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HomeMy WebLinkAboutCOUNTRY CLUB RESERVE - FDP180030 - - STORMWATER-RELATED DOCUMENTS (2) November 24, 2020 Blaine Mathisen Northern Engineering Services, Inc. 301 N. Howes Street, Suite 100 Fort Collins, CO 80521 RE: Country Club Reserve – Underdrain pipe flow capacity and misc. changes Dear Mr. Mathisen: As part of the City of Fort Collins’ review of Northern Engineering’s (NE) underdrain plans for the County Club Reserve development, the City requested an evaluation and statement about the adequacy of the slopes and size of the underdrains to convey the flow rates they are expected to collect from groundwater. Our prior report was finalized in August 2020, so we are providing this review as a separate document. Drain Pipe Conveyance Capacity. As noted in our main report, the model-projected discharge from the underdrain system is about 3 gallons per minute (gpm) based on the field data available to us and under the assumptions of our evaluation. Considering the uncertainties always involved in subsurface projections, we had recommended that infrastructure be designed for at least 10 gpm or more to be collected by and to discharge from the underdrains . Note that this flow rate is a cumulative number, meaning this rate would be found in the pipes only on the lower end (east side) of the system after most of the water has been collected. Flows are expected to be smaller on the upstream ends of the underdrain system. The typical slope for the underdrain plans which NE provided to us is about 1%. And the minimum slope, which we found on the lower end of the system we modeled, is at 0.33%. Using the Manning pipe flow equation, we calculate that a 6-inch diameter pipe has ample capacity even at the minimum slope. A well-maintained 6-inch perforated pipe at 0.33% slope is expected to convey about 30 gpm (0.07 cfs) while flowing only one-third full (calculation inputs attached). That is ten times higher than the model-projected flow rate, and three times higher than our suggested “safety factor” rate, and still with room to spare. Therefore, we see BLAINE MATHISEN NOVEMBER 24, 2020 P a g e | 2 no concerns about the slopes and size (6-inch diameter) of the underdrain pipes specified in NE’s plans. Note: the pipe flow rates obtained in the attached calculation are slightly lower than what one obtains with a typical Manning calculation. The calculation method we used increases the Manning’s “n” parameter as function of partial pipe flow depth and that slightly decreases the estimated flow rate. Minor Changes in Underdrain Depth and Path The underdrain plans we simulated in the groundwater model appear to be slightly different than the final plans, based on pages we reviewed from a Review Set dated 11/18/2020. We have compared the final plans to the plans in our model and we conclude tha t the changes do not require a revision to the groundwater flow model. The footprint of the final underdrain plan is very similar to the plan we modeled, so we confidently conclude our projection for the underdrain’s inflow rate will not change to within any level of significance. In other words, we projected 2.9 gpm before, and based on that we recommended you design for at least 10 gpm. A revised model would project a rate quite close to 3 gpm as well and our recommendation would still be to design for at least 10 gpm, so there is no reason to update the model. In this review, we noted that the underdrain depths on the lower end of the system appear to be a few inches higher than the plans we had last modeled. This means that our lot-by-lot projections of depth to water could also change by a few inches. Keep in mind, however, that our ability to project groundwater levels always has uncertainty for various reasons, and here we speculate our uncertainty is equal to at least one or two feet. Therefore, a change in underdrain depth of a few inches does not lead to a meaningful change in our estimates. Recommendations Forecasts, projections, and estimates about subsurface water behavior are always approximate and have some degree of uncertainty. We therefore recommend that some “as-built” verification be performed. For example, in our experience it is common practice for a builder to advance a geotechnical a boring on each lot before they make lot-specific decisions about foundations. We certainly recommend that kind of as-built confirmation here as well, to ascertain the degree of effectiveness of the underdrains and to check for unanticipated lot- scale variations. We also recommend that any basements have a redundant perimeter drain system to protect against stray water or lot-scale variations which are not within reasonable predictive capability. Partially Full Pipe Flow Calculations I. Calculation of Discharge, Q, and average velocity, V for pipes less than half full - U.S. Units Instructions: Enter values in blue boxes. Spreadsheet calculates values in yellow boxes Inputs Calculations Pipe Diameter, D =6 in Pipe Diameter, D =0.5 ft Depth of flow, y =0.1666 ft Pipe Radius, r =0.25 ft (must have y < D/2) Circ. Segment Height, h =0.1666 ft Manning roughness, nfull =0.011 Central Angle, q =2.46 radians Channel bottom Cross-Sect. Area, A =0.06 ft2 slope, S =0.0033 ft/ft Wetted Perimeter, P =0.6 ft Hydraulic Radius, R =0.09 ft Discharge, Q =0.07 cfs Ave. Velocity, V =1.24 ft/sec Calculation of n/nfull and n: y/D =0.333 n/nfull =1.28 n =0.014 Equations used for calculations: r = D/2 h = y R = A/P (hydraulic radius) Q = (1.49/n)(A)(R2/3)(S1/2) (Manning Equation) V = Q/A More Excel spreadsheets for partially full pipe flow at: Copyright © 2014 Engineering Excel Templates. All Rights Reserved. Equation used for n/nfull: n/nfull = 1 + (y/D)0.54 - (y/D)1.20 Source: Goswami, I., Civil Engineerign All-In-One PE Exam Guide: Beadth and Depth, 2nd Ed. EngineeringExcelTemplates.com (Eqn 303.32), McGraw-Hill, NY, NY, 2012