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Mr. Jeff Schwarz December d, 3001 Page 6 conditions which could be expected in the slough during various discharge rates. This cross section was developed using 2-toot contour mapping supplied by ICON Engineering. The attached results (Attachment .4) of the model indicate that levee would be overtopped at the north breach location (i.e.. May >, 2001 breach area) at a discharge of behveen 6000 and 8000 cfs (see attached HEC-RAS output). Correlation behveen the results of the normal depth analysis and HEC-RAS step- backwater is considered [o be good. Results from both modeling approaches also indicate that it is reasonable to assume that the original northern breach could have developed as a result of overtopping flows. Review of the HEC-RAS output data for cross section 30.5 present stage heights regarding backwater under various Flow conditions. During water years 1998, 1999, and 2000 where peak flows are known to have been in excess of 8,000 cfs. the levee would have been overtopped at the location of the northern breach. However, no issues appear to have been raised concerning erosion within the Bull Seep Slough. This can be associated with hvo factors: (1) the nominal amount of flow which moved through the slough and (2) the dampening effect of backwater where elevations are predicted to be approximately between 5026 to 5027 fez[. It is LA's understanding that the northern breach was repaired on or about May 21, 2001, but the southern breach was not repaired at this time. Mobile Premix Concrete applied for and obtained a Nationwide Permit to complete repair of the southern breach in 1999, bttt due to public opposition to the proposed construction, MPM did not pertotmt the repair. Following the event of May 5, 2001, MPM renewed the Nationwide Permit granted in 1999 and completed the repair of the southern breach in late July of 2001. On July 8 and 9, 2001 another event occurred which had a peak discharge of approximately 6,400 cfs occurred. Hydrologic data from this event is also included in the enclosures to this Letter. In this case the northern breach was repaired and the southem breach repair work was pending Corps approval. Damage to the slough and Bull Seep drain. due to this later, yet equivalent event was minimal. I hope the above and enclosed adequately addresses the DMG questions and adequately transmits the requested information. Please let me know if we can be of any other service. Sincerely. LIDSTONE AND SS CIATES, [NC. ~~.~ ~~ Christopher D. Lids[one, CPG President CDL/BRW:jab ce: John Hickman w/attachments Tom Schreiner w/o attachments (attachments previously delivered on November ;0, 2001) Allen Sorenson w/o anachments {attachments previously delivered on November 30, 3001) KbPEN\COLF102\COLF102 Dtw1G vansmn wpd