1. What are the features of the flood mitigation plan being delivered through this project?
This plan is meant to act as a guide for flood mitigation projects in the Lowe River. Please refer to our one-page summary located on the documents page of the project website.
2. Groins? Dikes? Levees? What is the difference and what do we have now?
In Valdez, we use all three terms interchangeably to describe embankments that hold water from dry land. However, there are differences between each of the terms. Traditionally, the following definitions apply:
- Levees protect land that is normally dry but that may be flooded when rain or melting snow raises the water level in a body of water, such as a river.
- Dikes protect land that would naturally be underwater most of the time.
- Groins are rigid structures built at an angle to the riverbank that interrupts or diverts the flow of water and limits the movement of sediment.
The current flood mitigation structures that protect the 10 Mile subdivisions on the Lowe River are technically groins. The groins are positioned to deflect and direct flows from entering the subdivisions without being hydraulically cut off from the river. This is important for the subdivisions as many streams that flow through the area need to drain into the river to prevent flooding.
3. What are the outcomes of this project?
The 2020 survey of Valdez residents indicates that maintenance of existing infrastructure is the most valuable use of public funds. Two construction projects may derive from the work completed through this project 1) gravel extraction in the Lowe River floodplain and 2) dike infrastructure improvements.
The gravel extraction in the Lowe River aims to encourage more of the water in the floodplain to flow away from the dikes at the 10 Mile subdivisions. Modeling indicates that this will reduce the water surface elevation at the dikes.
The dike infrastructure improvements will range from replacing worn rock or replacing rock displaced by storm events to strategically raising the dikes to make sure there is enough space between the top of the dikes and the water level during storms (freeboard).
4. Why is this project needed?
Historically, portions of the Lowe River floodplain have been susceptible to flooding. This project will protect areas of infrastructure through the development of new dikes and revetments, provide maintenance of existing dikes and levees, and identify areas that are susceptible to flooding that may not be preventable. Years that have had significant flood events include 1983, 1995, 2006, 2012, 2015, 2016 and 2018.
5. Why are our homes flooding?
Properties within the Alpine Woods Subdivision are taking on water from two sources: Lowe River and groundwater.
- Lowe River flooding: This is where rainfall or snowmelt causes high flows in the Lowe River that impacts the ability of the internal drainage behind the dikes to drain to the river, causing the water surface elevation to rise. It has also been seen during these events, that the river starts to backwater or ‘backup,’ causing water to enter the subdivision through the creeks.
- Groundwater flooding: This occurs when the groundwater table rises above ground level. Sources of water that contribute to groundwater flooding include water runoff from the mountains and hills surrounding the Lowe River and a proportion of flows from the Lowe River channels.
6. What processes are occurring on the Lowe River that influence the risk of flood and/or erosion?
- Channel migration: Due to the makeup of the sediment in the floodplain, it is relatively easy for the channel location and width to change throughout the year depending on the amount of water flowing in the river.
- Bank erosion: This is where bank material along the side of the floodplain is removed by fast moving water and deposited downstream. Bank erosion causes a loss of land and potential downstream issues related to the sediment accumulation.
- Groundwater flows: Subsurface water flows though the gravel. When water levels rise in the river, more water is available to flow through the ground as shallow groundwater. When the water table rises enough, it may appear above the surface of the ground. This causes areas of standing water and may contribute to groundwater flooding.
7. Why should I care?
- Risks to Alpine Woods Homeowners/Residents: Impacts of Lowe River flooding are mostly felt by the residents of Alpine Woods subdivision. Changes in the floodplain and groundwater are increasing the risks over time, but the new flood mitigation efforts kicking off with this project seek to mitigate risks associated with flooding as they currently exist.
- This is the conservative, sustainable approach: It is important to understand that for aggrading, braided floodplains, a comprehensive management approach is preferred over a piecemeal project-by-project solution because of the highly variable and consistently changing nature of water flows through the floodplain.
8. Who are the stakeholders in this project?
Everyone in Valdez is a stakeholder in mitigating the risk of Lowe River flood and erosion because of the public and private assets and infrastructure adjacent to the river that are subject to potential failure if that risk is not mitigated. A few of the stakeholders are:
- Alpine Woods residents/property owners
- Alyeska Pipeline Service Company
- City of Valdez
- Alaska Department of Fish and Game
- Alaska Department of Natural Resources
- Alaska Department of Transportation & Public Facilities
- U.S. Army Corps of Engineers
- Valdez public
9. What is a braided floodplain?
A braided floodplain is a floodplain that may have more than one river channel flowing through it and each of those channels have the potential to move within the floodplain. The movement of the channels depends on the type of material in the floodplain and the velocity and amount of water flowing through those channels.
10. What is an aggrading floodplain?
This is the term that describes if material is being deposited on to the riverbed or floodplain. The sediment that is carried by the river is either suspended in the water or rolls along the bottom of the riverbed. If the river is aggrading, it means that the material is settling out of the water to the bottom of the river/floodplain. This has the effect of raising the riverbed/floodplain over time.
11. What's so challenging about mitigating flooding on the Lowe River?
A lot! The Lowe River is an aggrading floodplain. The project corridor is a twelve-mile stretch of river with multiple stakeholders who each have different risks, costs, and possible benefits to weigh. In addition, rivers, and particularly glacial rivers influenced as heavily as the Lowe by seasonal precipitation and snow melt, are notoriously difficult to control.
12. What factors determine when a flood or erosion mitigation project will occur?
- Construction timing is based on design and permitting of the project, but also the project size, potential impacts to the Lowe River floodplain, and ensuring safe fish passage.
- Scope of the project needs to incorporate a lot of information: flow data from the river, what the flow does when it interacts with a structure, soil properties, groundwater data, what the river is doing on that day or in that season.
- Environmental considerations of a flood or erosion mitigation project include fish (what kinds, when they're in the river and for what reason), clean water, the impact of construction equipment performing a specific task in a specific location on the environment, impacts to the river downstream, etc.
13. Can the dikes near my home stop all water from the Lowe River?
No, the dikes are constructed to channel or direct water away from infrastructure. This is the case unless dikes contain an impermeable barrier that reduces the seepage through the dike. The amount of water that can seep through or under the dike is minimal compared to the amount of water that is deflected by the dike. The amount of water that can seep through the dike (permeability) is taken into consideration when designing the dikes.
14. What are some examples of other flood mitigation projects in Valdez?
- In 2019, the Alaska Department of Transportation and Public Facilities (DOT&PF) completed construction of the guide bank to the bridge at the Richardson Highway.
- Later in 2019 the City of Valdez extended the guide bank to Glacier Stream Haul Road by realigning the existing spur dike. This had the effect of creating a barrier between the floodplain of Glacier Stream preventing the flooding along the Richardson Highway as seen in 2017/2018.
- The new structures worked well during the annual jökulhlaup (glacial outburst event), where flow in the river rose from 2000 cubic feet per second to 21,600 cubic feet per second. The City of Valdez dike withstood a change in channel direction, meaning that the flow of water was directed at the dike, and not parallel along the dike. This was accounted for in the design and construction and worked well.
- Monitoring and maintenance to the Natural Resources Conservation Service (NRCS) dike on Copper Avenue.
- Based on a study completed in 2017, six locations were selected for mitigation measures to reduce the risk of flooding or erosion to the banks of Mineral Creek. In September 2019, work commenced to maintain and build new erosion control structures at the six locations with work being completed in October 2019. This reduced erosion and flooding risk along Mineral Creek Drive, Betty Cato Bridge, Blueberry Hill Road, Homestead Road and Homestead trail.
- With additional rock from the project the existing spur dikes on the west bank of Mineral Creek were refaced with new rock, extending the lifespan of the structures.
Riprap Maintenance Manual
- In response to the newly constructed structures on Glacier Stream and Mineral Creek, the City of Valdez requested a manual to guide the construction, maintenance and response to damage. The Maintenance Manual includes a recommended regular inspection schedule and enables the City of Valdez to have a method to consistently make repairs and perform maintenance on the structures in the future.