Model Results

General System Behavior

The first step in analyzing this dual drainage system is to simulate the model in its preliminary sized state for the 2-yr storm with Dynamic Wave routing to check its general performance. The hyetographs for both the 2-yr and 100-yr storms are the same as those used in Tutorial 01.

After simulating this model, the resulting Status Report shows that no nodes are either surcharged or flooded (see the Node Surcharge Summary and Node Flooding Summary sections of the report). Moreover, no conduit was surcharged too (see the Conduit Surcharge Summary section), which indicates that the system was well-sized. To understand surcharging and flooding, see Surcharge and Flooding of Nodes and Conduits in Appendix-B.

The drainage criteria (see System Design Criteria in Appendix-A) also require a minimum gutter grade of 0.4% and maximum average flow velocity of 10 ft/sec. Therefore, it is required to check that the minimum slope of the conduits representing gutters is larger than 0.4%. Table 7-5, taken from the Link Summary table of the Status Report, shows the slopes of the conduits representing the gutters. All the gutters have a longitudinal grade of 0.4% or greater and thus meet the design standards considered for this tutorial. The velocities in the gutters and all the conduits will be checked once the pipes of the system are properly sized.

Table 2.8 : Calculated gutter grades

Minor Storm Performance

When the model is simulated for the 2-yr design storm, as mentioned earlier, no conduits were surcharged. As a result, no resizing of the conduits is required. While designing conveyance system for storm water management, it is a common practice to design the conduits in a way so that the “Max/Full Depth” ratio remains 0.85, or, below than that. This 15% is considered as a safety factor to reduce the risk of surcharging. The Link Flow Summary of the Status Report shows that the values of the Max/Full Depth ratio of all the circular conduits are less than 0.85. According to the Link Flow Summary table, the highest velocities occur in conduits C16, C20, C17, C21, C3, C19, C7, and C8 (10.42, 10.48, 10.47, 10.99, 11.05, 12, 12.57 and 14.01 ft/sec respectively), all of them over 10 ft/sec. Standard drainage criteria define a maximum allowable velocity in pipes and culverts of 15 to 18 ft/sec (CCRFCD, 1999; Douglas County, 2008). Thus, the maximum velocities predicted for pipe C8, C7, C19, C20, C16, C17, C3 and C21 are acceptable, and no re-sizing of the conveyance system is required. The peak velocity for all the street conduits is less than 10 ft/sec, the maximum allowed by the drainage criteria.

On the contrary, according to the drainage criteria, no curb-topping should occur for the minor storm in any street. Figure 2.5 shows that curb-topping implies a Max/Full Depth ratio of 0.5/1.3 = 0.38. The Link Flow Summary in the Status Report for the final pipe sizing shows that the highest value of the Max/Full Depth ratio for the street conduits is 0.32 for conduit C6_Street. Thus, the final design of the system is appropriate in terms of the flows in the streets.

Major Storm Performance

Finally, the model is simulated for the 100-yr storm by changing the rainfall time series used by its rain gage. The Status Report’s Node Surcharge Summary and Node Flooding Summary tables show that no nodes are surcharged or flooded during the 100-yr storm. Additionally, the Conduit Surcharge Summary shows that only the conduits C1, C2 and C6 are surcharged. According to the Link Flow Summary table, the highest velocities occur in conduits C3, C7, C13, C19, C20, C21 (17.13, 17.54, 17.78, 17.17, 17.37, 17.38, ft/sec respectively). As previously mentioned, standard design criteria for maximum allowable velocities range from 15 to 18 ft/sec. So, the maximum velocities predicted for pipe C3, C7, C19, C13, C20 and C21 are acceptable, and no re-sizing of the conveyance system is required. The peak velocity for all the street conduits is less than 10 ft/sec, the maximum allowed by the drainage criteria. On the other hand, the Link Flow Summary table shows that the maximum velocities of all the street conduits are less than 10 ft/sec and the Max/Full Depth value are less than one. Therefore, the water depth in the streets never reaches the crown or the highest point of the sidewalks, and the requirements for the 100-yr storm are successfully satisfied.

Another way to visualize the behavior of the dual drainage system is with Profile Plots. Figure 2.7 contains two such plots stacked on top of one another. They depict the surcharge condition that occurs between nodes J2 and J7 around 37 minutes into the 100-yr storm. The lower plot applies to the below-ground sewer pipes C6 and shows that the pipe is surcharged. The upper plot is for the streets C6_Street that lie above C6. The water level at junction J7 is high enough to cause water to flow out of the sewer pipe C6 and onto the street, but not high enough to flood the street. On the other hand, the invert elevation of junction J2 is low enough so that the surcharged pipe C6_Street does not create street flooding and instead, street flow re-enters the sewer system there.

Major Outcomes

The major outcomes from the analyses carried out in this tutorial are presented below:

• The three-dimensional structure of dual drainage systems can be modeled by using manhole junctions set below ground that connect parallel pairs of sewer pipes and gutter/street channels, where the latter are offset to ground elevation.
• These systems are designed so that the below-ground sewer system will flow only partly full during the more frequent minor storm events and will surcharge into the street channels during the larger, less frequent major events without causing any overtopping of the street.
• Most of the results needed to evaluate the performance of a dual drainage system can be found in the various tables produced by SWMM’s Status Report.