TR55 Graphical Method

Overview
For details on calculating CN see previous section. When catchment area has Impervious areas (no water penetrates the ground), connected and/or unconnected, the TR55 graphical method can be used to calculate the new CN for the area affected.

Connected impervious areas — An impervious area is considered connected if runoff from it flows directly into the drainage system. It is also considered connected if runoff from it occurs as concentrated shallow flow that runs over a pervious area and then into the drainage system.

If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions in CN tables are not applicable, use the first table on the TR55 Graphs to compute a composite CN. For example, CN tables give a CN of 70 for a 1/2-acre lot in HSG B, with assumed impervious area of 25 percent. However, if the lot has 20 percent impervious area and a pervious area CN of 61 (from open space, grass cover >75%), the composite CN obtained from first table on the TR55 Graphs, is 68. The CN difference between 70 and 68 reflects the difference in percent impervious area.

Unconnected impervious areas — Runoff from these areas is spread over a pervious area as sheet flow. To determine CN when all or part of the impervious area is not directly connected to the drainage system, (1) use second table on the TR55 Graphs if total impervious area is less than 30 percent or (2) use first table on the TR55 Graphs if the total impervious area is equal to or greater than 30 percent, because the absorptive capacity of the remaining pervious areas will not significantly affect runoff.

When impervious area is less than 30 percent, obtain the composite CN by entering the right half of second table on the TR55 Graphs with the percentage of total impervious area and the ratio of total unconnected impervious area to total impervious area. Then move left to the appropriate pervious CN and read down to find the composite CN. For example, for a 1/2-acre lot with 20 percent total impervious area (75 percent of which is unconnected) and pervious CN of 61, the composite CN from second table on the TR55 Graphs is 66. If all of the impervious area is connected, the resulting CN (first table on the TR55 Graphs) would be 68.

More Resources on TR55 Method
Download TR55 Graphs
Example of TR55 calculation and analysis

Your Task
Work with the Land Use data to calculate the Curve Number using the Runoff Curve Number tables and Composite CN for unconnected and connected impervious areas. Download and open this WORD file and use it as a template to record your answers.

1. Consider the historical land-use map, locate the most northern urban part and record its area in km2.
   
2. Using the CN tables, look up the corresponding CN for this part of the watershed (assume Urban → Residential 1/3 acre). Assume soil group is B for all and soil condition is good.
   
3. If the lot has 20% connected impervious area and Grass with good condition (>75%), calculate the composite CN. ( Use first table on the TR55 Graphs)
   
4. If the lot has 20% unconnected impervious area and Grass good condition (>75%), of which 75% is unconnected, calculate the composite CN. ( Use second table on the TR55 Graphs)
   
5. If the lot has 40% connected impervious area and Grass good condition (>75%), calculate the composite CN. ( Use first table on the TR55 Graphs)
   
6. If the lot has 40% unconnected impervious area and Grass good condition (>75%), calculate the composite CN. ( Use first table on the TR55 Graphs)
   
7. Based on the total rainfall calculated from rain gauge # 6 and using the CN from question 6, what would be the runoff?
   
8. If the antecedent condition was AMC I, what would be the runoff for them?
   
9. If the antecedent condition was AMC III, what would be the runoff for them?
   

Once finished, click Next to continue.

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