2015-01-07_REPORT - M1989029 - Laserfiche WebLink

Home Browse Search

73 Recommendations 73A REACH l Overview Reach 1 begins at the confluence of St. Vrain Creek with Boulder Creek and ends at the eastern corporate limits of Longmont. A small portion of the upstream limits of this reach is within BCPOS limits, while the remainder crosses through both privately -owned and City of Longmont property. This reach exists within a wide- bottom valley and is generally not confined geologically. There is very little urbanization adjacent to the channel corridor and the surrounding land use is generally comprised of agricultural and rural land uses with a significant amount of mining operation. Sand and gravel mining operations have been prominent in this reach over the past two decades and have encroached upon the channel resulting in various degrees of channel realignment. Drainageway crossings within this reach exist at: » Highway 119 (downstream) » Pedestrian bridges adjacent to the Sandstone Ranch Community Park » Weld County Road 1 » North 119th Street 'Ihe confluence of Dry Creek with St. Vrain Creek occurs near the eastern boundary of the private residence that exists to the northeast of Quicksilver Road and Weld County Road 1. Additionally, the Spring Gulch ditch flows through Sandstone Ranch Community Park from the north and connects with St. Vrain Creek just to the west of the damaged pedestrian bridge. Historical aerial photography was reviewed to help identify the conditions that existed prior to flooding in this reach. The riparian corridor adjacent to the channel was densely vegetated with a combination of native and non - native species and ranged in width from 300 feet to 500 feet. The channel was highly sinuous and appeared to generally be in a stable state with no visible signs of vertical or horizontal instabilities. Although the adjacent floodplain has been historically connected to the channel, sand and gravel mining operations have been present over the past two decades which have altered the natural floodplain function. During the September 2013 flood, almost all of the gravel mining sites to the south of this reach were breached. This sequence of breaches resulted in the St. Vrain Creek being diverted from the historical flowpath to a path that currently flows through the gravel mining sites. The confluence with Boulder Creek currently exists approximately 1,400 feet upstream on Boulder Creek of the pre - flood confluence. Although the new flowpath does not adversely affect infrastructure, the stream is unstable and the connectivity is largely nonexistent. Groundwater and sub - surface flows are still infiltrating into the pre -flood channel alignment resulting in several areas of ponded water and providing some wetland habitat. Assessment riparian areas. The primary recommendation from the ecological assessment is to ensure that all future bridges and /or culverts that arc placed within the creek corridor span the active (low -flow) channel to encourage ecological connectivity. The pre -flood channel alignment was filled with minor to moderate amounts of deposition while the post -flood alignment experienced severe erosion. The planform for this reach of St. Vrain Creek has been relatively stable over the past 60 years. However, there has been some natural lateral migration across the floodplain during this time period. The results of the geomorphic assessment show that the alignment of the channel changed drastically between pre- and post -flood conditions as a result of an avulsion through the gravel mining sites. As a result, the planform geometry for this reach is less sinuous and steeper when compared to pre -flood conditions. The primary recommendation from the geomorphic assessment is to return the channel to the pre -flood alignment. However, it is noted that this recommendation may be cost prohibitive. Infrastructure damage from flooding occurred to the County Road 1 Bridge, the pedestrian bridge adjacent to Sandstone Ranch Community Park, and the driveway leading to the private residence that exists to the northeast of Quicksilver Road and Weld County Road 1. There are a few private residences that are at risk for future flooding during a 100 -year event. Plan Recommendations Two different alternatives were evaluated for this reach, both of which are described in more detail in Appendix D. General descriptions of each alternative are outlined below. Since the development of the Alternatives Analysis Report, the City of Longmont issued a Request for Proposal titled "St. Vrain Creek Improvement Project" to solicit design services for the completion a natural channel restoration for this portion of the St. Vrain Creek. City of Longmont voters approved a $20 million Storm Drainage Bond to help fund these proposed improvements. In addition, the City has worked with FEMA on flood damage relief for City owned property and improvements along these sections of the St. Vrain Creek and has created a Public Assistant Alternative Procedures Project (PAAP). Additional City Funds have also been identified for this project, and the City has applied for Army Corp of Engineers, FEMA and Community Development Block Grant - Disaster Recovery (CDBG -DR) funds for this project. This project will be completed under a separate contract and will perform a more detailed alternatives analysis in order to determine the appropriate design strategy for this reach. Once the design strategy is determined, construction documents will be prepared. This contract has been awarded and design will begin by the end of 2014. As a result, neither alternative that was presented in the Alternatives Analysis Report was carried forward into a conceptual design as a part of this project. Alternative 1— Split Channel The purpose of this alternative is to preserve the pre -flood channel alignment and add an additional channel to the south of the pre - flood channel. Flow will be delivered to the pre -flood channel with a baseflow diversion, which will help sustain the existing riparian vegetation throughout this corridor. The new channel alignment to the south will convey a majority of the flow in this reach. Flood conveyance is improved with this alternative by eliminating constrictions in the floodplain and implementing floodplain culverts to help convey flood flows through embankments. It is important to note that a significant amount of fill will be required to eliminate the risk of a future avulsion through the gravel mining sites and to allow for the construction of the confluence of the parallel channels near the downstream end of this reach. Alternative 2 — Minor Stabilization This purpose of this alternative is to implement only the improvements required to protect infrastructure and property that are at risk of being damaged and to implement site - specific projects to protect against major modes of failure. Major modes of failure in this reach consist of headcutting through the gravel mining sites and a potential avulsion through the pre -flood channel alignment. Most of the channel will generally remain in the post -food alignment and continue to cascade through the gravel mining sites. It is important to note that the channel will continue to erode and migrate in the absence of any stabilization measures and continued erosion could eventually encroach upon Boulder Creek and adjacent private property. Work that is in the process of being completed consists of the bridge and road design at Weld County Road 1. General restoration recommendations consist of: The ecological assessment did not encompass the segment of St. Vrain Creek downstream of County Road 1 because it was inaccessible at the time. However, based on observation during field work, this reach would likely be classified as being in Poor or » Incorporate /stabilize a low flow /bankfull channel section with the following general design parameters: Fair condition because of channel instability, eroding banks, barriers to movement of fish and aquatic organisms, and disconnected