05_TRBM_Appendix_C_-_Provisional_Inflow_Data_Development

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APPENDIX C –

Provisional Inflow Data Development

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The current methodology for developing model inflow data does not lend itself well to
frequent updates that will be necessary for real-time position analysis. The current
methodology requires a large amount of input gage data (using 11 gages); impairments
from reservoir operations, water supply, wastewater returns, and agricultural
withdrawals; correction to negative inflows that could otherwise cause model
infeasibility; and scaling of gains to ensure that filled-in data for gages with missing data
preserves the known volume of flow at downstream gages. Obtaining impairment data
alone (which are necessary to unimpair the gage flows) is the most time-intensive part of
the updating process.

HydroLogics has developed a simplified, provisional procedure that will enable weekly
or monthly updates to be made, later overridden by periodic annual updates using the
current methodology. It is meant to provide a representative inflow to key points in the
basin, most notably Tar River Reservoir, which is central to real-time management of the
basin. The calculations for the provisional update are handled automatically within the
GUI upon entry of data as shown on the Update Record tab.

To simplify the update as much as possible, the procedure eliminates the need for most
impairments and the concern over negative inflows that could lead to model infeasibility.
The assumption is that most of the net impairments (withdrawals less discharges) in the
basin are small and occur within a reach and therefore are not likely to have much effect
on the natural inflow.

As an example, the Tar River gages below Tar River Reservoir (e.g., Tarboro and
Greenville) are significantly affected by the operation of the reservoir during low flow
conditions. However, by computing the gains between the reservoir (using measured
releases) and Tarboro, and between Tarboro and Greenville, the effect of the reservoir
operation is removed. Only the net impairments within those reaches affects the natural
inflow, and as long as the net impairments are small, it can be assumed that the difference
in gaged flows in these reaches is the natural inflow.

Negative inflows can occur at Tar River Reservoir since they are back-calculated from
the historic releases. They can also occur when the downstream gage flow is less than
the upstream gage flow (which is usually due to time of travel issues or, in the case of
Greenville, tidal influence). In either case, these only pose a modeling problem if there is
not enough water in the river or reservoir to handle them, which is rare. As a precaution,
when gains are negative, the model’s OCL is used to filter them to maintain model
feasibility.

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The simplified procedure is detailed below in the following steps. As noted, all
calculations are automatically done within the model.

Step 1: Obtain from the USGS web site the daily gage data (in cfs) for the following
gages: Tar River near Tar River (Tarr), Tar River at Louisburg (Loui), Swift Creek at
Hilliardston (Swif), Little Fishing Creek near White Oak (Lfsh) Fishing Creek near
Enfield (Fish), Tar River at Tarboro (Tarb), and Tar River at Greenville (Gree).

Step 2: Obtain the following data for Rocky Mount:

Daily elevation for the Tar River Reservoir, expressed as inches above (+) or below (-)
the spillway, daily precipitation at Rocky Mount (in inches), daily release (in cfs) from
the reservoir, and daily net withdrawal (in mgd) from the reservoir.

Step 3: Compute the net evaporation for all reservoirs in the basin using the precipitation
data collected above and a monthly evaporation pattern.

Step 4: Back-calculate the (impaired) total inflow to Tar River Reservoir by adding the
reservoir release, net withdrawal, net evaporation on the lake, and the change in storage.

Step 5: Set the Tar River near Tar River flow equal to the Tarr river gage flow. Set the
Louisburg gain to be the difference between the Louisburg gage flow and Tarr flow
(lagged by 1 day).

Step 6: Compute the Cedar and Taylor Creek inflows using a drainage area adjustment of
the Swift Creek gage (Swif).

Step 7: Compute the Tar Reservoir gain by subtracting the Louisburg and Cedar and
Taylor Creek flows from the total reservoir inflow. To better reflect the unimpaired
inflow in this reach, an average of 10 cfs is added to this gain to account for the estimated
agricultural withdrawal. The remaining net impairment in this reach is small and thus
ignored.

Step 8: Set the flows for Swift and Little Fishing Creeks

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to the gage flows (Swif and Lfsh). Set the gain to Fishing Creek as the Fishing Creek
flow (Fish) minus the Lfsh flow. Compute Conetoe Creek flow based on a drainage area
adjustment of the Swift Creek gage.

Step 9: Compute the Tarboro gain by subtracting the Tar River Reservoir release (lagged
1 day) and the flows for the Swift and Fishing Creeks from the Tarboro gage flow (Tarb).
Compute the Greenville gain by subtracting the Tarboro gage flow (lagged 1 day) and
Conetoe Creek flow from the Greenville gage flow (Gree).