2015-08-31_PERMIT FILE - C1981010 ‎(59)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 8/31/2015
• Doc Name: Reconstructed Drainage Channel Design Model
• Section_Exhibit Name: Appendix Q Section XXXVII
• Media Type: D
• Archive: No

the start of the storm. This allows for a small amount of temporary storage at each check
dam.
By repetitively modeling this case, and gradually increasing the contributory drainage
area for each structure, the downstream channel flows and velocities likewise increase.
As the total drainage area becomes too large, the flow velocities exceed the
recommended limit for the unvegetated, erosive channel. In this case, where a channel
slope of 10% is used, the series of 10 identical structures can successfully detain runoff
from 100 acres before the limiting downstream flow velocity in the lower channel is
exceeded. This is equivalent to each check dam treating about 10 acres of drainage.
A similar analysis based on a curve number of 72 has also been prepared and is included
as Appendix E. This would be representative of the entire watershed consisting of 2 year
reclamation. With all other values unchanged, the ten structures can handle 356 acres
versus 100 acres in the prior analysis. This is equivalent to each check dam treating
about 36 acres of drainage. This demonstrates that the need for a large number of check
dams quickly decreases as the reclamation within the watershed becomes more mature.
Thus, mature reclamation requires significantly fewer check dams for the same
watershed.
A similar analysis based on a curve number of 78 and channel slope of 15% was also
modeled and is presented as Appendix F. As noted previously, the storage capacity of
check dams is only about 57 percent of a similar check dam in a 10% channel. With all
other values unchanged, the ten ponds can handle 75 acres, equivalent to each check
dam treating about 7.5 acres of drainage. This demonstrates that as the channel slope
increases, the need for a larger number of check dams quickly increases.
Using this same method, all possible combinations of 5%, 10%, 15%, 20%, and 25%
channel slopes under four curve number conditions (67, 72, 78, and 83) were similarly
modeled in SEDCAD. Table 2 presents the results of this, were the maximum number of
acres per check dam is identified.
Table 2, Results of String of Pearls Model showing acres per check dam at various curve
numbers and channel slopes
This same data is also presented graphically in two different formats. Figures 6a, 6b, 6c,
and 6d provide a recommended minimum number of check dams based on the channel
slope and drainage area. Notice the linear nature of these figures. Also notice that for
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This same data is also presented graphically in two different formats. Figures 6a, 6b, 6c,
and 6d provide a recommended minimum number of check dams based on the channel
slope and drainage area. Notice the linear nature of these figures. Also notice that for