HEC-RAS Unsteady Flow: Storage Areas 2

[KatiLynSki]

I have a very simplistic model which would be easy to model in PondPack unfortunately the state agency reviewing the model will not accept PondPack and requests that the hydraulic model be done using HEC-RAS to provide water surface elevations due to the existing and proposed culverts.

There is a road "causeway" which passes between two sections of a lake (16 acres on the east/upstream and 143 acres on the west/downstream). The downstream outlet to the lake is a dam/wier. The existing opening is a combination (in series, NOT parallel) of a 14'(w) x 8.5' (h) bridge that necks down to a 4' diameter metal culvert. The proposed structure is a double box culvert with each culvert being 7' (w) x 8.5' (h). (In HEC-RAS I am assuming the full length of the existing culvert is the 4' diameter metal culvert.)

In Pondpack I am currently modeling it as follows:

Geometry File:
--Connected to the top of the river reach model: Upstream Storage Area (LakeWEST)with Elevation vs Volume Curve
--River Reach (Road/Culvert) with 4 cross sections and the culvert, 2 cross sections upstream of culvert and 2 downstream of culvert
--Connected to the bottom of the river reach mode: Downstream Storage Area (Lake EAST) with Elevation vs Volume Curve

Unsteady Flow File:
--Each Storage area has a boundary condition type of "Lateral Inflow Hydrograph" with the upstream storage area having the hydrograph associated with the watershed to just that portion of the lake (west) and the downstream storage area having the hydrograph associated with the watershed to the whole lake (east and west)
--Initial Conditions, initial flows are set to 0, initial elevations are set to typical water surface elevation

QUESTIONS:
1. Currently I can not find a way to provide an outlet to the lake. I have a stage flow curve for the outlet weir but no way to connect it to the downstream storage area. As such the water surface elevations computed by the model are excessively high since there is only inflow. Any suggestions as to how to model the outlet of the storage area into HEC-RAS?

2. Any flaws in my logic of setting the storage areas and the river reach in between?

We are meeting with the state agency on Thursday to review the HEC-RAS model. Any guidance is appreciated.

 

[Drew08]

Hec-Ras is intended to model river reaches, not necessarily storage. I would suggest using HEC-HMS which is more versatile than RAS for modeling storage areas. A stage-discharge curve developed in a spread sheet model representing the inter-pond connection can be defined at the outlet of the upstream impoundment, and a stage-discharge curve representing the dam can be added at the outlet of the downstream impoundment. Watersheds can be added to both impoundments.

If you must use HEC-RAS, you may want to define the upstream and downstream impoundments using instream cross-sections, instead of off-line storage areas, with a dam in between, and a dam downstream.

Or, create a single "imaginary" river reach with two off channel connections to both the upstream and downstream storage areas, and connect the two storage areas using a levee (essentially a circular path). Add an "imaginary" instream dam in between the two off channel connections to limit the discharge through this channel forcing the majority of the inflow hydrograph to the upstram storage area.

 

[gbam]

Question, can you use pondpack to establish flow rates along the reach then model the hydraulics through the reach using steady state HECRAS? Will the State accept that? I mention this approach because I see your scenario as an interconnected pond condition, unless I misunderstood.

Good Luck.

 

[MattZ99]

There are two ways to approach this problem:

1) with your current model set up, add another river reach downstream of the downstream storage area. Add in some cross section. To model the outlet structure, put a culvert or inline weir in the downstream river reach.

2) Model the whole system as one river reach. Model the storage areas with cross sections instead of Storage Areas. I have done this with success many times. You will need several, closely spaced cross sections to take into the cross sectional area changes.

I am new to the forum, so if there is a way to private message me, please do so and I will review your model.

 

[KatieTX]

Sorry, off topic --

MattZ99 -- Do you work in southeast Texas?

 

[KatiLynSki]

MattZ99, I actually had come to the same conclusion of your first suggestion shortly before you posted.

My final solution was to add another small reach out of my downstream storage area with the rating curve of the dam. Success! Thank you for your valuable post.

 

[MattZ99]

KatieTX: Yes, I work in Houston at AECOM. Do we know each other?

KatiLynSki: I am glad that you were able to get your model to run. Sometimes, models with storage areas can be challenging.