Basic Hydraulics: Storm Water Management concepts

Storm Water Defined in Construction General Permit as Rainfall runoff; Snow melt runoff; Surface runoff and drainage. Defines in the HDM 13-2 as Overland flow Flow in ditches Flow in storm drain

Storm Water Management Structural (ditches, ponds, etc.) and non-structural measures to Control quantity (how much) Arrival time (when) Quality (how dirty)

Best Management Practices Practices to mitigate quantity and quality issues Processes Street cleaning Scheduling Devices Detention ponds Rock filter dams Vegetative buffer

Best Management Practices Practices to mitigate quantity and quality issues Processes Street cleaning Scheduling Devices Detention ponds Rock filter dams Vegetative buffer

Best Management Practices Water quality issues tend to dominate design practice. Practices need not be “devices” Construction activities Erosion control

Storm Drain Systems EPA NPDES municipal separate storm sewer systems influence BMP selection Edwards Aquifer Recharge Zone (TNRCC) also influences BMP selection Division of Environmental Affairs

Erosion Control Turbidity (cloudiness) and Total Suspended Solids are used as regulated parameters for Storm Water Management – Erosion control greatly influences these parameters

Erosion Control Flat side slopes in channels, swales, etc. Protection at culvert outlets Berms and similar protective devices Rock filter dams are temporary sediment controls that can be left in place and eventually become a berm that self-seeds and provided hydraulic detention as well as vegetative buffering Ground cover selection

Erosion Control Select route where erosion is unlikely Design slopes flatter than required by soil considerations Reduce area of unprotected soil exposure Reduce duration of unprotected soil exposure Protect bare soil with vegetative cover, mulch, erosion resistant materials Retard (delay) runoff with engineered devices (ponds)

Channels and Chutes Surface channels to carry storm water with minimal erosion – velocities are important Chutes (steep channels) carry water at high velocity – energy dissipation is vital

Quantity (How Much) Management Storage Delays arrival at a location Infiltration Increases rate of water flow into soil

Ditches, curbs, etc. Drains or inlets appearing in ditches and medians are usually grate inlets and are also termed “drop inlets.”

Ditches, curbs, etc. Curbs are the usual roadway bounding feature in urban areas. They may vary in height from negligible (in cases where a roadway has been repeatedly overlaid), and 8”. Curbs serve multiple purposes; they provide minor redirection for errant vehicles, as well as serving as a bounding feature for water running in the roadway as an open channel. Curbs may also provide constraint that allows them to become a part of inlets.

Hydraulics as pre–requisite Generally the hydraulics of storm water management is accomplished by various approaches discussed earlier in the course There is some added effort in velocity management (to control erosion) and sizing storage facilities