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Section 4: Quantity Management

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Impacts of Increased Runoff

For TxDOT applications, storm water quantity management mitigates the potential effects of increased runoff rates and volumes that can often accompany development, including highway construction. These effects include increased erosion and sedimentation, increased pollutant loads, and increased flood levels and velocities. By assessing the potential for increased runoff volume and, if necessary, taking measures to offset such increases, the department can minimize the potential for detrimental impact due to storm water runoff.

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Storm Water Quantity Management Practices

Storm water runoff can be collected and disposed of through an integrated system of facilities. Storm drain systems collect the runoff water initially, and it is then handled by the following:

The primary options for handling or mitigating increased runoff are detention, retention, outfall appurtenances, and outfall channels.

Measures for controlling urban storm runoff can be classified as structural or non-structural. Structural measures require the construction of certain facilities, such as detention basins for temporarily storing storm runoff, thus reducing and delaying runoff peaks. The hydrologic methods for analysis of detention and retention systems are detailed in Chapter 4, Reservoir Versus Channel Routing. Chapter 10 details storm drain system planning and design considerations, Chapter 11 gives pumping stations design and operation considerations, and outfall channel design and operation considerations and procedures are detailed in Chapter 7, Channel Analysis Methods.

Non-structural measures include such practices as land use management to strategically locate impervious areas so that the resulting total hydrograph peak is less severe. TxDOT rarely is involved in non-structural measures in association with transportation projects. Table 13-1 lists some of the measures for reducing and delaying urban storm runoff recommended by the NRCS.

Anchor: #LILENKNITable 13-1: Measures for Reducing and Delaying Urban Storm Runoff

Area

Reducing runoff

Delaying runoff

Large flat roof

Cistern storage

Rooftop gardens

Pool storage or fountain storage

Sod roof cover

Ponding on roof by constricted downspouts increasing roof roughness:

  • Ripples roof
  • Gravelled roof

Parking lots

Porous pavement:

  • Gravel parking lots
  • Porous or punctured asphalt

Concrete vaults and cisterns beneath parking lots in high value areas

Vegetated ponding areas around parking lots

Gravel trenches

Grassy strips on parking

Grassed waterways draining parking lot

Ponding and detention measures for impervious area:

  • Rippled pavement
  • Depressions
  • Basins

Reservoir or detention basin

Residential

Cisterns for individual homes or group of homes

Gravel driveways (porous)

Contoured landscape

Groundwater recharge:

  • Perforated pipe
  • Gravel (sand)
  • Trench
  • Porous pipe
  • Drywells

Vegetated depressions

Planting a high delaying grass (high roughness)

Gravel driveways

Grassy gutters or channels

Increased length of travel of runoff by means of gutters, diversions, etc.

General

Gravel alleys

Porous sidewalks

Hed planters

Gravel alleys



Of the measures listed in Table 13-1, detention basins or ponds, either dry or wet, are the most commonly used practices for controlling storm runoff. These facilities serve to attenuate flood peaks and flood volumes. Retention basins also are used in some instances when the total runoff volume can be stored permanently.

Refer to Chapter 4 for details of hydrograph routing by the Reservoir Versus Channel Routing. The extent to which storage is provided is left to engineering judgment. You should aim to balance the risk of impact with the costs of providing storm water quantity control.

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