HEC-RAS Unsteady Flow: Storage Areas
HEC-RAS Unsteady Flow: Storage Areas
(OP)
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.
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.





RE: HEC-RAS Unsteady Flow: Storage Areas
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.
RE: HEC-RAS Unsteady Flow: Storage Areas
Good Luck.
RE: HEC-RAS Unsteady Flow: Storage Areas
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.
RE: HEC-RAS Unsteady Flow: Storage Areas
MattZ99 -- Do you work in southeast Texas?
RE: HEC-RAS Unsteady Flow: Storage Areas
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.
RE: HEC-RAS Unsteady Flow: Storage Areas
KatiLynSki: I am glad that you were able to get your model to run. Sometimes, models with storage areas can be challenging.