Sea Level Rise Flood Projection Modelling due to High Tides

ClintonDotse

Project Overview:

I am currently developing a project aimed at simulating the potential flooding effects due to high tides. The study area includes agricultural land and other vulnerable regions protected by a set of discontinuous dikes. These dikes are intended to mitigate the impacts of tidal flooding, which would otherwise inundate the area during high tides. To aid in flood risk analysis, I have acquired a high-resolution (1 meter) Digital Elevation Model (DEM) of the study area and intend to generate a series of maps depicting the extent of inundation at varying water levels.

Project Goals and Requirements:

The primary goal of this project is to produce flood projection maps for a 100-year period, considering a sea-level rise scenario of 2 meters above the current Mean Higher High Water at Large Tides (MHHWLT) value. Specifically, I seek a detailed, step-by-step procedure for generating these maps, including methods for representing both the current water surface and the MHHWLT in my model.

Specific Questions and Areas of Concern:

1. Generation of Water Surface and MHHWLT Layers: How can I create a layer that represents the current water surface, as well as one that represents the MHHWLT for the study area?

2. Incorporating the Impact of Tides: What is the best approach to incorporate tidal fluctuations into this model, given the dynamic nature of tides and their contribution to the inundation extent?

3. Consideration of Sea-Level Rise: How should I factor in a 2-meter sea-level rise scenario into the flood model, and what methodologies can be applied to project its impact over the 100-year period?

Software and Tools:

I am currently using ArcGIS Pro 3.2 for this analysis and would greatly appreciate guidance on utilizing this software effectively for the tasks outlined above.

Conclusion:

I would greatly appreciate any recommendations or advice that could help streamline the development of accurate and reliable flood projection maps. Thank you for your time and support.

danielri

Hello,

1. Generation of Water Surface and MHHWLT Layers: When working with water elevations the first step is to have a full understanding of the conversion of observed water level vs. topographic elevation. The land elevation will be based on a vertical datum above a geoid (NAVD88) while water elevation will be height above or below a known point (hopefully referenced to NAVD88 at a tide gauge). Based on that you will be able to determine the NAVD 88 height that equates to a given tidal elevation. 

2. Incorporating the Impact of Tides: the answer to this is dependent on where your wetland area is. In the Pacific Northwest tides can be +/- 5 meters while in southern Florida tides are less than +/- 1 meter.  Recall that tide is just one part of the water height model and that all areas have the concept of Mean Higher High water. This means if you use a basic flood model based on a course DEM you will not have significant land loss until your modeled height exceeds MHHW. Once you exceed this threshold, unless you hit a cliff, is when you will see 'land loss'.

3. Consideration of Sea-Level Rise: When determining SLR don't just throw on a 2 m elevation rise. You first need to determine the per/year regional trend in elevation (uplift vs. subsidence) to add or subtract to the 2 m value for your study area. Remember, the projections from the IPCC are Global', by accounting for your regional trend you will convert the global to a regional estimate that is valid for your study.

4. Other considerations for your100 year period is how you expect the tidal estuary and wetlands to respond in your area. Will they gradually disappear with increased ponding as in areas of coastal Luisiana that is made up of silt/mud/clay OR will waves rework existing sand sediments and eventually build a barrier (could be a above water dune or a sand bar) that will reduce wave energy and the speed of land loss? 

You can see, this is science so you can go as in-depth as you want. If you want VALID results even with the simple bathtub model you MUST address regional trends in elevation of the Ground before applying a "depth" to the model. 

Last comment, in the past we often selected 0.5 m, 1 m, 1.5 m and 2 m because they matched with the elevation accuracy of the underlying DEM we were going to apply the SLR 'on'. For example, public DEMs produced prior to 2000 generally only have an accuracy of +/- 0.1 m at best with most being in the 0.25 m. range. The horizontal grid size selected also plays into the land loss projections (is it a 1 x 1 m vs. a 10 x 10 m DEM).