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Home My WebLink About1.00 General Application Materials_PartIStormwater Analysis for Oak Meadows Ranch PUD – Filing 4 – Phase III January 14, 2022 Page 1 of 5 January 14, 2022 Glenn Hartmann Garfield County – Community Development 108 8th Street, Suite 401 Glenwood Springs, CO, 81601 RE: Oak Meadows Ranch PUD – Filing 4 – Phase III – Stormwater Analysis. Dear Glenn: This drainage report is prepared on behalf of Gary Johnson in conjunction with the application to Garfield County for a Preliminary Plan Minor Modification of the proposed development of Oak Meadows Ranch, Filing 4, Phase 3. Location and Proposed Improvements: Oak Meadows Ranch PUD is located south of the City of Glenwood Springs within the Fourmile Creek drainage basin. Within the PUD, Phase III of Filing 4 is located on the west side of Oak Way North (CR 168A) north of Phase II of Filing 4 and south of the dedicated school district parcel. The proposed development improvements in the area analyzed include the following: 1. The development of 25 single-family residences with appurtenances such as driveways, patios, etc. 2. The construction of the development infrastructure necessary to serve the proposed development including roads, utility systems, and drainage infrastructure. Hydrologic Areas: The overall hydrologic tributary area which is relevant to the proposed development is approximately 132.88 acres. The significant majority of this area, 118.25 acres, lies southwesterly and uphill of an existing private single- family residential driveway which traverses the subject project (Pre- Development Tributary Areas 5, 6 and 7 as depicted on the attached 200- SCALE PRE-DEVELOPMENT TRIBUTARY AREA MAP), or are located down gradient of the private driveway and up gradient from an existing abandoned irrigation ditch which also traverses through the property (Pre-Development Tributary Areas 4A, 4B, 4C, 4D1, 4D2 as depicted on the attached 200-SCALE PRE-DEVELOPMENT TRIBUTARY AREA MAP). There are no proposed development improvements or development impacts within these tributary areas, but because these areas ultimately drain toward the development site, it is necessary to analyze the storm runoff from this area to determine any Stormwater Analysis for Oak Meadows Ranch PUD – Filing 4 ‐ Phase III January 14, 2022 Page 2 of 5 possible impacts to the stormwater infrastructure design for the proposed development. Specifically, stormwater runoff from Pre-Development Tributary Areas 5, 6 and 7 discharge into the roadside ditch adjacent to the private driveway. This ditch then diverts the stormwater runoff toward the development site. There are several existing culverts along this driveway which discharge stormwater runoff across the driveway and down gradient toward the development area. The discharge from these culverts, along with the stormwater runoff from Pre-Development Tributary Areas 4A, 4B, 4C, 4D1, 4D2 is then captured by an existing abandoned irrigation ditch which traverses the property. Based on this hydrologic analysis of these areas which are tributary to the irrigation ditch, the hydraulic capacity of the existing irrigation ditch is sufficient to divert 100% of the peak flows generated by the 100-year storm event past the developed areas. The remaining 14.6 acres of tributary area are the areas which are directly impacted by the proposed development (Pre-Development Tributary Areas 01, 02, 03 and 10 as depicted on the attached 200-SCALE PRE-DEVELOPMENT TRIBUTARY AREA MAP and the attached 60- SCALE PRE-DEVELOPMENT TRIBUTARY AREA MAP). The same general tributary area divided into the post-development basins (Post-Development Tributary Areas 01A, 01B, 01C, 01D, 01E, 02 and 03) are depicted on the attached 40-SCALE POST-DEVELOPMENT TRIBUTARY AREA MAP. Hydrologic Criteria and Methodology: Runoff flows for the different pre and post development basins were calculated using the U.S. Department of Agriculture Soil Conservation Service Technical Release No. 20 methodology. The hydrologic calculations, reach attenuation calculations, and detention pond routing analysis calculations were performed with HydroCAD release 8.00 software by HydroCAD Software Solutions LLC. The following data and calculations for both pre-development and post-development conditions for each of the drainage basins are provided in the appendices: the drainage basin areas hydrologic curve numbers times of concentrations peak discharges calculated hydrographs for the pre-development conditions calculated hydrographs and proposed mitigation for post-development conditions The peak rate of runoff and the stormwater hydrographs for the 25-year and the 100-year design frequency storm events were computed for each tributary area using the rainfall values for the 24-hour Type II storm from the NOAA Atlas 14, Point Precipitation Frequency Estimates. Attached is a printed report from the NOAA Atlas 14, Volume 8, Version 2 which provides the point based precipitation frequency estimates for this specific location. Specifically, based on this data, the 10-year 24-hour rainfall is determined to be 1.86 inches, the 25-year 24-hour rainfall is determined to be 2.21 inches, and the 100-year 24-hour rainfall is determined to be 2.81 inches. The times of concentration for each tributary area are determined by separating the overland flows into the different categories of flow types based on the site specific conditions. Typically the overland flow is divided into three flow types, sheet flow, shallow concentrated flow, and channel flow. The calculations for the times of concentration for each tributary area are provided in the pre-developed and post-developed HydroCAD analyses attached to this report. The T.R. 20 method requires the calculation of the times of concentration (Tc) for each tributary area as well as a subsequent travel time (Tt) from the outfall of the tributary areas to the composite watershed outfall point, where applicable. Those drainage basins that are not directly connected hydraulically to the primary basin outfall have been routed through “reaches” in order to provide for the travel time. The values of Tc as well as the Reach calculations for all tributary areas are provided in the HydroCAD analyses attached to this report. The TR-20 method also requires the calculation of composite Runoff Curve Numbers (RCN) for each tributary area. Typically, the RCN is based on the hydrologic soil type as well as the land use and conditions. The hydrologic soil type (A, B, C or D) is determined by the U.S. National Resources Conservation Service (NRCS) and provided on NRCS maps of the specific area. NRCS soils data for this site is provided in the attached NRCS report. According to the NRCS report, the soils within the defined tributary areas are exclusively hydrologic soils group C. Stormwater Infrastructure Improvements: Generally, stormwater infrastructure improvements for this development have been designed to safely convey the peak stormwater flows generated from the 100-year storm event through the project without risk of damage to public or private improvements. The stormwater infrastructure has also been designed to mitigate the peak downstream stormwater flow rates for both the 25-year and the 100-year storm events to less than the pre-development peak stormwater flowrates for these same storm events. Sheet D1 of the Preliminary Plans for the proposed development provides a preliminary design for all of the proposed stormwater infrastructure improvements for the development. Those improvements are further described below. As noted above, under existing conditions the majority of the area which is tributary to this property discharges into the abandoned irrigation ditch which ultimately discharges its stormwater flows to the north of the proposed development. While the existing abandoned irrigation ditch currently has sufficient hydraulic capacity to convey the peak flow from the 100-year storm event away from the proposed development, since this feature is proposed to continue to function as an element of the stormwater infrastructure for the development, we are recommending the implementation of some maintenance measures on the irrigation ditch as part of the stormwater infrastructure improvements for the proposed development. In order to avoid an increased peak stormwater flow rate generated from post-development Area-03, it will be necessary to construct a minor stormwater diversion channel along the northeast boundaries of lots 83, 84 and 85. This stormwater division channel diverts the post- Stormwater Analysis for Oak Meadows Ranch PUD – Filing 4 ‐ Phase III January 14, 2022 Page 3 of 5 Stormwater Analysis for Oak Meadows Ranch PUD – Filing 4 ‐ Phase III January 14, 2022 Page 4 of 5 development stormwater runoff from these lots into a proposed storm sewer system as described below, which then discharges into the proposed stormwater detention basin. In order to safely convey and mitigate the stormwater runoff from the areas proposed to be developed, it will be necessary to construct certain storm sewer culverts and storm drain systems to collect and safely convey the stormwater runoff through the proposed development. Several of the proposed storm sewer improvements direct the post- development stormwater runoff into the proposed stormwater detention basin. Two culverts, specifically STORM-02 and STORM-04, are designed to convey existing pre-development stormwater flows which are generated outside of the proposed development, through and around the proposed development. Finally, as noted above, a stormwater detention basin is proposed to be constructed at the discharge from post-development Area-01 (labelled on the attached 40-SCALE POST- DEVELOPMENT TRIBUTARY AREA MAP as Areas 01A. 01B, 01C, 01D and 01E). This stormwater detention basin is designed to capture the increased stormwater runoff from post-development Area-01 and release the stormwater flows at flowrates less than or equal to the pre-development flowrates. Results: This analysis demonstrates that the proposed stormwater infrastructure for this development safely conveys the peak stormwater flows generated from the 100-year storm event through the project without risk of damage to public or private improvements, and mitigates the peak stormwater flow rates from this development for both the 25-year and the 100-year storm events to less than the pre-development peak stormwater flowrates for these same storm events. Following are the results of the Hydrologic Analysis for Oak Meadows Ranch PUD – Filing 4 - Phase 3. Table – 1 Peak Stormwater Discharge Rates Tributary Area / Point of Discharge Pre‐ Development 25‐Year Peak Flow (CFS) Pre‐ Development 100‐Year Peak Flow (CFS) Post‐ Development 25‐Year Peak Flow (CFS) Post‐ Development 100‐Year Peak Flow (CFS) Pre-Development Tributary Areas 4A, 4B, 4C, 4D1, 4D2, 5, 6 and 7 / Abandoned Irrigation Ditch [REACH-05I] 0.79 CFS 2.22 CFS 0.79 CFS 2.22 CFS Stormwater Analysis for Oak Meadows Ranch PUD – Filing 4 ‐ Phase III January 14, 2022 Page 5 of 5 Tributary Area / Point of Discharge Pre‐ Development 25‐Year Peak Flow (CFS) Pre‐ Development 100‐Year Peak Flow (CFS) Post‐ Development 25‐Year Peak Flow (CFS) Post‐ Development 100‐Year Peak Flow (CFS) Tributary Area-01 2.00 CFS 3.89 CFS 1.78 CFS 2.23 CFS Tributary Area-02 1.56 CFS 2.89 CFS 1.35 CFS 2.04 CFS Tributary Area-03 1.97 CFS 3.59 CFS 1.96 CFS 3.42 CFS Summary: This analysis demonstrates that the stormwater infrastructure for the site is adequately sized to be able to convey the peak flows from the 25 and 100-year storm events and to detain post- developed stormwater rates of runoff to levels less than or equal to the pre-developed rates of runoff for the 25 and 100-year storm events. If you have any questions, or if you need additional information, please do not hesitate to contact us. Sincerely, Gamba & Associates, Inc. __________________________________ Michael Gamba, P.E. & P.L.S. 28036 H:\03385\2020\Design\Storm\20210628 Drainage Report-r01.doc