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HomeMy WebLinkAbout18C Valley West Sub. Drainage Plan - Part 1 DRAINAGE PLAN Valley West Subdivision Planned Unit Development BOZEMAN, MONTANA Prepared for: Aspen Enterprises, L.L.C. M 0-R-R-1 S 0 N INC. An Employee-Owned Company Prepared by: Morrison-Maierle, Inc. 901 Technology Blvd. Bozeman, MT 59718 Project Engineers: Greg Stratton, P.E. Jim Ullman, P.E. Introduction The Valley West Subdivision is primarily a residential development located in the City of Bozeman south of Durston Road and west of Ferguson Avenue as illustrated on the Vicinity Map. The development plan calls for mixed density traditional neighborhood housing with limited neighborhood commercial land uses on 310 acres. Prior land uses on this parcel are agricultural. More specifically pasture and hay production. This type of use is consistent with the surrounding area. The Major Drainage System in the Valley West Subdivision will be used to convey runoff from the development. The Maynard-Border Stream-Ditch is the primary drainage for the east portion of the property. Baxter Creek drains the west portions on the property. Baxter Ditch will be used to convey runoff from along Cottonwood Road to Baxter Creek. The Ferguson Drain, a storm drain pipe to be constructed paralleling Ferguson Avenue will convey runoff from the eastern boundary of the property to the Maynard-Border Stream Ditch. These drainage ways are illustrated on the exhibit entitled "Valley West Subdivision— Major Drainage System." The minor drainage system for the Valley West Subdivision is made up of streets with curb & gutter, drainage swales, inlets, valley gutters, and detention ponds. Runoff is collected in the streets and drainage swales, transported to detention ponds, and then released to the major drainage system. During larger storm events site grading provides a designated path to convey runoff to the receiving steams without inundating structures and minimizing erosion to the channel. MAJOR DRAINAGE SYSTEM Maynard-Border Ditch Flood Hazard Evaluation The Maynard-Border Stream-ditch is the primary drainage conduit conveying runoff from the site. The characteristics of this stream-ditch and the effects of the development on the existing drainage system need to be examined to effectively mitigate and control post development runoff. This section studies the runoff characteristics of the Maynard-Border stream- ditch. The impacts of the development on the existing drainage system are considered. A drainage plan to mitigate and effectively control runoff from the development and minimize the effects to the existing stream channel is provided. Existing Basin Characteristics The existing drainage system of the Maynard-Border stream-ditch flows to the north and eventually drains into the East Gallatin River. The Maynard-Border stream-ditch is not mapped by the National Flood Insurance Program for the 100- year flood plain. The drainage basin is less than twenty-five square miles and 1 of 15 H:\3384\003\Dr3inage\Va11ey West Drainage Study.doc therefore does not require flood hazard evaluation under chapter 16.20 of the City of Bozeman Interim Subdivision Regulations. The Maynard-Border stream-ditch originates approximately 4 miles south of the Valley West Subdivision. The drainage basin south of Durston Road is 1165 acres more or less. The exhibit entitled "Drainage Basin for Maynard-Border Stream-Ditch" provides an illustration of the existing drainage basin. Current land uses include agricultural, commercial, and a residential mobile home park. The agricultural uses include a combination of crops (hay and grain) and pasture. A summary of the current land uses is: Agricultural 89% 1038 acres ± Commercial 3% 34 acres ± Residential 8% 93 acres ± In order to effectively evaluate the drainage a summary of the land uses at various locations along the Maynard-Border stream-ditch basin channel is necessary. The following provides a summary of the existing basin characteristics at several key locations along the basin. @ Proposed Cascade Street Crossing: Agricultural 750 acres± Residential 33 acres ± Total Basin Area 783 acres ± @ Existing Babcock Street Crossing: Agricultural 722 acres± Residential 33 acres ± Total Basin Area 755 acres ± @ Existing Highway 191 Crossing: Agricultural 603 acres± Residential 33 acres ± Total Basin Area 636 acres ± @ Existing Farmer's Canal Crossing: Agricultural 585 acres± Total Basin Area 585 acres ± 2 of 15 HA3384\003\Drainage\Va11ey West Drainage Study.doc Channel Characteristics Examining the characteristics of the existing Maynard-Border Stream-Ditch channel is an important step in the evaluation of the drainage system. The flow characteristics of the channel and existing culverts along the channel are assessed to determine their capacities. The capacities of these culverts are compared to the estimated post-development runoff along the channel. This comparison is used to estimate the actual flow in the channel and the water surface profile for the 100-year storm event in the Maynard-Border Stream-Ditch after development. There are numerous manmade features on the channel that interrupt flood flows within the natural basin. They include roadways, irrigation facilities, and culverts. The first major feature is the Farmer's Canal. This irrigation canal intercepts runoff draining from 585 acres of agricultural land to the south. Runoff flows directly into the canal or ponds on the south side of the canal. A diversion allows flow to be conveyed from the canal into the Maynard-Border stream-ditch and continue flowing to the north. The diversion consists of a 30" CMP with a headgate. Flow in the Maynard-Border stream-ditch drainage basin is conveyed under Durston Road by three culverts. Primary flow in the ditch is conveyed under Durston Road by a 53"x41" CMPA and continues to the north. A 28"x20" CMPA carries additional irrigation flow across Durston Road and into a separate channel flowing west. Flow into both of these culverts can be controlled by headgate structures at the inlet of the pipes. An 18" CMP with a headgate allows excess flow into the ditch along the south side of Durston Road flowing to the east. This flow is combined with runoff draining from the east portion of the drainage basin and is transported under Durston Road by a 35"x24" CMPA. The drainage area at this location incorporates a larger area as flows from the east side of Ferguson Avenue are diverted under that road to this culvert. Excess flow from the Maynard-Border Stream-ditch will flow over the west bank of the channel and to the west in the south ditch of Durston Road to Cottonwood road. The Exhibit entitled Durston Flow Scheme illustrates the irrigation and runoff flow scheme at Durston Road. A list of the size and capacities of culverts at various other locations along the channel is provided below. Culvert capacity calculations are located in the Major Drainage Plan Appendix. Maynard-Border Stream-Ditch Existing Culverts and Capacities Location Size Capacity Farmer's Canal 30" CMP 39 cfs Highway 191 . 36" CMP 50 cfs Babcock Street 582@x36" RCP 99 cfs 3 of 15 HA3384\0030rainage\Valley West Drainage Study.doc Post Development Basin Characteristics The rational method and the City of Bozeman Storm Water Master Plan are used to calculate the peak runoff for a given storm event. A table identifying typical runoff coefficients is included in the Minor Drainage Basin Appendix. Runoff Coefficients used in this report are: Land Use Runoff Coefficient (C) Agricultural Land 0.20 Agricultural Suburban Development 0.28 Residential (Medium Density) 0.35 Commercial Neighborhood 0.70 The Valley West Drainage Plan is primarily designed to provide public convenience and safety by accommodating frequent storm events. The return period for the minor drainage system is 10 years with the 25-year event used for perennial stream crossings at streets and the major drainage system. The ability of the major drainage system to convey larger storm events up to the 100-year event is also considered. Development increases downstream peak flow by increasing runoff volumes and decreasing times of concentrations within the basin. The minor drainage system incorporates detention ponds to detain the 10-year storm event. These ponds reduce the downstream impact to levels existing prior to creation of the subdivision. The ponds will also provide settlement time for sediment and other pollutants to be removed. The Valley West Preliminary Drainage Plan is included in the Minor Drainage Plan Appendix. Design takes advantage of natural features that contribute to the management of storm water. Engineering techniques preserve and enhance the natural features and processes of the site. This in turn maximizes post- development economic and environmental benefits, particularly in combination with open space and recreational uses. Our design improves the effectiveness of the natural systems, rather than negates, replaces or ignores them. The Major Drainage System The backbone of the major drainage system in the east portion of the subdivision is the Maynard-Border stream-ditch. The drainage design utilizes it's channel capacity to convey runoff generated by return periods up to 100 years without inundating structures. Culverts placed within the waterways and storm drain pipes are designed for the 25-year return period. Installing culverts to convey runoff greater than the 25-year event results in excessive culvert costs with minimal reduction in the potential for adverse consequences. The applicant's funds are better invested in site grading and specifying floor elevations for lots adjacent to waterways. Site grading is designed so that storm events between 25 and 100 years pass over roadways or through emergency overflow areas without inundating building structures or causing significant erosion. 4of15 H:\3384\003\Drainage\Valley West Drainage Study.doc Post development flow estimates account for existing flows within the drainage basin as well as the effects of development in the basin. Estimating runoff for the post-development 100-year storm event is the first step. The existing basin is largely undeveloped. Future development of the basin is considered when determining the post development runoff. A limiting factor to runoff entering the Maynard-Border Stream-Ditch is the Farmer's Canal. A 30" CMP conveys irrigation flow and excess runoff from the canal to the ditch. The capacity of the culvert is estimated at 39 cfs. The estimated runoff for the 100-year event in the basin at the Farmer's Canal is 65 cfs corresponding to the future agricultural suburban development. The more conservative number of 65 cfs is used to insure adequate drainage capacity to account for downstream development of the major drainage system. Existing development between the Farmer's Canal and Highway 191 consists of a residential mobile home park and some rural residential home sites. Two livestock watering ponds, that obtain water from the Maynard-Border Stream- ditch, are constructed directly north of the canal. It is unlikely that use of this property will be significantly modified in the future. The estimated runoff for the 100-year event at the crossing of Highway 191 is 67 cfs. The capacity of the existing culvert under Highway 191 is estimated at 50 cfs. This corresponds roughly to the estimated runoff for the 25-year event at this location. This culvert significantly restricts flow to the downstream channel during the 100-year event. Based on visual field inspections excess runoff ponds in the ditch along the south side of Highway 191. To remain conservative and account for the estimated runoff within the entire basin 67 cfs is used in analysis of the major drainage system. Future commercial development in the basin area between Highway 191 and Babcock Street is considered. Zoning in this area is Residential-Office. It is anticipated that development in this area will be held to similar design criteria required of the Valley West #1 Subdivision. Based on this assessment, the basin characteristics are modified for the post-development 100-year event as follows: @ Babcock Street Crossing: (including basin areas to the south of Highway 191) Commercial 119 acres + C = 0.70 Residential 51 acres + C = 0.35 Agricultural Suburban 585 acres ± C = 0.28 Total area 755 acres + C = 0.35 Similar considerations are made for the proposed crossing of Cascade Street and existing crossing of Durston Road. Zoning in the area between Babcock Street and Durston Road is Residential (R-3). The 100-year runoff from the Valley West Subdivision is to be routed to the Maynard-Border Stream-Ditch through drainage swales and streets, as shown of the drainage plan. Based on these zoning and flood routing considerations, the basin characteristics for the post-development 100-year event are modified as follows: 5 of 15 HA3384\003\Drainage\Val1ey West Drainage Study.doc @ Proposed Cascade Street Crossing: Commercial 119 acres ± C = 0.70 Residential 79 acres ± C = 0.35 Agricultural Suburban 585 acres ± C = 0.28 Total Basin Area 783 acres ± C = 0.35 @ Durston Road Crossing: Commercial 153 acres ± C = 0.70 Residential 427 acres ± C = 0.35 Agricultural Suburban 585 acres ± C = 0.28 Total Basin Area 1165 acres ± C = 0.36 A summary of the estimated post-development 100 - year runoffs is: Maynard-Border Stream-Ditch Outfall Q25 0100 Durston Road Crossing 113 139 Cascade Street Crossing 76 94 Babcock Street Crossing 76 93 Highway 191 Crossing 54 67 Farmer's Canal Crossing 52 65 The culvert to be installed at the crossing of Cascade Street is sized to pass the 25-year storm event. The culvert sizing analysis is provided in the Major Drainage Plan Appendix. The required culvert is a 581/2"x 36" RCPA. The investigation assesses the characteristics of the Maynard-Border Stream- ditch channel to estimate the 100-year water surface elevations along the channel adjacent to the Valley West Subdivision. This is to establish minimum finished floor elevations for the lots within the subdivision. The 100-year water surface is estimated using the typical cross sections of the channel through the Valley West property. The land use plan for Valley West Subdivision calls for a park/open space corridor along the ditch. This buffer is considered in developing estimated surface water depths along the channel. The assessment and proposed channel cross section are included in the Major Drainage Plan Appendix. The estimated water surface depth for the 100-year storm event in the Maynard- Border Stream-ditch channel through the Valley West Subdivision is 2 feet. The finished floor elevations of the properties adjacent to the open space corridor surrounding the ditch need to be set at 3 feet above the channel bottom providing 1 foot of freeboard during the 100-year event. The grades for roadways adjacent 6of15 HA3384\003\Drainage\Valley West Drainage Study.doc to the Maynard-Border Stream-Ditch will be established to achieve these minimum elevations. The overtopping of Cascade Street during the 100-year storm event is also considered in setting minimum finished floor elevations. The culvert conveying flow in the Maynard-Border Stream-ditch across Cascade Street is sized to pass the 25-year event. Flows exceeding the 25-year event will pass over Cascade Street on the west side of the Channel. Excess runoff overflowing Cascade Street will flow to the Community Pond to be created directly west of the channel and north of Cascade St. Ferguson Avenue Drain Pipe Evaluation The Ferguson Drain is an existing ditch running along the west side of Ferguson Avenue. It is part of the Maynard-Border Drainage Basin. This ditch was constructed concurrently with the Ferguson Avenue Roadway to convey existing flow from the south to the Maynard-Border Stream-Ditch at Durston Road. It also drains the extreme eastern portion of the Valley West Subdivision. This ditch will be replaced with drainage pipe with the development of the property. The exhibit entitled "Drainage Basin For Ferguson Drain" illustrates the existing basin conveying runoff to the Ferguson Drain. An existing culvert crossing under Highway 191 controls the amount of flow entering the Ferguson Drain from south of the highway. The capacity of the culvert is 36.6 cfs based on a headwater depth of 3 feet. Field inspection of the property directly upstream from the culvert show that 3' is reasonable for the maximum headwater elevation. Analysis of the drainage basin flowing into the culvert yield the following runoff flows: Return frequency Pre-development Q Post Development Q cfs cfs 5-Year 6.7 20.8 25-Year 10 31.1 100-Year 12.3 39.2 This analysis uses a pre-development C factor of 0.20. Future development of the basin is accounted for assuming residential development at a C factor of 0.35. The residential land use corresponds to the City of Bozeman 2020 Plan land use designation. The pre-development time of concentration is determined using Figure 22 of the City of Bozeman Storm Water Master Plan to estimate and overland flow velocity of 0.4 fps. Post development time of concentration is estimated using and average flow velocity of 1 fps, assuming a combination of overland and gutter flow to the culvert. As can be seen the existing culvert capacity roughly corresponds to the estimate 25-year runoff event from the south. The capacity of the culvert will be used as a limiting factor. 7 of 15 HA3384\003\Drainage\Val1ey West Drainage Study.doc The assumption is made that flow entering the pipe between Highway 191 and Babcock St. is limited to runoff from Ferguson Avenue. The grading plan for the Spring Creek Village Resort Subdivision shows flow from this property into the Ferguson Drain to be negligible. The Ferguson Avenue street area is approximately 2.3 acres. The flow will cross Babcock St. in the existing Valley Gutter and flow to an inlet in Ferguson Avenue directly north of the proposed Cascade St./Ferguson Avenue intersection. The drainage area of Ferguson Avenue to the north of Babcock St. is also considered. Using time of concentration adjustments along Ferguson Avenue the runoff at the inlets north of Cascade St. is estimated @: Return frequency Post Development Q cfs 5-Year 4 25-Year 6 100-Year 7.9 A similar calculation for the inlets at Durston Road on Ferguson Avenue yields the following: Return frequency Post Development Q cfs 5-Year 5.1 25-Year 7.7 100-Year 10.5 Using Time of Concentration adjustments for the entire basin we calculate the runoff in the Ferguson Drain at the inlets north of Cascade Street including runoff from the Valley West Subdivision as follows: Return frequency Post Development Q (cfs) 5-Year 23.8 25-Year 35.7 100-Year 44.9 A similar calculation for the inlets at Durston Road on Ferguson Avenue yields the following: Return frequency Post Development Q cfs 5-Year 24.1 25-Year 36.1 100-Year 45.4 The Ferguson Drain needs to provide a flow capacity of 36.1 cfs to drain the 25- year event. The Valley West Drainage Plan calls for a 36" CMP @ an overall slope of 1% to be installed in the Ferguson Avenue Parkway. The parkway will be owned and maintained by the Valley West Subdivision Homeowner's Association. The capacity of the pipe is 39.3 cfs. 8 of 15 HA3384\003\Drainage\Valley West Drainage Study.doc Baxter Ditch Flood Analysis The Baxter Ditch is an existing ditch running along the west side of Cottonwood Road. This ditch was constructed concurrently with the Cottonwood Road improvements and as part of the Wetland Mitigation Plan for the Valley West Subdivision. It will be utilized to convey runoff from south of Babcock St. and portions of the Valley West Subdivision east of Cottonwood Road. As the ditch is part of an approved wetland mitigation plan is will remain an open ditch for drainage purposes. Culverts passing under Highway 191 and Cottonwood Road control flow entering the Baxter Ditch from the south. The culvert under Cottonwood Road accounts for flows passing under Highway 191 and include runoff between the highway and Babcock St. The capacity of the existing 26 5/8" x 43 3/a" culvert under Cottonwood road will therefore be used to assess the flow in the Baxter Ditch_ The capacity off the culvert is 45.3 cfs. The total area contributing to the Baxter Ditch downstream of the culvert is 56.8 acres. This includes the area within the Valley West Subdivision east of Cottonwood Road including Cottonwood Road from Babcock St. to Durston Road (49.6 acres), and the Cottonwood Road right-of-way from Highway 191 to Babcock St. (7.2 acres). Using Time of Concentration adjustments to account for development in Valley West yields the following runoff flows generated from this contributing area. Return frequency Post Development Q. cfs 5-Year 15.6 25-Year 22.7 100-Year 29.5 A conservative approach regarding the total flow in the Baxter Ditch is to neglect the Time of Concentration adjustment from the contributing area south of the 26 5/8" x 43 3/4" culvert in Cottonwood Road and add the capacity of this culvert directly to the flow from the contributing area below this culvert. This approach yields the following: Return frequency Post Development Q (cfs) 5-Year 60.9 25-Year 68.1 100-Year 74.8 Using a typical cross-section and the slope of the existing Baxter Ditch channel we can estimate the capacity of the channel for varying flow depths. A flow depth in the Baxter Ditch of 2.8 ft yields a capacity of 109.81 cfs. This is adequate to drain the Valley West Development. The finished floor elevations of the properties facing the Baxter Ditch will be set at 4.5 ft above channel bottom. 9of15 HA3 384\003\Dra inage\Val ley West Drainage Study.doc Baxter Creek Flood Analysis This section evaluates the potential for flooding along Baxter Creek in the western portions of the subdivision. The One hundred (100) year water surface elevations and the 100-year high water limits for Baxter Creek are shown on the Valley West Drainage Plan. Hydraulic calculations and the water surface profile for Baxter Creek shall take into account the upstream and downstream culvert capacities. The Valley West Subdivision Drainage does not direct flow directly to Baxter Creek. Extensive wetlands will be developed to effectively detain or retain runoff minimizing the effect on Baxter Creek. The following evaluation estimates 100-year water surface elevations and high water limits for Baxter Creek accounting for existing basin characteristics including upstream and downstream culvert capacities, and the effects of the Farmer's Canal. The evaluation considers existing conditions only. In reviewing the open space and wetland improvements surrounding Baxter Creek within the Valley West Subdivision, additional analysis of the basin is not necessary. Existing Basin Characteristics Baxter Creek originates approximately 4 miles south of Highway 191 (Huffine Lane). The drainage area south of Durston Road is approximately 1060 acres more or less. Current land uses are mainly agricultural with the exception of J.C. Billion auto plaza at the intersection of Cottonwood Road and Huffine Lane. In order to effectively evaluate the drainage a summary of the land uses at various locations along the Baxter Creek basin is necessary. The following provides a summary of the existing basin characteristics at several key locations along the basin. • Existing Farmer's Canal Crossing Agriculture 495 acres Total Basin Area 495 acres • Existing Highway 191 Crossing Agriculture 528 acres Total Basin Area 528 acres • Existing Durston Road Crossing Agriculture 1,040 acres Low Density Commercial 20 acres Total Basin Area 1,060 acres The rational method and City of Bozeman Storm Water Master Plan are used to calculate the peak runoff for the 100-year flood event in Baxter Creek, not 10 of 15 HA3384\003\Drainage\Va11ey West Drainage Study.doc accounting for the effects of the Farmer's Canal, existing culvert, or other manmade features along the stream. The basin characteristics to estimate runoff represent existing basin conditions. Basin characteristics to evaluate these conditions are as follows: @ Existing Farmer's Canal Crossing Agriculture 495 acres + C = 0.20 Total Basin Area 495 acres + C = 0.20 @ Existing Highway 191 Crossing Agriculture 528 acres + C = 0.20 Total Basin Area 528 acres + C = 0.20 @ Existing Durston Road Crossing Agriculture 1040 acres + C = 0.20 Low Density Commercial 20 acres + C = 0.70 Total Basin Area 1060 acres + C = 0.21 Calculations to estimate the pre-development 100-year runoff at each of the above locations are provided in the Appendix. A summary is shown below: Baxter Creek Stream Ditch Outfall Q100 Farmers Canal Crossing 38 cfs Highway 191 Crossing 38 cfs Durston Road Crossing 65 cfs An examination of the characteristics of the existing Baxter Creek channel and drainage basin is an important step in evaluating the potential for flooding. The flow characteristics of the channel and existing culverts along the channel are assessed to determine their capacities. The capacities of the culverts are compared to the estimated 100-year runoff along the Channel. This comparison is used to estimate the actual flow in the channel and the water surface profile for the 100-year storm event. There are numerous manmade features, along the channel, that interrupt flood flows within the natural basin. They include roadways, irrigation facilities, and culverts. The first major feature is the Farmers Canal. This irrigation canal intercepts runoff draining from 495 acres to the south. Runoff flows directly into the canal or ponds on the south side of the canal. A portion of the water flowing 11 of 15 11:\.3384\003\Drainage\Valley Nest Drainage Study.doc in Baxter Creek to the north is diverted from the canal. The Farmer's Canal Company can also use the creek as a blow-off at any time. A diversion allows flow to be conveyed from the canal into Baxter Creek through a 30" CMP culvert. The capacity of the 30" pipe is estimated at 39 cfs (see Appendix). Other considerations include steam modifications between the Farmer's Canal and Highway 191. Baxter Creek flows through three ponds constructed in line with the channel. These ponds will effectively act to detain a significant amount of runoff during large storm events. A limiting characteristic to the flow in Baxter Creek is the crossing of Highway 191. Baxter Creek crosses Highway 191 through a 54" CMP culvert. Water flows through this culvert until the headwater reaches a depth two feet above the top of pipe. Excess water flows in a roadside drainage ditch into adjacent fields or through a 21" CMP culvert crossing under Highway 191 approximately 830 feet to the west. The flow crossing through the 21" culvert eventually re-enters Baxter Creek on the north side of Highway 191. Under extreme conditions these culverts act to limit the flow crossing Highway 191 and into Baxter Creek where it crosses Valley West Subdivision. The capacity of these culverts is determined using information from the Montana Department of Transportation and onsite inspection. Culvert capacity calculations are included with this report (see Appendix). The capacity of the 54" culvert is 150 cfs with a headwater elevation 2 feet above the top of pipe. The capacity of the 21" culvert is 15 cfs. The total capacity of the piping under Highway 191 is 165 cfs. Flow in Baxter Creek is conveyed under Durston Road by a 54" CMP culvert. The capacity of this culvert is estimated at 150 cfs. Excess flow ponds on the south side of Durston road in a natural wetland area to be primarily located within the boundary of Bronken Park. Estimating 100-year runoff in Baxter Creek through Valley West Subdivision An estimation of the 100-year event in Baxter Creek must account for the various constraints and characteristics of the existing channel. When accounting for the effects related to the Farmer's Canal and the constraints associated with the culverts crossing of Highway 191, it is reasonable to assume that during the 100- year event, flow to the north of the highway will be limited to the capacity of the culverts. The capacity of the culverts is estimated at 165 cfs. Our analysis of the 100-year runoff at this location is 39 cfs, not accounting for the potential of an emergency discharge from the Farmer's Canal. The capacity of the discharge pipe that diverts water from the Farmer's Canal into Baxter Creek is estimated at 39 cfs. Comparing these potential flows, the 165 cfs capacity of the culverts crossing Highway 191, represents an over-conservative estimate for the total flow at this location. 12 of 15 H:\3384\003\Drainage\Valley West Drainage Study.doc Based on the existing channel characteristics, the 100-year runoff in Baxter Creek, at Highway 191, is estimated to be 39 cfs. The capacity of the culvert that diverts water from the Farmer's Canal is also 39 cfs. Combining these flows results in an estimated 100-year runoff of 78 cfs at Highway 191. This represents a conservative estimate in that the effects of the ponds to the south of the highway are not considered. These ponds will effectively act to detain runoff during significant flooding and reduce the flow in the steam. To obtain the estimated 100-year event at Durston Road, runoff from the contributing area between Highway 191 and Durston Road is added to the estimated flow at Highway 191. Accounting for time of concentration adjustments between Highway 191 and Durston Road, the estimated runoff for the 100-year event from this contributing area is 36 cfs. Based on the above assumptions, the total estimated 100-year runoff at Durston Road is 114 cfs. This flow is used to estimate the 100-year water surface elevations and 100-year high water limits for the existing Baxter Creek basin. Calculations to estimate the 100-year water surface elevation at various locations along the channel are in the Appendix. The estimated 100-year water surface limits are illustrated on the Final Plat. MINOR DRAINAGE SYSTEM The Valley West Subdivision is divided into 4 Drainage Areas for evaluation of the minor drainage system. The 4 drainage areas are illustrated on the Valley West Drainage Plan. Detailed minor drainage plans for each of the 4 areas are also provided. The minor drainage systems for the Valley West Subdivision are made up of streets with curb & gutter, drainage swales, inlets, valley gutters, and detention ponds. Runoff is collected in the streets and drainage swales, transported to detention ponds, and then released to the receiving channel or facility. The ponds are less than 1 .5' deep and designed so that the visual impact to the surrounding area is minimal. The ponds detain the 10-year post-development runoff volume. Runoff is released from the detention basin at a rate equal to or less than the pre-development rate for the 10-year storm event. During larger storm events site grading provides a designated path to convey runoff from the developed areas without inundating structures and minimizing erosion to the channel. Drainage Area 1 consists of seven minor drainage basins located east of the Maynard-Border Stream-Ditch. The eastern portion of this area drains to the Ferguson Drain. The western portion drains to the Maynard-Border Stream- Ditch. Detention facilities are located on park areas to be owned and maintained by the homeowner's association. Retention ponds will be used to retain flow from existing inlets along Babcock St. The detention and retention facilities are illustrated on the "Drainage Area 1 — Minor Drainage System." 13 of 15 H:\3384\003\Drainage\Valley West Drainage Study.doc Drainage Area 2 consists of eight minor drainage basins located between the Maynard-Border Stream-Ditch and the Baxter Ditch. The eastern portion of this area drains to the Maynard-Border Stream-Ditch. The western portion drains to the Baxter Ditch. Detention facilities are located on park areas to be owned and maintained by the homeowner's association. Retention ponds will be used to retain flow from existing inlets along Babcock St. The detention and retention facilities are illustrated on the "Drainage Area 2 — Minor Drainage System." Drainage Area 3 consists of six minor drainage basins located between the Baxter Ditch and Baxter Creek. The majority of this area drains to the wetlands to be created adjacent to Baxter Creek. The wetlands will effectively detain the runoff prior to discharge into Baxter Creek. Collection and discharge locations are illustrated on the "Drainage Area 3 — Minor Drainage System." Drainage Area 4 consists of six minor drainage basins located west of Baxter Creek. The majority of this area drains to Adjacker Creek located just east of the property. The eastern portion drains to the Baxter Creek. The majority of this area drains to the lake to be create with the development. The lake will effectively detain the runoff prior to discharge into the receiving stream. Collection and discharge locations along with detention facilities are illustrated on the "Drainage Area 4 — Minor Drainage System." Inlet locations are an important aspect of the minor drainage plan. Inlets will be located to limit the spread of water in the street and curb and gutter to allow for one 12' travel lane during the 10-year event. City of Bozeman Standard inlets, chases, and/or detention pond outlet structures will be utilized. The capacity of the inlets must be considered to insure an adequate number are provided to drain the area. Inlets will be placed at the approximate locations shown on the drainage plan to handle the anticipated 10-year event runoff for the subdivision. Storm drain piping is used where necessary to carry flows from inlet locations to the detention basins. Detention pond and inlet piping will be sized to carry the 10-year runoff events as determined in this report. Major drainage ways and pipes will be sized to accommodate 25-year runoff events. The Storrs Water Maintenance Plan The maintenance of the onsite storm water collection, detention and discharge release facilities will be integrated as part of the duties of City of Bozeman maintenance personnel responsible for storm drainage facilities. Duties shall include routine inspection to ensure that debris, yard waste, and seasonal ice does not impede operation of the detention ponds and discharge structures. It is recommended that these inspections occur after major runoff events and on a monthly basis throughout the year. Detention ponds located within open space/parks owned by the Valley West Homeowners Association will be mowed as part of the regular ground 14 of 15 H:\3384\003\Drainage\Valley West Drainage Study.doc maintenance for the park facility. The City of Bozeman parks ground maintenance crews will maintain detention ponds within parks dedicated to the City of Bozeman. Detention ponds shall be mowed on a regular basis during the growing season and be maintained free of vegetation that might impede their storage capacity by the homeowner's association. 15 of 15 HA3384\003\Drainage\Valley Nest Drainage Study.doc TO GALLATIN _ , ATo GALLATIN ARL ER .... ,.- . . �i�Ail RIVER .f�; `� •�•. -• - - - .1,y 9--. �3��: � -y I TTTT _ :, _ 'r}�..•,i�� _K` � _ ��, .. 1I� a �� � �_-___ __ —^",«�v`��• �' - i; � '�a � is. � - a W 'I IWO !2 41A a: SPRING / m {� 3 Riy 1. J ,• LL :a V cr _ t C) �- o z w a ir cr V 0 F— w � � � � x•- .. ,��. �7 at4 ya }� j1.� � IDS }} • ,1 it ,fc ; rat• �y tiY $t' s + FROM FA r FARMER'S < s,.; / ` CANA �, 4 CANAL VALLEY WEST SUBDIVISION MAJOR DRAINAGE COURSES IN ASSOCIATION WITH: IMORRISON '/<SEC. SECTION TOWNSHIP RANGE �dJ a P R u G H MAIERLE,iNe. .. 0 00 20=�0 L E NON `•"K�"�'+'+�.��' '�,rl��' �(W m-' PRINCIPAL MERIDIAN, MONTANA aspen CLIENT: GALLATIN COUNTY, MONTANA SCALE:1'=209 enterprises,I.I.c. R PLOTTED DATE Nov 16 2001 (This Orawin0 Hae Been ReOUCetl1 FELD WORK DATE:10/15/01 1/T��Mwn Stml %I--" MT 59715 S DRAWING NAME: DRAWN BY: KSS CALE:1'=200' x:\ales\ooz\ac+e\ce�en\u sueums�\ mt- a.9 _ CHECKED BY�PF PROJ/•AM 002 SHEET 1 OF 1 DRAINAGE BASIN FOR EXHIBIT 1 BAXTER CREEK ',--L "ILL -1 J"' I • 1 � ', rI ­-71r • I 11 �'A ER CR�E � OVE AND FL . . I\ h D�A NAGE BIASI a o - _ - - ----'- --- � T CHA "NEL FLO EJN ,' PONDS NSTRU ED AXTER CRE K CHA ELNOT SOWN -T�.SC, E) ti RMER' , CANAL ROBBING SIN A A = 49 BASIN AR A = j 10 0 ACRES@ •• . • • h --_ • . 0 h i LXI TIN 3 F RME AL OV FLOW a tl HIG WA 19 CR SS N- BASN � — f CIA MORRISON SURVEMIS MAIERLE,INC. GRAPHIC SCALE ro rr II13 IM 1„a•ry sw bmM a SM •n.s pal s om RX(4"N-nn REWSION DATE: REMS70N DATE: DRAYN BY: TJG DATE.01/27/00 1 Ima- !'ODOR H:\2885\010\dwg\EXHIBITI—Baxter floodplain.dwg Nov/06/2001 CHECKED BY: GJS DRANING NO.: EXHIBITI.DWG JOB NO.: 28MO09 040 0310 SHEET 1 OF 1