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Home My WebLink About3.0 Drainage StudyDRAINAGE STUDY FOR LOS ADOBES SUBDIVISION FINAL PLAT SUBMITTAL HCE JOB NO. 90042.003 FEBRUARY 8, 1993 TIM THY P. BECK, P.E. 923 Cooper Avenue • Glenwood Springs, CO 81601 Telephone: 303-945-8676 • FAX: 303-945-2555 • 1 TABLE OF CONTENTS Introduction Vicinity Map (8 1/2" X 11") Hydrology Drainage Plan Basin Map (8 1/2" X 11") Map of Area South of Highway (8 1/2" X 11") Appendix: Calculations • • INTRODUCTION The proposed Los Adobes Subdivision project is located on the east side of Highway 82 approximately one-half mile south of the Carbondale intersection. See the enclosed Vicinity Map for site location. The storm runoff flows for individual lots on this site will, in general, be quite diffuse. However, flows will be concentrated in the "major" drainage ways as they historically have been. HYDROLOGY The hydrologic methods for this study are outlined in the Soil Conservation Service publication "Procedures for Determining Peak Flows in Colorado" (1980). Peak flows in this area will be primarily rainfall derived since the whole site is well below 8000 feet in elevation. Therefore, the storm drainage system will be more than adequate to handle spring snowmelt runoff. DRAINAGE PLAN General: In general, this drainage plan attempts to prevent the disturbance of the historic flow paths and peak flows, as much as reasonably possible. m/ 111 • no. fr//7/-4$ 11 m 11 //0• U• OZZ9 • N .a •I 1 • • The drainage basins are shown on an enlarged portion of the U.S.G.S. quadrangle for this area showing Section 27. The proposed subdivision is located nearly in the center of said Section 27. The drainage basins affecting the project have been outlined in heavy dashed lines, and the boundary and lot lines are shown in lighter solid lines. Calculated flows are shown in the Appendix. Calculations were completed for the 25 -year and 100 -year return frequency events as required by County regulations. Drainage Improvements: The proposed culvert across the main access road is located at the point where the dividing line between drainage basins was naturally located. This is so that the natural drainage pattern will not be altered by the new access road. In fact, you will note that the only natural basin really affected by the new road is the 4 -acre one, and it actually ends up draining to the same location as historically, but not by exactly the same paths as historically. Therefore, Basins C & D will drain to the southeast corner of the property, while Basins A, B & E will drain to the southwest corner of the property. There are two existing culverts under Highway 82. The westerly one is a 24" corrugated metal pipe, and the easterly one is an 18" corrugated metal pipe. They are located approximately where one would expect them to be to serve the two main drainages crossing the subdivision. 2 • • These culverts are adequately sized to pass the expected 100 -year water flows with a headwater depth of less than twice the diameter of the culvert. Thus, expected outlet velocities and erosion should be tolerable. Another proposed item of improvement is to remove the fill that was placed near the highway for the construction of the original driveway. The removal of that fill material will provide another path for flow from the two large basins on the west side of the project. Once that fill is removed, flow can continue westerly along the north side of the highway and this will provide an additional safety outflow path in case the capacity of the existing culvert under the highway is exceeded, or if it should become plugged with debris. Detention: No actual detention is proposed for this development because of the large lot sizes and the diffuse flows. The increase in flow due to development on any particular lot would be insignificant. Any difference would be due to the influence of the proposed residences. If one assumes a 6,000 square foot house with approximately 2,000 square feet of garage/outbuildings, etc., each home site would add approximately '/a acre of impervious area to the watershed. The other areas adjacent to the house should be lawn or landscaped or revegetated, and would thus have less runoff than natural conditions. So the westerly drainages will have approximately 3' homesites associated with them, which would amount to less than 1% of the total drainage area 3 being impervious. The easterly drainages might have approximately 3% new impervious area. However, the increase in flow would not likely be even the 1% to 3% since the landscaped areas added would have less runoff than existing conditions, and thus would tend to cancel out the added impervious areas. Therefore, one can see that the change in runoff from the existing conditions to developed conditions should be quite minor. Attempting to channel the diffuse flows to detention areas would make construction difficult and would disturb more area and cause more erosion. It appears that any positive results of detention areas would be heavily outweighed by negative results. Erosion Control & Debris Flow Miti ation: In general, erosion control will consist of disturbing as little existing vegetation as possible and therefore, reclamation of as little area as possible. It is anticipated that areas that must be disturbed will be seeded with an arid area grass as recommended by the Soil Conservation Service. We have shown proposed sedimentation basins and their relation to the existing culverts on the "Topography and Proposed Drainage Improvements" (24" x 36" drawing). Note that these sedimentation basins are mainly intended for control of erosion from areas disturbed by construction, and not intended for mitigation of a significant storm event. However, the basin proposed for the west side of the project, if left in place after construction is completed, and if 4 • properly maintained, is large enough to provide a reasonable amount of mitigation for even a fairly large storm event. The flat slope from the basin mouth to the culvert, along with the basin cut below existing grade, should effect a reduction in debris carried, and thus allow the culvert to function better. Boulders and/or scrub oak placed at the base of the steep slope would further improve the performance, but access would be required to maintain the catchment area. The other "basin" is temporary, for the control of construction -related sedimentation, and will be composed only of straw bales. However, while in place, it would be large enough to provide some mitigation for the 8 -acre basin, but it would not be large enough to have any significant mitigating effect on the 22 -acre basin. Regarding debris flows specifically, this area has historically experienced many debris flows, and it will continue to experience debris flows in the future. In fact, this development should have very little effect on either the magnitude or the frequency of the debris flows experienced. However, debris flows have been considered in the planning and design for this development. The best mitigation technique is simply avoidance, and you will note that no significant construction has been proposed in the very steep and/or channelized portions of any of the basins. A berm has been in place, historically, across the southeasterly corner of the site. This earth fill was placed to protect the house built on the property at the southeast corner of the site. No substantial changes to this berm are proposed, due to liability reasons, however, it is suggested that the berm be extended slightly to the east. This would afford slightly more protection to the existing house, but still leave the original access road (and main drainage path) 5 • • unobstructed. Near the southwesterly corner of the development, a small basin is proposed, as mentioned above. This should help with initial sedimentation as well as later debris flow events, if properly maintained. The volume is not likely to have a significant effect on very large flows. However, it appears that it will at least provide some improvements over the existing conditions. Areas South of Highway 82: The enclosed map shows the relationship between the proposed subdivision and the areas south of Highway 82. As noted above, the difference between pre -development and post -development flows should be unnoticeable. Late summer thunderstorms that drop a significant amount of water quickly will still cause debris flow events, and they will still drain out through the existing culverts and occasionally across the highway. However, the influence of the development should be negligible. SUMMARY Natural drainage paths have been left essentially intact. This ensures that, in general, overland flow will follow the historic paths. The drainage plan for the Los Adobes Subdivision includes only minimal drainage 6 18" DIA. CMP 0 0 tiiJ h 1 RINE 580,902 1,516,095 x 610 5.7 • • improvements, but this should be adequate in view of the small hydrology impact of the development and they generally diffuse flows. 7 • • APPENDIX/CALCULATIONS f # Job No 9 vx-�42A662- Job Title�y,�DS DOc3t S bye% r? date 2/8` ck'd by date Subject page / of 923 Cooper Avenue • Glenwood Springs, CO 81601 Telephone: 303-945-8676 • 303-920-3669 • FAX: 303-945-2555