Dames & Moore, 1999 - USDA Forest Service

truly be distinguished. However, assuming that the observed measurements reflect actual conditions, it
appears that there is a loss of flow between RC-I and RC-4 during periods of low flow in the early spring
and in the fall. Referring to Figure 4.3-3b, the historical Railroad Creek channel intersects the present
Railroad Creek channel at the approximate midpoint of this reach. Flow measurements collected on May 26,
1997. exhibited an apparent gain in flow between RC-I and RC-2. This may reflect the overall recharge
conditions resulting from snow melt. Similarly, flow between RC-4 and RC-2 appears to exhibit the least
gain during fall and the greatest gain during spring. The flow ratios with Copper Creek do not appear to
have seasonal variability, indicating that flow patterns at this station follow the seasonal flow patterns in
Railroad Creek. This makes hydrologic sense because the basin of Copper Creek is similar, albeit smaller,
than Railroad Creek and would be expected to behave similarly given the proximity of the two drainages.
The seasonal variability evident between stations at the Site and RC-3 at Lucerne are large, and therefore the
difference in flow ratios was considered to be distinguishable from error. A relatively large seasonal
variability in flow pattern is expected between the Site and Lucerne because of elevational differences.
Snowmelt begins and ends earlier in the lower basin, and precipitation is lower; therefore, flows at Lucerne,
which include a large percent of drainage fiom the lower basin, will reflect this contribution.
The flow relationships between RC-4 and RC-3 developed from Table 4.3-4 were used to estimate the
relationship between monthly average flows measured during 1997 at RC-4 (continuous record for part of
April, May, June, July, August and September) and the long-term record at Lucerne. A monthly average for
April was estimated for 1997 by assuming the flows measured in mid-April were representative of flow
early in the month, and daily staff readings maintained by Dave Harris of Holden Village from April 19
through May 19 were used to estimate flow at RC-4 fiom the rating equation. Table 4.3-5 shows the
estimated historical record at RC-4 based on flow relationships between Lucerne (RC-3) and RC-4.
Table 4.3-5 also shows the average of measured flows for 1997.
Comparison of 1997 Flow to Historical Data
The historic flow record for Railroad Creek is 46 years in length. The 1997 flows were larger than the
estimated average monthly values for the Site. Based on a review of the historical record at Lucerne, the
1997 monthly average flows would be the highest on record for May, July and September (see Appendix H)
and near the maximum for June and August. The estimated peak flow in 1997, to be approximately 1,100
cfs (which occurred June 16 at RC-4), would be approximately a 10-year flood event, based on comparison
with the historical record at Lucerne. Thus, flow in 1997 was relatively high in total volume of flow,
consistent with the large snowpack, but not extreme with respect to peak instantaneous floods due to
relatively moderate melt rates.
Predicted Flood Event Flows
To evaluate flood potential at Holden, the historic flow record at Lucerne and historic precipitation data for
the Cascades was used to calibrate a KEC-1 model of the Railroad Creek watershed (AppendixH).
Previous flood estimates for Railroad Creek, developed from the historic flow data at Lucerne, indicate that
the 100-year flood for Lucerne is approximately 3,500 cfs to 3,900 cfs (see Appendix H). The 50 to 100-
year flood at Holden has been estimated to be greater than 3,000 cfs (ORB, 1975; PNL, 1992). The HEC-I
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