Predicting Excess Water Accumulation in an Urban Area Using ArcHydro
Over the past decade there has been an increase in frequency of powerful storms and cyclones that have hit the Eastern continental United States. Not only are these storms more powerful, they are slower moving and dump large amounts of water causing unprecedented flooding deeper inland. This increase in storm magnitude can be attributed to climate change and warmer ocean temperatures causing the storms to grow extremely wide while forming in the Atlantic. It is predicted that these storms are only going to intensify and the effects of inland flooding will worsen.
September 17, 2018, hurricane Florence dumped record amounts of rainfall onto the state of NC causing nearly $17 billion in damages. It was named the wettest tropical cyclone on record to hit the Carolinas. The storm produced a 9 to 13 foot storm surge and 20 to 30 inches of total rainfall, inundating the coast and causing massive amounts of inland flooding even into the piedmont. Areas like Raleigh and Chapel Hill accumulated 6 to 10 inches of rainfall during the duration of the storm. These urban areas are not used to large amounts of rainfall and flooding here can be costly. Therefore I am interested in analyzing the physical characteristics of these urban inland areas so that i can predict flooding.
At the beginning of 2020 I moved to an apartment off Hillsborough street in downtown Raleigh after I graduated. The goal of this project is to use ArcHydro and overlay analysis to determine areas that accumulate water based on normal rain patterns so that predictions of flooding during heavy rains can be made. I wanted to know if my apartment would be in danger of water damage from inland flooding due to heavy rains from a hurricane.
Methodology
Three major factors were used to determine the areas were accumulation of water would be highest; slope, land use, and flow accumulation. These layers would be overlaid using raster calculator in ArcMap to create a suitability map.
Location of study site and surrounding areas.
The first step was to obtain the DEM data for the area of study. In this case the area was 9,800 by 10,000 square meters of downtown Raleigh including my apartment complex. I downloaded quality level 2 bare earth LiDAR data points in LAS file format from the NC Spatial Data Download website. This format came as a very dense point cloud that had about 150 points per square meter and needed to be converted into a usable DEM format such as a TIFF. The Q2 LiDAR data was loaded into ENVI LiDAR software so that the points could be converted into a continuous DEM.
Slope was used as a factor in analysis because it would show the potential for water to move across the surface from a higher elevation to a lower one. The DEM that was created before was inputted into the Slope tool in ArcMap and a raster layer for slope was created. The slope values were displayed in natural breaks as percent rise from 0-5, 5-13, 13-26, 26-47, 47-135. These ranges for slope were very large and the area was relatively flat so I changed it to manual breaks to display smaller increments of change in slope to increase the detail of the model. Slope values after manual break change; 0-2, 2-5, 5-15, 15-35, 35-135.
After this layer was created, a reclassification was done in order to sow the areas were higher potential for water flow were occurring.
Land use was used in order to determine the impermeability of the ground within the study area. Land use data was obtained from the National Land Cover Database and clipped to the same extent as the DEM file as well as projected into its coordinated system. The land cover data displayed 15 different land covers ranging from open water to agricultural fields. It also contained categories showing varying intensities of developed areas which I used to designate the degree of permeability of the ground.
The flow accumulation is the most important factor in this overlay analysis. It will tell me where the water would flow and accumulate based on the DEM. I will also be inputting the annual NC precipitation in inches for the area so that I can get an idea of water flow across the land and not just from inputs from nearby bodies of water. First step is to create the flow direction from the DEM using the flow direction tool. This shows what direction a drop of water would flow out of cell, and it calculates this for every cell in the raster. Next the flow direction is inputted into the flow accumulation tool and weighted using the annual NC precipitation in inches for the area to create the flow accumulation map.
Map Algebra
The final step was to give weight and overlay the layers for a final map product. Both the slope and the land use layers were given weight of .25 and the flow accumulation was given .5 because it is the most important factor in determining areas that would be flooded. The layers were added together in the raster calculator tool to output the final map product displaying the accumulation of excess water in downtown Raleigh, NC.
Final Product
Georefferenced overlay of the final map product on google maps with my apartment complex indicated by red circle.
Results
The final map product had a low value of 3 and a high value of 13 with 3 being the areas least likely to accumulate water during heavy rains. I overlaid the map onto satellite imagery in order to visualize what areas where are more likely to flood than others. My apartment complex is in no danger of being flooded, however the parking lot where I will be keeping my car will accumulate a lot of water. There are many other parking lots that seem to accumulate water and are hazard zones for flooding. A larger construction area north of my apartment complex will accumulate a lot of water as well as the residential area surrounding it. The area south west known as centennial campus will see lots of accumulation of water, mainly around the administrative buildings.
Concluding Statements
It seems that my original question was answered and the apartment I will be living in will be okay during times of heavy rain, however I found that many of the buildings on NC state’s campus will most likely see larger amounts of water accumulation. Many of the buildings and parking lots on the centennial campus could have problems of water damage if we were to see more devastatingly wet hurricanes like Florence. This is probably due to the high occurrence of impermeable surfaces in this area. This map could be used to predict where to build water barricades around NC state’s campus or where rework the landscape to cope with the lager amounts of water to reduce potential water damage.
Several large construction sites seem to be accumulating a lot of water. This could pose as a problem because large machinery could be damaged. Most of the time there is exposed dirt which could be washed away and cause sedimentation of the nearby water bodies such as lake Raleigh.
Most residential areas seem to be relatively safe from high amounts of water accumulation except for one area around the Cameron village shopping center. This area is a retail shopping center surrounded by single family homes and it could pose a problem if it were to be flooded.
This map generated using high resolution geospatial data is an exceptional tool in predicting flooding, even in urban areas. It is however only a prediction and we do not know how accurate this method of flood mapping will be. This method should be tested using lower resolution spatial data to see if you need to use such high resolution because of how time-consuming working with the data can be. Further analysis should be taken such as on-site surveying after a heavy rain storm in order to validate flood prediction from the map.