Evaluating Flooding Potential in Urban Areas
We can analyze existing drainage structures and how they perform by evaluating historic flooding events or predicting effects from future, less frequent, storm events that have not occurred to-date. The process of assessing flood events has to start with watershed delineation so the contributing area is known. There are several steps involved in delineating a watershed or sub-drainage. We use ArcGIS tools to help delineate watersheds and/or sub-drainage areas within the watershed. The following steps are generally used in delineating the watershed:
Download digital elevation model (DEM) data to define the surface contours in the watershed (Figure 1);
The National Map is usually a great place to start when looking for DEM data.
Identify the downstream terminus of the area being evaluated;
Use ArcGIS Spatial Analyst Hydrology tools to generate a watershed;
Review the computer-generated watershed for reasonableness and provide manual adjustments if necessary;
Generate final watershed outline figure to determine the size and geometry of watershed.
Figure 1: Topographic contour map generated from DEM data.
Figure 2: The contour map showing surface flow accumulation lines.
In assessing flooding potential, we generate a number of maps as intermediary steps when delineating watersheds. Figure 1 is a topographic contour map generated from the DEM raster data. This contour map is developed to be able to visualize the magnitude and direction of surface flows. Using the Figure 1 contour map, Figure 2 is generated from that map and shows surface flow accumulation lines, where larger connected areas will accumulate more flow. The darker red a line is, the more flow lines are connected to this location and the more locations can contribute to flow at that point. Sub-watershed areas were then developed based on the different flow accumulation lines from Figure 2 and, therefore, show the sub-drainage areas.
In the example shown in Figure 3, related to the sub-watershed areas, the parcels outlined in black experienced flooding while other nearby parcels did not. After generating sub-drainage flow patterns (Figure 3), including flow control conduits and drains to route storm flows, as shown in Figure 3, if a low-frequency storm were to produce significant surface runoff in the orange and red outlined areas, then these properties could flood. The four southernmost parcels would have a much greater flooding potential than the northern and middle properties, since the northern sub-drainage areas (shown in blue and green) have a lesser potential to contribute surface runoff to these properties due to these drainage areas being much smaller than the southern sub-watersheds (orange and red, Figure 3). Depending on where the most intense storm area occurred for a specific storm, and the duration of the storm, flooding could be evaluated with these methodologies, taking into account anthropogenic alterations to the sub-watersheds.
Figure 3: The contour map showing subdrainage flow patterns.
For example, the red and orange drainage areas have been made larger and have concentrated flows on the upstream end of the sub-watersheds due to the anthropogenic effects related to routing of storm flows through underground flow conduits. Without the conduits concentrating surface runoff flows, the water from the southern mountains would not be routed to these properties, but would flow further to the east.
If you have potential flooding concerns, or questions about any surface water or groundwater issues, please contact Lytle Water Solutions (LWS) at 303-650-4090 or lws@lytlewater.com, or contact the individuals below.
Anna Elgqvist, EI anna@lytlewater.com
Bruce Lytle, P.E. bruce@lytlewater.com
