Laserfiche WebLink
Dec-04-O1 O2:24P <br />r <br />Mr. !eff Schwarz <br />Veccmhcr 4, 2p01 <br />Page fi <br />conditirnts which could be expected in the slough during various discharge rates. This cross <br />section was developed using 2-fool contour mapping supplied by 1CUN Engineering. <br />'L'he attached results (Attachment A) of file model iltdicatc that levee would be overtopped at the north <br />breach location (i.e., May S, 2001 breach area) at a discharge of between 6000 altd 8000 cfs (see attached <br />IiEC-RAS output). Correlation between the results of the normal depth attalysi.s anti HEC-RAS step- <br />backwater is considered to be good. Results front 6olh modeling approaches also indicate that it is <br />reasonable to assume that the original norlhem breach could have developed as a result of overtopping <br />flows. <br />Review of t}tc HEC-RAS output data for crass 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 excess of 13,OU0 cfs, the levee would have beep overtopped al the location of file noltltern <br />breach. However, no issues appear to have been raised concerning erosion within file Bull Seep Slough. <br />This can be associated with two factors: (1) the nominal amount of flow which moved through the slough <br />and (2) the dampening effect of backwater where elevations are predicted to be approximately between <br />502ti to 5027 feet. <br />It is LA's understanding that tho northern breach was repaired on or about May 21, ZUU1, but the southern <br />breach was not repaired at this time. Mohilc Premix C'uncrete applied for and obtained a Ntlionwide <br />Permit to complete repair of the southcm breach in 1999, but due to public opposition lu the proposed <br />cottstnlction, MPM did nor perform the repair, tullowing the event of Msy 5, 2001, MPM renewed file <br />Nationwide Permit granted in 1999 and completed lire repair of the sotltltcrn breach in tale July of 2001. <br />On July S and 9, 2001 another event occurred wltielt Itad a peak discharge of approximately 6,400 cfs <br />occurted. l Iydrologie data from this event is also included itt the enclosures lu this letter. In this case the <br />northcrtt breach was repaired and the southern hreach repair work was pending Corps approval. Damage <br />to the slough and Aull 5ccp drain, due to this later, yet equivalent event was minimal. <br />1 hope the above and enclosed adequately addresses the DMG questions altd adcyualcly transmits file <br />requested ittforlttation. Please let me know i f we can be of any other service. <br />Sincerely, <br />LIDSTONE AND SS CIATES, INC. <br />~~.~ ~~ <br />Christopher D. L,idstone, CPG <br />President <br />CDL/RRW:,jab <br />cc: John I lickmmt w/ attactvncnls <br />Tom Schreiner w/n attachments (attaeluneltts previously delivered on November 30, 2001) <br />Allen Sorenson w/o attachments (attachments previously delivered on November 30.2001) <br />P.O7 <br />K 1ppL7.nCOLI'IU?TC'r)LI'In? DMCi vast+ntit wpA <br />