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  • 1.  Hydrologic condition of concern (hcoc)

    Posted 07-16-2014 12:04 PM
    Hello,

    I am currently working on an industrial development project in which the project site is located within an area of hydrologic condition of concern (hcoc). By using underground storage chambers or infiltration basins, I am able to control the volume of storm water captured and stored but I am not sure what techniques I could use to mitigate the time of concentration (Tc) and the flowrate (Q).

    Hope anyone familiar with this could assist me.

    Thank you.

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    Luis Prado, EIT
    Design Engineer
    Santa Fe Springs CA
    (323)513-3087(323)513-3087
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  • 2.  RE: Hydrologic condition of concern (hcoc)

    Posted 07-17-2014 09:11 AM
    Infiltration and/or alternate use will take care of the volume; the time of concentration and flow rate can be managed using any variety of outlet structures (weirs, orifices, riser pipes, etc.) The usual menu includes a small low-flow outlet, then adding structures to push and pull various segments of the post-FFQ curve to the existing condition. There are lots of resources out there for this, check out for instance the San Diego County Drainage Design Manual that we updated in around 2005 and subsequent stormwater manuals from the county and Caltrans; jurisdictions in the northwest (Seattl/King County, Washington and Portland OR) also have good resources for this. Marc Schulte P.E., D.WRE, CPSWQ Project Manager St Paul MN (858) 614-5010 -------------------------------------------
    Original Message


  • 3.  RE: Hydrologic condition of concern (hcoc)

    Posted 07-18-2014 11:13 PM
    Thank you all for your helpful responses! ------------------------------------------- Luis Prado S.M.ASCE Santa Fe Springs CA (323)513-3087 -------------------------------------------
    Original Message


  • 4.  RE: Hydrologic condition of concern (hcoc)

    Posted 07-29-2014 02:43 PM
    I have designed a system of retention ponds (22 in all), swales, soakaways, infiltration trenches, all interconnected to form a system of drainage network for a development in Fleet, Hampshire, England. Elvetham Heath is a natural conservation area of an area about 63ha, with a large wetland pond at the downstream end that received greenfield runoff.
    The entire system mimic the original greenfield state, the biggest challenge being the design of roadway with proper drainage to road gullies which then connect either to soakaways or swales. The pipe flows connecting intermediate interconnecting ponds are regulated by hydrobrakes and of course pipe sizes and the storage volumes.
    The software I used was Microdrainage, but there are many other software out there that can do similar job.
    The system works very well as you can see from this report attached.

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    Johnny Lee P.Eng, CP, M.ASCE
    Kitchener ON
    1519 8948591
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    Original Message


  • 5.  RE:Hydrologic condition of concern (hcoc)

    Posted 07-17-2014 11:53 AM

    One solution to mimic the undeveloped condition's Tc is to route the developed runoff flows along a long path - so grade the site to take all flows the long way around the site to the designed outlet. Even better - use grassed Swales or channels which slow the flows and provide opportunity for some infiltration to groundwater and sediment trapping in the grass.
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    David Bauer P.E. A.M.ASCE
    Fort Collins CO

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    Original Message


  • 6.  RE: Hydrologic condition of concern (hcoc)

    Posted 07-17-2014 11:54 AM
    The storage of the chambers and/or basins in combination with the outlet structure allow you to adjust the OUTFLOW Tc and Q.  The inflow to the storage is determined by the developed conditions. 

    The outflow structure limits how fast the flow leaves the storage so you can approximate the pre-development hydrograph.  Normally a low flow drain is used, sized to not clog due to debris.  The opening may be large enough to pass a can or liter bottle or else have a trash rack.  Two or more openings would pass various sized storms by limiting the max flow to the pre-development peak.  Storage volume would have to be sized to hold the design storms.  Definitely an iterative process as changes in storage depth may change outflow rate.  Outflow design should be coordinated with structural design for buildable structure - simpler is better. 

    Depending on size of storage, many example designs are available at Federal, State, large municipalities as well as some pre-cast or other vendors.  Many structure vendors are listed in stormwater related publications.

    Item usually forgotten is that extended moderate flows (in channel) may cause more erosion downstream than passing the post-development peak (overbank flows for short period). 

    Check for timing of peaks downstream to avoid flooding due to multiple extended discharges from other storage areas.  This problem cam be an issue in our area since multiple green field developments may discharge to the same stream network.

    Remember every structure will need maintenance (sediment & debris removal, mowing), repair (infiltration layer replacement), and eventual major overhaul and/or removal.   Allow for access & inspection.  Don't allow buildings, etc. on top of anything that may eventually collapse - pipes, storage/infiltration units, etc.

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    David Moughton P.E., D.WRE, M.ASCE
    SR PROJ MANAGER
    XCG Consultants
    Cincinnati OH
    (513) 231-1931
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    Original Message


  • 7.  RE: Hydrologic condition of concern (hcoc)

    Posted 07-17-2014 02:37 PM
    Luis: Tc is mainly the result of the length, surface roughness, and slope of overland sheet flow.  Paving, grading, and adding drain inlets and pipes results in shorter, smoother, steeper sheet flow, and shorter Tc, which in turn gives you higher peak Q.  Onsite detention is a common solution, and if you can add a gravity outfall to your underground storage chambers or infiltration basins, they can work just like surface detention, to attenuate peak Q.

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    Martin Farber P.E., D.WRE, M.ASCE
    Consultant
    Sacramento CA
    (707)253-9606
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    Original Message


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