ENFORCE27394

• Permit No: M1978052
• IBM Index Class Name: Enforcement
• Doc Date: 12/4/2001
• Doc Name: FAX TRANSMITTAL
• From: LIDSTONE & ASSOCIATES INC
• To: DMG
• Media Type: D
• Archive: No

Dec-Q4-01 O2:24P • • P_O5
Mr. Jell' Schwari
Uecem6cr 4, 2nn 1
Page 4
O.OOlS fVft, which was slightly greater than the measured bed slope (5.001 ft/ft: established using the
Epp and Associates survey data). LA's divergence from tneasurcd bctl slope is justified in that the water
surface energy slope rather than the channel slope is an essential element in this calculation. Under lower
flood tlow conditions, the effect of the Branhter will also eel lv steepen the energy ,lope. The decision to
use a slope of 0.0015 is further validated by tield evidence that supports cltanncl's cunlaittment of events
Lower titan 6000 cfs. Al a slope of 0.001, tltc river wvt+ld overtop the bank at 5000 cfs, rather titan 6500
cfs. Historic evidence doesn't support this.
Assuming there are no pre-existing "breaches" in the cltattttcl bank, stage discharge curves were
developed at the nvrlhem and southern "breach cross section location" (Tahlcs 1 and 2). The ttorntal
depth analysis fur the northern breach indicates that the levee begins to overtop al 6,SOU cfs (Reference
Table 1 where a stage Iteigltt of 10 feel is cyual tv 5038.6 feet). Normal depth analysis of the svuthcm
breach indicates that the Ievice will nut be overtopped wail the dischsrge is in excess of 8,000 cfs (please
reti:rettce Table 2 where a stage height of 9 fi:et equates l0 5038 feet). In January 2001, a photo was taken
by Llrban Drainage (Figure 3.1) which indicates the presence of art erosion cut (i.e., the area ot'tlte May 5,
2001 breach) at the north Ivcalivn. LA has estimated that this "cut" was approxima[ely four (4) feet deep
and 10 feet wide (see attached photo). Assuming the top of bank is elevation of 5039, the invert of the
cut is 5035. This would suggest that flow would begin to enter tltc "cut" and discharge from the South
Platte River to the Bull Seep Slvugh, when the main channel is flowing around 2600 ct's.
Tables 3 and 3A present hydraulic analyses of the assumed dimensions initial breach using the weir
equaticn CI,H`n. Depending on the assumed sii4 of the erosion cut (4 versus 6 feel deep), somewhere
from 266 cfs to 490 cfs stay have passc<I through the "erosion eul" at the northern breach location
assuming that the geometry of the "erosion cut" remained unchanged during the May 5, 2001 event. This
is an unlikely scenario and LA concludes that as mute and more Clow passed through the May 5, 2001
breach, the erosivtt cut progressively enlarged wail failure was achieved. Because the levee was
constructed of nun-engineered fill material, once water started tlowittg through the breach, failure
occurred quite rapidly. Llltimatcly the breach enlarged to an opening apprvxitnately six (6) to seven (7)
feet deep by 100 feet Inng based on Geld observations of the breach repair which occurred at the Cnd of
Mey, 2001. Table 4 presents a rating table fvr tltc hrcach utilizing these enlarged dimensions.
Normal depth analyses were perfbrnted for the Bull Seep Drain et cross sections approximately 200 fact
upstream and 800 tear downstream oC the north breach (Table 5 grid 6 rnspectively)• To complete the
normal depth analysis, a roughness coefficient of 0.035 and an average slope of 0.003 fl/fl was assumed.
Table 5 identifies the actual pre-flood capacity (400 cfs) of the Bull Secp drain, Yertnit M-78-052
(M!'M's reclamation permit) required s capacity of 350 cf's. Table 6 reflects the conveyance capacity of
the pre-flood Aull Seep Slvugh below the Nortlt Breach. Neither the existing trot the pcmtittcd Hull Seep
T.)rsin ('!'able S), upsttrrutt of the Bull Seep Slough could convey the flood even! of May 5, 2001 (>I500
cfs: Table 6). Witlt this said, the locstivn of the Hu11 Seep Drain wuultl not make arty difference with
respect to dowttstreartt material damage within the Bull Seep Slough.
It is 1.A's uttderstattdi+tg that a hrcach did exist at the southern location at the time of the May 5, 2001
event. The dimensions of this breach at the time of the May flood evert are unknown but field evidence
indicates that; (I) the ittveR of this breach was higher tlton the nnrlhem breach; (2) the conveyanec
capacity of the river cross section at this location was greater than its downstream counterpart; and (3)
due to the geometrical configuration of this hresch (off-channel from the South Ylatte) that development
of the opening would be limited. The potential impact of the southern breach an the Bull Seep Slough as