Dames & Moore, 1999 - USDA Forest Service

3.11.3 Surface Water Data
3.11.3.1 Basin-wide Climatic Water Balance
A general basin-wide, climatic water balance or budget was developed for the entire Railroad Creek
watershed for use in analyzing flow characteristics and predicting minimum riprap sizing to protect the
lowermost portions of the tailings piles From erosion by Railroad Creek. The results of the basin-wide
water budget are discussed in Section 4.3.5.
3.1 1.3.2 Hydrologic Modeling
The results of the basin-wide climatic water balance were utilized to evaluate flood conditions in Railroad
Creek using HEC-I and HEC-2 (incorporated into a package for Windows called HEC-RAS) hydrologic
models. These models were developed by the Army Corps of Engineers Hydrologic Engineering Center
(HEC) and have been in use for more than 20 years. Their use is accepted by government agencies
including FEMA and the USGS. HEC-I employs statistical meteorological and watershed characteristics
to develop flood estimates, and can be calibrated using actual flow data from the watershed. The Railroad
Creek HEC- 1 model was calibrated using flow data from the USGS Lucerne gage collected between 19 1 1
and 1946.
The HEC-2 model uses uniform flow assumptions (Mannings equation) to route water through prescribed
channel geometry to estimate water levels and velocities during flood events. The model uses channel
geometry data from field measured cross-sections, and then interpolates-between sections. Cross-sections
for Railroad Creek were measured in the field at 12 stations from RC-6 to RC-2 along the Site to
characterize the channel geometry. Values of roughness coefficients used by the model were estimated
from actual flow measurements in the field for each station. The results of the analyses are presented in
Section 4.3.6.
3.11.4 Groundwater Data
Techniques for groundwater data analysis are presented below. The results of the analyses are presented
in Section 4.4.
3.11.4.1 Groundwater Flow Net Analysis
Flow nets are graphical representations of groundwater flow systems. Flow nets are'constructed by
drawing lines of equal groundwater elevation (equipotential lines) and from those equipotential lines
drawing lines of anticipated groundwater flow. In cases where the permeable material through which
groundwater flows has the same hydraulic properties in all directions, flowlines are drawn perpendicular
to equipotential lines.
Flow nets were constructed based on groundwater elevation maps for the Site from May and September
1997. Hydraulic conductivity values from slug tests at site wells are used to provide ranges of values for
discharge calculations. The thickness of the zone providing water to Railroad Creek is estimated based
on evaluation of site water level measurements, boring logs, and geophysical data.
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17693-00s-019Vuly 19. 19992:07 PM;DRAFT FINAL RI REPORT