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Section 3: Runoff

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Hydrologic Considerations for Storm Drain Systems

As inlet locations are established, the hydraulic designer can indicate intermediate drainage boundaries, and must show schematically or otherwise describe contributing watersheds. See Chapter 3 for discussion of preliminary design activities, and see Chapter 4 for hydrologic considerations.

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Flow Diversions

A storm drain system should mimic the natural drainage pattern. Diversion of flow from one watershed to another should be avoided. When diversion is unavoidable, the impacts of the flow diversion must be considered. DES-HYD should be involved in the design.

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Detention does not change the total runoff volume; however, detention does change the rate of discharge, depending on the characteristics of the runoff and the detention facility. Such facilities may be in the form of holding reservoirs, large borrow ditches, or underground storage.

In the past, detention was not typically incorporated into the design because the Department's policy was to remove and dispose of runoff as quickly and effectively as possible. However, with increased development in Texas, runoff rates and volumes have increased, causing the need for larger and more costly drainage structures. The greater rates and quantities of runoff may also damage downstream development.

A detention facility may decrease facility costs or diminish potential damages due to the increased runoff rates and volumes. With this intent, many municipalities, counties, and other entities in Texas have begun to require detention as an integral part of drainage design. While not specifically required to comply with municipal and county requirements, hydraulic designers for the Department should consider the need for detention in the design to avoid the risk of damage to adjacent properties. A detention system may also be necessary for water quality control.

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Determination of Runoff

The first step in designing a storm drain system is to determine the peak runoff flow. The Rational Method, discussed in Chapter 4, is the method that applies to the vast majority of watersheds for storm drains.

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 (tc, also called inlet time) and the travel time (tt)as the water flows through the storm drain line under consideration (travel time through a conduit). See Time of Concentration in Chapter 4 for more information. The hydraulic designer must be careful to document the actual inlet time and travel time for each segment, because the total time is summed through the system and used for sizing the conduit. The temptation to use a default tc of 10 minutes for every drainage area usually results in the flowrate in the conduit being underestimated.

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Other Hydrologic Methods

On occasion, a special hydrologic method may need to be used. 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 procedures. (See References for information on contacting this agency.) In other situations, the use of a unit hydrograph procedure may be in order. Refer to Hydrograph Methods in Chapter 4 for detailed information on the NRCS methods.

Where considerable storage is required in the storm drain system or detention is being designed, hydrologic routing methods should be employed to accommodate peak flow attenuation. Refer to Chapter 4 for information on Hydrograph Methods.

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