Coastal Inlet & Stream Mouth Tidal Sizing Guidance - Efforts

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Introduction

Restoration practitioners in the Puget Sound have limited guidance on geomorphically appropriate geometric design for restoration of small to medium-sized open coastal inlets and stream mouths. These systems are typically funnel-shaped, extending from a wider, open inlet to the inland limit of tidal influence (head-of-tide). Between the opening and the head-of-tide, the width and depth of the estuary varies along its axis. However, little is understood about the relationship between the geometry of the system and physical parameters that serve as predictors for the geometric response. The rate at which the width varies along the length is important to understand for restoration efforts that seek to restore estuary processes in an altered estuary and for the design of new or modified crossing infrastructure. In the absence of guidance information on geomorphically appropriate tidal channel geometry, restoration designers may overly rely on sizing calculations based solely on hydraulic capacity and risk under sizing channels.

For estuaries and stream mouths that have been extensively modified (e.g., by nearshore fill, shoreline armoring, etc.), the appropriate estuary dimensions are even more difficult to discern, as there may no longer be evidence of the system’s natural form visible on today’s landscape. Historical information and maps can provide information about the natural planform shape of an estuary; however, there is limited historical mapping of small to medium-sized inlets and stream mouths in Puget Sound. In addition, many restoration projects and crossing modifications are focused on these types of estuaries.

To date, typical evaluation for tidal crossings has relied primarily on hydraulic design for engineering safety (i.e., sizing the crossing such that it can pass the 100-year return period flow with freeboard and without excessive scour), and for juvenile salmonid velocity criteria. While both evaluation criteria are important, considering only these factors can result in crossings that are too narrow to fully restore geomorphic processes and generate full ecological connectivity.

For this project, ESA will evaluate factors that can be used to estimate estuary widths for small funnel-shaped, coastal inlets and stream-mouth estuaries throughout Puget Sound. This research and analysis will be used to establish tidal connectivity sizing recommendations to inform the design of future ESRP program investments. The study will fill a significant data gap regarding the appropriate width and depth of an estuary at a given location between head of tide and estuary outlet based on considerations of geomorphology and ecological processes. The focus is open coastal inlets and stream-mouth estuaries, including streams draining into barrier embayments. Thus, the project complements ESRP’s investments in barrier embayment outlet channel geometry (16-2283) and marine fish access (16-2282).

The project is divided into two phases with the current grant covering Phase 1. In the first phase, ESA will develop a systematic process for identifying and evaluating sites that meet criteria for inclusion. We will then conduct a desktop data analysis, compiling available data and assessing additional factors such as wind wave exposure, lower stream and valley slope, beach and offshore slope, and tidal statistics. These factors will be related to the estuary width and depth to evaluate which external physical parameters best predict estuary width.

In a future phase of the project (a proposal has been submitted to ESRP), we will conduct fieldwork to refine the Phase 1 analysis and confirm desktop data in the field. We intend to measure the geometry of approximately 40 undisturbed or low-degree of disturbance small to medium open coastal inlets and stream mouths. Since estuary geometry cannot always be determined from LiDAR and not all sites are likely to have publicly available bathymetry datasets or digital elevation models, we will conduct brief site visits to collect accurate data. The primary field objectives will be to measure estuary transects and thalweg elevations, map the ordinary high-water mark, evaluate the predominant bed material, and a check of the estimated head-of-tide. The relationships between estuary width and depth will be updated based on the new data, and recommendations for evaluating estuary sizing will be developed.