Geometry Data

The cross-sections of the Mississippi River (MR) from Tarbert landing to the Passes were taken from the 2010 HEC-RAS study (Davis et al. 2010). For components of the geometry of the model, you can refer to Mallory Davis thesis (Davis et al. 2010). In the previous model, the outflows such as Fort St. Philip and Bohemia Reach were using simplified rectangular cross- sections. These outflows flow estimates were not calibrated. Also, various small outflows in Ostrica reach as well as between Fort St. Philip and Baptiste Collette were not included into the model due to lack of data. These outflows were improved using survey data provided by Lake Pontchartrain Basin Foundation (LPBF) and with the use of Google Earth.

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Figure 5-1 shows the outflows in the Lower Mississippi River (LMR) with the Ostrica and 7-cut weir.

The Bohemia reach extends from RM 31 till RM 44. The Bohemia reach consists of an elevated road built on a natural levee along the east bank of MR until the Ostrica Lock. Most of the reach has irregular height of land which acts as a natural levee allowing flows to discharge during the high stages (discharges) in the Mississippi River. The Bohemia reach was surveyed by LPBF. Figure 5-2 shows the height of land survey which represents the Bohemia reach. The survey data from LPBF was imposed as weir in the Bohemia reach. The Bohemia reach was divided into 3 channels i.e. Bohemia Upstream, Bohemia Intermediate and Bohemia

Downstream. The division was done in the model for easier management of the structure and also to have control over the weir coefficient. The Bohemia Upstream was further divided into 8 smaller channels which corresponded to the existing equivalent channels behind the bohemia weir as captured in the Google Earth. In the previous models, the bohemia reach was used as a lateral structure with the capability to withdraw flows from the MR however it lacked the capability to model for flows coming from the bohemia reach into the river during extreme conditions such as hurricane. Currently, the Bohemia reach has been modeled as an inline structure with the capability to allow flow in to the open water and vice versa.

Figure 5-2: LPBF Survey for Bohemia Reach where the distance is measured from upriver (RM 44) to downriver (RM 31).

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Bohemia U/S Structure 3 Bohemia U/S Structure 4

Bohemia U/S Structure 5 Bohemia U/S Structure 6

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Figure 5-3: Bohemia weirs installed in HEC-RAS Model corresponding to LPBF Survey Data.

Figure 5-4: Bohemia reach with the survey data path plotted in Google Earth image (RM 31 to RM 44).

Bohemia Intermediate Structure Bohemia D/S Structure

Bohemia U/S

Bohemia Intermediate.

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Ostrica extends from RM 22 to around 26. There are couples of cuts that branch off from the MR and have the capability to extract flows. These cuts were not included in the previous models. For the current model, the study focused on a better estimate of flows extracted from MR and thus all major cuts capable of extracting flows were included. Google Earth images were used to measure the length and width of the cuts located in these areas. Then, Lacey’s Regime equations were used to calculate the depth.

In Lacey’s Equation, width is represented by the wetted perimeter (Pw),

2 / 1 67 . 2 Q Pw  Equation 5.1

As the width is found from Google Earth, estimated discharge (Q) can be found for every cut. Lacey uses a silt factor to give the effect of sediment.

 





The D50 is used as 0.18mm as it represents an average particle size in the LMR. The depth (D) is represented by the hydraulic radius (R) which is given by:









The velocity in the channel can be calculated by:

 

17 . 1 f R

V  _s Equation 5.4

This form of Lacey’s equations is in US customary units.

From equation of continuity, the cross-sectional area can be calculated by: RV

P V Q

A /  _w Equation 5.5

For the equivalent channel, an average length of the channel is calculated based on each individual measured data. The cumulative width and depth of the channels are calculated to achieve the equivalent channel.

Figure 4-5 shows the Google Earth image of Ostrica reach with the cuts marked. Figure 4-6 show the HEC-RAS model cross-section used to represent Ostrica.

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Figure 5-5: Ostrica outlets marked on Google Earth image (RM 22 to RM 26).

Figure 5-6: Ostrica equivalent channel cross-section in HEC-RAS model. Ostrica

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Figure 5-7: Fort St. Philip cuts from Google Earth image(RM 18 to RM 21).

Figure 5-8: Fort St. Philip equivalent channel cross-section in HEC-RAS model. Fort St. Philip

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The Fort St. Philip cuts are located around RM 18 to RM 21 in the MR. Figure 5-7 shows the Google Earth image of the cuts in the Fort St. Philip reach. The outlet was again measured for length and width using Google Earth map and the channel dimension was estimated using Lacey’s regime equations. The channel was divided into 3 equivalent outlets which were compiled as a single cross-section for the HEC-RAS model. Figure 5-8 shows the cross-section of Fort St. Philip used in the HEC-RAS model.

The 7-Cut weir extend from RM 11 to RM 18. This reach located between Fort St. Philip and Baptiste Collette contains multiple cuts with 7 significantly visible channels and was thus named as 7-Cut weir. Similar approach of equivalent channels and Lacey’s regime equations was applied for this channel too obtain a useable geometric cross-section in the HEC-RAS model. Figure 5-9 shows the Google Earth image of the cuts in the Fort St. Philip reach. Figure 5-10 shows the cross-section of Fort St. Philip used in the HEC-RAS model.

Figure 5-9: 7-Cut weir outlets marked on Google Earth image (RM 11 to RM 18). 7 Cut weirs

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Figure 5-10: 7-Cut weir equivalent channel in HEC-RAS model.

Figure 5-11: Southwest pass marked with cuts on Google Earth image.

Joseph and Burrwood cuts are equivalent channels included in the model to represent the cuts located in South-west Pass. The upper cuts were combined to form Joseph outlet which is located around 4.5 miles downstream Head of Pass (HOP). Burrwood is the equivalent channel

Joseph (RM -4.5)

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for cuts located at lower portion of the pass which is located at around 14.5 miles downstream of HOP. It is important to include these cuts as they extract significant amount of flow from the pass affecting the head and energy present in the pass.