Control of surface water
a. Runoff of surface water from* areas surrounding the excavation should be prevented from entering the excavation by sloping the ground away from the excavation or by the construction of dikes around the top of the excavation. Ditches and dikes can be constructed on the slopes of an excavation to control the runoff of water and reduce surface erosion. Runoff into slope ditches can be removed by pumping from sumps installed in these ditches, or it can be carried in a pipe or lined ditch to a central sump in the bottom of the excavation where it can be pumped out, Dikes at the top of an excavation and on slopes should have at least 1 foot of freeboard above the maximum elevation of water to be impounded and a crown width of 3 to 5 feet with side slopes of IV on 2-2.5H.
b. In designing a dewatering system, provision must be made for collecting and pumping out surface water
so that the dewatering wells and pumps cannot be flooded. Control of surface water within the diked area will not only prevent interruption of the dewatering operation, which might seriously impair the stability of the excavation, but also prevent damage to the construction operations and minimize interruption of work. Surface water may be controlled by dikes, ditches, sumps, and pumps; the excavation slope can be protected by seeding or covering with fabric or asphalt. Items to be considered in the selection and design of a surface water control system include the duration and season of construction, rainfall frequency and intensity, size of the area, and character of surface soils.
c. The magnitude of the rainstorm that should be used for design depends on the geographical location, risk associated with damage to construction or the dewatering system, and probability of occurrence during construction. The common frequency of occurrence used to design surface water control sumps and pumps is a once in 2-to 5-year rainfall. For critical projects, a frequency of occurrence of once in 10 years may be advisable.
d. Impounding runoff on excavation slopes is somewhat risky because any overtopping of the dike could result in overtopoping of all dikes at lower elevations with resultant flooding of the excavation.
e. Ample allowance for silting of ditches should be made to ensure that adequate capacities are available throughout the duration of construction. The grades of ditches should be fairly flat to prevent erosion. Sumps should be designed that will minimize siltation and that can be readily cleaned. Water from sumps should not be pumped into the main dewatering system.
f. The pump and storage requirements for control of surface water within an excavation can be estimated in the following manner:
Step 1. Select frequency of rainstorm for which pumps, ditches, and sumps are to be designed.
Step 2. For selected frequency (e. g., once in 5 years), determine rainfall for 10-, 30-, and 60-minute rainstorms at project site from figure 3-6.
Step 3. Assuming instantaneous runoff, compute volume of runoff VR (for each assumed rainstorm) into the excavation or from the drainage area into the excavation from the equation
Vr = cRA = c – 43,560A (cubic feet) (4-14)
c = coefficient of runoff R = rainfall for assumed rainstorm, inches A = area of excavation plus area of drainage into excavation, acres
(The value of c depends on relative porosity, character, and slope of the surface of the drainage area. For impervious or saturated steep excavations, c values may be assumed to range from 0.8 to 1.0.)
Step 4. Plot values of VR versus assumed duration of rainstorm.
Step 5. Plot pumpage rate of pump to be installed assuming pump is started at onset of rain.
This method is illustrated by figure D-10.
g. The required ditch and sump storage volume V is the (maximum) difference between the accumulated runoff for the various assumed rainstorms and the amount of water that the sump pump (or pumps) will remove during the same elapsed period of rainfall. The capacity and layout of the ditches and sumps can be adjusted to produce the optimum design with respect to the number, capacity, and location of the sumps and pumps.
h. Conversely, the required capacity of the pumps for pumping surface runoff depends upon the volume of storage available in sumps, as well as the rate of runoff (see equation (3-3)). For example, if no storage is available, it would be necessary to pump the runoff at the rate it enters the excavation to prevent flooding. This method usually is not practicable. In large excavations, sumps should be provided where practicable to reduce the required pumping capacity. The volume of sumps and their effect on pump size can be determined graphically (fig, D-10) or can be estimated approximately from the following equation:
Qp = Q – V/T (4-15)
Qp = total pump capacity, cubic feet per second JQ= average rate of runoff, cubic feet per second V = volume of sump storage, cubic feet T = duration of rainfall, hours