6.0 Alternative Analysis
6.1 Alternative Development Process
An Alternative Analysis was completed to develop flood mitigation solutions for the problem areas identified in the previous section. Goals for mitigation focused on solutions to reducing flooding on insurable structures. Consideration was also given to reducing infrastructure sizing and costs by incorporating detention.
Alternatives were considered first for open channel and then for piped systems. Open channels are more cost effective, provide additional ecological benefits, mimic or sometimes restore the natural environment, but require additional stabilization and maintenance to provide long term benefits. Piped systems are far more hydraulically efficient and allow for more flexible land uses in and around the system, but do not benefit the natural environment and can be significantly more costly to install, particularly in underlying rock soils.
For example, the development of downtown Lyons over the historic drainageway has limited the ability to convey the major storm events through an open channel system. Since an open channel system is not feasible, large storm drain infrastructure is needed to reduce the flood hazards throughout the study area. The storm drain infrastructure must limit the amount of flow in the street to provide emergency vehicles access during storm events.
The analysis aimed to develop stormwater solutions first within the Town of Lyons. When those alternatives were exhausted as cost prohibitive or unfeasible, then the project team considered alternatives outside of the Town limits. The cost of land acquisition, land use regulations, and extra-territorial jurisdiction can add significant costs to stormwater projects outside of the Town limits. Conversely, knowing about potential out of town solutions to in-town risks can be prudent to future planning exercises in annexation, development referrals, and multi-agency coordination.
6.2 Criteria and Constraints
As noted above, goals for the baseline alternative plan was to provide storm drainage capacity to meeting currently defined drainage criteria set forth in the Town of Lyons Storm Drainage System Criteria. The minor storm frequency for the Town of Lyons is the 2-yr design storm. The major storm frequency for the Town of Lyons is classified as the 100-yr design storm frequency. The only inlet type allowed on streets is CDOT Type R inlets.
6.3 Evaluation of Detention
Detention is a common approach to reduce peak flows, optimize pipe size, and save on downstream infrastructure costs. Review of the previous master plan confirmed that previously master planned detention facilities were drafted to meet this goal. If detention is installed in the upper watersheds of the Town, there is a significant cost savings to downstream stormwater infrastructure. However, upon closer examination of the previous master plan assumptions, field inspection of the proposed detention sites, and conceptual pond grading and layout there were additional constraints on the detention pond locations.
6.3.2 Steep Topography
The steep slopes on the north end of Steamboat drainage require significant grading operations to achieve a functional detention volume even approaching 0.5 acre feet. This fact alone may still be a viable alternative since excavation is generally less expensive than long lengths of large storm sewer pipe material, utility crossings, and installation. However, given the geology of the upper Steamboat drainage the depth to bedrock is known to be very shallow in many locations. Rock excavation for a detention facility is prohibitively expensive. Excavation of the rock in a quarry scenario would be one economically viable means of providing sufficient detention volume at a reasonable cost for Steamboat drainage.
6.3.3 Multiple Hillside Ponds
Staged detention facilities staggered in stair-step fashion up the steep slopes is another alternative to achieve the benefits of detention without significant excavation. However, the extensive footprint of disturbance for multiple embankments, overflow spillway design, and permanent impact on property makes this a challenging alternative. If future development, annexation, and stormwater infrastructure funding allows this alternative can be studied in greater detail. For the immediate future, this is a difficult alternative to consider for near term funding.
6.3.4 Property Acquisition
The natural valleys between 4th and 5th Streets and 3rd and 4th Streets could provide suitable detention facilities if several existing constraints are mitigated. First, the private homes in the adjacent parcels would be impacted by detention depths of more than approximately 5 feet. Purchase and demolition of homes affected by a proposed detention facility would be required to achieve a useful detention volume. Second, the existing embankments between 4th and 5th Street should be reconstructed to replace the rock and native soils that were used to build what is reported to be an old railroad embankment. Until the compaction and stability of that embankment can be verified, it should not be relied upon to safely detain stormwater. It is unclear what the overflow path may be should the outlet under the embankment fail, collapse or clog. However it is likely the overflow could lead to additional erosion and scour of the embankment flanks and expedite complete failure of the embankment. Third, the valleys near the cemetery are also candidates for detention, but the steep slopes make each valley individually difficult to achieve a reasonable volume. However, purchase of a uniquely shaped parcel of land slicing between the valleys could allow an embankment to be constructed that joins the two valleys and creates a reasonable detention volume.
6.4 Alternative Categories
For the purpose of this study, alternatives were generally developed to mitigate the flooding hazards identified in Section 5.0. Specific details related to individual alternatives follow in subsequent report sections.
6.5 Alternative Hydraulics
Alternatives were modeled using EPA SWMM to determine the size of the structures necessary to convey the design flow. The reduction in flooding potential throughout the basin was evaluated for the minor and major design storm frequencies using FLO-2D.
6.6 Alternative Costs
Alternative cost estimates were developed using UDFCD's master planning cost estimating spreadsheet UD-MP COST, version 2.2. 2012 unit cost values were adjusted to present value using the Colorado Construction Cost Index 2016 Second Quarter Report. A rolling four-quarter inflation rate of 1.2673 was used to adjust unit costs.
Operation and Maintenance was also included within the UD MP Cost worksheet. Maintenance to remove sediment and debris and conduct structural repairs on storm drain manholes and inlets was assumed to occur one every year. Maintenance to remove tree and weeds and sediment and debris in open channel alternatives was also assumed to occur once every year. Costs for maintenance of the hydraulic drops for the Red Hill Gulch West Channel was assumed to occur once a year.
Inlet quantities were calculated assuming an inlet interception capacity of 1 cfs / foot of inlet.
Dewatering, Traffic Control and Utility Coordination / Relocation were assigned based on the following percentages of capital costs: Dewatering (1%), Traffic Control (5%), Utility Coordination / Relocation (10%).
Special items that were added to the UD-MP COST spreadsheet include:
Asphalt Repaving: $40 / S.Y.
Curb and Gutter: $30 / L.F.
No alterations were made to default values calculated as a percent of Capital Improvement Costs, such as Engineering, Legal / Administrative, Contract Administration / Construction Management, and Contingency.
6.7 Alternative Plans
A - Red Hill Gulch Overflow Channel East
A diversion channel west of Lyons Valley Park Subdivision is proposed to intercept overflow from Red Hill Gulch before the discharge damages property. Any flow exceeding the capacity of the South Ledge Ditch continues north spilling into Bohn Park and Lyons Valley Park Subdivision. This uncontrolled spill flow risks the structural integrity of the ditch and poses a hazard to homes along the west side of Lyons Valley Park. The Red Hill Gulch Overflow Channel East would intercept flows along the east side of Bohn Park preventing these flows from diverting into the Lyons Valley Park Subdivision. The grading associated with the overflow channel would change the surface conditions of the east side of the park but still allow for parking and multi-purpose uses. The multiple uses of this eastern side of the park require the channel to be wider and flatter that typical conveyance channels in order to maintain the function of the space for parking and exhibitions.
In general, this option intercepts and conveys large storm events from Red Hill Gulch before those flows encroach on private property. The alignment utilizes existing town property and requires no additional easements or property acquisition.
However, this alternative compromises existing functions of the park land and assumes upstream development will not have an impact on the drainage conditions. In other words, solving the Red Hill Gulch drainage this far down in the watershed costs a lot when compared to what upstream alternatives may do to reduce stormwater impacts.
B - Red Hill Gulch Overflow Channel West
An alternative to the east channel at the downstream end of the watershed is a western channel. When the Western Corridor is annexed into the Town of Lyons the overflow channel alignment should be evaluated to intercept the flows upstream of Bohn Park eliminating any disturbance to the park. The flows would be conveyed west of Bohn Park and discharged into South St. Vrain Creek. This is a smaller channel used for a singular stormwater purpose – it does not have to share uses with a park or parking lot. The western alignment also conveys water around Bohn Park and Lyons Valley subdivision.
However, the channel would require an easement or land acquisition from the adjoining land owners. And, because of the smaller footprint, the hydraulics of the channel require drop structures to control erosive velocities.
C - Red Hill Gulch Cut-Off Channel
Outside of the Town, there is another problem area and option for managing Red Hill Gulch stormwater runoff.
A third alternative for Red Hill Gulch drainage intercepts the gulch flows at the Picture Rock Trailhead and diverts those flows west into the South St. Vrain. This alternative has the advantage of working a stormwater solution upstream of existing development and mitigating flood risk as high up in the watershed as feasible. This is the most hydraulically efficient alternative in that the diversion distance to the South St. Vrain is short and the diversion volume is reduced when compared to downstream alternatives.
However, the constraints on this alternative are compounded by the need for easement or land acquisition on both public and county open space property. The details of a diversion in this location will require careful design and analysis. The surface flows are spread over a large area, but the right grading could allow just enough water to drain north through the park area and intercept the remainder to safely divert it west to the South St. Vrain. Easements and reconstruction of Red Gulch Road would be required. Existing irrigation ditches in the area compound problems rerouting surface flows. Any diversion would still have to cross Ledge Ditch and Meadow Ditch.
There are several roadside ditches and ditch crossings that should be investigated further. The stormwater flows into Ledge and Meadow ditches complicate the flow paths for this drainage. Upstream and downstream of the irrigation ditches, the roadside swales are an important part of conveying stormwater but need to be maintained to ensure positive drainage away from the roadway.
D - Steamboat Drainage Culvert Replacement
The historic flowpath of Steamboat Valley Watershed conveys flow south between Fourth and Fifth Avenue through private property. The existing confined channel and culvert roadway crossings could be replaced with a reinforced concrete box culvert conveying the entire 100-year design flow. The proposed culvert replacement along the existing alignment uses the natural topography to convey the runoff to the new culvert. However, implementing this alternative would require extensive easements as the historic culvert runs through the downtown area underneath development on private properties.
E - Steamboat Drainage Interceptors
The Steamboat Drainage Interceptor Alternative proposes to intercept flow just downstream of the private railroad embankment at McCalley Alley. Flows in excess of the existing channel and roadway crossing capacity would be intercepted and conveyed west to Fifth Avenue. The storm drain system would continue south where the system would outfall into North St. Vrain Creek. This alternative proposes a lateral in Fourth Avenue from Stickney Avenue to North St. Vrain Creek to intercept local runoff in the lower portions of the Steamboat Valley Watershed.
F - Third Avenue Inlets and Storm Drain
To mitigate flood hazards associated with the Third Avenue Watershed, a storm drain is proposed along Third Avenue from Cemetry Circle to North St. Vrain Creek. A lateral along Stickney Avenue is proposed to intercept flow from the east as it enters the residential area. Continuing the storm drain system south of Main Street would mitigate some of the flooding hazards within the South Second Avenue Watershed. Any flows in excess of the existing storm drain in the Third Avenue Watershed currently spill in the South 2nd Avenue Watershed impacting properties.
However, it is important to note that excavation in the northern end of the Third Avenue basin is likely to encounter rock and other earthwork complications. Depth of inlets and storm sewers will be limited by excavation cost, which limits the hydraulic head and increases pipe size. As pipe size increases, depth to cover the pipe must increase and become a costly design loop to determine a feasible storm sewer alignment.
G - Lower 3rd Avenue Inlet
Excess surface flows on 3rd Avenue could be conveyed to a large storm sewer inlet at the existing storm sewers on the southeast corner of 3rd and Broadway. The existing grated inlets on the west flowline of 3rd Avenue at Main Street could be improved, but flows exceeding the capacity of those inlets will continue south on 3rd Avenue. The proposed lower 3rd Avenue inlets would capture flows from both gutter flowlines to maintain safe street capacity on the south end of 3rd Avenue during storm events.
The west flowline is also one of the locations where surface flows could be routed through a minor storm weir into a small volume water quality pond in the east end of Sandstone Park. The pond would treat ‘first flush' or the initial runoff volume with larger flows continuing south.
H - Third Avenue Drainage Interceptors
Similar to the Steamboat drainage alternative, the surface conveyance on Third Avenue could be improved instead of excavation and installation of inlets and pipes. The surface conveyance improvement contains flows within the Town ROW, gutters, and directs flow south to the river. This reduces many of the overflows into private property that exceed the existing roadway swales or smaller curbs. This alternative also reduces surface flows intercepted at cross street intersections that drain water into private property and complicate street flows entering Steamboat drainage.
This alternative does not resolve major storm overflows. The minor storms are handled with the interceptor gutters and cross pans. However, when those conveyance paths are at capacity, the overflow will continue down the natural topography. An inlet and pipe system could be sized to convey a much larger portion of the major storm event. But, as a relatively low cost alternative, the benefits are tangible.
I - Eastern Lyons - Second Avenue Gutter Interceptor
Recent development within the Eastern Lyons watershed has increased the runoff from the hillside causing local flooding problems. For larger events the existing roadway conveyance and roadside swale along 2nd Avenue does not adequately convey the runoff south along 2nd Avenue instead diverting flow to the west into the Third Avenue Watershed. Formalizing the street conveyance by installing curb and gutters and cross pans in 2nd Avenue would encourage runoff from this watershed to continue south along 2nd Avenue and not impact properties the Third Avenue Watershed.
J - Eastern Lyons - Second Avenue Inlets and Storm Sewer
To ensure no runoff diverts from the Eastern Lyons Watershed, inlets and a storm drain pipe can be installed along 2nd Avenue. The storm drain inlets and pipe in combination with curb and gutter would intercept the flow and convey the flow south to Main Street. South of Main Street the storm drain would convey runoff to the Second Avenue and Park Street intersection before turning east to the outfall location into the St. Vrain Creek.
K - Lyons Valley Inlet Improvements
Runoff from the upper portions of the subwatershed sheet flows in a general northeast direction through the subdivision to St. Vrain Creek. The storm drain infrastructure installed with the development of Lyons Valley Park Subdivision is a minor drainage system and lacks capacity to convey major storm events. This alternative proposes to increases the storm drain along the existing alignment to convey the design flow for the 100-year storm.
L - Lyons Valley South Ditch Improvements
The South Ledge Ditch located south of Lyons Valley Park Subdivision intercepts runoff from the subwatershed. Formalizing the ditch to convey stormwater east towards St. Vrain Creek would reduce the tributary area contributing to the flooding hazards of Lyons Valley Park Subdivision.
This alternative should be developed in coordination with the future development of Lyons Valley. A drainage feature around the south end of the currently platted lots would benefit the new construction as well as the existing homes in the neighborhood.
The greatest limitation of this alternative is the operation and maintenance of the irrigation ditch and the coordination with any future development. Breaches and overtopping of the existing ditch are likely to continue to occur given the earthern embankment construction of the ditch. As the platted future development moves into the final plan approval process, careful coordination should identify the benefits to both existing and future homes in the area. And, when the cost of perimeter surface water conveyance (i.e. swale) is compared to sizing interior storm inlets and pipes sized to safely capture and convey the offsite flows, the cost-benefit should resolve any further limitations of the concept.
M - Lyons Valley McConnel Drive Culvert
The impervious area of the school campus leads to nuisance drainage issues along McConnel Drive. It is unclear how the stormwater runoff is treated within the school campus, but the roof drains and parking along the east side of the campus are direct discharge to the west gutter of McConnel Drive. On-site water quality and detention facilities, perhaps conversion of part of the irrigated turf areas, would collect, treat, and attenuate on-site discharges. A pond near the northeast corner of the site could connect to a new underground storm sewer tied into the Type R inlets at 2nd Avenue. This would move surface flow, particularly nuisance flows dribbling winter melt water that freezes overnight, into the on-site pond and underground, safely conveyed in a pipe system.
N - Ewald Ave - Corona Hill Diversion Ditch
As witnessed in the 1994 event, the Ewald Avenue Watershed poses a significant flooding hazard to downstream properties within the watershed. High velocity runoff flow off Corona Hill can carry debris into the residential neighborhood. Intercepting the flow upstream of the development and conveying the flow to the south would prevent debris flow from impacting private properties. The diversion channel south of the development would require significant energy dissipation to prevent the discharge from reaching high velocities and eroding the hillside south of the development. The steep slopes along the hillside would make construction of any diversion channel difficult.
O - Ewald Ave - Corona Hill Street Inlets
An alternative to intercepting flows upstream of the residential development is formalizing street conveyance and installing storm drain infrastructure along Prospect Road. The storm drain infrastructure in combination with curb and gutter improvements would confine the runoff from Corona Hill to the street limiting the impact to private properties. The storm drain system would continue east along Prospect Road to the confluence area between North and South St. Vrain Creek
P - 2nd Avenue – Kelling Drive Private Improvements
There are past stormwater runoff events that have led to private property damages (fencing, landscaping, and structural flooding). Magnitude of the damages has not be investigated as part of this study. The subject properties, residential structures on the north side of Kelling Drive and commercial structures on the south side of Kelling Drive, are near the bottom of a steep southwestern facing slope. Runoff from the upstream property impervious area is controlled through downspouts, gutters, landscape swales, street gutters, and storm sewer systems.
The northern properties appear to have all or most of their foundations above adjacent grade, positive drainage away from the structure. However, additional property line swales could convey surface flows around the structures. The southern properties appear to have adverse drainage slopes near the building and could benefit from more formalized drain pans along top or toe of the retaining walls on the north side of the property. An existing grated inlet in the north flowline of Kelling Drive could be improved to increase surface water capture, but only reduces surface flows already in the street.
Support from the Town in terms of permitting, review, and easement terminology could resolve drainage issues in the vicinity of Kelling Drive. The installation of swales, inlets, pipes, moving existing outbuildings, and re-landscaping perimeter swales on each of the affected lots can be highly effective private improvements that mitigate the flood risks in this area. However, the work would be on private property and therefore require special agreements (easements, cost-sharing, access requirements, or code updates, etc.) for the Town to participate in design, construction, operation, maintenance, or financing. Therefore, the town's assistance could come in the form of coordinating the private efforts to ensure a comprehensive solution is achieved.
Q - Stone Canyon Outfall Improvements
Downstream of Stone Canyon Road, private development has infringed on Stone Canyon Drainageway. The existing roadway crossing at Ute Highway lacks the capacity to convey the 100-year discharge without overtopping the roadway. A significant box culvert roadway crossing structure would be required to convey the 100-year discharge without overtopping. The conveyance of Stone Canyon Drainageway downstream of Stone Canyon Road to the St. Vrain Creek should be analyzed in further detail when the Eastern Corridor expansion is finalized.
Table 6-1: Cost Summary
|Alt ID||Description||Capital||Easement / ROW||Engineering||Legal / Admin||Contract Admin / CM||Contingency||Total Capital Cost||Annual O-M Cost||50-Year O-M Cost|
|A||Red Hill Gulch Overflow Channel East||$435,644||$-||$65,347||$21,782||$43,564||$108,911||$675,248||$2,580||$81,073|