Assessing alternatives to alleviate suburban flooding

February 2013 » Features » PROJECT CASE STUDY
Dynamic hydrologic and hydraulic model of stormwater infrastructure is used to analyze the extent of flooding problems and evaluate solutions.
Justin Avent, P.E.

Project
Lateral J Drainage Improvements, Collierville, Tenn.

Civil engineer

Smith Seckman Reid Inc.

Project application

Dynamic stormwater modeling using Bentley SewerGEMS software helps the town of Collierville, Tenn., plan and design a regional stormwater detention basin.

The town of Collierville, Tenn., recently began construction on a regional detention basin designed to reduce flooding. Collierville is a suburb of the Memphis metropolitan area and has a population of approximately 44,000 residents. The town has experienced several major storm events in the last few years, and several homes within the Lateral J basin have reported flooding. Rather than looking at each flooding issue as an isolated problem, the town decided to investigate the root of the problem by using computer software to create a hydrologic and hydraulic (H&H) model of the stormwater infrastructure in the basin.

The Lateral J basin has a drainage area of approximately 1,600 acres (2.5 square miles) and drains into the Wolf River, which in turn drains into the Mississippi River. The basin is comprised of mostly residential areas although there is a small amount of undeveloped and commercial area. The study area comprises the entire Lateral J drainage area, including several sublaterals and other drainage networks that originate in nearby residential areas (see Figure 1).

Figure 1: Lateral J drainage basin

The main channel of Lateral J generally flows north through an open drainage system, with the exception of road crossings, where culverts are used. There are several sublaterals that discharge into the main channel at different points. There are some smaller streams and several drainage pipe networks that discharge into the sublaterals or the main channel. The main goals of the study were to analyze the existing conditions during several storm events and assess alternatives to determine ways to alleviate flooding. Based on historical flooding records, the culvert crossing at White Road just west of Rillbrook Drive was of particular interest.

The specific tasks performed for this project included the following:

  • Collect and incorporate all pertinent data required for the development of the H&H models. Data sources used for the drainage study include storm sewer maps from Collierville's records, 2-foot-interval contours derived from LiDAR data provided by Shelby County, aerial photography, and zoning maps. Survey information was collected, as needed, to supplement the available information.
  • Conduct field investigations at known flooding sites and along the study reaches. Field investigations were performed to supplement the information available in Collierville's storm sewer maps.
  • Develop a basin-wide integrated H&H model to estimate the extent of the flooding problems occurring throughout the study area.
  • Model alternative improvements to the drainage network intended to reduce the severity and extent of flooding problems in the study area.
  • Meet with town officials at regular intervals to discuss progress, review the model-selection process, review modeled scenarios, and review the final report.
  • Prepare a report that provides a narrative, flooded-area maps, and water surface elevation estimates.
  • Transfer the computer model to the town of Collierville and provide training on the modeling software for town staff.

Creating the model
The model was developed in Bentley SewerGEMS using the city of Memphis Drainage Manual as guidance for input parameters. The Shelby County 2-foot-interval contours were used extensively in sub-basin delineations. In addition to the 2-foot contours, aerial photography and storm sewer layouts were used to provide the most realistic drainage area boundaries as they relate to the storm sewer system.

Copies of the storm sewer maps for the drainage networks (provided by Collierville) were reviewed and pieced together to form a comprehensive but incomplete overall picture of the existing stormwater drainage system. Survey data was used to supplement the information available from the plan drawings to develop a complete drainage network. Survey data consisted of cross sections of the open channels, culvert data, and information on any pipe 36 inches in diameter or larger.

Curve Numbers and the time of concentration were calculated for each sub-basin and were input into the model for hydrograph generation. The SCS Unit Hydrograph Method was used to calculate excess rainfall and runoff hydrographs for each sub-basin and storm event. The two-, 10-, 25-, 50-, and 100-year storms were analyzed. Rainfall amounts were obtained from the city of Memphis Drainage Manual and distributed according to a 24-hour, Type II rainfall distribution.

With regard to hydraulic analyses, Bentley SewerGEMS allows for open and closed conduits with an infinite variety of cross sections. The Bentley SewerGEMS hydraulic model consists of a network of links and nodes. The links represent conduits such as culverts and open ditches. Nodes are placed at the junction of two or more links and represent manholes, inlets, open-channel cross sections, or the point of change from one conduit to another. Also, nodes serve as the point of inflow for storm runoff from the hydrologic component of Bentley SewerGEMS. The flow from each sub-basin enters the model at a node.

The horizontal alignments of the open channels were established using a combination of field survey data and State of Tennessee GIS data. The horizontal alignments of the channels represent the flowline of the channel.

The 2-foot-interval contours provided by the county were used to make an existing ground terrain surface in AutoCAD Civil3D. Once the surface was created, surveyed information such as cross sections, points, and breaklines was merged with the surface to create a composite surface that consisted of surveyed data within the banks of the open channels and LiDAR contour data for any area outside the top of banks. This allowed for a more accurate digital terrain model in open-channel sections. Using the surface model, alignments were created and cross sections cut for open-channel sections. The cross sections include the concrete-lined portions and large over-bank sections on each side of the open channel. These large cross sections allow for more accurate modeling of storms that exceed the capacity of the drainage system. The cross-section data was entered into the model, along with all culverts along the alignment, and any pipe 36 inches in diameter or larger.

Figure 2: Existing condition profile (10-year storm)

Existing condition results
The model verified what Collierville engineers intuitively suspected. The 72-inch concrete pipe under White Road was undersized and causing water to back up into the upstream channel, causing flooding to adjacent property owners. Figure 2 shows the profile of the area in question for the 10-year storm. The portion of the channel directly upstream of the pipe is a vertical-wall, concrete-lined channel. This channel has a 3-foot vertical drop right before the pipe. The flooded-area map in Figure 3 shows the limits of flooding during the two-year and 100-year storms.

Figure 3: Existing condition flooded-area map

One thing the model did not show was flooding on the north side of White Road along Rillbrook Drive, but we know from historical records that flooding did occur in this area. This is a great example of why it is important to do field work, talk to homeowners, and have a working knowledge of past flood events. Inside the model, the road is considered an overflow weir. When the pipe surcharges to an elevation higher than the overflow weir (White Road), the overflow gets discharged to the next downstream node, which in this case is the open channel at the downstream end of the pipe under White Road. By speaking with homeowners, it was determined that what actually happens is water overtops White Road, then flows in a northeast direction toward the intersection of White Road and Rillbrook Drive. It then flows north along Rillbrook Drive, flooding several properties along the way before finally flowing back into the main channel (Figure 4).

Figure 4: Flooded area along Rillbrook Drive
Figure 5: Profile of improved condition (100-year storm)

Solution
The solution that was accepted by the town was to increase the size of the culvert under White Road to a 9-foot by 8-foot box culvert. The vertical-wall concrete channel upstream of the culvert also was redesigned to remove the drop near the downstream end. These improvements obviously increase the amount of water released downstream, so a detention pond was located immediately downstream of the new culvert. Figure 5 shows a profile of the improvements during the 100-year storm. Figure 6 shows the limits of flooding during the two-year and 100-year storms.

Figure 6: Improved condition flooded-area map

This location also provided the town with an opportunity to solve a bank stabilization issue. The property owners along Rillbrook Drive have seen their backyards slowly fall into the channel during the last several years as erosion took its toll. This project has allowed Collierville to adjust the alignment of the channel and reconstruct the east bank to give the property owners their yards back.

The town also plans to use this area as a common open space. There are low-flow channels that meander through the area to carry the flow from smaller storm events, but most of the time the area will be dry. There will be an asphalt walking trail around the top of bank, which will allow residents to enjoy the area.

Modeling the basin allowed Collierville to provide cost-effective solutions to difficult problems. The model allows engineers to make changes easily and can simultaneously solve multiple scenarios for multiple storm events. This allows engineers to analyze the performance of any design and determine the impacts that any proposed improvements may have to other parts of the basin. In short, modeling the basin allowed the town to make more informed decisions about how to spend taxpayer money to construct improvements to their stormwater infrastructure.

Justin Avent, P.E., is a senior engineer in the Memphis, Tenn., office of Smith Seckman Reid Inc., an engineering design and facility consulting firm headquartered in Nashville, Tenn. (www.ssr-inc.com). He can be contacted at javent@ssr-inc.com.

Upcoming Events

See All Upcoming Events