PERMFILE127799

• Permit No: M2001001
• IBM Index Class Name: Permit File
• Doc Date: 11/14/2003
• Doc Name: Witnesses and Exhibits of Marilyn Boynton
• Media Type: D
• Archive: No

ESTIMATION OF DOLORES RIVER FLOODS
NEAR LINE CAMP GRAVEL PITS
UPSTREAM OF DOLORES, COLORADO
19 December 2001
William Jobin, Sc.D.
SUMMARY
Because of concerns about apparent inaccuracies in the US Army Corps of Engineer's
Flood Hazard Information Report of September 1978 which was used in granting a permit
for gravel mining along the Dolores River at the Line Camp upstream of Dolores,
Colorado, and also the FEMA Flood Map for the Dolores River, we completed a study of
the river and its floodplain in October and November 2001. Topographic surveys of the
river and its floodplain were made at two transects upstream of the town of Dolores in the
reach containing the Line Camp gravel pits. We calculated the roughness coefficients for
use in the Manning flow equation at the two transects, calibrating our measurements with the
reported flow for late October of 65 cfs at the USGS river gage in Dolores. Transect A was
upstream of the Line Camp and just downstream of Twin Spruces gravel pits, and Transect
B was just downstream of the Line Camp gravel pits. From these calibrations and our
transects, the estimated flows for bank-full conditions and the horizontal extent of the
floodplain for the 100-year flood of about 12,000 cfs, were calculated with the Manning
equation, with appropriate adjustments for flood flows.
Our calculations indicated that the USACE flood hazard information was seriously inaccurate
for this reach of the Dolores River. At Transect A near Twin Spruces, we estimated that the
bank-full flow would be 3,700 cfs, compared to the USACE estimate of 6,000 cfs, which
they also estimated to be the 10-year flood. From our topographic survey we estimated
that the floodplain during the 100-year flood of 12,000 cfs would extend westward all the
way to the highway embankment at this transect, making it nearly twice as wide as the
USACE map indicated. There were similar discrepancies at Transect B immediately
downstream of the Line Camp, where we estimated that the floodplain for the 100-year
flood was more than twice as wide as the USACE estimate. Inspection of their 1978
topographic maps also revealed serious discrepancies in their ground elevations at this
transect, a possible reason for their erroneous flood estimation. Based on our analysis and
having used our current field data to verify the more theoretical USACE simulations of
1978, we recommend that the entire relevant portion of the Dolores River and floodplain
should be carefully remapped to obtain more accurate and current information for evaluating
the impact of the gravel pits at the Line Camp.
These inaccuracies in the USACE report are apparently due to large discrepancies in the
topographical maps used in 1978, compared to current conditions. They are a serious
matter because the flooding and course of the Dolores River might be drastically altered by
the Line Camp gravel pits. The consequences of these inaccuracies might mean that: (1)
larg2 and damaging floods occur more frequently than the USACE estimated for this reach,
(2) that the Line Camp pits would be completely inundated during the 100-year flood and
even during smaller floods, and (3) that the overbanking river flow would move at velocities
higher than the USACE estimated in their 1978 report, thus leading to greater risk of erosion
and of shifting in the river course. If the river were to shift its course during large floods, it
could pose serious risks to homes, property and life downstream of the Line Camp gravel
pits.
Do/ores River 1 1/19/02