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Watershed Stormwater Master Plans: South Tributaries Watershed Study

Recommended CIP Projects


Recommended Capital Improvement Projects

Haines Branch Watershed Study Area Master Plans

The major driver of stream instability in the Haines Branch Watershed is incision (downcutting of the channel bottom). Recommended CIP projects are strategically located to prevent the channel from cutting deeper. Bank stabilization is recommended to stop bank erosion and grade control structures are used to protect the channel immediately upstream from further erosion.

Middle Creek Watershed Study Area

Knickpoint at Debris Jam
Knickpoint at Debris Jam

The major drivers of instability in the Middle Creek Watershed study area are widening and plan form adjustment (channel meandering) along the main stem and incision on the tributaries. Many of the Middle Creek tributaries are managed channels at the tributary confluence with the main stem that have prevented the propagation of incision upstream. For those tributaries that are not managed, incision has substantially moved up the tributary, creating deeply cut channels and leaving crossing roadway culverts perched. In many cases, roadway culverts have stopped the progression of tributary incision at the road crossing.

No structures are threatened by widening or stream bed shifting on the main stem. Instead, the main stem CIPs recommended are grade control projects to stabilize knickpoints in the channel.

Several of the CIPs address culverts and storm pipe outfalls. Stilling basins and outlet scour protection are recommended to protect the channel, banks and culvert outfalls.

Bank stabilization is also included in the CIPs to address bank erosion and mass wasting in areas where infrastructure is threatened.

South Salt Creek Watershed Study Area

The major drivers of instability in the South Salt Creek Watershed study area are widening on the main stem and incision on the tributaries and Wagon Train main stem.

There are no structures threatened by the widening and, therefore, no CIPs to address widening along the main stem. However, CIPs are recommended to construct grade controls at existing observed knickpoints along the main stem to stop these knickpoints from moving upstream.

Grade control projects for the tributaries and for the Wagon Train main stem are included in the CIPs to halt active incision and protect the reaches upstream of the grade control. Bank stabilization is also included in the CIPs to address bank erosion and mass wasting in areas where infrastructure is threatened.

Capital Improvement Project Types

High Bank from Erosion
High Bank from Erosion

The recommended projects generally fall into one of three categories of projects:

  1. Grade Control Projects in the channel bed to prevent downcutting (see Figure 2) – construct grade controls along the main stem and tributaries at select locations to stop incision at these locations.
  2. Bank Stabilization Projects on channel banks to protect against erosion (see Figure 3) – construct engineered bank stabilization to protect identified structures from continual bank erosion. Each bank stabilization project includes grade controls at the project limits to protect the stabilized bank from potential future incision and add to the systemic stability of the watershed study area.
  3. Outlet Stilling Basin and Scour Protection at culvert outfalls to protect against erosion (see Figure 4) – construct a stilling basin or scour protection at the pipe or culvert outfall to protect the pipe from erosion and undermining due to outfall scour.
Scour Hole Approximately 3 Feet Deep
Scour Hole Approximately 3 Feet Deep

Water Quality Considerations

Water quality impacts are included in the evaluation of potential CIPs. Project evaluation and development took into account the potential water quality impacts which may occur as a result of project construction. Consideration was also given to the benefit the project would provide by reducing channel erosion and protecting stable reaches, therefore lessening sediment loading.

Special Area Considerations

The process of evaluating the potential CIPs also included a consideration of the potential impacts each project may have on the special areas. Consideration was given to avoiding potential negative impacts during project construction and to the benefits each project provides in terms of protecting the local special areas from unmanaged system instability. Special areas are those that contain ecological, archeological, cultural and/or other community assets that merit consideration when developing CIPs.

Prioritization

The recommended CIPs were prioritized using the categories from the Prioritization Methodology Report for Watershed Master Planning Projects PDF, City of Lincoln, Nebraska, 2006. This methodology was developed for the City to set priorities and implement Capital Improvement Programs for watershed master planning each year. The following prioritization categories were used for project ranking:
  • Flooding Impacts: This category identifies the impact of floodwater encroachment on structures, public or private property, parking lots, public utilities or other infrastructure.
  • Stream Stability: This category identifies the impacts of channel erosion, plus the transport and undermining of soil by stream flow or overland flow. Channel erosion can threaten structures, public property, parking lots, public utilities or other public infrastructure. Channel erosion can also endanger streams, wetlands, lakes, conservation easements, buffer zones or other natural resources.
  • Water Quality: This category identifies the impacts of water quality. A number of geomorphic mechanisms can adversely affect water quality through increased pollutant loading.
  • Safety Factor: This category identifies benefits to mitigate potential threats to public health and safety. The potential for loss of life or bodily injury may include individuals trapped in structures during flooding or vehicles being swept away by floodwater. A safety factor is generally associated with projects addressing structural or non-structural flooding, although safety may be associated with stream stability or water quality projects.
  • Miscellaneous Factors: This category identifies various other miscellaneous factors and additional considerations that have not been addressed in the previous four categories. Examples of other factors include but are not limited to: project location, development status, adjacent projects, complaints and outside funding opportunities.

This ranking system was specifically developed for Capital Improvement Projects proposed as part of the on-going watershed master planning efforts. Ranking worksheets were used to assign points under each category, with the goal of developing an overall score. The projects with the highest point score are considered a higher priority.

What's Next

An open house is set for Thursday, May 8th, at Roper Elementary School, where the CIP projects for the three basins will be presented and the consultants along with the city staff will be available for questions. After the open house, the CIP projects will be finalized and a final report will be prepared for each basin and presented to the City Council for consideration for adoption.

Validate

South Tributaries Watershed Study