Canal Requirements, Seepage and Sloping

A canal reach or canal is defined as a continuous section of canal bounded by control structures and contains one or more inflow and/or outflow points. A canal network is a system of canals served by one or more outlets of a storage area or reservoir, composed of a single canal reach or a complicated system of canal reaches.

To estimate surface water requirements for a canal network, the following information is required for each canal reach in the canal network.

Total head drop of water surface, HDC -- remains constant throughout the simulation

Average width of each canal reach in the network, WIDTH Desired minimum stage (level) CRMIN [ft NGVD] -- set to -9.5 (the critical minimum level) if canal not maintained

Canal-aquifer conductivity coefficient, CHHC [ft/day/ft-head] -- needed for calculating groundwater interaction

Index for upstream canal reach discharging into canal reach of interest, IFF Number of downstream outflows simulated for water supply, NBRANCH -- <= outflows in canal

For routing water from a storage area through the canal system to meet water requirements, the following information is needed for each canal reach.

Index of the canal reach of interest, N
Total number of outflow structures simulated in the canal reach of interest, NSTRCTR
Array of indices identifying the downstream structures, (ISTRC(N,I), I=1to NSTRCTR)
Indices identifying the canals receiving the water from each outlet structure,

and for each canal network, the following branch information is needed for the algorithm

The total number of branches in the canal system, NSTART
The most downstream canal reach for each branch in the canal network,

Using this information the algorithm calculates the total volume of water required to maintain any number of canal reaches with a canal network by starting at the most downstream canal reach of a canal network and following its tributaries when the canal branches. The maximum levels of recursion the model simulates is two levels of receiving (downstream) canals or tributary for each canal reach or tributory. The SFWMM allows rivers to define slopes and exact distance to 3 points (downstream or upstream). SFWMM represents through the ordering and it provides an option to enter slope.

To estimate the surface water requirements for a canal reach, the mass balance approach is used. The volume of water needed to maintain a canal reach at a desired minimum level, VOLj, is simulated by:

where j is the index of the canal of interest (the jth canal input in the canal definition file), CSTGj is the simulated downstream stage in canal j at the beginning of the time step., SEEPi is seepage (the canal-groundwater interaction), and OVLNFi is the canal-surface water interaction at the ith grid cell. A negative value for VOLj represents the excess water available in canal j that can be used to meet the needs downstream. To calculate the seepage, SEEPi :

canal stage

and to calculate overland flow, OVLNFi :

where

The total volume of water required for water supply through any structure within a canal network, DQUj, where j equals the canal number immediately downstream of the structure is the sum of:

DQUj = 0 if VOLj < 0.

that is, the sum of the volume of the water required to maintain the canal reach immediately downstreeam of the structure (VOLj ), and the total volume of water required for all canal reaches downstream of canal j. When VOLj is negative, the volume of available water in the canal of interest meets or exceeds the downstreamm surface water needs, therefore no water is required through the structure upstream of canal j. The total water supply requirement for the structure is then set to zero.

To estimate the surface water requirements for a canal network the needs at the farthest downstream reach that is maintained in the main branch of the canal network is calculated. The farthest downstream reach is the last reach to receive water and the first to drop below and the first to drop below it desire minimum level when there is not sufficient amount of water available to meet demand. Starting with this reach the requirements upstream along the main branch are accumulated until one of the following occurs:

Condition 1 The canal of interest (canal j) branches into at least one tributary whose water level(s) has (have) to be maintained. The number of tributaries equals NBRANCHj-1; or

Condition 2 The branch terminates with the canal of interest. For the main branch this implies IFFj refers to a canal reach immediately upstream of the branch of interest. If the total needs for an entire canal network has been determined without occurrence of Condition 1, then the canal network either has no tributaries or none of the canal reaches in the tributaries need to be maintained.

If Condition 1 occurs (NBRANCHj > 1), the water requirements, VOLj, for the canal reach of interest are determined. and then from the next most downstream canal reach being maintained (stops if canal = 0), the needs along this tributary are accumulated. Initially, the number of occurrences of Condition 1 before Condition 2 corresponds to the number of levels beyond the main branch.

When Condition 2 occurs add the total needs for the tributary, DQUiupsc, where inpsc is the canal number of the most upstream reach, plus the volume of water required to maintain canal immediately upstream of iupsc,VOLj. Then, continuing to the remaining canal reaches in the main branch of the canal network, determine the total needs for each of the remaining tributaries. Condition 2 occurs when the needs for all the canal reaches in the branch of interest have been determined. The total number of branches in a canal network corresponds to the number of occurrences of Condition 2.

Each time Condition 1 occurs before Condition 2, accumulate the needs along the tributary in thenext level, beginning with the most downstream canal reach maintained. The number of occurrences corresponds to the number of levels beyond the level at which Condition 2 last occurred.

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