System Approach to Watershed Planning Cottonwood Creek Drainage Basin Planning Study

by Lucas Babbitt, PE, CFM, Matrix Design Group, Inc.
Background

Conventional watershed planning has consisted of identifying infrastructure deficiencies and flood control facilities required to mitigate the increased runoff caused by development. While these components are a critical part of watershed planning, other components important to a complete system plan tend to be neglected. Recent history has shown that other watershed amenities tend to suffer when flood control is the only priority.
Matrix Design Group, Inc. is in the process of finalizing the update to the Cottonwood Creek Drainage Basin Planning Study (DBPS) for the City of Colorado Springs (City). The main stem of Cottonwood Creek is approximately 12 miles long and has a drainage area of approximately 19 sq. mi. at its confluence with Monument Creek. Previous studies have been completed for this watershed with a focus on flood attenuation and conveyance which has left the watershed in a state of instability. Current concerns within the watershed consist of the release of higher than historic flows downstream, channel instability, habitat destruction, and water quality issues.
Project Approach
The goal of the Cottonwood Creek DBPS was to develop a sustainable watershed plan by utilizing a system approach to satisfy multiple objectives. A system approach to watershed planning consists of evaluating everything within the watershed that is inextricably linked. This includes infrastructure deficiencies, hydrology and hydraulics, economic impact, public safety, flooding, wildlife habitat, water quality, floodplain preservation, and aesthetics. The objectives of the Cottonwood Creek DBPS were to:

1. Identify and evaluate infrastructure deficiencies
2. Provide flood control and safety
3. Provide an amenity to the community by restoring the aesthetic value of the corridor
4. Protect the environment by restoring habitat and implementing water quality
5. Provide an economic benefit to the City by implementing cost-effective and low maintenance solutions

The underlying philosophy towards the development of this watershed plan was to preserve floodplains, incorporate natural channel design to the maximum extent possible, and manage low flow hydrology. Floodplain preservation is paramount in developing a sustainable watershed plan for multiple reasons. Floodplains facilitate the effective conveyance of floods by distributing shear stress across a wide channel section, provide habitat, provide water quality by promoting storage and infiltration, and provide an aesthetic value to the community. Natural channel design focuses on providing, or maintaining, channel connectivity to the adjacent floodplain and is designed based on low flow hydrology or, more specifically, channel forming flow. Natural channel design uses geomorphology-based design approaches in combination with habitat restoration and revegetation to establish a stable system. Managing low flow hydrology in a developing watershed is critical because the quantity of runoff increases along with development. Low flow hydrology consists of the flows that are most responsible for shaping channel dimension over time and, if this increase in runoff goes unmanaged, then the restored natural channels will be at risk of failure due to the change in flow regime. The benefits of natural channel design include:

1. Lower construction costs by balancing the need for large grade control structures
2. Habitat creation and preservation by restoring connectivity to adjacent floodplains and wetlands
3. Wildlife corridor connectivity
4. Water quality by promoting storage and infiltration into floodplains and limiting channel erosion
5. Improved channel function by restoring connectivity to adjacent floodplains and effectively conveying sediment for a full spectrum of flows
6. High aesthetic value

Information collected from extensive field investigation, hydrologic and hydraulic modeling, public feedback, and client input was synthesized to create a multi-criterion alternatives screening process to evaluate both detention and reach alternatives.
A full range of detention pond alternatives was evaluated that consisted of 2-yr flood control detention, 100-yr flood control detention, and full spectrum detention for all possible locations within the watershed. Online ponds were avoided where possible because they impair the conveyance of sediment which results in clear water scour downstream of the pond and maintenance costs for sediment removal within the detention pond. Detention pond locations were optimized based on how effectively they reduced peak flow with preference given to offline ponds located higher in the watershed. The criteria that were used evaluated the type of detention included pond costs, channel improvement costs, habitat, water quality benefit, 2-yr peak flow reduction, 100-yr peak flow reduction, lot premiums, and operations and maintenance costs. The result of the analysis favored the use of full spectrum detention for all proposed detention ponds and recommended that the outlet structures of existing detention ponds be retrofit to provide full spectrum. Full spectrum detention provides low flow hydrology management, flood control, and incorporates water quality.
A suite of channel improvement solutions were prepared to work in concert with full spectrum detention while focusing on implementing natural channel design everywhere possible. In order for natural channel design to be implemented the proposed channel reach had to have a calculated sheer stress of less than 2 lbs/sq ft, a longitudinal slope of less than 2 percent, and a belt width less than the available corridor width. If natural channel design could not be implemented for a given channel reach then the next preferred channel improvement was a selection of vertical drop structures ranging from 3 ft to 6 ft in height which was based on the required horizontal spacing.
Summary
The developed watershed plan will restore the aesthetic value of the corridor by:

1. Implementing natural channel design everywhere possible
2. Preserving existing floodplains
3. Preserving existing wetlands
4. Preserving trail networks and recreation areas important to the community

The developed watershed plan will protect the environment, restore habitat, and implement water quality by:

1. Providing 9.5 miles of directly connected wildlife corridor to the Black Forest which is considered a high biodiversity area
2. Incorporating water quality into 13 of 20 detention ponds
3. Implementing natural channel design where viable
4. Preserving existing floodplains
5. Preserving existing wetlands

The developed watershed plan will provide an economic benefit to the City by:

1. Providing a low cost and self-maintaining system through the use of natural channel design
2. Avoiding the use of on-line detention ponds

The developed watershed plan will provide flood control and safety by:

1. Providing infrastructure planning guidance as a result of the hydraulic evaluation of over 10 miles of storm sewer
2. Providing infrastructure planning guidance as a result of the hydraulic evaluation of 13 bridge crossings
3. Providing designs for 9 additional full spectrum detention ponds
4. Providing retrofit full spectrum detention solutions for 8 existing detention ponds
5. Reducing the 100-yr peak flow at the watershed outlet from 13,000 cfs to 9,900 cfs