Easter Lake Park Streambank Stabilization

Easter Lake Park Streambank Stabilization Project

In every urban neighborhood there is a network of pipes underground to control stormwater runoff when it rains. The engineered storm drain network is designed to quickly collect and convey stormwater runoff keeping streets free of large quantities of water during rainstorms. However, have you ever thought about where these pipes lead to?

In most cases, storm drains outlet into a river, lake, or other body of water without any treatment. During rainstorms, the storm drainpipes rapidly transport millions of gallons of water into stream channels and can cause severe stream bank erosion. While erosion is a natural process, urban development and our storm drain network has accelerated this process leading to environmental degradation. For example, did you know that since its creation in 1967 Easter Lake lost 30% of its volume and 60% of the sediment in the lake was attributed to stream bank erosion?

In a natural system, healthy soils help infiltrate water into the ground, and wetlands or other features store water reducing the quantity of surface runoff in any given rain event. In urban environments, impervious surfaces such as parking lots, and storm drainpipes have replaced the natural system. Consequently, instead of a system where most water is stored in the landscape with slower transport of water into stream channels, we have larger quantities of water traveling faster through storm drains placing much more stress on our streambanks.

In 2016, a basin was constructed near the historic covered bridge to capture sediment coming into Easter Lake from an unnamed tributary. While working on the sediment basin, project coordinators discovered severe erosion originating upstream in the tributary. Hidden by trees and overgrowth of invasive plants, the tributary was found to be very actively eroding sending large amounts of sediment directly into Easter Lake’s center arm.

In 2017, project coordinators surveyed the eroding channel and found two large nick points with drops of an immediate 10 feet in elevation. Overall, the channel dropped nearly 30 feet in elevation over 110 yards. Honeysuckle, and other invasive species had taken over the area making the soil easily susceptible to erosion.

Pictured to the left is the sediment basin located in Easter Lake Park

Photos of Unnamed Tributary Erosion Prior to Stabilization

The unnamed tributary at the south entrance of the park is fed by stormwater and urban runoff from a 34-acre drainage area. The drainage area consists of mostly single-family residential lots and streets with a large percentage of impervious surfaces. Stormwater from the area is conveyed into a single pipe that outlets on the east side of the south park entrance.

Water that flows from the channel comes from this 34-acre drainage area outlined in red (Photo credit: ISG) and enters the channel through this pipe located on the north side of Easter Lake Drive on the east side of the park’s south entrance.

Planning

In spring of 2018, the Polk County Conservation Board (PCCB) accepted a proposal from ISG to complete designs for a stream stabilization project. Working closely with PCCB and the watershed project, ISG designed a solution to stabilize the area, treat, filter, and slowdown stormwater.

ISG’s design incorporates green infrastructure practices including a bio-retention cell and native plantings along with step-pools and floodplain restoration, both practices found in the Iowa Department of Natural Resource’s (IDNR) River Restoration Toolbox to stabilize the tributary.

In 2018, the IDNR released the River Restoration Toolbox, a set of proven engineering practices with emphasis on incorporating natural materials such as logs, stone, and native plantings, to assist designers with stream stabilization and restoration projects. This stabilization project planned for Easter Lake Park will be one of the first projects in Iowa to use the toolbox since its release in 2018.

Pictured here are example views of a step-pool system. Acting like stairs, the step pools help stabilize steep changes in elevation in a stream channel. Credit: ISG

In the design, the red line shows the boundary of the new floodplain. Credit: ISG Once completed, the channel will be stabilized to prevent further streambank erosion and protect water quality in Easter Lake for years to come.

Design

Among the dozens of techniques described in the River Restoration Toolbox, the relative size and steepness of the channel and other characteristics of the tributary suggested a step pool system, incorporation of a floodplain, regrading and native plantings would provide the grade control needed to stabilize the site. After water exits the storm drain pipe it will enter a sediment basin that will help slow down the water and help capture initial sediment and other pollutants present in the water.

Once leaving the sediment basin, water will travel through a series of step-pool structures helping the channel decrease in elevation in a more stable way. Step-pools are grade control structures formed by large boulders used to control elevation drop and disperse energy in stream reaches with a steep slope.

One of the most beneficial aspects of the design is the incorporation of a floodplain. Along the length of the channel, the stream will be connected to a floodplain on each bank to provide energy dissipation during large rain events. Beyond the floodplain on either side of the channel the area will be regraded to a stable slope and will be planted with Iowa native plants.