In a feasibility study, I have one of the objectives to estimate quantum of recharge from a natural channel at various discharges to be released in the channel. I have performed infiltration tests in the bed and leveling survey of x-sections at appropriate locations.
Would this estimation of recharge to groundwater be possible in HEC-RAS?If yes, steady or unsteady model runs would be appropriate?
Kind guidance is required from my dear colleagues.
I dont think HECRAS can do this. This is typically addressed with water balance or study of water movement with wells, Darcys law, substrate hydraulic conductivity, etc. Some stream channels are more connected as direct conduits and interchange than others. Confined aquifer may have recharge areas large distance from location.
HEC-RAS is a model to address hydraulic issues. To address ground water recharge is applicable a water balance model like Wasim-ETH or Coupmodel (there are a lot models more).
HEC-RAS is a one-dimensional steady/unsteady flow hydraulic model designed to aid in channel flow analysis and floodplain determination. The basic computational procedure is based on an iterative solution of the energy equation. On the other hand, for groundwater computation is based on Darcy's Law, which soil conductivity is the main parameters in the computation. This parameter is not included in the HEC RAS calculation. So, I think HECRAS can not be used to estimated groundwater recharge.
You are absolutely right and I am grateful for your replies.
I am planning to find out the steady state discharge that can be accommodated in the channel and then by using wetted perimeter and infiltration rate I would be able to estimate recharge to the aquifer at various discharges in the the channel.
Thanks all for the guidance.
Steady model runs should be sufficient for finding the wetted perimeter for a given discharge unless your channel geometry is incredibly complicated or you need to determine the wetted perimeter over the course of a storm hydrograph. I would also acknowledge what others are saying here, that without understanding the interaction between the water table and the channel it will be difficult assess infiltration rates.
I agree to the approach as mentioned by Poyom Riles (thanks and appreciations) that with steady runs for different discharges would help in estimating respective wetted perimeter. The watertable is deep enough that infiltration below the channel would be governed by hydraulic gradient of 1, i.e. vertical flow under gravity. Therefore, filed measurements of infiltration rates would help in estimating the recharge to groundwater. The important question arising here is that the bank infiltration would certainly be less than bed infiltration. So, what factor be utilized for bank infiltration to make it representative of the filed situation.
I haven't done previous research on this but I would think you would vary the infiltration rate based on the head pressure at the soil water interface. This would mean that infiltration rate would increase as the water gets deeper.
Mohammad, you say the groundwater table is deep enough to assume that the vertical infiltration can be treated as gravity flow. In this case, the rate of infiltation will depend only on three things: the wetted area of the river, the depth of water in the river (the driving head), and the capacity of the river bed to infiltrate the water.
The wetted area and water depth are obviously highly transient conditions that depend on the flow rate in the river. A series of steady state simulations plus a time estimate of the relatve periods may be sufficient for a first approximation.
However, the capacity of the riverbed to infiltrate may also be dynamic. During low flows, sediments will accumulate in the river bed limiting infiltration. During high flows, these sediments may be mobilized resulting in higher infiltration rates until the bed sediment can re-establish itself.
Further complicating matters, the infiltration rate may vary across the cross-section. Areas that are normally dry will develop macropores that will be highly conductive. If the dry soils have a high clay content, then the soils may crack, resulting in high infiltration initially, and deceasing infiltration as the soils wet.
Finally, you need to carefully evaluate your initial assumption of vertical gravity flow. If the infiltration rate is high enough, then it is likely that a local groundwater mound will develop beneath the river. This could significantly reduce total infiltration during the wet season, and may even reverse the flow direction for a period of time. That is, the groundwater may discharge to the river for some weeks or month after the end of the wet season.
Thanks Graham for such a detail. I have taken all your points.
My study area is plain with a slope of 1 in 4000, and deep groundwater levels. Also, With such a smaller recharge from the channel there is no possibility of reversal of flow direction (from groundwater to channel).
I am grateful to every body for their valuable contributions to my question.
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