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  1. env21tech

    WALLER METHOD

    Not sure what the Waller Method is, just keep in mind that for most cases it is adequate to detain about 75% of the rainfall for the 10-yr,1-hr duration. Don't go crazy with computer modeling because the IDF correlations are just exrapolations of very limited rain gage data.
  2. env21tech

    Detention pond routing with variable tailwater - program?

    We use the FHA culvert analysis program that allows tailwater depth to increase with flow rate.
  3. env21tech

    Time of Concentration

    The overland travel time (time to reach the inlet) for a one acre parking lot will be about 100 ft/2fps = 50 seconds. For this reason computer models that require a minimum Tc of 5 minutes are inherently invalid. The Rational Method was never intended to be used with Tc --this misconception was...
  4. env21tech

    Storm Sewer Inlet Calculations

    One guideline for inlet spacing regards accumulation of debris along gutters. Closer spacing will result in a smaller debris pile that needs to be flushed to the inlet. In other cases the spacing is dictated by the terrain -- flat terrain usually results in more PVI's and subsequently closer...
  5. env21tech

    Storm Sewer Inlet Calculations

    The local highway department may have guidelines for maximum inlet spacing. There is no hydraulic criteria -- just Good Engineering Practice. Be careful using the FHA computer Models -- the hydrology may not be valid.
  6. env21tech

    Stormwater time of concentration

    hello blue oak -- my thinking is that we should not use Tc to predict how hard it is going to rain. This prediction should be based on IDF curves.
  7. env21tech

    New to Storm Water - some basic questions

    psmart --the five minute duration would be used for sizing storm sewers and I agree that a longer duration (e.g. 1-2 hrs) is needed to estimate detention. I ususally assume the rainfall hydrograph = the inflow hydrograph because the site will have been saturated by the leading edge of the storm...
  8. env21tech

    Stormwater time of concentration

    Lcruiser -- you are correct. I should have typed " there is no relation between rainfall intensity (what is happening in the sky) and Tc (surface water flow patterns).
  9. env21tech

    Stormwater time of concentration

    There is no relation between rainfall intensity (what is happening in the sky) and Tc(what is happening on the ground). The raincloud will act the same whether it is raining on a parking lot or on a heavily wooded forest. The water generation capability of rainclouds is available in IDF curves...
  10. env21tech

    New to Storm Water - some basic questions

    By all means use the Rational Method and IDF curves (Intensity = average intensity during the duration). Avoid software that requires a relation between Tc(what is happening on the ground) and average rainfall intensity (what is happening in the sky).The raincloud will act the same whether it...
  11. env21tech

    Can you creat IDF Curve for specific storm types?

    (1)IDF data is based on raincloud precipitation and is independent of watershed travel time (what is happening on the ground). (2)Hydro-35 precipitation data appears to be based on sampling durations of 5, 15, and 60 minute. Average rainfall intensity = depth collected /duration of sampling...
  12. env21tech

    Rational Vs NRCS for ponding depth.

    Step 1- assume you will be detaining during the IDF 60-minute duration. The average intensity associated with longer durations is usually a drizzle. Step 2 - rainfall depth for this duration = 60 min x average intensity. This is what you would need to detain if you had no outflow pipe. This is...
  13. env21tech

    Floodway, FEMA, and garage doors!!!

    A flexible rubber flap on the bottom 12" of the garage door would allow floodwater(but bnot critters)to enter the garage, thereby equalizing the water elevations with that outside the garage. This would prevent the garage walls acting like a dam (until they finally buckle).
  14. env21tech

    Curve Numbers for Specific Land Uses

    Depends on the type of vegetation on the forest floor,the slope of the forest floor, and the soil codition (dry, moist, wet). Any CN you use will be an engineering estimate. Flip a coin. Heads = 40, tails = 60.
  15. env21tech

    TR-55 Storm Sewer Program w/ Hydraulic Calc

    The Rational Method will do fine. Use the 5-minute rainfall duration -- this provides 500-1000 acres of raincloud size. Average rainfall intensity for the cloud is given in IDF curves. If your site is relatively flat assume the best you can do is a flow velocity of 3-4 ft/sec. Do not go above...
  16. env21tech

    Modified Rational vs NRCS?

    There is no need to use Tc execept to adjust the runoff coefficient used in the Rational Method. Rainfall intensity will be determined by atmospheric conditions. These conditions are available from rain gage data for 5,15,and 60 minute durations. This data is used to generate IDF Curves. A...
  17. env21tech

    the retention time of oil in Buffer tank?

    (1) assume droplets are 100 microns = droplet rise velocity of 0.10 cm/sec = 0.003 ft/sec (2) 0.003 ft/sec = 0.003 cf/sf-sec = 1.2 gpm/sf =1600 gpd/sf (3) Provide minimum of 0.8 sf of water surface area for every 1600 gpd of flow (4) Use 2.0 sf/gpd to allow for turbulence and poor flow...
  18. env21tech

    CT rainfall 3hr totals storm design

    To do a 3-hr hydrograph use the local IDF curves that plot average rainfall intensity vs duration of the raincloud. Volume = raincloud duration x average intensity. You can do a several durations to develop the hydrograph. For the 3-hr base use 1.6 hrs for the leading edge, 1.4 hrs for the...
  19. env21tech

    Voodoo Hydrology

    Voodoo has apparently prevented drainage engineers from looking up in the sky. Rainclouds are huge - a 5-minimum duration raincloud moving at 12 mph is one mile long!! IDF curves provide the average rainfall intensity for these rainclouds. Engineers are creating site-specific micro-clouds that...
  20. env21tech

    Hydrology Programs

    Your comments are valid.These computer models (TR-55, others) assume there is a relation between concentration time (what is happening on the ground) to rainfall duration(what is happening in the sky). Rainclouds do not care what is happening on the ground. Raincloud behavior (IDF curves) has...

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