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Mr. Jeff Schwarz December 4, 2001 Page 6 conditions which could be expected in the slough during various discharge rates. This cross section was developed using ?-toot contour mapping supplied by ICON Engineering. The attached results (Attachment A) of the model indicate that levee would be overtopped at the north breach location (i.e., May ~, 2001 breach area) at a discharge of between 6000 and 8000 cfs (see attached HEC-RAS output). Correlation between the results of the normal depth analysis and HEC-RAS step- backwater is considered to be good. Results from both modeling approaches also indicate that it is reasonable to assume that the original northem breach could have developed as a result of overtopping Flows. Review of the HEC-RAS output data for cross section 30.E 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 northem breach. However, no issues appear to have been raised concerniltg erosion within the Bull Seep Slough. This can be associated with two factors: (1) the Itominal 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 X027 feet. It is LA's understanding that the northem 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, but due to public opposition to the proposed construction, MPM did not perform the repair. Following the event of May ~, 2001, MPM renewed the Nationwide Permit granted in 1999 and completed the repair of the southern breach in late July of 2001. On July 3 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 southern breach repair work was pending Corps approval. Damage to the slough and Bttll 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 ASS CIATES, INC. ~,~.-~ ~J Christopher D. Lidstone, CPG President CDL/BRW:jab cc: John Hickman w/ attachments Tom Schreiner w/o attachments (attachments previousl}' delivered on November 30, 2001) Allen Sorenson w/o attachments (attachments previously delivered on November 30, 2001) I: \OPEN\COLFIO'_\COLf 10? DMG iransm¢.upd