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Section 3: Runoff

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Hydrologic Considerations for Storm Drain Systems

Show watershed boundaries on the schematic. As inlet locations within the established system are finalized, you can indicate intermediate drainage boundaries. Either show schematically or otherwise describe component parts of contributing watersheds (subareas). See Chapter 4 for discussion of field surveys, and see Chapter 5 for hydrologic considerations.

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Flow Diversions

Generally, a storm drain system should accommodate the natural drainage area. Avoid diversion of flow from one watershed to another. Where diversion of flow has already occurred, you may need to consider the implications of accommodating the diversion. However, it is not the usual practice or aim of TxDOT to divert runoff flows from one major watershed to another. If and when it is unavoidable, you must consider the impacts of flow diversion. You may be required to coordinate with the Texas Commission on Environmental Quality (TCEQ) in many instances, and you should investigate this early in the planning and design process. (See Reference for information on contacting the TCEQ.)

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Detention does not change the total volume of runoff. However, the runoff rates change depending on the characteristics of the flood and the detention facility. Such facilities may be in the form of holding reservoirs, large borrow ditches, and underground storage sumps.

TxDOT has not usually incorporated detention into designed systems because the department’s chief aim is to remove and dispose of runoff as quickly and effectively as possible. With increased development in Texas, greater runoff rates and quantities have occurred, causing the need for larger and more costly drainage structures. The greater rates and quantities may also damage downstream development.

You may incorporate a detention facility into a design for drainage systems to decrease facility costs and diminish possible damages due to the increased runoff rates and quantities. With this aim, many municipalities, counties, and other entities in Texas have begun to require detention as an integral part of drainage design. Additionally, you may need to design a detention system for multiple use, especially for storm water quantity and quality control.

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Determination of Runoff

In a storm drain design, first determine the peak flow runoff. The Rational Method, discussed in Chapter 5, is the method that applies to the vast majority of the types of watersheds that storm drains handle.

The time of concentration in a storm drainage design is comprised of the time required for water to flow from the most distant point of the drainage area to the inlet (called inlet time) and the travel time as the water flows through the storm drain line under consideration (travel time through a conduit). See Procedure to Estimate Time of Concentration in Chapter 5 for more information.

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Other Hydrologic Methods

For the urban area under consideration, the TxDOT designer may need to use a special hydrologic method because of some funding arrangements. For example, if a city is funding the surface drainage facilities, that city may insist on using its own specific hydrologic method. Usually, such special methods are similar to the Rational Method with some minor variations.

Some situations may require the use of some variation of Natural Resources Conservation Service (NRCS) hydrologic estimating methods such as the NRCS TR-55 or TR-20 procedure. (See References for information on contacting this agency.) In other situations, the use of a unit hydrograph procedure may be in order. Refer to NRCS Runoff Curve Number Methods in Chapter 5 for detailed information on the NRCS methods.

Where considerable storage is required in the storm drain system, employ hydrologic routing methods to accommodate peak flow attenuation. Refer to Chapter 5 for information on flood hydrograph routing methods.

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