yes some parts are standing water. you are right - some of the area will not contribute until the wetland depressions are filled up. when the depressions are full then the entire area will contribute. I am having a hard time trying to account for storage etc when determining a "C" factor. I would like to simplify the process somewhat since it would be impractical to survey/model the wetlands. These wetlands feed into a proposed subdivision drainage system and its detention pond.
thanks for your help.
In Florida I've come across two different schools of thought regarding wetlands. Use a high C of 0.9 or 1.0 but allow for some storage (hard to do with the rational method) or estimate a relative C and adjust the Time of concentration.
Open water wetlands (standing water) would be 1.0, However most natural wetlands are a mixture of open water and boggy areas. If you talk to an environmental scientist, they also include "transitional wetlands" which never have standing water (but that's another issue!).
I have used a C of 0.6 to 0.7 with an increased Tc (results in a lower intensity). But you also have to evaluate what your design parameters are. If you are accepting wetland discharge INTO a subdivision system and pond, BE CONSERVATIVE and use a short Tc and C of 0.9 or 1.0. Remember if a large storm hits during the wet season, there would be little or no storage available in the wetlands.
If you are discharging into a wetland system or need to calculate a joint discharge at a point downstream you have a little more fexibility.
Remember rule #1 for subdivision drainage design, would you let your family live there?
I've done some wetland research, and an important determinate in wetland discharge and storage capacity is the outlet configuration. Understand the outlet and you may be able to make simplifying assumptions.
If the outlet is a pipe, you may be able to estimate the hydraulic capacity and use that as an assumed peak discharge, if the storage is adequate to accomodate the design storm without overtopping an embankment.
If the wetland has a "base" water level, you could estimate the stage of water once it has received the runoff volume from the design storm in its catchment area, assuming C=1.0 for standing water and saturated soils. Then, using the "high" water level, compute the discharge through the outlet using culvert nomographs or open channel equations. Broad crested weir could be the approximation if it is a broad outlet that overtops.
Understanding what the outlet consists of and how it functions is key to obtaining any degree of accuracy.
It is much better to use a time series analysis, even if a very simple water budget for a design storm; analyzing wetlands is really way beyond what the Rational Method is intended for or calibrated to.
If this scenario where submitted to me for review, I would disallow any credit for storage or Tc decrease for wetlands, unless storage volume where provided and flow was routed through it. The reasons?
1) You are using this flow to design a detention basin downgradient. You must err conservative. Why not divert the outflow around the basin? Careful though; if this is a perenial stream, then it is waters of your state or province and therefore (in the US) under USACOE juristiction. You can't just divert it.
2) As previously pointed out by lsp01, during times of flood flows, this area will be full (or very soon full) and provide little storage...unless you can demonstrate it through stage storage data. I would not necessarily require survey points, but it would be on you to show storage, so I don't see how else you would do it.
3) As touched on by bltseattle, only if the wetlands has a culvert outfall could you really claim a significant decrease in Tt. Weir flow over a berm could be analyzed, but you will find that instantly and with very little head (far less than a foot for all but the largest watersheds) the peak out will equal, or indeed exceed the inflow. Agian, if there is significant storage, you could see a time lag, but I would insist on seeing it routed.
4) A higher C value through wetlands would have to be shown through hydrogeologic analysis. Why not use SCS methods? There is allowance for soil type, land coverage and ponding built in to SCS methods.
1) Use a C=1.0 for the design HWL of the wetlands and an 0.2 for the balance.
2) Use Water Management District aerial maps with contours to define the wetland, supplement with limited survey data of critical points.
3) Model the wetland system as a standard ponded system, (I use a modified rational method - PULS)
4)Use the output from the wetlands system as part of the input to your larger model - ie use the same time steps so it is easy to add the wetland output to you regular CIA input in a rational method spread sheet. (I use this method to input a Retention Pond A's outflow into my input to Retention Pond B)
You will find that if you use a long broad berm discharge from the wetlands then you will get high discharges and limited or no attenuation when the wetland is full. Much is dependant on your antecedant conditions and the wetland geometry. If you wetland has a culvert outfall, then you could consertively assume peak flow based upon peak stage or model it as previously suggested.
You will find much truth & wisdom in the previous comments.
