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Dec-Q4-O1 02:24P • • P.07 <br />Mr. !eff Sehwatz <br />Ueccmbcr 4, 2001 <br />Yage 6 <br />conditions which could be expected in tfte slough during various discharge rates. This cross <br />section was developed using 2-fool contour mapping supplied 6y 1C[1N Engineering. <br />'L'he attached results (Attat:hment A) of the model indicate that Irvee would be overtopped al the north <br />breach location (i.e., May 5, 2001 breach area) at a discharge of between 6000 and 8000 cfs (see attached <br />13EC-RAS output). Correlation between the results of the normal depth analysis anti HEC-ItAS step- <br />backweter is considered to be good. Results front both modeling app+'oacltes also indicate that it is <br />reasonable to assume that the original northern breach could have developed as a result pf overlapping <br />tlnws. <br />Review of the I-lEC-1tAS output data for cross section 30.5 present stage heights regarding backwater <br />under various flow conditions. During water years 1998, 1999, and 2000 where peak flows are known to <br />have been in execs of 8,000 cfs, the levee would have been nvenoppcd al the location of the no+tltern <br />breach. However, no issues appear to have beet raised cnrtccrning erosion within the Bull Secp Slough. <br />This ea,t be associated with two factors: (I) the ttn+ninal amount of ^ow which moved tltrnuglt tltc slough <br />and (Z) the dampening effect of backwater where elevations are predicted to be approximately between <br />5026 to 5027 feet. <br />It is LA's uttderstandittgtbat the northern breach was repaired on or about May 21, 2UU1, but the snutltern <br />breach was not ropaired at this time. Mobile Premix Concrete applied for and obtained a Nationwide <br />Permit to complete repair of rite southern breach in 1999, bttt due to public opposition to the proposed <br />coustntction, MPM did trot perform the n;pair, following the event of May 5, 2001, MPM renewed the <br />Nationwide Permit gratttcd in 1999 and completed ilte repair of the sot+tltcrn breach in late Jttly of 2001. <br />Qn July 8 and 9, 2001 another event occurred wltieltltad a peak discharge of approximately 6,400 cfs <br />occurred. 1lydrologie data from this event is also included itt Ute enclosures to this letter. In this case the <br />northern breach was repaired and the southern breach repair work was pending Corps approval. Damage <br />to the slough and Aull Sccp drain, due to this later, yet equivalent event was minimal. <br />1 hope the above attd enclosed adequately addresses the DMG questions and adequately transmits the <br />reyuested information. Please let me know if we c.m be of any other service. <br />Sincerely, <br />LID.STONE AND SS CLATES, INC. <br />~~~ ~~ <br />Christopher D, l,idstnne, CPG <br />Presidem <br />CDI./RR W:,j ab <br />cc: John I liekmnn w/attachments <br />Tom Schreiner w/n attachments (attaclunett[s previously dcliverc:d on November 30, ZD01) <br />Allen Sorenson w/o attachments (attachments previously delivered on November 30, 2001) <br />x torrrttcou +uzu~ot.rmz oM<t ff.~,.,,,,+, wed <br />