the flood way would not allow them to retain all run-off as required. The ponds on the down hill side will be flush to the ground allowing flood waters to pass over them. The ponds may silt in and need cleaning to restore them to a functioning level. Area#3 will need three retainage ponds and area#7 will need two retainage ponds to limit the ditching to reasonable lengths. Area#6 may need a second retainage pond if the creek is too tight to the bluff. The total planned volume of all the ponds is 21,060 cu.ft. and required run-off volume needed to be retained is 20,963 cu.ft. The ditching to the ponds will be shallow swales 2' deep and flowing with the contours and located just above the creeks high water marks and/or property lines. Sincerely, George D. Paulsberg, Colo. P.E. # 35359 Enclosures: Calculations, graphs, tables, contour map and rainfall intensity curves. 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