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HomeMy WebLinkAbout09-24-21 Public Comment - D. Sauther - Canyon GateFrom:Diana Sauther To:Jacob Miller Cc:Ross Knapper; Agenda; Jack Sauther Subject:Canyon Gate proposal -- storm water concerns Date:Friday, September 24, 2021 5:24:07 PM Attachments:Historical drainage route vs current developments.pngSTORMWATER MASTER PLAN-pplat.pdfSTORMWATER MASTER PLAN-color-May 2006.pdf CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe. Hi Jacob, I understand that you are the planner assigned to the Canyon Gate proposed development atthe corner of Story Mill and Bridger. I live in the neighboring Legends neighborhood. Are you aware that the Legends subdivision's Storm Water Master Plan to channel run-off andoverflow from Bridger Creek (and save homes from flooding) ultimately drains onto the Canyon Gate parcel? The design, approved by the City, retained the historical flows, whichmoved from the Legends area onto the Canyon Gate parcel. In fact, it appears that three of the four historic outfalls along the western boundary of the Legends flow onto the Canyon Gateparcel. What provisions are being made to accommodate this flow? Given the unpredictability of our rapidly warming climate with its bigger and more devastating storms,proper plans and facilities for accommodating what used to be 100-year storms is even more important. Attached are both the preliminary and final plans -- as well as a side-by-side screenshot I mocked up to better visualize the drainage plan with the developed neighborhood. Below ismy layman's summary of the the key points. Any water that overtops the banks on the south side of Bridger Creek is expected toflow initially south and then west toward the Legends property line. Of the 4 historicoutfalls along the western boundary of the Legends, three of them flow onto theCanyon Gate parcel. "When large precipitous of flood hazard events occur, it islikely these existing flow paths or routes represent the major drainage system forthe area." (p.4 of May 2006 report) I've attached a screen shot comparing the flows depicted in the report side by side with the Legends as developed. The Legends was designed with a substantial system of culverts, swales, retention ponds, etc., to: reduce the downstream peak runoff to levels that existed pre-subdivision build off of the existing topography, taking advantage of the many naturalfeatures the contribute to storm water management create three retention ponds designed to retain the 10-year post-developmentrunoff volume while releasing excess flow at a rate not to exceed the 10-year pre- development rate. Pond A is 36,000 cubic ft. -- 34 acres -- and abuts the Canyon Gateproperty.Ponds B and C are in the creek corridor near the playground area -- 6,132 and 1,157 cubic ft, totaling 5.9 and 1.1 acres respectively.Overflows for precipitation events exceeding the 10-yr 2-hour willproceed downgrade along historical flow patterns. road culverts within the Legends greenspace corridors are designed to conveyrunoff from a 25-year event without overtopping the roadway Storms greater than a 100-yr event are expected to pass over roadways or throughemergency overflow areas Bottom line: Any development on the Canyon Gate parcel MUST take into account thehistorical flows and be able to accommodate them through proper storm water management. The proposal the Canyon Gate developer gave to the Legends HOA board does not seem toaddress this -- I see no retention ponds, and only 2 acres of "parkland" to be used as a trail. On a separate note: The developer shows a new Canyon Gate Lane road tying intoNorthview. To do this, he would have to cross over Legends HOA open space; he doesn't own that land, and thus has no right to it. Best, Diana -- Diana Sauther Top Hand Realty Advisors, Inc. 403 W. Main Street, Suite 1 Bozeman, MT 59715 Office: 406-586-0356 Cell: 406-599-3433 Fax: 406-551-1008 Diana@TopHandAdvisors.com www.TopHandAdvisors.com N:\3959\005\Design Docs\Storm\FPLAT\doc\STORMWATER MASTER PLAN FPLAT.doc STORM WATER DESIGN REPORT For The Legends at Bridger Creek II Subdivision May 2006 Prepared by: Storm Water Design Report for The Legends at Bridger Creek II Page 2 of 7 Table of Contents I. Introduction II. The Proposed Drainage System A. Major Drainage System B. Minor Drainage System C. Ponds D. Culvert Inlets and Street Flow III. Recommendations and Additional Thoughts IV. Storm Water Maintenance Plan Appendices A. Hydrological Calculations B. Hydraulic Calculations Storm Water Design Report for The Legends at Bridger Creek II Page 3 of 7 I. Introduction The proposed Legends at Bridger Creek II Subdivision is located in the south half of Section 32, Township 1 South, Range 6 East. More specifically, it is located east of Story Mill Road and the Legends at Bridger Creek I, along Boylan Road extended. The development includes 127 single family lots on total area of approximately 57.3 acres. The project also has approximately 23.7 acres of open space with the Bridger Creek running through the north end of the property. The neighboring subdivision to the east does not convey runoff into the subject property. However, recent modeling and historical photographs suggest that spring runoff down the Bridger Creek Drainage can make its way through the subject property. Since the neighboring Bridger Creek is a natural, meandering stream perched on alluvium, it is not uncommon to have areas adjacent to the stream that are actually at or below the elevation of the active stream bed itself. This phenomenon can be seen just west of the subject property in the Vicinity Map below. And of course the site itself generates runoff as well. Vicinity Map II. Proposed Drainage Plan The Legends at Bridger Creek II Storm Water Plan consists of two drainage systems- a Minor and Major System. The Minor Drainage System is designed to provide public convenience and to accommodate moderate, relatively frequent runoff events. The Major Drainage System has the task of conveying significantly more water while protecting structures from inundation. The Major Drainage System begins operating when the rate or volume of runoff exceeds the capacity of the Minor Drainage System. The storm return period for the Minor Drainage System for this project shall be 25-years for conveyance structures and 10-years for ponds. The Major Drainage System shall convey the 100-year event without inundating building structures. Development is commonly known to increase downstream peak flows by increasing runoff volumes and by decreasing the time it takes the runoff to travel downstream. As part of the minor drainage system, Storm Water Design Report for The Legends at Bridger Creek II Page 4 of 7 retention pond areas designed for the 10-year return period shall be implemented to reduce the downstream peak runoff to levels that existed before the subdivision was constructed. The pond areas will also provide sufficient time for sediment and other storm water pollutant loads to settle out. Design of the Drainage Plan was based on survey data collected across the subdivision site using sub- centimeter GPS equipment. The data was used to produce a 1’ contour map of the existing ground. The proposed plan builds off of the existing topography and takes advantage of the many natural features that contribute to the management of storm water. Significant lot grading and first floor elevations are proposed as part of the Stormwater Plan to facilitate proper drainage and manage the risk of potential flooding as the result of historical rapid snow melt runoff events in this area and the proximity of the proposed subdivision to Bridger Creek. For the purposes of this study, the modified rational method was implemented to estimate peak runoff rates from the 10, 25, and 100 year storm events. Supporting calculations can be found in the appendices. The Major Drainage System The backbone of the major drainage system throughout the proposed Subdivision is the Legends Greenway Corridor. This greenway corridor has the capacity to attenuate and convey the runoff generated by the 100-year event without inundating any building structures within the subdivision. All road culverts placed within the corridor shall be designed to convey runoff from the 25-year event without overtopping the roadway. Site grading shall be completed so that storm events greater than 100-year event will pass over roadways or through emergency overflow areas without causing significant erosion. The street sections in front of the proposed homes are designed to provide additional runoff conveyance away from the homes, down to the streets and eventually into the Legends Greenway Corridor. The Grading and Drainage Plan shows the necessary finished surface grading for each lot. It is noted that at a minimum, first floor elevations are approximately 0.75 feet above the finished grade contours as shown sheet ST-1. Finished ground and first floor elevations shall also be located on the final plat and in the subdivision covenants. The hydraulic performance of the major drainage system including conveyance capacity and backwater conditions above road crossings were analyzed using HecRas and FlowMaster design software. The finished ground and first floor elevations of lots were adjusted to account for increases in water surface elevations resulting from backwater conditions. Historically, the subject property has drained to the west in multiple broad meandering flow paths that are generally parallel to Bridger Creek. These flow paths route to four separate, existing and historic outfalls along the west boundary of the subject property and continue west towards Story Mill Road where it begins to overtop the road at an elevation of ~4712.5 (COB Datum). When large precipitous or flood hazard events occur, it is likely these existing flow paths or routes represent the major drainage system for the area. Runoff that overtops Story Mill Road will proceed across a parking area just north of the NRCS building where it crosses through a hole in a railroad bed thence flowing northwest in an existing swale through the golf course and eventually back to Bridger Creek. A more thorough discussion of the existing drainage system, FEMA floodplain, onsite/offsite historical flow paths and potential hazards can be found in the preplat Storm Water Plan dated February 2006. The Minor Drainage System The minor drainage system is comprised of roads, culverts, swales, and retention ponds. The ponds shall be designed to retain the 10-year post-development runoff volume while releasing excess flow at a rate not to exceed the 10-year pre-development rate. If larger storm events are encountered, site grading and the emergency pond overflow structures will provide a designated path for the overflow to pass into the creek corridor or natural drainage path without causing significant erosion to the pond. The modified rational method was used to size all retention ponds. Refer to sheet ST-1 for the noted retention areas. The 10-Yr, 2-Hr storm event shall be routed into each pond. Ponds Three retention ponds are proposed to retain storm water runoff generated during the 10 year, 2 hour event as per COB March 2004 design standards. Pond locations are shown on sheet ST-1. Pond A is located in the southwest corner of the property and is a retention pond with approximately 36,000 cubic feet (cft) of Storm Water Design Report for The Legends at Bridger Creek II Page 5 of 7 storage and a mean depth of 1.6’. The contributing area to this pond is approximately 34 acres. Pond B is located in the northwest corner of the property and is a retention pond with approximately 6,300 cft of storage. The pond has enough capacity to completely contain the runoff from a 10-yr, 2hr storm event. The contributing area to this pond is approximately 5.9 acres and is shown on sheet ST-1. Pond C is pond located in the northwest corner of the property and is a retention pond with approximately 1,500 cft of storage. The pond has enough capacity to completely contain the runoff from a 10-yr, 2hr storm event contributed from a subarea of 1.1 acres. As with all retention ponds, overflow for precipitation events exceeding the 10 year 2 hour will proceed downgrade along historical flow patterns Table 1. Retention Ponds ID Pond Location Contributing Area (ac) Est. 10Yr/2Hr Retention Volume (cft) Proposed Pond Volume (cft) Mean Depth (ft) Release Rate (cfs) A Southwest corner 34 35,098 35,500 1.6 NA B Northwest Corner 5.9 6,132 6,300 1.2 NA C Northwest Corner 1.1 1,157 1,500 1.0 NA - This area intentionally left blank - Storm Water Design Report for The Legends at Bridger Creek II Page 6 of 7 Culverts, Inlets, Valley Gutters and Street Flow Inlets will be needed at low points and where street conveyance capacities are exceeded. This project has inlets in both sump and on grade conditions. Culverts are also proposed where the Legend’s Greenway Corridor intersects with Boylan Road and Northview Road. Culvert and inlet locations are denoted on sheet ST-1. The proposed sizes, estimate of capacities and number of inlets can be found in Table 3 below. For this particular site, street conveyance capacities using a 9.5 foot spread from the face of curb were adequate and it appears no additional storm water main runs or piping will be necessary. If street intersections produce a low point or sump condition in the curb return, then flow-through concrete valley gutters are proposed as shown on the Grading and Drainage Plan. Table 2. Typical Street Flows Street Name Est. Max Capacity1 (cfs) Est. Post Q25 (cfs) Conveys Post Q25 In Allowable Street Spread Boylan Road @ 1% 11.1 10.1 Yes Northview Road @ 0.6% 8.6 2.1 Yes Midfield Street @ 0.7% 8.6 3.9 Yes Typical 30’ Alley at 1.3% 2.4 1.1 Yes Maiden Spirit Street @ 1% 10.5 1.1 Yes Medicine Wheel Lane @1.3% 12.7 0.7 Yes Atsina Lane @ 0.6% 7.8 3.2 Yes Blackfeet Lane @ 0.6% 7.8 2.7 Yes Cree Lane @ 1.3% 12.2 2.4 Yes Richau Lane @ 0.9% 9.9 1.8 Yes 1 The maximum Capacity for street sections is measured from 0.15’ below the top back of curb resulting in 9.5’ of allowable spread on both sides. Alleys are measured edge of pavement to edge of pavement. Table 3. Culverts and Inlets ID Culverts Description Est. Post Q25 (cfs) Slope (ft/ft) Hydraulic Head (ft) Actual Capacity (cfs) # of Inlets2 C-1 Boylan Road Greenway Crossing1 1- 18.0” X 28.5” 10.1 0.01 1.5 11.0 4 Inlets in Sump C-2 Northview Greenway Crossing1 2- 22.5”X 36.25” 13.6 0.004 2.0 41 NA C-3 Intersection of Northview & Midfield 1- 18” Lead pipe 12” 6.3 0.01 2.5 10 3 Inlets in Sump C-4 Northview Low Point @ Sta4+25 1-12” 1.7 0.01 2.5 4.0 2 Inlets in Sump C-5 Northview Existing Drainage Crossing1 1-12” 3.0 0.005 2.0 3.4 NA C-6 Maiden Spirit @ West Boundary to Pond A. 1-12” 1.1 0.005 1.5 3.0 2 Inlets on Grade. C-7 Pond-B Inlets 1-15” 4.6 0.005 1.5 5.2 4 Inlets on Grade. C-8 Pond-C Inlets 1-12” 2.0 0.005 1.5 3.0 2 Inlets on Grade. 1 Discharges greater than the 25-yr event are designed to overtop roads and flow down the Greenway Swale. 2 Inlets are assumed to be single NEENAH R-3067 Diagonal or equivalent. 3 All culverts within public right-of-way are Class IV RCP unless otherwise noted. Storm Water Design Report for The Legends at Bridger Creek II Page 7 of 7 III. Recommendations and Additional Thoughts There are always inherent risks when developing land next to streams. The benefits can outweigh the risks if the risks are managed properly. It is recommended that the Grading and Drainage Plan be adhered to and that minimum first floor elevations be enforced. All lots that are immediately adjacent to Bridger Creek shall have a first floor elevation that is at least 2 feet above the Base Flood Elevation or 100-yr event water surface elevation. Further refinement of the Grading and Drainage Plan may be necessary as this project progresses and final design commences. It should be disclosed to prospective buyers that this area is next to a stream and that lands in this area may become inundated during large events. Although no subdivisions lots are in the 100-year floodplain as published by FEMA, it is recommended that prospective lot buyers consider obtaining flood insurance due to proximity. As the result of short lot setbacks and the proposed lot grading, driveways may tend to be steeper than usual if a step between the first floor of the residence and the level of the garage floor is not implemented. Not all lots will need this proposed step. However some lots that may benefit from such a step are 25, 30, 39, 51, 121, & 122. This is not a complete list and it may not be definitive if a particular lot needs a step until the some of the building details (such as driveway location) are established. Due to the acute angle formed between North View Road and the Legend’s Greenway Corridor, it is possible that a small landscape retaining wall may be needed for the area west of lot 39 and east of North View Road. Creative landscaping may also accomplish the task of blending the 4 foot difference in elevation. Again it is imperative that the lot finished grade and finished floor elevations be adhered to. There are areas along the south side of Bridger Creek that possess over-steepened banks, with relative high rates of erosion. These areas are for the most part hundreds of feet away from residential lots. However, it may be prudent to complete a channel stability assessment and provide any necessary natural or soft bank stabilization efforts such as plantings now or in the near future. IV. The Storm Water Maintenance Plan The maintenance of the onsite storm water collection, detention and discharge release appurtenances shall be integrated as part of the duties of maintenance personnel responsible for normal grounds keeping. Duties shall include routine inspections to ensure that debris, yard waste, and seasonal ice does not impede operation of the detention ponds and discharge structures. These inspections shall occur after each major runoff event and on a continued monthly basis throughout the year. All results shall be recorded and kept on file for future verification by regulation agencies. In addition to the routine inspections, annual inspections of appurtenances shall assess and mitigate, if necessary, performance of the detention ponds, and release structures. These inspections shall address accumulation of sediment and debris in the detention ponds and release structures as well as any weather or vandalism-related damage. Detention areas and swales shall be mowed on a regular basis during the growing season so as to remain free of vegetation that might impede their conveyance capacity or ability to function. N:\3959\005\Design Docs\Storm\PREPLAT\STORMWATER MASTER PLAN - PREPLAT.doc PRELIMINARY PLAT STORM WATER PLAN for The Legends at Bridger Creek II Subdivision February 2006 Prepared by: Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 2 of 16 Table of Contents I. An Introduction to the Existing Drainage System A. Existing Historical Flow Paths B. Bridger Creek 100-yr Floodplain II. The Proposed Drainage System A. Major Drainage System B. Minor Drainage System III. Recommendations and Additional Thoughts IV. Storm Water Maintenance Plan Appendix A. Photos B. Hydrological Calculations C. Hydraulic Calculations Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 3 of 16 I. An Introduction to the Existing Drainage System The proposed Legends at Bridger Creek II Subdivision is located in the south half of Section 32, Township 1 South, Range 6 East. More specifically, it is located east of Story Mill Road and the Legends at Bridger Creek I, along Boylan Road extended. The development includes 127 single family lots on total area of approximately 57.3 acres. The project also has approximately 23.7 acres of open space with the Bridger Creek running through the north end of the property. The neighboring subdivision to the east does not convey runoff into the subject property. However, recent modeling and historical photographs suggest that spring runoff down the Bridger Creek Drainage can make its way through the subject property. Since the neighboring Bridger Creek is a natural, meandering stream perched on alluvial gravels, it is not uncommon to have areas adjacent to the stream that are actually at or below the elevation of the active stream bed itself. This phenomenon can be seen just west of the subject property in the Vicinity Map below. And of course the site itself generates runoff as well. All of the abovementioned sources and potential hazards are further explored within the pages of this report and are considered to be major components of the existing drainage system within subject property. Vicinity Map Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 4 of 16 Existing Drainage Basin The subject property has historically drained to the west, paralleling Bridger Creek as depicted with the cyan colored drainage arrows in Figure 1. This figure illustrates how the creek is perched when compared to land adjacent to it. The existing contours show an undulating surface comprised of farming modifications mixed with ancient channel meander scars. If water is spilled from the existing left or south bank of the creek, it will flow away from the creek and proceed in a westerly fashion towards Story Mill Road. The cyan arrows along the west boundary of the subject property indicate locations where drainage will likely leave the property. Figure 1 – Onsite historical flow paths. The historical flow paths offsite are represented with dark blue drainage arrows as seen in Figure 2 on the next page. Field surveys and observations in and around the low point of Story Mill Road were unable to locate an existing roadway culvert. It appears that the lands east of Story Mill Road provide an area for natural ponding of excessive runoff from Bridger Creek with no flow through a culvert under the road at this location. Story Mill Road acts as a barrier creating this historical depressional storage behind it. The limit of this storage volume is controlled by the overtopping elevation necessary for the conveyance of runoff over Story Mill Road. Our calculations estimate this overtopping water surface elevation to be in the 4712.5 to 4713.5 range (City of Bozeman Datum). This historical backwater effect created by Story Mill road will obviously have an impact on lands immediately adjacent and east of the road. However, it will have no effect on the subject property. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 5 of 16 Figure 2 – Offsite historical flow paths. After the excessive runoff overtops Story Mill Road, it then proceeds across a parking area just north of the NRCS building where it encounters an old railroad bed. The bed has a hole cut through it allowing flow to cross to the west side of the bed. This rail road bed shows up on the FEMA Flood Plain Map as well as the USGS Quadrangle Map. The barrow areas along the side of the existing railroad bed are considerably lower than native ground and act as channels routing flood flows to the northwest and ultimately back to Bridger Creek. The subject property and proposed subdivision will not change this existing drainage pattern but rather use it as it historically has. The proposed subdivision will detain or retain its own storm water onsite as required by the City of Bozeman Storm Water Regulations. It is noted that during large runoff events, the proposed subdivision will convey runoff in the same fashion that has historically been conveyed. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 6 of 16 The Bridger Creek 100-yr Flood Plain The current regulatory floodplain designation for the subject property is published in the 1988 FEMA Flood Insurance Study (FIS). According to the study and Flood Insurance Rate Map (FIRM) panels, the subject property is designated as Zone AE along the primary floodway with shaded Zone X on the fringe and un-shaded Zone X towards Bridger Drive to the South. The floodway is defined as the stream channel plus any adjacent floodplain areas that must be kept free from encroachment so that 100 year flood can be conveyed with less than a 0.5 foot surcharge to the 100 year base flood elevation (BFE). The floodway fringe is the remainder floodplain area(s) between the designated floodway boundary and the ground-truth floodplain boundary. The floodway fringe is the portion of the floodplain that can be completely obstructed without increasing the 100 year BFE by more than 0.5 foot. Essentially Zone AE is a special flood hazard area and indicates that the BFE has been established along Bridger Creek using hydraulic modeling techniques, while shaded Zone X designates either areas of 500 year flood or areas of 100 year flood with average depths less than 1 foot. A significant portion of the property is designated as un-shaded Zone X, indicating land outside of the 500 year floodplain but still within the scope of the FIS. Please see Figure 3 containing the FEMA FIRM map 300028-0008C. - This area intentionally left blank - Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 7 of 16 Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 8 of 16 While approximate boundaries are published on the FIRM panels based on coarse survey data, FEMA methodology requires that the actual boundary for the edge of the AE zone be determined based on the intersection of the natural ground with the published BFE. A summary of this delineation and regulatory BFEs are shown in Figure 4. Figure 4 – Ground Truthed BFE Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 9 of 16 It should be noted that two gaps exist along the south floodplain boundary indicating areas where flows may spill out of the creek bank. These areas are denoted with red arrows on Figure 4. These areas do exhibit signs of previous inundation which has also been alleged in verbal historical accounts by property owners. Because Bridger Creek is a perched alluvial stream, a significant portion of the land to the south has natural ground elevations that are lower than the regulatory BFE. Any water that overtops the banks on the south side of Bridger Creek is expected to flow initially south and then west towards the property line as shown in previous figures. Since the FIS was published in 1988, planform and grade changes within the stream reach have changed the hydraulic function of Bridger Creek. In order to verify the FIS boundary with more recent floodplain data along this particular reach of Bridger Creek, MMI constructed an updated hydraulic model based on current geometric data. This model used a 100-Yr event flow of 1150 cfs and was calibrated using rating curves interpolated from the FEMA FIS and a recent Montana Department of Transportation study related to the Highway 86 Bridge replacement. The model results indicated a smaller floodplain boundary with narrower regions of potential spillage along the South side of Bridger Creek than the original FIS model. Another notable difference between the regulatory FIS and the MMI model is in the area immediately upstream of the east property boundary where MMI model elevations were approximately 2 feet lower than those published in the FIS. This difference is significant in that the model predicts water will not spill out of the south side of Bridger Creek on the adjacent land just upstream of the subject property. Summary of the Existing Drainage System Based on the Current FEMA study and the MMI modeling results, it is possible that some water will overtop isolated portions of the creek bank along the south side of Bridger Creek and flow through the proposed development during significant events such as the 100-yr event. This potential spillage will most likely be low velocity water with depths less than one foot. This description sounds more like a shaded Zone X rather than an unshaded Zone X. Based on site visits, topographic and airphoto analysis, it does appear that two primary preferential flow paths do exist on the subject property. The subdivision will take these paths into consideration and incorporate them into the proposed layout. During large runoff events, spillage from the Bridger Creek will likely use these primary flow paths and proceed off-site through other historical flow paths resulting in the overtopping of Story Mill Road as mentioned earlier in this report. The source and timing of these large rapid snowmelt events is obviously not produced on or modified by the subject property and historical outfalls will continue to be utilized by the subject property. Therefore, the development of the subject property will have no measurable effect on the historical magnitude or route of the 100-yr food events in this area. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 10 of 16 II. Proposed Drainage Plan The Legends at Bridger Creek II Storm Water Plan consists of two drainage systems- a Minor and Major System. The Minor Drainage System is designed to provide public convenience and to accommodate moderate, relatively frequent runoff events. The Major Drainage System has the task of conveying significantly more water while protecting structures from inundation. The Major Drainage System begins operating when the rate or volume of runoff exceeds the capacity of the Minor Drainage System. The storm return period for the Minor Drainage System for this project shall be 25-years for conveyance structures and 10-years for ponds. The Major Drainage System shall convey the 100- year event without inundating building structures. Generally speaking, development tends to increase downstream peak flows by increasing runoff volumes and by decreasing the time it takes the runoff to travel downstream. As part of the minor drainage system, retention and detention pond areas designed for the 10-year return period shall be implemented to reduce the downstream peak runoff to levels that existed before the subdivision was constructed. The pond areas will also provide settlement time for sediment and other storm water pollutant loads to drop out. Design of the Drainage Plan was based on survey data collected across the subdivision site using sub-centimeter GPS equipment. The data was used to produce a 1’ contour map of the existing ground. The proposed plan builds off of the existing topography and takes advantage of the many natural features that contribute to the management of storm water. Significant lot grading and first floor elevations are proposed as part of the Stormwater Plan to facilitate proper drainage and manage the risk of potential flooding as the result of historical rapid snow melt runoff events in this area and the proximity of the proposed subdivision to Bridger Creek. For the purposes of this study, the modified rational method was implemented to estimate peak runoff rates from the 10, 25, and 100- yr storm events. The hydrological calculations can be found in the appendix. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 11 of 16 The Major Drainage System The backbone of the major drainage system throughout the proposed Subdivision is the Legends Greenway Corridor. This greenway corridor has the capacity to attenuate and convey the runoff generated by the 100-year event without inundating any building structures within the subdivision. All road culverts placed within the corridor shall be designed to convey runoff from the 25-year event without overtopping the roadway. Site grading shall be completed so that storm events greater than 100-year event will pass over roadways or through emergency overflow areas without causing significant erosion. The street sections in front of the proposed homes are designed to provide additional runoff conveyance away from the homes, down to the streets and eventually into the Legend’s Greenway Corridor. The Grading and Drainage Plan shows the necessary finished surface grading for each lot. It is noted that at a minimum, first floor elevations are approximately 0.75 foot about the finished grade contours as shown on the Grading and Drainage Plan. Finished ground and first floor elevations shall also be located on the final plat and in the subdivision covenants. Particular conveyance sections were modeled using Hec-Ras and FlowMaster design software. As the runoff encounters known obstacles like roadways, back-water conditions were created. The finished ground and first floor elevations have been adjusted accordingly with backwater effect in mind. Please see Figure 5 for the location of a few modeled street and swale sections. A summary of the results can be found in Table 1. Table 1. Est. Max1 Est. Post Conveys ID Street & Swales Capacity(cfs) Q100(cfs) 100-Yr Event 1 Boylan Road @ 1% 95 10 Yes 2 North View Road @ 0.6% 50 5 Yes 3 Typical 30’ Alley @1.4% 30 3 Yes 4 Legend’s Greenway Corridor 300 - Yes 1 Max Capacity for street sections is measure from back of sidewalk to back of sidewalk. Alleys are measured edged of pavement to edge of pavement. As with any development, the possibility of experiencing a 500-Yr flood event or the failure of a stream bank can exist. The probability of it occurring in any given year is slim, somewhere on the order of a 0.20 to 0.99 percent chance. Despite the improbability, Morrison-Maierle has looked at the ability of this subdivision to convey significantly large flood flows. For example, the Legend’s Greenway Corridor can convey upwards of 300 cfs if necessary, without inundating building structures. These types of measures should help manage the risk associated with Bridger Creek. . Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 12 of 16 Figure 5 – Typical Conveyance Sections Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 13 of 16 The Minor Drainage System The minor drainage system is comprised of roads, culverts, swales, and retention/detention ponds. The ponds shall be designed to detain the 10-year post- development runoff volume while releasing flow at a rate not exceeding the 10-year pre- development rate. If larger storm events are encountered, site grading and the emergency pond overflow structures will provide a designated path for the overflow to pass into the creek corridor or natural drainage path without causing significant erosion to the pond. The modified rational method was used to size all retention/detention ponds. Please see Figure 6 for the noted detention/retention areas. The 10-Yr storm event shall be routed into each pond and through the outlet structure using a “delta time step & mass balance” method. All outlet structures are modeled to operate in the inlet control condition with the weir and orifice equations used to model the release rate with respect to pond depth. The design of the pond outlet structure shall be simple, visually unobtrusive, and cost effective. It is proposed that small diameter high density polyethylene (HDPE) culvert pipe will accomplish these two tasks at once. HDPE pipe shall be the pond’s release culvert and at the same time, provide control of the release rate. Ponds Three ponds are proposed to retain or detain storm water runoff from the site and can be seen in Figure 6. Pond A is located in the southwest corner of the property and is a detention pond with approximately 13,800 cubic feet (cft) of storage. The contributing area to this pond is approximately 29 acres and is shaded orange in Figure 6. The pond releases at the 10-yr post-development rate of no more than 4.6 cfs. This is accomplished through an outlet structure comprised of three ~ 10 inch pipes laid at a slope of 0.4%. The outfall of the pond is to lands west of the subject property in an existing drainage swale. For storm events that are greater than the 10-yr return period, the runoff proceeds over the outlet structure emergency overflow without inundating any buildings in the proposed subdivision. Pond B is located in the northwest corner of the property and is a retention pond with approximately 9,000 cft of storage. The pond has enough capacity to completely contain the runoff from a 10-yr, 2hr storm event. The contributing area to this pond is approximately 6.8 acres and is shaded pink in Figure 6. If a larger storm event is encountered, then runoff spills back into Boylan Road and proceeds west through Legends I Subdivision. Pond C is pond located in the northwest corner of the property and is a retention pond with approximately 1,700 cft of storage. The pond has enough capacity to completely contain the runoff from a 10-yr, 2hr storm event. The contributing area to this pond is approximately 1.3 acres and is shaded yellow in Figure 6. If a larger storm event is encountered, then runoff spills back into Boylan Road and proceeds west through Legends I Subdivision. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 14 of 16 Figure 6 – Pond & culvert locations Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 15 of 16 Inlets, Culverts and Street Flow Inlets will be needed at low points and where street conveyance capacities are exceeded. This project has inlets in both sump and on grade conditions. Culverts are also proposed where the Legend’s Greenway Corridor intersects with subdivision roads. Culvert and inlet locations can be seen in Figure 6 and are denoted as items I through 6. The proposed sizes, estimate of capacities and number of inlets can be found in Table 2 below. For this particular site, street conveyance capacities using a 9.5 foot spread from the face of curb were adequate and it appears no additional storm water main runs or piping will be necessary. If street intersections produce a low point or sump condition in the curb return, then flow-through concrete valley gutters are proposed and can be seen on the Grading and Drainage Plan. Table 2. Est. Post Description Slope Head Act. Cap. # Inlets ID Culverts Q25(cfs) (ft/ft) (ft) (cfs) 1 Boylan Road Greenway Xing1 9 1- 27”X 44” 0.009 3 24 2 “HH” in Sump 2 North View Greenway Xing1 10 2- 22”X 36” 0.004 4 70 NA 3 Int. North View & Maiden Rock 6 1- 18” 0.01 2.5 10 3 “H” in Sump 4 North View Low point 2 1- 12” 0.01 2.5 4 2 “H” in Sump 5 North View Existing Drainage Xing 3 1- 12” 0.005 2 3 NA 6 West Boundary 5 1- 18” 0.008 2.5 9 6 “H” on Grade 7 Pond C inlets 2 1- 12” 0.005 2.5 3 2 “H” on Grade 1 Flows greater than the 25-yr event are designed to overtop roads and proceed down the Greenway Swale. Preliminary Plat Storm Water Plan for The Legends at Bridger Creek II Page 16 of 16 III. Recommendations and Additional Thoughts There are always inherent risks when developing land next to streams. The benefits can outweigh the risks if the risks are managed properly. It is recommended that the Grading and Drainage Plan be adhered to and that minimum first floor elevations be enforced. All lots that are immediately adjacent to Bridger Creek shall have a first floor elevation that is at least 2 feet above the Base Flood Elevation or 100-yr event water surface elevation. Further refinement of the Grading and Drainage Plan may be necessary as this project progresses and final design begins. It should be disclosed to prospective buyers that this area is next to a stream and that lands in this area may become inundated during large events. Although no subdivisions lots are in the 100- year floodplain as published by FEMA, it is recommended that prospective lot buyers consider obtaining flood insurance due to proximity. As the result of short lot setbacks and the proposed lot grading, driveways may tend to be steeper than usual if a step between the first floor of the residence and the level of the garage floor is not implemented. Not all lots will need this proposed step. However some lots that may benefit from such a step are 20, 21, 22, 26, 40, 121, & 100. This is not a complete list and it may not be clear if a particular lot needs a step until the some of the building details (such as driveway location) are established. Due to the acute angle formed between North View Road and the Legend’s Greenway Corridor, it is possible that a small landscape retaining wall for the south west corner of lot 100 may be needed. Creative landscaping may also accomplish the task of blending the 4 foot difference in elevation. Again it is imperative that the lot finished grade and finished floor elevations be adhered to. There are areas along the south side of Bridger Creek that possess over-steepened banks, with relative high rates of erosion. These areas are for the most part hundreds of feet away from residential lots. However, it may be prudent to complete a bank stabilization assessment and provide any necessary natural or soft bank stabilization efforts such as plantings now or in the near future. IV. The Storm Water Maintenance Plan The maintenance of the onsite storm water collection, detention and discharge release appurtenances shall be integrated as part of the duties of maintenance personnel responsible for normal grounds keeping. Duties shall include routine inspections to ensure that debris, yard waste, and seasonal ice does not impede operation of the detention ponds and discharge structures. These inspections shall occur after each major runoff event and on a continued monthly basis throughout the year. All results shall be recorded and kept on file for future verification by regulation agencies. In addition to the routine inspections, annual inspections of appurtenances shall assess and mitigate, if necessary, performance of the detention ponds, and release structures. These inspections shall address accumulation of sediment and debris in the detention ponds and release structures as well as any weather or vandalism-related damage. Detention areas and swales shall be mowed on a regular basis during the growing season so as to remain free of vegetation that might impede their conveyance capacity or ability to function.