Culverts Are A Problem

Let’s face it, most of the culverts in your Town are old. And quite often there is no record of when they were installed. Plus, in the past 50 years or more, the surrounding landscape has changed. That means runoff has changed too. At Indian Landing on Irondequoit Creek in the Town of Penfield NY (Monroe County), you can now walk across the creek on most days. During the colonial period, boats would sail up the Bay and dock at the landing to trade, hence its historic name. What has changed? The answer is simple: Land use. Changes in the landscape have affected runoff and erosion all across New York state. Today, that means culverts designed to move runoff away from roads, houses, and business are often under siege with more water than they were designed to handle. And the problem is not limited to culverts. Catch basins and associated stormwater pipes are also often undersized for today’s peak flow rain events.

Evaluating culverts and other drainage infrastructure one piece at a time is time consuming and expensive. Planners and Engineers need better tools to assess drainage and its supporting infrastructure. The good news is, there are new tools and data out there to help.  And the result is better informed decisions at the local level about resource allocation toward improved community resiliency.

Change in land use have resulted in change in runoff.

A significant amount of our infrastructure is undersized for current conditions. 

Peak runoff during storms, or even during “regular” rainfall events, can cause localized flooding and road over-wash. This damages roads and pavement while destabilizing embankments and causing significant erosion. In recent years, local highway departments have been reacting to flood damage with emergency repairs. But instead of being reactive, Towns can save money and improve design by being proactive in prioritizing infrastructure repairs.

As examples, the Town of Woodstock in Ulster County NY has been working with the County Department of Environment to become proactive in identifying culverts that are potentially undersized, and hence pose the greatest risk of failure. The Town of Greece in Monroe County has been working with local engineers to assess and improve nuisance flooding along the lakefront, which has been inundating roads and residences, and overwhelming existing stormwater infrastructure.

There is Lots of Data to Help

Extremely high-resolution elevation mapping via Geographic Information Systems (GIS) is now available nearly ubiquitously across New York State courtesy of the NYS GIS Program Office. This mapping data, called LiDAR (for Light Detection and Ranging), is extremely useful for municipalities because it provides the ability to analyze details that were previously only thought possible through custom engineering and surveying projects. Using publicly available LiDAR for engineering and survey work has the potential to save municipalities thousands of dollars in engineering and surveying fees. Studies have shown that in many cases, existing LiDAR data can support detailed drainage analysis and design.

 

New Tools Make It Easy

In addition, researchers with the New York Water Resources Institute at Cornell University have been working to develop GIS based tools to help evaluate culverts and drainage infrastructure. A recent model, dubbed the Cornell Culvert Model, was designed to help identify culverts that are likely to be inundated during heavy rain events, and prioritize those in need of replacement or repair.

In the eastern part of New York, the Hudson River Estuary Program has been supporting communities in using both LiDAR and the Cornell Culvert Model as part of their Culvert Management Plans.  These Culvert Management Plans require the creation of a culvert inventory that includes a basic set of field information regarding the size, shape, and condition of each culvert. Using a standard culvert inventory database, such as the one developed by the North Atlantic Aquatic Connectivity Collaborative (NAACC), the Cornell Culvert Model can analyze culverts to determine both their existing capacity and the likelihood of failure for each culvert. 

 

A Case Study

Originally built to use “traditional” (older) lower resolution USGS digital elevation models as its base topographic data, the Cornell Culvert Model has now been adapted to use the more modern, high resolution LIDAR data. And the results are impressive. GroundPoint Engineering recently ran the Cornell Culvert Model using LIDAR for a portion of Ulster County covering areas in the Towns of Woodstock and Saugerties. The study looked at 274 culverts, all at one time. Depending on which of the two different elevation model was used (LiDAR or the older, non-LIDAR), the difference between the calculated culvert drainage areas and the resulting peak flows is significant. 

As a mapping exercise, when comparing areas calculated using the two different data sources, the measured differences (in acres) and the percent difference (as a % change between the two) are important to understand. For example, a culvert with a very small drainage area may show a 100% change in area between the two approaches, but the overall area of that change may still be…very small. Conversely, a culvert with a very large drainage area may show a relatively small change as a percent of the overall area, but the changed area may actually be quite large.

When comparing culvert catchment areas calculated with the “traditional” elevation model to the newer LIDAR elevation model, over 84% of the culverts experienced a change in drainage area larger than 6 acres. 6 acres was chosen as a size that is significant in changing the amount of runoff.  On a percent basis, over 50% of the culverts experienced a change of at least 10% or more in total drainage area, and 36% (over a third!) of the culverts experienced a drainage area change of 25% or more. What this means is that there is real opportunity to improve drainage infrastructure design with better data.

84% of catchments changed by at least 6 acres.
36% changed in size by more than 25%.

Regardless of how one goes about it, the whole point is to get the mapped drainage areas to each culvert or catch basin correct, so that runoff calculations and the resulting sizing and design will also be correct. The data that is used in the calculations can have a significant impact on the results.

With some manipulation of the LIDAR data, this can be accomplished in a matter of hours for an entire municipality, where traditional field survey would take weeks at best, and may not even be physically possible.

 

Conclusions

The GIS nature of the Cornell Culvert Model allows municipalities to look at all their mapped infrastructure together, not each in isolation.  The ability to not only visualize all that on a map but prioritize and sort based on the highest likelihood of failure, is a game changer when it comes to being able to evaluate and prioritize municipal infrastructure.

The use of LiDAR data over more “traditional” topographic data is also a game changer. While the LIDAR data does require some expertise to use and process effectively, the results are undeniable. Using LiDAR offers better results, faster and cheaper, than is currently possible with traditional engineering and surveying methods.

Using the Cornell Culvert Model in conjunction with NYS LiDAR data can allow Towns to evaluate the drainage infrastructure across their entire municipality, and support design projects intended to reduce flooding and improve community resiliency.

Ben Houston is a licensed Professional Engineer in New York and Principal at GroundPoint Engineering, PLLC. GroundPoint Engineering is a NYS certified Service-Disabled Veteran Owned Business (SDVOB). New York Law Article 17-B requires the use of SDVOB certified firms on all NYS funded contracts.