19  Ordinary Highwater Mark RAT

🪛 Practical

Purpose

The Floodplain Connectivity Web Application is an interactive, browser‑based tool for assessing hydraulic connectivity between a river channel and its adjacent floodplain. It provides a structured workflow for entering site data, computing connectivity metrics, and summarizing results to support the design and evaluation of floodplain restoration and management projects.

The application is intended for fluvial geomorphologists, hydraulic engineers, river restoration practitioners, and water‑resources planners.

Access and Platform

Scope and Capabilities

At a high level, the application:

  • Accepts user‑defined topographic, hydraulic, and geometric inputs describing a river reach and its floodplain.
  • Determines the flow conditions under which different portions of the floodplain become inundated or hydraulically connected to the channel.
  • Computes connectivity metrics such as thresholds for floodplain activation (e.g., water surface elevation, discharge, and associated return interval when available).
  • Produces tabular and graphical outputs to support screening, comparison, and documentation of floodplain connectivity conditions.

Typical Workflow

1. Project Setup

Users begin by defining the analysis context:

  • Enter project or reach identifiers, analyst name, and date.
  • Specify the objective (e.g., existing conditions assessment, alternative comparison, or restoration scenario evaluation).

2. Input Data

The application typically requires the following information (exact fields depend on the specific configuration):

  • Topography and Geometry
    • Channel bank elevations and floodplain surface elevations.
    • Cross‑section data and/or representative station–elevation points.
    • Delineation of floodplain units or surfaces, if applicable.
  • Hydraulic Information
    • Stage–discharge relationship or rating curve for the reach.
    • Design flows or a range of discharges for analysis (e.g., annual exceedance probabilities or specific return intervals).
    • Optional roughness or land‑cover parameters when needed.

Input fields are provided through numeric entry boxes, drop‑down selectors, and file upload controls (if the application is configured to accept external data).

3. Connectivity Computation

Based on the entered data, the application:

  • Identifies the water surface elevation and discharge at which the main channel first spills onto adjacent floodplain surfaces (onset of connectivity).
  • Tracks the progression of inundation or connectivity across different floodplain units as flow increases.
  • Associates connectivity thresholds with return intervals or exceedance probabilities, when appropriate flow–frequency information is supplied.

4. Metrics and Diagnostics

The application computes and reports metrics such as (where applicable):

  • Elevation and discharge at initial floodplain activation.
  • Connectivity thresholds for individual floodplain units or surfaces.
  • Incremental changes in inundated area, depth, or connectivity status across the analyzed flow range.
  • Indicators of disconnection (e.g., surfaces that are only inundated during high‑magnitude, low‑frequency events).

These metrics help characterize how frequently, how deeply, and under what conditions the floodplain is connected to the channel.

5. Visualization and Output

The tool provides visual and tabular summaries to support interpretation and documentation, including:

  • Plots of water surface elevation vs. discharge with floodplain activation thresholds highlighted.
  • Longitudinal or cross‑sectional views showing channel banks, floodplain elevations, and connectivity stages.
  • Tables listing connectivity thresholds (elevation, discharge, and, where available, return interval) for each defined floodplain unit or location.

Results can be copied or exported (e.g., as CSV tables or images) for inclusion in reports, figures, or further analysis.

6. Scenario Evaluation and Iteration

Users can:

  • Modify topographic or hydraulic inputs to represent alternative conditions (e.g., proposed grading, levee removal, or reconnection features).
  • Re‑run the analysis to compare connectivity metrics across scenarios.
  • Iteratively refine restoration or management designs to achieve desired floodplain activation frequencies and depths.

Intended Use and Limitations

The Floodplain Connectivity Web Application is intended as a decision‑support and screening tool. It is not a substitute for detailed hydraulic modeling or professional judgment.

Key assumptions typically include:

  • Simplified hydraulic representation, often based on one‑dimensional stage–discharge relationships.
  • Generalized characterization of floodplain topography, constrained by the resolution and accuracy of available data.
  • Focus on connectivity thresholds and patterns rather than full two‑dimensional floodplain flow dynamics.

Users should:

  • Verify that input data and assumptions are appropriate for the project scale and objectives.
  • Cross‑check key thresholds and metrics against field observations, detailed hydraulic models, or additional analyses when required.
  • Document input sources, processing steps, and scenario definitions for transparency and reproducibility.