Section 4: Quantity Management

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.

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:

pumping stations

detention systems

retention systems

sedimentation basins

hazard spill tanks

bio-filtration systems

outfall appurtenances

outfall channels

man-made wetlands.

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

Table 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.