
HECRAS ANALYSIS OF ARROYO ALAMAR REHABILITATION PROJECT


The Instituto Municipal de Planeación (IMPlan), Tijuana, Baja California,
has among its current projects the rehabilitation
of Arroyo Alamar, a tributary of the Tijuana river. The main objective of the project is to
rehabilitate the Arroyo Alamar and its flood plain to encourage planned land use and preserve
primary hydroecological functions. The project will satisfy a host of urbanplanning needs,
including the preservation of riparian areas, flood management, planned land use, recreation,
landscaping, a green corridor, replenishment of groundwater, improvement of water quality,
and compliance with federal stream zoning regulations.
The project encompasses the 10km reach of Arroyo Alamar, located between the bridge on the toll
road to Tecate to the east, and the channelized reach near the confluence with the Tijuana river to the west (Fig. 1).

Fig. 1 Arroyo Alamar: Project location.


To perform the hydrological and hydroecological design of the rehabilitated channel
includes the determination of flood discharges for selected design frequencies,
and the calculation of watersurface profiles.
The U.S. Army Corps of Engineers
HECRAS (Hydraulic Engineering Center  River Analysis System) model was used to
calculate the watersurface profiles.

2. DESIGN FLOOD DISCHARGES

A hydrological study to determine design flood discharges for 2 to 1000yr frequencies
has been performed by Ponce (2000). In addition, the Gumbel method
was used to calculate the 5000yr and 10000yr flood discharges (Alamar Research Group, 2003).
The adopted design discharges are shown in Table 1.

Table 1. Adopted design discharges.

Return period (yr) 
Design discharge Q (m^{3}/s) 
10 
680 
50 
1140 
100 
1310 
500 
1600 
1000 
1720 
5000 
2140 
10000 
2290 
The HECRAS model calculates the watersurface profile of a channel system, when presented with appropriate upstream
and downstream boundary conditions.
3.1 Length and elevation data
The Alamar Project has established a tentative project alignment with a total channel length of 9880 m.
The upstream point, with invert elevation 80 m, is at the bridge on the toll road to Tecate.
The downstream point, with invert elevation 40 m, is at the confluence with the channelized reach near the confluence with the Tijuana river.
This provides an average channel slope of 0.004048.
3.2 Riversystem schematic
The riversystem schematic is developed by drawing and connecting the various reaches of the system within the geometric data editor.
The project reach, of length 9880 m,
was subdivided into reaches at an equidistance of 988 m,
for a total of 10 reaches and 11 cross sections.
River stations are numbered from 0 to 10.
3.3 Crosssection geometry
A prismatic channel of a chosen crosssectional geometry was adopted for design. The channel consists of
a main channel and right overbank channel (flood plain). The bottom width of the main channel
is 40 m, and the main channel depth is 3.8 m.
The side slopes of the main channel are 2 horizontal to 1 vertical.
The overbank channel is 40 m wide each, with channel depth 2.8 m. and side slopes 2 H:1 V.
The overbank channels drain into the main channel with a 1% transversal slope. Figure 3 shows the channel design.

Fig. 3 Typical cross section in Arroyo Alamar.


In HECRAS, crosssectional data is entered from left to right, looking in the downstream direction.
The left channel bottom xcoordinate was specified as 100 m, and the corresponding crosssectional coordinates
were calculated using a spreadsheet. The xcoordinate left overbank limit is 92.4 m, and the xcoordinate right overbank limit is
147.6 m.
Figure 4 shows typical cross sections generated by HECRAS for the discharge of 680 m^{3}/sec, at cross section 0 (downstream end)
and cross section 10 (upstream end).
Figure 5 shows typical cross sections generated by HECRAS for the discharge of 1720 m^{3}/sec, at cross section 0 (downstream end)
and cross section 10 (upstream end).

Fig. 4 Typical cross sections generated by HECRAS for
the flood discharge of 680 m^{3}/sec.


Fig. 5 Typical cross sections generated by HECRAS for
the flood discharge of 1720 m^{3}/sec.


3.4 Energy loss coefficients
Several types of loss coefficients are utilized by HECRAS to evaluate energy losses. These are
the Manning's n for friction (boundary) loss, contraction and expansion coefficients to evaluate transition
loss, and bridge and culvert loss coefficients. At the present level of approximation,
all secondary energyloss coefficients have been neglected.
Appropriate values of Manning's n are significant to the accuracy of the calculated
water surface profiles. The Manning's n value depends on a number
of factors, including surface roughness, amount and type of vegetation, channel irregularities, channel alignment, scour
and deposition, presence of obstructions, size and shape of channel, stage and discharge, seasonal changes, temperature,
and bed material load.
Manning's n values for inbank channel flow was estimated at 0.035, and for overbank flow at 0.075. These values are consistent
with established practice.
The values of Manning's n recommended by HECRAS
are shown in Table 2.
3.5 Boundary conditions
The boundary condition is necessary to establish the starting watersurface elevation at either
end of the channel system. Since the flow is subcritical, the boundary condition is specified at the
downstream end of the channel system. The average channel slope S_{o}= 0.004048 has been
specified as downstream boundary condition in the present study.
3.6 Flood discharge
Flood discharge information is required at each cross section in order to compute the watersurface
profile. The flow value is entered at the upstream end of the reach, and it assumes
that the flow remains constant until another flow value is encountered with the same reach.
The flood discharges corresponding to 10, 50, 100, 500, 1000, 5000 and 10000yr return
periods were used to calculate watersurface profiles with HECRAS.

The HECRAS model results are shown in Table 2.
Flow depths vary from 3.7 m to 6.9 m; inbank flow velocities vary from 3.87 to 5.77 m/s;
overbank flow velocities vary from 0.76 to 1.64 m/s. In inbank Froude numbers vary from 0.69 to 0.76.
The total Froude numbers vary from 0.71 to 0.79.
For the 10yr design flood discharge, the freeboard is 0.1 m (main channel only).
For the 1000yr design flood discharge, the freeboard is 1.02 m.
For the 10000yr design flood discharge (freeboard hydrograph), the freeboard is 0.1 m.

Table 2. HECRAS results
showing flow depths, mean velocities, Froude numbers, and freeboard for corresponding return periods.

Return period (yr) 
Discharge (m^{3}/s)

Flow depth (m) 
Inbank flow mean velocity (m/s) 
Overbank flow mean velocity (m/s) 
Froude number (inbank) 
Froude number (total) 
Freeboard (m) 
10 
680 
3.70 
3.87 
 
0.69 
0.69 
0.10 
50 
1140 
4.87 
4.62 
0.76 
0.72 
0.79 
2.13 
100 
1310 
5.22 
4.84 
0.95 
0.73 
0.76 
1.78 
500 
1600 
5.77 
5.16 
1.20 
0.74 
0.73 
1.23 
1000 
1720 
5.98 
5.28 
1.29 
0.74 
0.73 
1.02 
5000 
2140 
6.67 
5.65 
1.55 
0.76 
0.71 
0.33 
10000 
2290 
6.90 
5.77 
1.64 
0.76 
0.71 
0.10 
A hydrological design has been accomplished for the Arroyo Alamar rehabilitation project, Tijuana, Baja California.
The design used the U.S. Army Corps of Engineers' HECRAS model to determine flow depths, mean velocities, inbank and total Froude numbers, and freeboards
for a typical crosssection featuring a compound prismatic channel with right overbank side channel (flood plain) (Fig. 3).
The flow depths and mean velocities obtained from the model, shown in Table 2,
are consistent with established practice.

http://ponce.sdsu.edu/protected42/alamar_sustainable_architecture_hecras_six_point_run.html
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