Conewago Creek Stream Restoration – Dauphin and Lancaster Counties, PA

Herbert, Rowland & Grubic, Inc. (HRG) helped two townships devise a cost-saving approach to reduce pollution in the Conewago Creek, which subsequently reduces pollution in the Susquehanna River and the Chesapeake Bay. Millions of people rely on these bodies for drinking water and recreation. The particular portion of the creek being addressed by this project is a designated trout stocked fishery (TSF) and is located within a National Fish and Wildlife Federation Eastern Brook Trout habitat priority watershed.

Due to significant documentation of pollution in the creek, the Pennsylvania Department of Environmental Protection had developed a Total Maximum Daily Load (TMDL). As MS4 communities that drain to the creek, Londonderry and Mt. Joy Townships were required to reduce sediment and nutrient pollution in order to meet a targeted wasteload allocation.

HRG recommended they cooperate on stream restoration in order to reduce costs and produce better results. The stream is divided across both municipalities, so cooperation was necessary to address the entire stream as a unit. Cooperation would also allow the communities to share costs and enjoy economies of scale.

HRG’s role on the project involved:

Conducting a joint stream assessment to fully document existing conditions.
LandStudies, Inc. measured bank heights, recorded flow depths, noted vegetative conditions, and installed bank pins to measure erosion rates. HRG surveyed the assessed stream reach and developed hydraulic models of the channel to pinpoint the highest priority areas in need of restoration. This stream assessment was conducted using the BANCS method.

Determining what projects could restore the stream channel to its natural, stable dimensions and re-establish the processes that reduce erosion, capture pollutants, and reduce flooding.

Working with the townships on prioritizing which projects to pursue.

Preparing conceptual designs and cost estimates for priority projects.

Negotiating an inter-municipal cost-share agreement.

Obtaining more than $1 million in grant funding to support design and construction.

The project being designed involves creating a stable, low-lying floodplain bench with adjacent banks graded at a shallow slope and stabilized with extensive native plant cover. This is accomplished by removing legacy sediment from the surrounding floodplain to restore the whole area to its historic conditions. This floodplain bench will create a low bank height for the stream. During high- flow events, the floodplain bench will allow increased flows to escape the channel and access the additional flood storage provided by the restoration area, greatly reducing flow velocity by spreading the volume of water over a greater surface area. This additional flow area will mitigate otherwise potentially erosive flow conditions and reduce the potential of severe flooding downstream.

The approach provides additional capacity for flood flows and reduces erosive energy in the channel while minimizing the impact to adjacent agricultural fields. Thanks to the available increased flood storage, adjacent farms will be inundated less frequently, keeping valuable seed and nutrients on the field (and out of the creek).